‘Apparent’ Stern-Geary coefficient calculated from Rt resistance performed in 0.6 M NaCl solutions.
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But for people fighting their fatigue after brain injury day after day, fatigue is a major problem. This post-injury mental fatigue is characterized by limited energy reserves to accomplish ordinary daily activities. Persons who have not experienced this extreme exhaustion which may appear suddenly, and without previous warning during mental activity, do not understand the problem. This is especially difficult to understand as the fatigue may appear even after seemingly trivial mental activities which, for uninjured persons, are regarded as relaxing and pleasant, as reading a book or having a conversation with friends. A normal, well-functioning, brain performs mental activities simultaneously throughout the day, but after a brain injury, it takes greater energy levels to deal with cognitive and emotional situations.
In this chapter, we highlight mental fatigue after TBI. In the case of long-lasting mental fatigue, it could be the only factor that keeps people from returning to the full range of activities that they pursued prior to their injury with work, studies and social activities. We describe mental fatigue and suggest diagnostic criteria and we also give a theoretical explanation for this. At the end of the chapter, we discuss treatment strategies and give some examples of possible therapeutic alternatives which may alleviate the mental fatigue.
Normally, the brain works in an energy-efficient manner and prominent energy reserves are present. This is due to well-functioning ion channel and amino acid transport systems and other effective physiological processes. After brain injury, some of these systems are down-regulated, and when mental energy requirements are high the physiological processes do not function to their full capacity; these cease to function efficiently with a resultant energy loss. This may be an explanation as to why the mental fatigue appears.
Annually, about 100-300/100 000 individuals sustain a TBI, and most of the injuries are mild in severity [1]. A majority of patients recover within one to three months following mild TBI [2, 3].
Fatigue is one of the most important long-lasting symptoms following TBI, and is most severe immediately after head injury. However it is difficult to arrive at any clear figure as to how common fatigue or, in particular, mental fatigue is. The reason for this is that different results have been obtained, and these are attributable to differences in definitions and differences in the methodology in the various studies. In follow-up studies, the frequency of prolonged fatigue varies from 16 up to 73 % [4-6]. There is no correlation between persistent fatigue and severity of the primary injury, age of the person at injury or time since injury [7, 8]. For those suffering from fatigue 3 months after the accident the fatigue remained relatively stable during longer periods [9]. In particular, for those subjects who were suffering from the syndrome one year after the accident improvement in the fatigue was limited [10].
In the above reports, fatigue is discussed in terms of a single construct, i.e. not differentiated between the physical or mental aspects. In this chapter, we consider mental fatigue as a separate construct and we discuss its relationship to cognitive and emotional symptoms.
Mental fatigue is not an illness, rather it represents a mental sequel, probably due to a disturbance of higher brain functions, either physical or psychological in origin. It is included in, and defined within the diagnoses Mild cognitive impairment (F06.7), Neurasthenia (F48.0) and Posttraumatic brain syndrome (F07.2) [11].
A typical characteristic of pathological mental fatigue after TBI is that the mental exhaustion becomes pronounced during sensory stimulation or when cognitive tasks are performed for extended periods without breaks. There is a drain of mental energy upon mental activity in situations in which there is an invasion of the senses with an overload of impressions, and in noisy and hectic environments. The person feels that their brain is overloaded after a tiny load. Another typical feature is a disproportionally long recovery time needed to restore the mental energy levels after being mentally exhausted. The mental fatigue is also dependent on the total activity level as well as the nature of the demands of daily activities. Fatigue often fluctuates during the day depending on the activities carried out. Thus, this fatigue is a dynamic process with variations in the mental energy level. The fatigue can appear very rapidly and, when it does, it is not possible for the affected person to continue the ongoing activity. Common associated symptoms include: impaired memory and concentration capacity, slowness of thinking, irritability, tearfulness, sound and light sensitivity, sensitivity to stress, sleep problems, lack of initiative and headache [12].
For many persons, this mental fatigue is the dominating factor which limits the person’s ability to lead a normal life with work and social activities. For most people, fatigue subsides after a period of time while, for others, this pathological fatigue persists for several months or years even after the brain injury has healed. Interestingly, however is that as many as 30% of family or friends interpreted fatigue as laziness [9].
