Electrochemical Corrosion Study of Cold-Rolled AA8015-Alloy Processed by Reversing Cold Rolling Mill at Varying Surface Roughness

Abstract

AA8015-alloy is a general-purpose aluminium alloy having a wide range of applications. Electrochemical corrosion testing of cold-rolled AA8015-alloy processed by reversing cold-rolling mill at different surface roughness in natural seawater were investigated. The AA8015-alloy utilised in this study was cold-rolled in a reversible Achenbach cold-rolling mill in four number of passes to a gauge thickness of 1.2 mm. This industrial cold rolling process was achieved at Tower Aluminium Rolling Mill, Sango-Ota, Nigeria. The different surface roughness’s each with three cold mounted samples, 1.54 μm, 0.83 μm, 0.18 μm and 0.04 μm were achieved on automated polishing machine using 320-grit, 800-grit, 1200-grit and diamond abrasive MD-Mol respectively. Electrochemical corrosion experiments were conducted on the samples in natural seawater using a computer-controlled potentiostat in an open polarisation cell set-up at room-temperature. The corrosion behaviour on surface morphologies of the samples was observed by high-mega-pixel camera and scanning electron microscope. Findings reveal asymmetric polarisation curves, and the polarisation resistance increases as the surface roughness decreases. Consequently, corrosion-rate reduces as the surface get smoother and EDS elemental analysis shows the existence of insoluble sulphate and chloride complexes formed on the surfaces. Conclusively, surface roughness affects the corrosion resistance of cold-rolled AA8015-alloy in natural seawater.

1. Introduction

The useful lifespan of any component or device is always established at its design and manufacturing stage [1]. In extractive metallurgy, large billets have to be reduced by mechanical deformation processes such as forging, rolling and extrusion for further reduction and change in their shapes. There are three basic temperature ranges in metal forming at which the metal (workpiece) can be formed which are hot working, warm working and cold working [2]. Cold working is a strengthening mechanism that involves plastic deformation. This strengthening mechanism is mostly utilised in ductile metals. In addition, cold working takes place when the processing temperature of the mechanical deformation of the ductile metal is below the recrystallization temperature [3, 4, 5]. Cold working in mechanical rolling process is eminent as compared to pressing, drawing, spinning and extruding.

The cold working in rolling process, known as cold rolling, involves deforming the ductile metal by using rolls at low temperatures, especially at temperatures below the recrystallization temperature of the specific metal. Cold rolling processes are achieved by using rolling mills to produce metal sheets of a certain required thickness. The vast majority of cold rolled metal is in the form of flat rolling. Worth noting is the fact that cold rolling has enormous benefits in its ability to manufacture products from relatively large pieces of metal at very high speed in a continuous manner with good surface finish, highly accurate tolerances and stronger products. In addition, cold rolling process eliminates shrinkage effect, increases hardness and elastic limit. It also promote decrease in ductility due to strain hardening effect [2, 4, 6, 7].

There has been advances in research on corrosion behaviour of cold rolled metal-alloys in various solutions reported. Liu et al. [8] carried out electrochemical measuring technique on metastable Cr-Mn-Ni-N austentic stainless steel in acidic medium. Their findings shows that Fe and Cr dissolutes on the stainless steel surface in the process of corrosion which is clearly facilitated by the cold rolling deformation. Studies by Ma et al. [9] demonstrated that high strained cold rolling deformation such as 70 and 90 percent reduction in thickness in Ta-4 W alloy improves corrosion resistance due to preferential crystallographic orientations. More studies [10, 11, 12, 13], also confirms that cold rolling deformation effect in metal-alloys greatly influence the corrosion current density and corrosion potentials.

Moreover, open literature reports evidence of aluminium and its alloys corroding under typical applications. To mention a few. Rao et al. [14] in their literature review on stress-corrosion cracking, confirm that 2xxx, 5xxx and 7xxx aluminium alloys are susceptible to stress-corrosion cracking. Moreover, a review on the corrosion inhibition performance evaluation for aluminium and its alloys in chloride and alkaline solutions by Xhanari and Finsgar [15], confirms evidence of corrosion in aluminium. Further scholarly articles [16, 17, 18, 19, 20, 21, 22, 23, 24, 25] reported also indicate that aluminium and its alloys corrode, despite its formation of a natural oxide layer that is chemically inert. However, corrosion study on aluminium 8-series especially aluminium 8015-alloy is yet to be reported.

Conducting effect of corrosion on varying degree of surface roughness in cold rolled metal reduction has been a grey area of research. Therefore, this research work focus on an extensive in-depth experimental investigation on the corrosion behaviour of cold-rolled AA8015-alloy in natural seawater solution at varying surface roughness condition.

