Sample size of the Forestry Community Training Center and The Staff of Baluran National Park (BNP).
\\n\\n
Released this past November, the list is based on data collected from the Web of Science and highlights some of the world’s most influential scientific minds by naming the researchers whose publications over the previous decade have included a high number of Highly Cited Papers placing them among the top 1% most-cited.
\\n\\nWe wish to congratulate all of the researchers named and especially our authors on this amazing accomplishment! We are happy and proud to share in their success!
\\n"}]',published:!0,mainMedia:null},components:[{type:"htmlEditorComponent",content:'IntechOpen is proud to announce that 179 of our authors have made the Clarivate™ Highly Cited Researchers List for 2020, ranking them among the top 1% most-cited.
\n\nThroughout the years, the list has named a total of 252 IntechOpen authors as Highly Cited. Of those researchers, 69 have been featured on the list multiple times.
\n\n\n\nReleased this past November, the list is based on data collected from the Web of Science and highlights some of the world’s most influential scientific minds by naming the researchers whose publications over the previous decade have included a high number of Highly Cited Papers placing them among the top 1% most-cited.
\n\nWe wish to congratulate all of the researchers named and especially our authors on this amazing accomplishment! We are happy and proud to share in their success!
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Alteration of different protein kinases can result in remarkable changes in these processes. Moreover, these protein kinases are frequently recognized as oncogenic and can be crucial for the survival and spread of cancer cells. Because of the fundamental role of protein kinases in cell biology and their function in numerous sarcomas and cancers, an intensive search for new kinase inhibitors in academia and industries has been enduring for the last two decades. Protein kinase has become the most imperative and commercial class of drug target which is attracting pharmaceutical industries to spend 30 % of their current research investments only in developing kinase inhibitors for various therapeutic implications. This is exemplified by the fact that 75 drugs targeting protein kinase have been clinically approved to date. More than 100 kinase inhibitors are in the final stages of development and likely to be approved in the coming years. 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His major area of research interests are drug-design, polymer-drug conjugates for targeted delivery to cancerous cells and CNS, antimalarial therapeutic agents, natural product chemistry and green chemistry approaches for chemical synthesis. Dr. Singh has over 15 years of teaching experience and guided 01 PhD and 18 PG students. He is currently guiding 02 PhDs and 01 M. Pharmacy students. He has published more than 60 peer-reviewed SCI-indexed scientific research papers of total impact factor more than 100 in various Chemistry and Pharmacy Journals including the European Journal of Medicinal Chemistry, Biorganic Chemistry, Biomedicine and Pharmacotherapy, European Journal of Pharmaceutical Sciences, Mini-Reviews in Medicinal Chemistry, Pharmaceutical Research, Medicinal Chemistry, Bentham, J. Enzyme Inh. Med Chem. (Informa Healthcare), Res. Chem. Intermediate (Springer), Arabian Journal of Chemistry, etc as main or corresponding author. He currently serves on Editorial Advisory Board Member of 09 Peer-reviewed International Journals including SCI indexed MRMC and PeerJ. He has received Publon Award 2016, 2017 for top reviewer and 'Publon Excellent Peer Reviewer Award” for outstanding reviewing more than 150 research papers of different International Journals of ACS, RSC, Springer, Elsevier, Dove, Informa, and Bentham of Impact Factor varies from 1.0 to 9.5. He has also to his credit 46 National and International Conference Abstracts, 2 Book, 5 Best Paper Presentation Awards, 1 Travel grants to attend Int. Conf. and 5 Research Projects funded by Govt. Agencies, India. He is also on the panel of International Reviewer for reviewing Research Proposal for Royal Society Grants. He is also serving as PUBLON ACADEMY MENTOR and BENTHAM BRAND AMBASSADOR.",institutionString:"Punjab Technical University",position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"0",totalChapterViews:"0",totalEditedBooks:"0",institution:{name:"Punjab Technical University",institutionURL:null,country:{name:"India"}}}],coeditorOne:null,coeditorTwo:null,coeditorThree:null,coeditorFour:null,coeditorFive:null,topics:[{id:"6",title:"Biochemistry, Genetics and Molecular Biology",slug:"biochemistry-genetics-and-molecular-biology"}],chapters:null,productType:{id:"1",title:"Edited Volume",chapterContentType:"chapter",authoredCaption:"Edited by"},personalPublishingAssistant:{id:"194667",firstName:"Marijana",lastName:"Francetic",middleName:null,title:"Ms.",imageUrl:"https://mts.intechopen.com/storage/users/194667/images/4752_n.jpg",email:"marijana@intechopen.com",biography:"As an Author Service Manager my responsibilities include monitoring and facilitating all publishing activities for authors and editors. 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Venkateswarlu",coverURL:"https://cdn.intechopen.com/books/images_new/371.jpg",editedByType:"Edited by",editors:[{id:"58592",title:"Dr.",name:"Arun",surname:"Shanker",slug:"arun-shanker",fullName:"Arun Shanker"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"878",title:"Phytochemicals",subtitle:"A Global Perspective of Their Role in Nutrition and Health",isOpenForSubmission:!1,hash:"ec77671f63975ef2d16192897deb6835",slug:"phytochemicals-a-global-perspective-of-their-role-in-nutrition-and-health",bookSignature:"Venketeshwer Rao",coverURL:"https://cdn.intechopen.com/books/images_new/878.jpg",editedByType:"Edited by",editors:[{id:"82663",title:"Dr.",name:"Venketeshwer",surname:"Rao",slug:"venketeshwer-rao",fullName:"Venketeshwer Rao"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"4816",title:"Face Recognition",subtitle:null,isOpenForSubmission:!1,hash:"146063b5359146b7718ea86bad47c8eb",slug:"face_recognition",bookSignature:"Kresimir Delac and Mislav Grgic",coverURL:"https://cdn.intechopen.com/books/images_new/4816.jpg",editedByType:"Edited by",editors:[{id:"528",title:"Dr.",name:"Kresimir",surname:"Delac",slug:"kresimir-delac",fullName:"Kresimir Delac"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}}]},chapter:{item:{type:"chapter",id:"66123",title:"The Effect of Forest Institution Connectedness, Incentive Participation Program, and Social Capital on Public Participation and Welfare as Mediators of Forest Management in Baluran National Park",doi:"10.5772/intechopen.84674",slug:"the-effect-of-forest-institution-connectedness-incentive-participation-program-and-social-capital-on",body:'A national park, as a forest conservation area, has a variety of flora and fauna which can be relied upon to ensure the human survival for now and future [1]. The majority of these parks have now faced threats and interferences such as encroachment, and illegal cultivation continues to increase over time [2]. Threats and disturbances in these areas are caused by various factors, namely (1) the institutions role in forest conservation management and local population participation level of the are still not optimum (especially in the case of those living around the forest); (2) the lack awareness about the conservation area is still very low among local people; (3) the education level of local people is low; and (4) there is a lack of agricultural land [1, 3].
The forest destruction in Baluran National Park includes: (1) forest fires in 2014, with 132 fires covering an area of around 2005.90 ha. Rather than natural factors, the main causes of forest fires are local people not acting responsibly, a lack of security personnel guarding the forest, and weak law enforcement. Forest fires impact heavily on the flora and fauna. (2) Clearing activities as a result of 400 ha being devoted to agricultural plants business. (3) Timber theft (as well as theft of firewood, fruit tart, hazelnut, gebang trees, ornamental fish and over grassing) especially in the Labuhan Merak resort. (4) Cattle grazing is a problem that is quite prominent, especially in the areas of Karangtekok, Labuhan Merak, and Balanan with about 3450 ha. Cattle grazing (cows and goats) is widespread, with an average of 1447 head of cattle per day. As a result of this illegal grazing, the soil becomes solid, which is harmful to plants and vegetation that could potentially be survival disruption of the park, as well as deer, antelope, and bison (the unique wildlife of Baluran National Park). (5) Local transmigration settlements since 1976, covering an area of 57 ha in Pandean area of Wonorejo village. (6) Illegal encroachment and the tilling of the soil. (7) Hunting of wildlife by people with firearms, snares, poison, and sap that often occurs during the dry season. Various factors affect the behavior and movement patterns of animals, including a limited source of drinking water for animals, especially in the dry season. Based on the above phenomena, this paper focuses on the damaged forest in the Baluran National Park, caused by the poor level of public participation [4, 5, 6].