Theories as to the mechanisms accounting for mental fatigue including our own theory, suggest that cognitive activities require more resources and are more energy-demanding after brain injury than usual [13, 14]. Thus, more extensive neural circuits are used in TBI victims compared to controls during a given mental activity [15]. This indicates an increased cerebral effort after brain injury.
Schematic representation of recovery of mental energy after TBI. The green line represents a full recovery while the blue and red lines represent impaired recovery in terms of the mental energy levels. Persons whose recovery follows the blue line recover partially. On their return to work and daily activities, they are not able to manage and they become exhausted. Persons whose recovery follows the red line do not recover and are not able to return to work and daily activities.
Therapist Luann Jacobs describes mild TBI and the lack of energy and lack of endurance that many can experience. As they are able to do what is normal and what appears normal, they run the risk that their symptoms will be misunderstood [16].
“Mild brain injury is a real misnomer, as it conveys the idea that nothing much is a problem when quite the opposite is more often true. It is called “mild” because, in fact, the mildly brain injured can walk, talk, eat and dress independently, often times drive a car, shop, cook, go to school, or even work.
What the term fails to account for is the inherent limits of how often, for how long (endurance), and the all-important, how consistently (e.g., every day, once a week) these activities can be performed. Even more elusive is the concept of how many of these daily activities can be done sequentially in a given day as is normal in the lives of people who are not brain injured.
The fatigue they feel defies description, going far beyond and far deeper than anything a non-brain-injured person would consider profound exhaustion.”
The cause of this extreme fatigue is not known. However, there are speculations that the symptom may be caused by dysfunction of the astrocytes, the most common supporting cells in the brain [17, 18]. As a consequence, nerve cell communications do not function properly.
Schematic drawing of a synapse with glutamate as the transmitter and an astrocyte with processes surrounding the synaptic terminal. After being released from the presynaptic terminal (pre-syn; this is shown in red in the figure), glutamate interacts with glutamate recognizing receptors on the postsynaptic membrane (post-syn; shown in green in the figure). After stimulation of the postsynaptic neuron, glutamate is taken up by glutamate transporting systems on the astrocyte processes. Glutamate is converted to glutamine in the astrocyte and transported back to the presynaptic terminal where glutamine is converted back to glutamate. During this process, and with decreasing ATP levels as the signal, glucose is taken up from the blood to supply neurons and astrocytes with energy.
Following TBI there is a neuroinflammation with down-regulation of astroglial glutamate transport systems. If this state is not restored completely, there will be an impaired extracellular glutamate clearing with slightly increased extracellular glutamate levels, slight astrocyte swelling and impaired glucose uptake. Neuronal activity, if long-lasting, may result in energy crisis.
Following TBI there is a low-grade neuroinflammation with down-regulation of astrocyte glutamate transporters and Na+/K+ ATPase activity [19, 20]. If these physiological systems are not restored completely there will be a dysfunctional support of the glutamate transmission. Glutamate signaling is essential for information processing, including learning and memory formation. Low levels and fine-tuning of extracellular glutamate are necessary to maintain high precision in information processing, and thereby high efficiency in the information handling within the CNS. Our hypothesis implies that such dysfunction could underlie the mental fatigue at the cellular level. From experimental data, the astroglial cells are considered the most important cells for clearing the extracellular space from glutamate during glutamate transmission. In addition, it is well-accepted from the experimental data that this clearing capacity is attenuated by substances or conditions associated with brain dysfunction or pathology (see [17]).
If the capacities of these processes are not fully restored, neuronal function is impaired in at least two ways: 1) extracellular glutamate levels increase upon neuronal activity leading to unspecific signaling and 2) lack of energy. In the event of a high mental load with high neuronal activity, these factors may lead to a metabolic collapse of neuronal circuits – we have previously called this a “dead-lock” situation, which may take a long time to restore.
We consider this metabolic failure as one probable explanation for the prominent and abrupt exhaustion that the TBI victims with mental fatigue can experience. The long restoring time at a cellular level corresponds to the long time it takes for the TBI victims to restore mental activity.
One way to restore this dysfunction is to stimulate Na+/K+-ATPase along the dopaminergic circuits which regulate attention and executive functions. Possible candidates are methylphenidate and the dopaminergic stabilizer OSU6162 (see below under the heading, ‘Treatment’).