2. Achenbach reversing mill and cold rolling process

The Achenbach reversing cold rolling mill utilised in this study is a 4-high single stand unit at Tower Aluminium Rolling Mill, Sango-Ota, Nigeria. The 4-high reversing mill detail description and pictorial representation are presented in Table 1 and Figure 1 respectively.

	Description
Type	Single Stand Reversing 4Hi Cold-Rolling Mill
Make	Achenbach
Strip width	1250 mm
Speed	485 m / minute
Inner coil diameter	600 mm
Outer coil diameter	1520 mm
Entry thickness	Max. 8 mm
Exit thickness	Min. 0.20 mm
Hydraulic System Pressure	40 Bar
Load Capacity	12,000 kN

Table 1.

Description of Achenbach 4-high reversing cold rolling mill at towers aluminium rolling mill.

Figure 1.

Pictures of Achenbach 4-high reversible cold rolling mill at tower aluminium rolling mill; (a) left-end of the loaded coil, (b) cold rolling process, (c) right-end of the loaded coil, (d) computer numeric control unit, (e) bigger view of the 4-high reversible cold mill, (f) end view of the rolls.

The annealed coiled AA8015-alloy with 7 mm sheet thickness is heated up to approximately 120°C before been loaded into the deforming rolls at industrial ambient temperature. The alloy sheet was cold rolled successively at thermal equilibrium maintained at about 70°C by passing it back and forth in four successive pass schedules in alternate directions until desired thickness of 1.2 mm is attained. The pass schedules were chosen to maintain reasonable constant drive power and rolling force during successive passes.

3. Experimental details

3.1 Material, sample preparation and electrolyte solution

The cold-rolled AA8015-alloy specimen was investigated using Bruker Elemental Optical Spectrometry and the obtained elemental composition is given in Table 2. The cold-rolled alloy specimen was square-cut using automated Mecatome T300 cutting machine embedded with a 10S25 cut-off wheel based on the inscribed markings on the surface having dimensions 10 mm by 10 mm. Twelve corrosion specimen samples were prepared according to ASTM standard G1–03 [26] and cold mounted using EpoFix Resin and hardener following Struers application note for cold mounting procedures [27]. Further surface treatment was done on the cold mounted samples using SiC emery paper of varying 320, 800 and 1200 grit and diamond polished with MD-Mol disc surface on automated grinding/polishing machine. Four different surface conditions were attained after cleaning with distilled water and acetone. The surface roughness (R_a) was determined with HOMMEL-ETAMIC TURBO roughness and contour metrology, for each surface conditions given in Table 3.

Element	Si	Fe	Cu	Mn	Mg	Cr	Zn	Ti	Al
Weight %	0.478	1.338	0.146	0.074	0.035	0.0067	0.072	0.011	balance

Table 2.

Chemical composition of AA8015-alloy in weight per cent.

Cold-rolled AA8015 alloy	Surface roughness value with 320 grit SiC paper (Ra)	Surface roughness value with 800 grit SiC paper (Ra)	Surface roughness value with 1200 grit SiC paper (Ra)	Surface roughness value with diamond polishing (Ra)	Units
Cold-rolled samples	1.54	0.83	0.18	0.04	μm

Table 3.

Surface roughness values of cold-rolled AA8015-alloy corrosion samples under different surface conditions.

A natural seawater electrolyte solution was utilised in the experiment with a pH of 7.04, taken from the Sea at Durban, South Africa. Table 4 depicts the chemical composition of the seawater at 3.5% salinity. The major ion in parts per million (ppm) is shown.

Element	PPM in sea water
Chloride (Cl^−)	18,980
Sodium (Na^+)	10,556
Sulphate (SO4^2−)	2649
Magnesium (Mg^2+)	1262
Calcium (Ca^2+)	400
Potassium (K^+)	380
Bicarbonates (HCO3^−)	140
Strontium (Sr^2+)	13
Bromide (Br^−)	65
Borate (BO3^3−)	26
Fluoride (F^−)	1
Silicate (SiO3^2−)	1
Others	—

Table 4.

Major ion composition of seawater at 3.5% salinity [28].