There are some previous studies which discuss public participation in the forestry management program. These include: (1) studies which explained the factors affecting public participation in forestry management, because of the role of forest institution connectedness by Baynes et al. [7], Muro and Namusonge [8] and Lise [9], (2) the quality of forest institutions as good governance will be able to create conditions of security, belief, trust, and economic welfare by Hans-Jurgen [10] and Akib et al. [11], (3) the effects of incentive participation program for social forestry management in increasing public participation by Adhikari et al. [12], Djamhuri [13], and Kaseya and Kihonge [14], (4) incentive participation program effects in relation to welfare Rahut et al. [15], William and Ayuk [16], Das and Sarker [17], (5) the social capital effect on public participation by Sara et al. [18] and Sharpe [19], (6) social capital effect on public welfare by Grootaet [20] and Narayan and Pritchett [21], (8) how welfare effect on public participation in development programs Rahut et al. [15] and Akamani and Hall [22].
The issue of differences in measurement of incentive participation program is a gap in this research. Adhikari et al. [12] measured incentive participation program with six indicators, they are: (1) access to forests and availability of forest products, (2) financial support to supplement household income, (3) social security and cohesion through local institution building, (4) investment in local community infrastructure and development, (5) well-defined & enforced property rights over forest resources assigned to the users, and (6) payment for environmental services. The research’s purposes of Adhikari et al. [12] are: (1) to determine the relationships between different incentive participation program and the level of public participation of user group members; (2) to explore how households might respond to any changes in the incentive participation program, in terms of their decision to participate in common property resource governance; and (3) to propose/recommend how organizational incentive participation program can be better integrated in order to induce more effective public participation of users in the governance and management of property resources. The indicators of public participation were measured based on (1) membership length; (2) representation on the executive committee; (3) level of public participation in meetings, (4) in decision-making, and (5) in implementation; and (6) overall benefits.
While Djamhuri [13] measured incentive participation program with seven indicators, they are: (1) forest village population); (2) villages forests/WPH; (3) number of forest village community (LMDH) trustee board members; (4) percentage of Tumpang Sari Farmers on the LMDH trustee board; (5) tree coverage on foundation of the LMDH; (6) current tree coverage; (7) trustee board members attendance of routine meetings. The indicative numbers of LMDH trustee board members and percentage of Tumpang Sari Farmers on LMDH trustee board consist of: (1) formal education; (2) household annual income; (3) use of feed/fodder from state forest land; and (4) use of firewood from forest land. Djamhuri [13] said Tumpang Sari is an incentive participation program which is traditional in forest management. Government and society integration provides a better incentive participation program in the hope that the public will be will contribute in the state forest management.
Kaseya and Kihonge [14] measured incentive participation program with three indicators, they are: (1) civic education, (2) financial incentives both transport and lunch allowances, and (3) scheduling of forums/meetings. The study result was corroborated by the findings from the open interview which indicated that 62.5% of the respondents concurred that financial incentives are offered to participants. Measurements of incentive participation program in this research refers to [12], but its indicators are based on research object conditions.
The second gap of this research is the differences of social capital’s measurements done by Grootaet [20] and Narayan and Pritchett [21]. Grootaet [20] measured social capital into six dimension of social capital, they are: (1) density of membership, (2) heterogeneity index, (3) meeting attendance, (4) decision making index, (5) membership dues, and (6) community orientation.
Narayan and Pritchett [21] measured social capital into six variables, they are: (1) heterogeneity members, (2) inclusiveness members, and (3) performances members. Social capital’s indicators consist of: (1) membership, (2) characteristic of membership; (3) values and individual’s behaviors.
Measurement of social capital in this study refers to Grootaet [20], who measures social capital as a factor in the reduction of poverty and increase in prosperity, but indicators of social capital of this research based on research object condition. Welfare provision would increase the role of public participation in development. Increased public participation will reduce transaction costs and the cost of control, raise output and further improve the welfare of the community.
Rahut et al. [15] suggested increasing public participation in collaborative forest management (CFM) while adding welfare as a mediation variable which will affect social capital and also affect public participation.
The novelty of research are: (1) to examine the integrative model with purposes to insure weather forest institution connectedness, incentive participation program, social capital and public welfare still have positive significant effect on public participation; (2) to analyze the level of public participation based on participation in planning program, implementation, benefit-sharing; and evaluation and monitoring of the forest conservation management in Baluran National Park.
After all, the purposes of this study that were presented here will be to analyze and explain: (1) effect of the forest institutions’ connectedness to public participation; (2) how welfare mediates forest institution connectedness to public participation; (3) the effect of incentive participation programs on public participation; (4) how welfare mediates the effects of incentive participation programs on public participation; (5) social capital’s effect on public participation; (6) how welfare mediates social capital’s effect on public participation; and (7) welfare’s effect on public participation.
Data were collected during July–December 2017 through interview, research questionnaires, and documentation. Interview was conducted to determine the respondents’ answers to a questionnaire relating to the variables that have been used in this study.
The sampling method is proportional random sampling. The unit analysis is the heads-of-household who are members of the forestry community training center (120 people) and the staff of Baluran National Park with 50 people. All of them are 170 respondents in total (see Table 1). The construct validity of reflective indicators were tested based on convergent validity, discriminant validity, composite reliability [23]. Variable with formative indicators were tested based on the values of full collinearity variances inflations factor.
The method of data analysis used in this study is structural equation modeling using WarpPLS 5.0. This research is based on working with numbers, and the data are tangible, analyzed using statistics to test hypotheses or answer specific research questions and to make predictions that a particular variable affects other variables [24].
To test mediation roles the causal-step approach of Baron and Kenny was used. The best way to test for mediation effects is by counting the Variance Accounted For (VAF) value, which can determine the indirect effect relative to the total effect [25].
According to Baron and Kenny [26] the causal step approach has four mediation effects, they are: (1) nonmediation, if VAF value < 20%; (2) partial mediation, if VAF value is around 20 ≤ 80%; (3) full mediation, if VAF value > 80%; and (4) suppressed mediation, if the direct effect sign changed after inclusion of the mediation variable.
The validation of reflective indicators was done through: (1) convergent validity; (2) discriminant validity; and (3) reliability test consists of (a) indicator reliability; and (b) consistency internal reliability both composite reliability and Cronbach alpha [23].
Convergent validity testing is performed to identify the items of instrument indicators as indicators from a latent variable (see Table 2). The convergent validity test result shown that all of the outer loading values are more than 0.6 (>0.6). At last, it can be seen that this research has met the requirements of the convergent validity [23, 25].
Sample size of the Forestry Community Training Center and The Staff of Baluran National Park (BNP).
A discriminant validity test (Table 3) was performed after those for convergent validity. It is to identify the validity of instrument items in a model [27]. The discriminant construct validity test will meet the criteria of the discriminant validity if the square roots of AVE are higher than the variable correlation score. KLM (X1) has a square root of AVE 0.793 is more than its correlation 0.669, 0.281, 0.669. ISN (X2) is 0.938, its correlation scores are 0.669, 0.299, and 0.681. MDS (X3) is 0.755, and its correlation scores are 0.281, 0.299, and 0.304. PAR (Z1) is 0.892, and its correlation scores are 0.669, 0.681, and 0.304. The criteria of discriminant validity are therefore met [23, 25]
Convergent validity test.