There is an abundance of scales for assessing fatigue in general and several of these scales are designed for use in different diseases [21, 22]. The scales include questions relating to feelings of fatigue, perceived impact on activities, affective feelings and mental or cognitive effects. Many of the scales are self-reported on a Likert or an ordered scale, with the following response alternatives: Never, Sometimes, Regularly, Often or Always.
We have developed and used the Mental Fatigue Scale (MFS) during the last five years. We decided to construct this scale since we were not able to find an assessment scale adapted to mental fatigue. The MFS is a multidimensional questionnaire containing 15 questions. It incorporates affective, cognitive and sensory symptoms, duration of sleep and daytime variation in symptom severity. The questions concern the following: fatigue in general, lack of initiative, mental fatigue, mental recovery, concentration difficulties, memory problems, slowness of thinking, sensitivity to stress, increased tendency to become emotional, irritability, sensitivity to light and noise, decreased or increased sleep as well as 24-hour symptom variations. The questions in the scale are based on common activities and we have related the estimation to exemplified alternatives. It is also possible to provide estimations in-between two alternatives. The intention was to make the scale more consistent between individuals and also between ratings for the same individual. The exemplified alternatives can help the person to respond in a similar way despite the present state of fatigue or emotional state. The MFS is designed in a similar way as The Comprehensive Psychopathological Rating Scale (CPRS). The CPRS also includes exemplified alternatives and it is used to record changes in psychopathology over a comparatively short period [23]. The questions included in the MFS are based on symptoms described following longitudinal studies of patients with TBI, brain tumours, infections or inflammations in the nervous system, vascular brain diseases, and other brain disorders, which indicates that an acquired brain injury or disorder can result in similar symptoms [24-26]. The scale is free to use and can be downloaded at www.mf.gu.se (both in Swedish and English). We have transcribed one of the questions in the MFS, below:
Mental fatigue
Does your brain become fatigued quickly when you have to think hard? Do you become mentally fatigued from things such as reading, watching TV or taking part in a conversation with several people? Do you have to take breaks or change to another activity?
0 | \n\t\t\tI can manage in the same way as usual. My ability for sustained mental effort is not reduced. | \n\t\t
0.5 | \n\t\t\t\n\t\t |
1 | \n\t\t\tI become fatigued quickly but am still able to make the same mental effort as before. | \n\t\t
1.5 | \n\t\t\t\n\t\t |
2 | \n\t\t\tI become fatigued quickly and have to take a break or do something else more often than before. | \n\t\t
2.5 | \n\t\t\t\n\t\t |
3 | \n\t\t\tI become fatigued so quickly that I can do nothing or have to abandon everything after a short period (approx. five minutes). | \n\t\t
Figure 4 shows how healthy controls and subjects suffering from mild TBI, TBI and stroke have rated separate questions on the MFS. The brain injury victims were divided into different groups according to their total rating on MFS. When a person rates low on one question, the total rating on most of the separate questions will also be low, while persons rating high on one question on the MFS, will also rate most of the questions on a high level.
Rating on separate items on the Mental Fatigue Scale for controls and brain injured subjects. Brain injured subjects are divided into groups according to their total rating on MFS.
The rating on MFS by healthy controls and people who suffered mild TBI or TBI did not reveal any significant differences between females and males, and there was no correlation between the results on MFS and age or education of the TBI victims (figure 5). Furthermore, we did not find any correlation for the TBI participants concerning time since injury and their rating on MFS. We have, in our studies worked with participants with mental fatigue lasting for six months or periods greater than six months. At this stage, we do not have any data relating to ratings early after TBI or mild TBI. This accounts for the fact that the rating may lack correlation to time since injury.
The control group rated MFS significantly lower than mild TBI and TBI victims. The participants included for the analysis were healthy controls and participants who had suffered mild TBI or TBI without major depression. The participants were between 20-67 years of age.
We recommend a cutoff score on the MFS at 10.5. A score of 10.5 on the MFS was found to deviate significantly from the control sample and is also above the 99th percentile for the control group. A score above 10.5 implies a problem for the person, although a serious problem is not always the case. However, such a score implies the need for the person to consider the current situation with their work and/or social life. The MFS had a high internal consistency and all separate items were rated significantly higher among brain injured subjects compared with healthy controls (see also figure 5).