Reliability test (Table 4) consist of indicator reliability and consistency internal reliability both composite reliability and Cronbach alpha. The reliability test shows that all of the outer loadings are >0.6, and p-value is <0.001 less than 0.05, which means all instruments are reliable [23, 25].
Discriminant validity test.
Consistency internal reliability was tested both for composite reliability, and Cronbach alpha. The consistency of internal reliability values in this study also more than 0.60 (>0.60), and are thus reliable (Table 5).
Indicators reliability test.
The composite reliability coefficients values in this research are more than 0.70 (see Table 5). All variables meet reliability requirements [23]. The value for KLM (forest institution connectedness) is 0.846, ISN (incentive participation program) is 0.967, MDS (social capital) is 0.868, and PAR (public participation) is 0.940.
Internal consistency test (Table 6) can be proved by the exact Cronbach alpha values. The Cronbach alpha are as follows: KLM (X1) is 0.770, ISN (X2) is 0.951, MDS (X3) is 0.809, and PAR (Z1) is 0.914. The criteria for internal consistency are therefore met [25].
Composite reliability coefficients.
The indicator reliability test (Table 7) was done in order to ensure the quality of variable with formative indicators. This test result can be gained from the significant of weights or indicator weights. All of the formative indicators have met the requirement of indicator reliability with a p-value less than 0.05 (<0.05) and all instruments are valid [23, 25].
Internal consistency test (Cronbach’s alpha of each variable).
Variable with formative indicators will meet the requirements of collinearity, if the value of variances inflation factor (VIF) is <3.3. KSJ (Welfare) is a latent variable of welfare with four formative indicators, and has the value of variance inflation factor is 2.527 less than 3.3.
Goodness of fit (inner model) can be evaluated based on R-squared, adj. R-squared, Cronbach alpha, Avg. Var. Ectrac, full collinearity VIF, and Q-squared value (see Table 8). R-squared with high value means the model is good and R-squared can be used for response variable.
Indicator reliability test of indicator weights.
The results of R-squared for the public participation (PAR) is 0.979 which means that the contribution of the variables incentive participation program (ISN), social capital (MDS), and welfare (KSJ) to the effect on public participation (PAR) is 97.9%, and the remaining 2.1% is attributable to another variable outside the research model.
Composite reliability value and Cronbach alpha can be used to evaluate research instruments. Based on the output, the composite reliability coefficients are 0.846 for KLM, 0.967 for ISN, 0.868 for MDS, 0.907 for KSJ and 0.940 for PAR. They are more than 0.60 and the Cronbach alpha coefficients are 0.770, 0.951, 0.809, 0.861, and 9.14. All of them are more 0.70 for all variables. Therefore, all variables in this research have met the reliability criteria.
The average variances extracted (AVE) is used to evaluate the discriminant validity, with the criterion that values must be >0.50. The AVE values are as follows: (1) forest institution connectedness (KLM) variable is 0.543; (2) incentive participation program (ISN) variable is 0.880; (2) social capital (MDS) variable is 0.571; (3) welfare (KSJ) variable is 0.713; and (4) public participation (PAR) variable is 0.795. All the variables met the AVE value criterion >0.50 and meet the discriminant validity.
Full collinearity VIFs is a complete collinearity test consisting of vertical and lateral multicollinearity. Lateral collinearity is a collinearity between a predictor latent variable and criteria variables and can be used to test the common method bias. The criterion for the full collinearity test values <3.3. This research has met the full collinearity requirements for all variables; they are 2.228 for KLM, 2.254 for ISN, 1.126 for MDS, and 2.527 for KSJ.
Q-squared is used as a predictive test of the relation between the predictor latent variables and the criterion variables. The Q-squared result can be negative, but the R-squared result must be positive. The estimation result of this output above shows good predictive value; at 0.708 and 0.852, values are more than zero (Table 8).
The outer loading values are used to know indicator’s weight of every variable. Indicators with high outer loading values show they are strong variable measures (Table 9). Forest institutions’ connectedness variable is consist of five indicators (accountability, transparency, belief-based relationship, forest rules, and information access) are categorized as not good condition (3.86 < 4.00). The highest outer loading is forest rules (0.803) and means score (4.00) is reflected as good condition. But the lowest mean score is accountability (3.71) is reflected as not good condition and effects the level of public participation.
Output latent variable coefficients.
Incentive participation program variable consist of four indicators (incentive participation programs of training, agriculture tools, cash payment, and agriculture land use) are categorized as not good conditions (3.81 < 4.00). The highest outer loading is the incentive participation program of agriculture tools (0.986), mean score (3.97), but it is still reflected not good condition (<4.00). The lowest mean score is the incentive participation program of training (3.67) is reflected as not good condition and effects the level of public participation, especially in developing the quality of human resources.
The social capital variable is consist of five indicators (reciprocity, social norms, network interaction, level of trust in the community group, and buffer villages group donations) are categorized as not good conditions (3.93 < 4.00). The highest outer loading is social norms (0.819) and means score is (4.21) is reflected as good condition. But the lowest mean score is buffer village group’s donation (3.64) is reflected as not good condition and effects the level of public participation.
Welfare variable is consist of five indicators (household income, household education, household health, and household supporting facilities) are categorized as not good condition (3.81 < 4.00). The highest outer loading is family income (0.877) and mean score (3.86) is reflected as not good condition and effects the level of public participation (<4.00). The lowest mean score is family supporting facilities (3.73) is reflected as not good condition and effects the level of public participation.
Public participation variable is consist of four indicators (participation in planning program, participation in implementation, participation in benefit-sharing, and participation in monitoring and evaluation) are categorized as not good condition with average score is 3.85 or less than 4.00. The highest outer loading is participation in implementation (0.915) and mean score (3.85). The lowest mean score is participation in planning program (3.68) is reflected as not good condition.
Path coefficients and p values (Table 10) and direct hypothesis (Table 11) that: (H1a) KLM (forest institutions’ connectedness) does not have a positive significant effect (0.087) on public participation, with p-value 0.166; (H1b) KSJ (Welfare) mediates the effect of KLM (forest institution connectedness) on public participation (0.552), with p-value <0.001; (H2a) INS (incentive participation program) has shown a positive significant effect (0.196) on public participation (p-value 0.013); (H2b) welfare (KSJ) mediates the effect of ISN (incentive participation program) (0.273) on public participation with p-value <0.001; (H3a) MDS (social capital) has a positive significant effect (0.141) on public participation, with p-value 0.056; (H3b) welfare (KSJ) mediates the effect of MDS (social capital) on public participation (0.177), with p-value 0.023; (H4) welfare (KSJ) has a positive significant effect (0.782) on public participation with p-value <0.001.
Outer loading value of variable.
Path coefficients and P values.
Forest institutions’ connectedness does not have a positive significant effect (0.087) on public participation, with p-value 0.166. Because p-value 0.166 is more than 0.05 (0.166 > 0.05), H1a is not accepted. This test result does not provide empirical support for the findings of Baynes et al. [7], Muro and Namusonge [8], and Lise [9].
The incentive participation program has a positive significant effect (0.196) on public participation, with p-value 0.013. Because p-value 0.013 is less than 0.5 (0.013 < 0.05), H2a is accepted. The results support the theory of incentive participation programs of Robbin [28], Adhikari et al. [12], Djamhuri [13], and Kaseya and Kihonge [14].
Social capital gives significant positive effect on public participation (0.1401), with p-value 0.056. Because p-value 0.056 is less than 0.05 (0.056 < 0.5), H3a is accepted. The test results support the theory of social capita [29], as well as supporting the empirical research of Sara [18] and Sharpe [19].
Path coefficient indirect effect (Table 12) shows that welfare mediates the effect of forest institution on public participation (0.552), with p-value <0.001. Because p-value <0.001 is less than 0.05 (<0.001 < 0.5), hypothesis H1b is accepted. The test results provide empirical support for the work of Hans-Jurgen [10] and Akib et al. [11].
Direct hypothesis.
Welfare mediates incentive participation program on public participation (0.273), with p-value < 0.001, less than 0.05 (<0.001 < 0.5). Hypothesis H2b is thus accepted. The test results provide empirical support for the work of Rahut et al. [15], William and Ayuk [16], Das and Sarker [17].
Social capital by the mediation of welfare has a positive significant effect on public participation (0.177), with p-value 0.023, less than 0.05 (0.023 < 0.5). Hypothesis H3b is therefore accepted. The results are related to the social capital theory [20]. In addition, Fukuyama [30] added that the social capital and the level of welfare are closely related in a community or nation [29]. This result provides empirical support for the research of Grootaet [20], Narayan and Pritchett [21].
Welfare contributes significant positive effect on public participation by 0.782 on public participation, with p-value <0.001. Because p-value <0.001 is less than 0.05 (<0.001 < 0.5), hypothesis H4 is accepted. This test provides empirical support for the research of Rahut et al. [15] and Akamani and Hall [22].
To test mediation effect, this research uses Baron and Kenny’s causal-step approach. Baron and Kenny [26] using causal step approach which has four mediation effects, they are: (a) first step, directional hypothesis if the results are significant/positive; (b) second step, the indirect hypothesis was tested whether it is significant/positive; (c) third step, test mediation effects using VAF (Variance Accounted For) with the criteria: VAF value >80% means full mediation, 20% ≤ VAF ≤80% means partial mediation; and VAF < 20% means no mediation. The mediation effect is significant/positive if p-value indirect effect is less than 0.05 [25].
Figure 1 shows that all of the direct effects are significant/positive because the p-values are less than 0.05. Then the indirect effects (mediation variables) are included, as shown in Figure 2.
Direct effect without including mediation.
Indirect effect including mediation.
Figure 2 shows all of the indirect effects are significant/positive because p-values are less than 0.05. The third step is to test mediation effect by using the VAF formula. The formula of VAF = (p12 × p23)/(p12 × p23 + p13). The results of the mediation test using the VAF method are as follows:
Forest institutions’ connectedness effect on public participation in the mediation of welfare is significant and positive with p-value <0.001 (<0.05)
VAF = (0.552 × 0.782)/(0.552 × 0.782 + 0.087).
VAF = 0.431/0.518.
VAF = 0.832.
VAF = 83.2%.
This means that welfare mediates the effect of forest institutions’ connectedness on public participation as a full mediation.
Incentive participation program’s effect on public participation in the mediation of welfare is significant and positive, with p-value <0.001 (<0.05)
VAF = (0.273 × 0.782)/(0.273 × 0.782 + 0.196).
VAF = 0.213/0.409.
VAF = 0.520.
VAF = 52.0%.
This means that welfare mediates incentive participation program’s effect on public participation as a partial mediation.
Social capital’s effect on public participation in the mediation of welfare is significant and positive, with p-value 0.023 (<0.05)
VAF = (0.177 × 0.782)/(0.177 × 0.782 + 0.141).
VAF = 0.138/0.279.
VAF = 0.494.
VAF = 49.4%.
This means welfare mediates social capital’s effect on public participation as a partial mediation.
Based on the descriptive analysis, both direct and indirect hypothesis results (Tables 11 and 12), for all variables can be summarized in Table 13.
Path coefficient indirect effect.
The analysis of public participation in this research is based on the characteristics of five buffer villages. They are Wonorejo, Sumber Waru, Sumber Anyar, Watu Kebo and Bajul Mati.
The buffer villages have potential to be developed into bigger villages. Management of regions is required in order to avoid disturbing the forest conservation in Baluran National Park.
The contribution of nontimber forest product (NTFP) to family income is around 19.79% up and 61.44% of their total annual income (Table 14). This research also empirically supports research from Baluran [5] and Syafi’i [31]. The total income of user’s forest product in buffer villages of Baluran National Park is about Rp. 100,900,000 a year.
Hypothesis test results, summary of direct and indirect effect.
The people in the buffer villages have not been widely involved in the forestry management planning (shown in Table 15). Their participation in decision making in the meeting of the forestry planning program by forestry community training center is 28.2%, and by Forest Institution of Baluran National Park is 20.6%.
Contribution of nontimber forest product (NTFP) on family income.
The members of Forestry Community Training Center participated by giving inputs of the forestry planning program in BNP (23.5%) and giving efforts and actions of the forestry planning program (17.6%) as shown in Table 16.
Public participation in the forestry planning program in Baluran National Park.
The members of the Forestry Community Training Center have participated in the benefit-sharing from forestry management to increase their family’s income with value 21.8%, and have participated in forest conservation management with value 17.1% (Table 17).
Public participation in implementing program.
Table 18 shows that 17.6% of members of Forestry Community Training Center has participated in the evaluation of Baluran Forest in Baluran National Park, and 22.4% have participated in the monitoring of forest conservation.
Public participation of benefit sharing.
According to Cohen [32], the level of public participation is high when people involved in four stages of the management process. They are (1) program planning participation; (2) actuating participation; (3) benefit-sharing participation; and (4) evaluation and monitoring participation. Scores > 21 indicate high level, 17–21 medium level, and <17 low-level participation [32].
Table 19 shows that public participation from the five buffer villages (Wonorejo, Sumber Waru, Sumber Anyar, Bajul Mati, and Watu Kebo) is low level at 47.1%, the middle level at 33.5%, and high level 19.4%.
Public participation in evaluation and monitoring.
Classification of public participation in forestry management in Baluran National Park.
Based on the research findings, several conclusions can be stated as follows. Public welfare is the most important factor in forest management. Especially in the Baluran National Park, that the public welfare is the main factor affecting public participation. That is why, the public participation will increase if: (1) there is a good relationship between the forest institution connectedness and local people and make them welfare and better in their life than before; (2) the forest institution provide incentive participation program that can increase the local people’s welfare and better in their life; (3) there is a good social capital that can increase the local people’s welfare because they are more having skills, experiences and productivity; (4) the public welfare always increase and make their life better; (5) public welfare is the most important factor to increase public participation in forest management; (6) the contribution of nontimber forest product (NTFP) to family income getting increased; (7) the forest institutions give opportunities to member of forestry community training center in the forestry planning program, implementing program, benefit sharing, and evaluation and monitoring. Classification of public participation in forestry management in the Baluran National Park shows that the public participation needs to be increased (19.4%).
Based on the findings of this study, the following recommendations are proposed:
The forest institution of Baluran National Park should improve its relationship between the institution connectedness and local people. Because of this, the institution should know what the local people needs.
The forest institution should increase the incentive participation program (incentive participation program of training), because it effects the level of public participation;
The social capital should be improved to create the local people’s welfare by giving incentive participation program of training in order to improve the skills, experiences and productivity of local people;
The forest institution of Baluran National Park should improve the public welfare by focusing it on each planning program of forest management;
The contribution of nontimber forest product (NTFP) to family income must be increased;
The forest institutions give opportunities to member of forestry community training center in the forestry planning program, implementing program, benefit sharing, and evaluation and monitoring. Classification of public participation in forestry management in the Baluran National Park shows that the public participation needs to be increased (19.4%).
Future research should develop socio-demography variable as a predictor variable of public participation in optimizing the level of public participation as many researchers using it in their study.
We first thank Indonesia Endowment Fund for Education and Training Ministry of Finance (LPDP), The Republic of Indonesia funding my study in doctoral management and business, University of Brawijaya. We thankful also to the professionals involved in forest management in the Baluran National Park, and the local key informants, who shared their views and experiences for this research.