Correlation with age and rating on MFS for healthy controls and subjects with long-lasting mental fatigue after brain injury.
It has been proposed that subjective mental fatigue after TBI or mild TBI correlates to poor performance in attention tests and reduced processing speed [13, 27, 29-34]. We also found that information processing speed, attention and working memory were significantly reduced for the brain injury victims (both mild TBI and TBI) compared to controls. Furthermore, the tests correlated significantly to the results on the MFS (figure 6). Among the cognitive functions, processing speed was found to be a significant predictor for the rating on MFS [27].
Correlation between Mental Fatigue Scale and information processing speed (Digit Symbol-Coding).
In the population of TBI victims, depression is elevated although there is a wide variation in frequency, depending on methodological differences [35-37]. In our studies, we have included participants who complained of mental fatigue after TBI and we excluded subjects affected by major depression, as it was our intention to explore the mental fatigue component. Despite this, we found, with the use of the CPRS/MADRS, that there was an elevation in the rating of depression items for TBI subjects compared to controls. The CPRS scale includes both a depression and an anxiety scale [23, 38]. The CPRS depression scale is also called the Montgomery-Åsberg Depression Rating Scale (MADRS) [39].
However, there are overlapping items in the MFS and CPRS. The overlapping items include the following: lack of initiative, concentration difficulties, irritability and decreased sleep. With a factor analysis, the items were separated into a mental fatigue component and a depression and anxiety component. Irritability was placed in the depression-anxiety component and the other three items in the mental fatigue component. With an analysis using the new components, we found that by adjusting the mental fatigue component this removed the difference observed between the brain injured subjects and controls in the depression-anxiety component. However, by removing the depression-anxiety component this did not have an effect on the difference observed between the brain injured subjects and controls in the mental fatigue component.
In this subject sample, we were able to demonstrate that a significant effect on the difference observed between the brain injured subjects and controls in the scores for depression can result in an overestimation if the effect of the mental fatigue component is not taken into consideration. This indicates that mental fatigue and depression must be treated as separate constructs and it is also important to make this distinction for the purposes of therapeutic strategies.
The diagnostic criteria for posttraumatic brain syndrome include most of the symptoms that are often present along with mental fatigue. However, we suggest mental fatigue to be a central symptom after a brain injury reflecting an inefficient support to the neuronal networks.
Mental fatigue is a lack of mental energy with impaired cognitive, emotional and sensory functioning. Mental fatigue is characterized by an unusual feeling of fatigue or malaise. There is a drain on the person’s mental energy upon mental activity. The result is a diminished attention and concentration capacity. Situations which involve high levels of external cues and an overload of impressions are strenuous. Failing energy levels and excessively long recovery times are the result of over-exertion. The condition impairs the person’s ability to function in their work, studies and gatherings with family and friends.
The mental fatigue has persisted for at least 1 month;
The sum of scores from the MFS is 10.5 points or above.
Typical symptoms include:
An unusually rapid drain of mental energy upon mental activity;
Impaired attention and concentration capacity over time;
Following over-exertion, a long recovery time disproportionate to the exertion level;
Diurnal variation of the fatigue symptom with the fatigue often being better in the mornings and worse in the afternoons and evenings; variations from one day to the next;
Usually one or several associated symptoms (see below):
The following additional or associated symptoms are common:
Mood swings, irritability and stress intolerance;
Trouble with memory;
Sleep problems;
Sensitivity to, or intolerance of light and loud noise;
Headaches following over-exertion.
Sleep problems most often occur in the following way: either a shorter duration of sleep with interrupted wake-ups or sleeping more than usual. If the person becomes more mentally fatigued, the sleep will most often become worse, and if the person rests for some days the sleep can become improved again. The emotional load may increase the severity of the fatigue, but if mental fatigue exists, it will remain even once the emotional components, as depression or anxiety have been treated. However, it is important to treat the emotional problems. In this way, the mental fatigue may, to some extent be relieved.
A total and almost paralyzing fatigue;
Longer periods of rest may be needed, often over several days;
A worsening of symptoms over time;
Situations in which there is an invasion of the senses with an overload of impressions, and noisy and hectic environments such as crowded events, also the hustle and bustle of shopping centers, and travelling by bus, etc.;
Reading books and newspapers;
Conversations with people – this becomes more of a struggle when more people are involved;
Unexpected events.