Magnesium (Mg) and Mg alloys are materials of great technological interest. Its low density (approximately 1.7 g cm−3) even lower than that of aluminum, combined with good mechanical properties makes them the lightest structural materials [1]. Mg and its alloys are used for a number of different applications such as biomaterial for biodegradable implants, electrode battery material for primary and secondary Mg batteries and casings for electronic devices, among others. Furthermore, the increasing demand of more environmentally sustainable transports, in particular the lightweighting of vehicles, makes Mg alloys excellent candidates for the automotive sector [1].
Despite the increasing technological possibilities of Mg alloys its use has been limited so far due to its high reactivity in aqueous solutions. Mg exhibits the lowest standard reduction potential among all structural metals (approximately −2.4 V vs. SHE). This results in very high corrosion rates when in contact with water [2].
There are a number of methods to assess the corrosion rate of Mg. As with most metals, the most common are weight loss measurements. This simple and well-established method is based in determining the difference in weight of the specimen under study before and after it is immersed in an aggressive electrolyte for a certain amount of time [3]. However, this method presents some important limitations [4]. The first one is that it only provides the average corrosion rate, not allowing for differences in the oxidation kinetics to be determined. Secondly, it can lead to inaccuracies in corrosion rate determination as a consequence of a poor or an excessive corrosion product removal post immersion.
Another common procedure to assess the corrosion rate of Mg alloys is the hydrogen collection method. In this method the rates associated with the HER in a quiescent electrolyte are determined [5]. The primary cathodic reaction in Mg dissolution in the absence of an external polarization is the hydrogen evolution reaction (HER). This is due to the very low corrosion potential (Ecorr) exhibited by Mg and its alloys in solution (between −1.3 and − 1.8 V vs. SCE), well below the reversible potential for the HER (Erev,H) that creates a large overpotential for the HER [1]. Under these circumstances the oxygen reduction reaction (normally diffusion limited) does not control the cathodic kinetics and the HER dominates the rates associated with the overall reduction process. Given that at the Ecorr the rate of oxidation is equal to the rate of reduction, the corrosion rate of Mg can be easily assessed by recording the evolution of H2 volume with the time of immersion. Please note that in this case, in contrast with the weight loss measurements, the Mg dissolution kinetics is determined indirectly by assessing the rates associated with the main cathodic reaction.
There are two main experimental arrangements for measuring the evolution of H2 from a dissolving Mg specimen, as shown in Figure 1. In the traditional setup (see Figure 1a) the H2 gas volume is collected by a burette attached to a funnel that are placed upside down over the sample and filled with electrolyte. As Mg dissolves the evolved H2 travels upwards to the top of the burette, displacing the solution so the volume of gas can be easily monitored over time [6]. Even though this setup has been common for a long time it presents important limitations [4]. Recently, Fajardo and Frankel developed a more sensitive setup originally proposed by Curioni [7] in which the evolution of hydrogen is monitored by attaching the specimen to a fully immersed container coupled with a high precision balance [5] (see Figure 1b). The gas that accumulates at the top of the container changes the apparent weight measured by the balance due to a variation in the buoyant force produced by the volume of solution displaced by the evolved H2.
Schematic of the experimental setups for hydrogen gas collection [1]: (a) volumetric method [6], and (b) gravimetric method [5].
Besides weight loss and H2 volume collection measurements, the electrochemical methods continue to be the most popular in corrosion science and Mg is no exception. This is essentially due to the electrochemical nature of aqueous corrosion. They represent a reliable, rapid and often simple way of assessing many aspects of the corrosion of metals such as kinetics, passivity, etc. Furthermore, the application of electrochemical techniques for the study of Mg corrosion can be used independently or simultaneously with the non-electrochemical methods previously described (among others). In this chapter the main electrochemical techniques that are commonly used in the study of Mg corrosion will be presented. Global electrochemical methods such as potentiodynamic/potentiostatic and galvanodynamic/galvanostatic polarization measurements will be described and critically discussed. Conventional electrochemical impedance spectroscopy (EIS) will be also covered. Furthermore, localized electrochemical methods have shown to be highly useful for the study of many aspects of Mg corrosion and corrosion protection. For this reason, localized electrochemical techniques such as the scanning electrochemical microscopy (SECM), the scanning vibrating electrode technique (SVET), localized electrochemical impedance Spectroscopy (LEIS) and the scanning Kelvin probe (SKP) will be reviewed, among others.
Mg corrosion in aqueous solutions is in nature an electrochemical process and as such, electrochemical techniques are a powerful tool to assess the corrosion rate of Mg and Mg alloys. In this section, the main global electrochemical techniques used in the study of Mg corrosion, either by the application of a direct current (DC) or an alternating current (AC), will be discussed.
In these electrochemical methods a unidirectional flow of charge is passed through the electrochemical cell either by controlling the voltage and monitoring the current, or vice versa. A three electrode configuration is commonly used. The specimen under study acts as the working electrode, a reference electrode is used to measure the potential of the working electrode, and a counter or auxiliary electrode allows to close the electrical circuit. Finally, a potentiostat/galvanostat is employed to control the electrochemical cell.
This is a voltage control technique in which the working electrode is polarized at a fixed rate over a range of potentials (PDP) or remains at a constant value during the entire time of experimentation (PSP). The current flowing through the cell in response to the generated electric field is then recorded. PDP allows to determine the kinetics of the total anodic and cathodic processes when the electrode potential is scanned at potentials above and below the Ecorr, respectively. This is clearly shown in Figure 2 where the PDP plots of a Mg specimen with 2 different purities are presented [2]. Furthermore, PDP measurements allow for any passive behavior to be detected. This is defined by the potential region in a V vs. log i plot (V being potential and I being current density) where the current remains constant with the increase in the applied anodic potential.
PDP curves of high purity and ultra-high purity (HP and UHP, respectively) Mg in NaCl solution [2].
If the electrochemical system is controlled by activation (i.e. the rate limiting step is the charge transfer reaction), it is possible to determine the corrosion current density (analog to the corrosion rate) by extrapolating the straight lines shown in the cathodic and anodic branches of a PDP plot to the corrosion potential. These lines are called Tafel lines and normally extend through a region in a range of about ±50 to ±250 mV from the Ecorr [8]. An example of this protocol is shown in Figure 3. In this case the Tafel extrapolation was carried out on pure Mg in phosphate-buffered saline (PBS) using a rotating disc electrode (RDE) [9].
Tafel extrapolation on PDP curves of pure mg under static and rotating conditions in PBS [9].
Instantaneous corrosion rate can also be determined by the linear polarization resistance (LPR) method [10]. In this case, the electrode potential is scanned over a much smaller range (normally ±10 mV vs. Ecorr) where a linear-like behavior is expected under activation controlled kinetics [8]. Figure 4 shows the linear polarization resistance plots of a Mg-Y-RE Mg alloy where the grain size varied in a range from 70 to 0.7 μm [11]. The linear regions expected close to the Ecorr are clearly shown. The slope of the linear region is defined as the polarization resistance (Rp) and is inversely proportional to the corrosion rate, which can be calculated then using the Stern-Geary equation (see Section 3.2) [8]. The major advantage of the LPR method is that it can be considered as non-destructive given the small polarization applied to the electrode in comparison to an extended PDP measurement where the Tafel regions want to be determined.
Linear polarization resistance plots of different conditions of Mg–Y–RE alloy in 3.5 wt.% NaCl [11].
However, Tafel-extrapolation and the LPR method are not trivial for Mg due to the increasing ohmic potential drop resulting from the continuously increasing flow of current and resistance associated with the HER (occurring both under anodic and cathodic polarization) [1]. At this point is necessary to comment that Mg and its alloys contradict standard electrochemical kinetics, showing increasing rates of HE with increasing anodic polarization [2]. This phenomenon, historically referred to as negative difference effect (NDE), opposes the expected behavior for an electrochemical system. According to standard electrochemical kinetics (as exemplified by the Butler-Volmer equation) the rate associated with an electrochemical half reaction should decrease exponentially with the increase in polarization of the opposite polarity [12]. As a consequence of this anomalous HE, ohmic potential drop effects dominate the shape of the PDP plot, difficulting the identification of the Tafel regions to carry out the extrapolation or distorting the linear-like behavior expected for a LPR measurement [3].
In PSP measurements a constant potential is applied to the working electrode. This method is normally less common than PDP. However, it is useful in some cases like, for example, for the study of the cathodic activation of Mg under anodic polarization [13]. Figure 5 shows the evolution with time of the measured current density for high purity Mg under PSP at an anodic potential (i.e. more positive than its Ecorr). As observed, the electrode undergoes a polarity reversal, exhibiting a net cathodic behavior after about 20 min of polarization even if the applied potential is anodic.
Net current density measured during PSP of high purity Mg at −1.6 VSCE in NaCl solution [13].
As opposed to the potential-controlled methods, GDP and GSP measurements are current control techniques. This means that current is either scanned over a certain range or applied at a constant value (i.e. GDP and GSP, respectively) and the potential is measured [8]. Please note that according to standard electrochemical kinetics potential and current are mutually dependent.
Even though galvanodynamic polarization is not a suitable method when the passive behavior of a metallic surface wants to be assessed, GDP and GSP measurements are particularly useful for the study of certain aspects of Mg corrosion. One example is the investigation of the anomalous HE exhibited by anodically polarized Mg. Current control allows for an easy comparison between the current associated with the evolution of H2 and the applied current density [2, 14, 15]. This has been traditionally evaluated in Mg and Mg alloys using GSP, where a constant anodic current density flows through the electrochemical cell and the hydrogen gas volume evolving from the Mg sample is collected. Figure 6a shows the volume of H2 measurements as a function of time determined from gravimetric measurements on ultra-high purity Mg in NaCl solution using GSP under the application of different anodic current densities [14]. From the steady-state rates during GSP the HE current density values can be calculated (see Figure 6). The reason for the preferential use of GSP instead of GDP is that it has not been until recently that a hydrogen collection method with a suitable temporal resolution has been available (see Section 1) [5]. Since the development of the gravimetric method for HE, GDP measurements have shown to reliably monitor the real-time evolution of H2 from a Mg surface. Using GDP instead of GSP allows for an easy determination of the HE current density in a single GDP measurement by interpolation in a HE current density vs. applied current density plot [16] (see Figure 7). This has shown to be particularly useful when short timescales are needed or when limited amount of material is available and a large range current densities need to be applied.
(a) Volume of H2 determined from gravimetric measurements as a function of time for an ultra-high purity Mg electrode in NaCl solution during a GSP measurement and (b) their corresponding HE current density values (right) [14].
H2 evolution rates for a high purity Mg electrode in 0.1 M citric acid buffer (pH 3) solution during anodic GDP as a function of the applied current density [16].
Another advantage of using current control methods such as GDP and GSP instead of potential control techniques is that the sample under study can be more reliably polarized before ohmic effects dominate the electrochemical response of the system. Under potential control the large ohmic potential drop associated with the intense HE exhibited both during cathodic and anodic polarization difficult an accurate polarization to a particular potential value. Furthermore, the high reactivity of Mg in aqueous solutions makes it behave like an ideally non-polarizable electrode (i.e. an increase of several orders of magnitude is shown within a small range of applied potentials above the Ecorr), limiting the region over which the electrode can be effectively polarized. Consequently, if current is controlled instead of potential it is possible to accurately expand the study of the electrochemical behavior of Mg and Mg alloys over a wider range of polarization.
It is also interesting to mention that, due to the anomalous HE exhibited by Mg during anodic polarization, the current densities registered by the potentiostat do not resemble the true dissolution kinetics of Mg. This evolution of hydrogen is a cathodic process and as such, consumes electrons. The source of electrons on the anodically polarized surface is the Mg specimen. Please note that these electrons are consumed at the electrode surface and do not flow through the potentiostat. Consequently, the Mg extra dissolution associated with the anomalous HE is not accounted in the current density measured in a simple polarization experiment. To accurately assess the true Mg dissolution kinetics, it is necessary to simultaneously account for the real-time HE occurring during polarization. This way, the true current density associated with Mg oxidation will be given by the sum of the net current density measured by the potentiostat (electrons that flow through the electrochemical cell) and the HE current density (consequence of the anomalous HE and measured using a suitable hydrogen collection method) [5] (see Figure 8).
GDP curve of a high purity Mg electrode in NaCl solution as a function of the measured potential [5].
In summary, direct current electrochemical methods are a powerful technique to evaluate the electrochemical behavior of Mg and its alloys. They allow for the reaction kinetics and corrosion rate to be determined as well as to assess passivity. Furthermore, they are useful when the evolution of hydrogen is investigated. However, it is necessary to be particularly careful with the ohmic potential drop that is difficult to compensate since they may distort the shape of a polarization curve leading to inaccurate results.
EIS is one of the most widely used technique for investigating in situ the corrosion mechanisms and surface films developed on Mg and alloys specimens in corrosive environments [1, 3, 4, 17, 18, 19, 20]. EIS studies are also useful to evaluate the corrosion rate of Mg [1, 3, 17] and to rank the corrosion protective ability of Mg alloys [20].
Its most outstanding and well-known advantages are its relative ease of use, the relatively short time taken for measurements, the use of relatively cheap and simple equipment, quantitative nature, high accuracy and reproducibility of the results [18]. Apart from the quantitative information provided by this technique, the occurrence of corrosion in Mg alloys can also be indicated by the emerging of EIS inductive characteristics long before visual changes can be observed using traditional exposure tests [21].
While polarization curve technique is destructive to the specimen and cannot serve for prediction of the long-term corrosion rates of the material [4], the surface properties of the metal or metal alloy remain similar after each EIS measurement since signals applied have low amplitude (the deviation from corrosion equilibrium was ±5 to ±10 mV) and do not alter corrosion potential [18, 21]. The non-destructive character of the impedance technique [19] allows continuous monitoring of the progress of corrosion process in situ, with instantaneous measurements of the corrosion rate and can provide information on the changes of the mechanisms of degradation for the Mg alloys during the immersion period [17, 18, 19]. EIS has been considered as an ideal method to evaluate the durability of these alloys [20].
A major shortcoming with conventional EIS measurements is associated with the lack of spatial resolution, as the measured impedance results are attributed to the electrochemical response of the global properties of the corroding system, reflecting an averaged electrochemical behavior of the whole electrode [22]. This is a major limitation in the investigation of localized corrosion processes such as passivity breakdown, pitting corrosion and the breakdown of coated systems [22].
Equivalent circuit models using passive electrical engineering and physics circuit elements have commonly been used in order to convert frequency response data to corrosion properties (e.g. resistance and impedance) [4]. However, one of the major weaknesses of this analytical approach arises in how to select the equivalent circuits that not only fit the values, as often multiple equivalent circuits may fit the same data [4], but also provide a meaningful interpretation of the studied interface with quantitative parameters [4].
In our EIS studies about the corrosion of Mg alloys, we reported Nyquist plots characterized by two loops (Figure 9a), a capacitive loop at high frequencies (HFs) and an inductive loop at low frequencies (LFs) [18] which can also be observed in corresponding Bode plots (Figure 9b and c). It is generally agreed that the capacitive loop at high frequencies is always related to the charge transfer resistance as well as the effect of ionic double layer capacitance of the electrode. The equivalent circuit shown in Figure 10a was used to simulate the two-time constant impedance spectra in Figure 9a. In this circuit, Rs represents the electrolyte resistance, R1 is the charge transfer resistance, and CPE1 represents the non-ideal capacitive behavior related to the electrical double layer [18]. An inductor (L) and a resistance (RL) have been included to represent the inductive response appearing at low frequency [17, 18, 19, 20].