The figure illustrates mental fatigue. Characteristic symptoms are seen on the blue circle and associated symptoms on the green circle. (The figure in the middle is illustrated by Kristina Edgren Nyborg).
There is currently no effective treatment for mental fatigue. For many people, there is an increased risk of doing too much and becoming even more fatigued. Today, the most important recommendations are to adapt to the energy available by doing one thing at a time, resting regularly and not overdoing things.
When mental fatigue is present, it is important to adapt work as well as daily activities to levels that the brain can manage. However, this is challenging for most people and it may take a long time, even years, to adapt to a sustainable level. It may also be difficult for the person to learn by himself/herself and it can take several years of considerable struggle, frustration, despair and depression, to find the right balance between rest and activity. Professional support is required but this can be hard or impossible to find especially when mental fatigue continues for many years.
The figure illustrates levels and fluctuations in mental fatigue measured with the MFS after TBI and variations over time. Most mild TBI victims recover completely (green field) and do not exceed 10 points on the MFS. People within the blue, yellow or red fields suffer from metal fatigue to varying extents. It is also shown that treatment strategies decrease the mental fatigue, while over-exertion leads to increased rates on the MFS.
Take regular breaks;
Encourage rest before becoming over-tired;
Try to work at a steady pace, taking one task at a time with short working periods, and prioritize the tasks;
Plan the days’ activities or the activities for the week in a diary or journal. Avoid over-exertion.
The use of strategies is important. By resting the brain as much as possible the mental energy will be alleviated. However, the brain and the individual also need positive experiences and stimulation to ensure wellbeing. It is difficult to achieve this balance between rest and stimulation.
When mental fatigue becomes a prolonged problem, it is essential to be able to alleviate the symptoms. We have reported on significantly reduced mental fatigue after treatment using the mindfulness-based stress reduction (MBSR) program [40, 41]. We have also reported on possible therapeutic strategies to reduce mental fatigue by means of pharmacological treatments, using neurostimulant substances as methylphenidate [42] which affects dopamine and norepinephrine signaling. We have also reported on a new substance not currently available on the market, (-)-OSU6162, which is a dopamine and serotonin stabilizer [43].
The MBSR program was tested on TBI and stroke victims suffering from long-term mental fatigue [40]. MBSR is a clinically-effective
method for a wide range of conditions as stress, depression, pain, and fatigue after cancer, with the potential to help individuals to cope with their difficulties [44-47]. MBSR is also suggested to be linked to improvements in attention and cognitive flexibility [48] and also to changes in brain neuronal connectivity [49].
MBSR includes a range of both formal and informal practices. The intervention is based on Kabat Zinn’s MBSR program [50]. The formal practices in MBSR are described by M. Cullen 2011 [51] and these include gentle Hatha yoga with an emphasis on mindful awareness of the body, a body scan designed to systematically, region by region, cultivate an awareness of the body without the tensing and relaxing of muscle groups associated with progressive relaxation, and sitting meditation with an awareness of the breath as well as a systematic widening of the field of awareness to include all four foundations of mindfulness: awareness of the body, feeling tone, mental states and mental contents. As such, the intention of MBSR is much greater than simple stress reduction. The program consists of eight weekly group sessions which are each approximately 2.5 hours long, one day-long silent-led retreat between sessions six and seven and home practice of about 45 minutes, six days a week. The participants receive guided instructions and CDs for home practice.
We found a significantly reduced mental fatigue after the MBSR program and participants improved their processing speed significantly compared to control on waitlist [40]. Improvement was independent of gender, time since injury and age. Another recent study with MBSR for mild TBI patients showed a similar result with significant improvement in quality of life, perceived self-efficacy, working memory and attention [52]. Furthermore, a small-scale study of 10 mild TBI subjects included in the MBSR program over a 12-week period also showed a significantly improved quality of life and decreased depression rating [53]. The effects were maintained one year later among the seven contactable participants. They also noted an improvement in reported energy levels at the follow-up [54]. However, after TBI, a short MBSR program over a 4-week period did not result in any cognitive or emotional changes [55].