(a, d) Typical Nyquist and (b, c, e, f) bode plots for Mg alloys.
(a) Equivalent circuit used for fitting two-time and (b) three-time constants impedance spectra.
Most of EIS studies about the corrosion of Mg alloys reported impedance diagrams rather complex, exhibiting three loops (Figure 9d), a capacitive arc in the high frequency (HFs) region, a capacitive arc in the middle frequency (MFs) region, and one inductive loop in the low frequency (LFs) region [23, 24, 25, 26]. This behavior can be confirmed using Bode plots (Figure 9e and f). In this case, an additional R2/CPE2 model element was introduced in the equivalent circuit (Figure 10b). Some studies suggested that the capacitive loop in the HFs may be ascribed to the charge transfer process at the double layer formed at the surface film while the capacitive loop in the MFs accounts for mass transport processes [21, 24, 25]. However, the interpretation of the two characteristic capacitive loops in the high and medium frequency ranges is controversial and unclear [23, 26].
Stern and Geary [10] shown that the that the polarization resistance (Rp) is inversely proportional to the corrosion current density in cases when metals behave as electrochemically active
where B is the Stern Geary coefficient that is a function of the cathodic and anodic Tafel slopes (βc and βa) [17, 19].
The use of the Stern-Geary equation (1) for the quantitative determination of the corrosion rate for Mg and its alloys presents two main challenges: the precise knowledge of the B constant and the value of resistance obtained from fitting of the EIS spectra selected for the estimation of icorr.
As already commented (Figure 9a and d), the impedance diagrams for Mg usually are characterized by a well-marked inductive loop at LF [17]. The inductive behavior is often associated with the occurrence of pitting corrosion caused by anodic dissolution of the Mg and linked to the relaxation of adsorbed intermediates [27]. The disappearance of the inductive loop is caused by the formation of protective corrosion layers on Mg surface which inhibit the pitting corrosion [28]. However, the precise chemical species responsible for the characteristic inductive loop are still unclear [17] and its interpretation remains controversial [19].
The Nyquist diagrams of AZ31 specimens in NaCl 0.6 M solution display one semi-circle followed by an inductive loop at lower frequency region (Figure 11a). After the first hour of immersion in 0.6 M NaCl solution, the charge transfer resistance values, Rt, increased markedly, as shown in Figure 11b related to the formation of a corrosion layer with major protective ability. Finally, Figure 11c compares the variations of EIS-estimated corrosion rate with immersion time obtained by integration of the current density (icorr) vs. time data determined by the evolution of reciprocal Rt over time and the B´ values calculated via correlation with gravimetric measurements (Table 1). It should be noted that corrosion rates determined from parameter Rt quantitatively agree with the corrosion rate calculated by using independent hydrogen evolution measurement over the whole duration of the measurement (Figure 11c).
(a) Variation in Nyquist plots for AZ31 sample with immersion time in 0.6 M NaCl solution; (b) evolution of the resistance (Rt) value with immersion time; and (c) variation in corrosion rate (mm.year−1) as a function of immersion time obtained from hydrogen evolution measurements and EIS during 14 days of immersion in 0.6 M NaCl.
‘Apparent’ Stern-Geary coefficient calculated from Rt resistance performed in 0.6 M NaCl solutions.
Corroding metallic surfaces present heterogeneous surfaces and that influence their electrochemical activity, including highly reactive sites and/or passive regions. This is the case of Mg and Mg alloys. So, in order to be able to study the different processes that take place on an Mg corroding surface at the micrometer scale (i.e. corrosion initiation, corrosion propagation, corrosion inhibition by smart coatings, etc.) the use of localized electrochemical techniques can provide crucial information not available using solely global electrochemical methods.
Mg, as previously mentioned, is a very reactive metal that readily reacts with water, producing large amounts of H2 gas. This H2 evolution leads to a change in the natural convection of the electrolyte and thus in the electrochemistry of the system under study. The exposition time to the aggressive medium is another important factor, since the increase in pH of the solutions, due to the corrosion processes, causes the precipitation of Mg(OH)2, leading to a further change in the electrochemical activity of the material. In this section the advantages and challenges faced by localized electrochemical techniques to characterize Mg corrosion, will be highlighted, as this remains a complex task.
At present, two different methodologies are applied for local electrochemical investigation:
Scanning techniques, where the whole metal surface is either immersed in aqueous electrolyte [Scanning Electrochemical Microscopy (SECM), Scanning Vibrating Electrode Technique (SVET), Localized Electrochemical Impedance Spectroscopy (LEIS)] or exposed at high humid atmosphere [Scanning Kelvin Probe (SKP)].
Methodologies where only small areas are exposed to the electrolyte using mini or micro capillary electrochemical cells [Scanning Droplet Cell (SDC), Flowing electrolyte type SDC (FT-SDC), Mini Cell System (MCS)].
The scanning techniques can perform a relatively rapid mapping of the studied areas but, generally, highly diluted electrolytes with low conductivity are required in order to achieve good local resolution. Furthermore, the scanned area cannot be locally polarized. The methods of the second group, on the other hand, have the main advantage that concentrate electrolytes can be used and the current resolution is improved down to the pA to fA range, allowing the study of local dissolution processes in the nm-range. Moreover, any DC or AC electrochemical technique can be used to study the surface with localized resolution. The main disadvantage of these methods is, in general, the slow mapping process. Furthermore, the surface conductivity of the studied metallic material plays an important role in the local resolution achieved. Let’s continue firstly, showing the applications of these capillary electrochemical techniques and secondly those of the scanning techniques.
These techniques basically consist in the miniaturization of a three electrode electrochemical cell. The SDC the electrochemical cell is formed by a micro reference electrode, micro counter electrode and a microcapillary filled with the electrolyte solution that will be in contact with a microarea of the working electrode, which is the metallic surface [29]. In certain designs, which are the case of the FT-SDC, the electrolyte solution can also flow through the microcapillary, allowing fresh solution to be brought to the substrate surface. In new configurations of the scanning droplet microscope (FT-SDCM) [30] the gas bubbles or reaction products formed on the surface of the working electrode can be removed using a constant electrolyte flow, which is extremely useful for the study of Mg and its alloys. This represents an improvement with respect to previous experimental setups, because the cell can operate with stagnant or flowing electrolyte. In the study of the heterogeneous microstructured AZ91 alloy, potentiodynamic polarization scans were recorded using microcapillaries of 5–10 μm diameter [31]. The scans recorded at lowest current were associated with the β phase, meaning that this phase was the most corrosion resistant. These results highlight the suitability of the SDC technique for monitoring the electrochemical activity of microstructural feature avoiding the interference from the bulk of the alloy. For the study of the new smart coatings developed for Mg corrosion protection, it is necessary increase the capillary diameter to reduce the big resistances (MΩ) provided by the coatings when measured in areas of 5–10 μm diameter. The Mini Cell System (MCS) with a geometry pencil like incorporates a three electrode cell with a capillary diameter of 700 μm which allows separate the kinetics of damaged and undamaged areas of a metal/coating system. The MCS has been proved to be extremely useful to characterize new electrochemically active sol-gel thin films applied onto metallic substrates [32, 33].
The Scanning Kelvin Probe (SKP) allows the study of localized corrosion processes underneath insulating coatings [34] relevant for the corrosion processes at the polymer-coated metal interface, like swelling and ion incorporation into the coating [35]; the initiation of localized corrosion in defects; and corrosion propagation, caused by anodic or cathodic reactions occurring on the underlying interface. The SKP measures the work-function of a sample using the vibrating condenser method. The major advantage of the SKP in comparison to conventional electrochemical devices is the fact that the SKP measures electrode potentials without touching the surface under investigation across a dielectric medium of high resistance. The measurements are performed in humid atmosphere but in absence of aqueous electrolyte.
The Scanning Electrochemical Microscopy (SECM) uses an ultra-microelectrode (UME) immersed in a liquid electrolyte that bathes the entire metal surface. The UME scans the substrate surface at a fixed height detecting the electrochemical activity on the electrode surface as a consequence of the electrochemical response that the probe experiences when in close proximity with the substrate surface measuring the diffusion-limited current of a redox-active couple (e.g., Fe2+/Fe3+). With a spatial resolution in the micrometer scale or below allows the discrimination between the activities of different electroactive species by polarizing the UME at a potential of interest [36]. The SECM has shown to be a powerful technique in the study of Mg alloys in simulated biological fluids [37] and the localized corrosion of Mg alloys in NaCl solutions [38, 39] and has been also used to study the phenomenon of anomalous HE [40].
Another non-destructive technique is the Scanning Vibrating Electrode Technique (SVET) that measures the electric field generated by a sample immersed in a solution using a vibrating probe with a fixed amplitude, frequency and height. The scanning vibrating probe detects the potential gradients ohmically produced by local currents originated from actively corroding surfaces immersed in an electrolyte. Scanning is done with a 3D micro-stepping motorized micro-manipulator [41]. The probe vibration is controlled by a piezoceramic displacement device allowing vibration amplitudes from 1 to 30 μm (perpendicular to the sample surface). Calibration then enables quantification of localized cathodic and anodic activity [41, 42].
Scanning Reference Electrode Techniques have increasing their use over the last 30 years in studying the phenomena associated to the localized metallic corrosion. The principles of SVET measurement have been extensively addressed in pioneer works written by Isaacs [43, 44, 45]. A basic equation used by Bastos et al. [41, 40], for calculating the current density in solution, i, and describing the operating principle of SVET is
which is a form of Ohm’s law, where κ is the solution conductivity, E is the electric field in solution and V is the potential difference between two points separated by the distance r in the direction of current flow.
Several examples of the use of SVET in studies of Mg and Mg alloys can be found in renowned works of Williams and McMurray [46]. They have used SVET for studying, for instance, the inhibition of localized corrosion occurring on unpolarized Mg samples immersed in uninhibited aqueous sodium chloride electrolyte [42]. These authors also have used SVET for studying the behavior of a range of potential anionic inhibitors, selected on the basis of their ability to form insoluble precipitates with aqueous Mg2+ ions [46]. Their results using SVET for investigating the source of hydrogen evolution from high purity Mg anodically polarized are also significant [14, 16, 47].
Other successful examples of the use of SVET in corrosion studies of Mg alloys are found in several works of Montemor and Ferreira. These authors studied the relation with corrosion protection of pre-treatments consisting in the application of cerium and lanthanum nitrate solutions on AZ31 Mg alloy substrates [48]. Very important contributions of these authors have been their studies with SVET of modified bis-[triethoxysilylpropyl] tetrasulfide silane films on AZ31 alloy and, inhibitor-doped sol-gel coatings (8-Hydroxyquinoline) also on Mg alloy AZ31 substrates [49, 50].
Finally, it is important not to overlook in this section the contributions of the Bierwagen group on the application of SVET to studies of cathodic corrosion protection performance of Mg-rich coatings on aluminum substrates. In such studies coating characteristics were assessed not just using SVET but also multiple electrochemical techniques as SECM, open circuit potential (OCP) measurement, potentiodynamic polarization measurement (PDP) and EIS [51, 52].
Isaacs and Kendig were pioneers in the development of the local electrochemical impedance spectroscopy (LEIS) [53, 54]. This technique is closely related with SVET [55]. Typically, the LEIS method used a five-electrode configuration (Figure 12) [56]; a typical three-electrode arrangement (working electrode, counter electrode, and reference electrode) was used to control the dc potential and excite the interface potentiostatically with an ac signal, while two microreference electrodes were used to detect the local potential gradient in solution above the sample surface [57]. The principles of the technique are similar to those used in the conventional bulk EIS; a small sinusoidal voltage perturbation is applied to a working electrode sample and the resulting current is measured to allow the calculation of the impedance. The applied voltage (ΔVapplied) is the potential difference between the working electrode and the reference electrode. The local impedance Zlocal is calculated by the relationship:
Schematic representation of the LEIS apparatus [56].
The local AC current density (ilocal) is calculated using the Ohm’s law:
where ∆Vlocal is the ac potential difference measured between the two probes positioned on and in a conical plastic holder which are separated a distance d, and κ is the conductivity of the electrolyte [56].
Two measurements modes are possible:
Full local impedance spectrum at one single location (LEIS)
Area maps of the local impedance of the sample at one frequency (LEIM)
Baril, Galicia and co-workers applied the first measurement mode (LEIS) to study corrosion behavior of pure Mg and AZ91 magnesium Alloy [24, 56, 58]. More recently Barranco, Galvan et al. have applied the second mode (LEIM) to study the corrosion behavior of sol-gel thin films modified with Zr4+ and Ce3+ ions on AZ9, and doped with organic corrosion inhibitors (benzotriazole and L-cysteine) on AZ31 and AZ61 magnesium alloys [59, 60]. Figure 13 shows as an example impedance maps (LEIM) recorded around an artificial defect at a fixed frequency (500 Hz) and variable soaking time in 0.006 M NaCl solution for (a) undoped sol-gel film and (b) 0.3% wt.% benzotriazole (BTA) doped sol-gel film deposited on AZ31 alloy [60]. For the undoped sol-gel films no remarkable differences were observed between the registered impedance maps obtained initially and after longer soak time. In contrast, for BTA doped sol-gel films the LEIS maps showed that around the artificial defect the impedance values increased with soaking time. This effect was ascribed to the gradual release of the BTA from the sol-gel film that caused inhibition of corrosion on the active areas of the magnesium substrate [60].
Impedance maps recorded at constant frequency (500 Hz) and variable soaking time in 0.006 M NaCl solution for (a) undoped sol-gel film and (b) 0.3% wt.% BTA doped sol-gel film deposited on AZ31 alloy.
Global DC methods, comprising potential and current controlled techniques, are versatile tools to assess the corrosion rate of Mg and Mg alloys. They provide relevant information on the reaction kinetics and passive behavior of these materials. Furthermore, DC methods are particularly suitable for the study of the anomalous hydrogen evolution on anodically polarized Mg when coupled with other non-electrochemical methods such as hydrogen volume collection. The main limitation of the global DC techniques for the study of Mg is that, as a consequence of the intense evolution of hydrogen gas (both under anodic and cathodic polarization) large ohmic potential drops normally dominate the electrochemical response not far from the Ecorr. For this reason, care should be taken when designing, performing and analyzing the results obtained using these methods.
EIS is a very powerful tool, non-destructive and sensitive electrochemical technique, to study corrosion processes over time, formation of corrosion products and monitor corrosion rates of Mg alloys. These measurements obtained in a very wide range of frequencies allow us to examine the different processes that determine the corrosion behavior of Mg alloys, from the faster processes that are under charge transfer control, to the slower ones, generally of mass transport control, and inductive characteristics due to adsorption species.
Localized electrochemical techniques, either that based on surface scanning; LEIS, SVET, SKP and SECM or those based on microcapillaries; SDC, FT-SDC, MCS, have proven to be valuable tools in the study of microareas with different electrochemical activity that exist in Mg alloys due to their microstructure.
Complementary studies based on multiple global and localized electrochemical techniques are the key to face the challenge of controlling the corrosion of reactive Mg and its alloys for their use in technological applications.
The authors gratefully acknowledge the financial support provided by the Ministry of Economy and Competitiveness of Spain (MAT2015-65445-C2-1-R) to carry out the present study. Santiago Fajardo expresses his gratitude to the State Research Agency (Ministry of Economy, Industry and Competitiveness of Spain), the Spanish Research Council (CSIC) and the European Regional Development Fund (ERDF) for the financial support under the Project MAT2015-74420-JIN (AEI/FEDER/UE).
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