The results demonstrate that mindfulness practice may be a therapeutic method well-suited to subjects suffering from mental fatigue after brain injury. One reason why MBSR was effective may be that this treatment offers strategies to better handle stressful situations appropriately and economize with mental energy. Despite the problem of ensuring that participants stay awake, which is one of the fundamental aspects of meditation, it was possible to adjust mindfulness to suit the needs of mental fatigue subjects and to improve their wakefulness as well as reducing their mental fatigue levels.
Methylphenidate inhibits dopamine and noradrenalin reuptake resulting in increased extracellular concentration of dopamine and noradrenalin [56]. Methylphenidate has been used for many years in the treatment of ADHD in children, in the first instance to increase wakefulness, attention and concentration capacity. Methylphenidate has also been tested on TBI victims with positive effects on information processing speed and, to some extent on working memory and attention [57-63]. Guidelines for use of methylphenidate for deficits of attention and processing speed after TBI have been suggested [64], while no such guidelines exist for fatigue following TBI.
In an open randomized study, methylphenidate significantly improved mental fatigue dose-dependently as assessed with the MFS [42]. The item, pain was also studied and we found that this item was rated high by most of the subjects in our study as the participants were recruited on the basis of the items, TBI and pain. However, no significant alleviation of pain was reported as a result of methylphenidate treatment. However, it is important to note that pain can hide posttraumatic brain injury symptoms or mental fatigue which is not always connected to the actual pain. We also found that there was no interaction between the pain and the mental fatigue in those participants treated with methylphenidate. These findings indicate that, not only is it necessary to treat patients for the pain for which they are primarily referred to the clinic, but also for the mental fatigue, if present.
Methylphenidate was well-tolerated by TBI subjects. However, tolerance of methylphenidate differed between subjects and we therefore recommend starting treatment with an initial low dose.
The monoaminergic stabilizer OSU6162 interacts with both dopaminergic and serotonergic systems. It appears to act as an antagonist on a binding site of the D2 receptor. More recent research has demonstrated that OSU6162 also exerts a stabilizing effect on serotonergic neuronal circuits, acting as a partial 5-HT2A agonist [65, 66].
In two randomized, double-blind and placebo-controlled studies we found statistically significant alleviation of mental fatigue after a stroke or TBI by OSU6162 during 4 weeks’ treatment with active drug [43]. However, the numbers of patients in these studies were small (21 TBI and 19 stroke victims). Further studies are needed, with a larger number of patients and, in particular longer treatment periods as mental fatigue may be long-lasting. Adverse reactions were mild and could be avoided by dose adjustment. Several patients experiencing such adverse reactions expressed the wish to receive continued treatment with the drug.
Similar results were detected for methylphenidate and OSU6162. These drugs were shown to have the effect of both alleviating mental fatigue and increasing information processing speed.
Mental fatigue can become a prolonged and distressing problem after TBI having considerable effect on life and wellbeing. It is important to acknowledge and assess mental fatigue when discussing the options regarding therapeutic methods as the mental fatigue has been the result of a TBI.
After TBI, mental energy levels are failing, and the brain needs to rest. It is not possible to improve the mental energy with training in order to perform more mental activities. In fact, training with a view to resting the brain is what is important. Suitably-adapted and energy-saving strategies are important and most patients need support in order to achieve an enduring balance between activities and rest as this is difficult, it takes a long time and may be frustrating.
The treatment studies we reported on are aimed at helping the person to manage their life better. However, it is important to stress that there is a risk that the medication can compel the person to do more than is appropriate. The reason for this is that, most often they want to carry out activities in a similar way as before the injury and have been longing for the chance to be able to do this. The problem is that, for most persons suffering from long-term mental fatigue after TBI, the activity levels are close to the threshold of what they are able to sustain. This makes them susceptible if they increase their activity levels too much. With mindfulness most participants reported on more energy, but they also became more pleased and happy with life. Mindfulness also gave them a tool to use and they could take command over their own lives; how it is here and now, not longing for a better life or ruminating over what has been. This also saves energy! A combination with neurostimulants and mindfulness may be a good therapeutic strategy.
In the future, research is warranted for early treatment with the intention to reduce the development of long-term mental fatigue. We also need to better elucidate and carry out an in-depth analysis of mental fatigue.
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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