3.2 Electrochemical measurements

Potentiodynamic electrochemical technique was utilised in determining the corrosion behaviour of the cold-rolled AA8015-alloy at varying surface roughness following ASTM standard G5 [29]. The potentiodynamic polarisation curves were generated using the Ivium Compact-Stat Potentiostat computer-controlled with accustomised Ivium corrosion analysis software to produce Tafel fit lines. Electrochemical experiments were conducted for three samples of the cold-rolled alloy at each surface roughness carried out at room temperature in an open glass cell containing 200 ml solution of natural seawater. This is important to confirm reproducibility and precision. Open circuit potential (E_oc) measurement of each sample of the cold-rolled aluminium 8015-alloy at varying surface roughness was first determined, according to ASTM standard G69 [30]. The polarisation cell set-up for determination of E_oc consist of connections of the working electrode (prepared cold-rolled aluminium 8015-alloy samples at different surface roughness) and the silver/silver chloride reference electrode in natural seawater electrolyte solution to the potentiostat. Subsequent potentiodynamic experiments carried out utilised a conventional three-electrode polarisation cell that includes a platinum counter-electrode. A linear potential sweep in the anodic direction was performed at a scan rate of 0.167 mV/s, starting from 250 mV below the E_oc and terminating at 250 mV above the E_oc. The scanning electron microscope images were recorded to ascertain the interaction of seawater medium with the alloy surface using TESCAN VEGA Scanning Electron Microscope with Energy Dispersive X-ray Spectroscopy (EDS). Figure 2 shows the experimental polarisation cell set-up accordingly.

Figure 2.

Electrochemical corrosion experimental set-up; (a) Ivium compact-stat Potentiostat, (b) an open glass cell system.

4. Results and discussion

4.1 Open circuit potential (E_oc) and potentiodynamic polarisation result

The measured potential of the working electrode (cold-rolled AA8015-alloy) at varying surface roughness is given in Figure 3, as a function of time for an hour duration. The variations of the open circuit potential against Ag/AgCl with the time plot for the three samples show a steady state variation almost throughout the time duration with relatively stable drifting within 0.1 V for samples with surface roughness 1.54 μm and 0.83 μm, and 0.05 V for samples with surface roughness 0.18 μm and 0.04 μm respectively. The open circuit potential (E_oc) values for the three samples at each surface roughness all show electronegative potentials, given in Table 5. These steady-state E_oc values were taken at the last 3600 seconds. The anodic and cathodic reactions on the alloy surface are in equilibrium at these potentials.

Figure 3.

Variation of open circuit potential with time for cold-rolled aluminium 8015-alloy at (a) 1.54 μm; (b) 0.83 μm; (c) 0.18 μm and; (d) 0.04 μm surface roughness’s in natural seawater.

Cold-rolled AA8015 alloy samples	Surface roughness Ra ≈ 1.54 μm	Surface roughness Ra ≈ 0.83 μm	Surface roughness Ra ≈ 0.18 μm	Surface roughness Ra ≈ 0.04 μm	Units
Sample 1	−0.739	−0.740	−0.713	−0.734	V
Sample 2	−0.714	−0.728	−0.723	−0.710	V
Sample 3	−0.740	−0.759	−0.703	−0.714	V

Table 5.

Open circuit potentials (E_oc) for cold rolled AA8015 with varying surface roughness.

Likewise, Figure 4 shows the polarisation curves generated for each potentiodynamic polarisation test for all the samples. Observations for each curve shows similarity in shape and are asymmetric. Furthermore, the Tafel behaviour on the cathodic side extends over a wider potential range than on the anodic side. In addition, the anodic current rose more steeply with changes in potential than the cathodic current. This indicates that reduction reaction occurs at slower rate than oxidation reaction. Therefore, cathodic reaction controls the electrochemical corrosion of the cold-rolled AA8015-alloy. The drifting occurrence in the polarisation curves confirms electrochemical noise. These suggest indication of sudden inert oxide-layer film rupture causing dissolution of the AA8015-alloy.

Figure 4.

Polarisation curves of cold-rolled AA8015-alloy immersed in natural sea water at surface roughness’s (a) Ra ≈ 1.54 μm; (b) Ra ≈ 0.83 μm; (c) Ra ≈ 0.18 μm; (d) Ra ≈ 0.04 μm.

4.2 Corrosion rate analysis

The Tafel plot analysis on the cold-rolled AA8015-alloy samples at each surface roughness condition are presented in Figures 5–8. The current-potential data obtained were plotted as logarithms of current against potential for both the anodic and cathodic branches. Straight lines that best fit the data at high potentials were achieved at selected potential range markers on both anodic and cathodic curves. The point of intersection yields the corrosion current density (I_corr) and the corrosion potential (E_corr). The corresponding electrochemical kinetic parameters and corrosion rate for each surface roughness condition of the cold-rolled AA8015-alloy in natural seawater solution were computed and given in Tables 6–9.

Figure 5.

Tafel analysis of cold-rolled aluminium 8015-alloy at R_a ≈ 1.54 μm. (a) Sample 1; (b) sample 2; (c) sample 3.

Figure 6.

Tafel analysis of cold-rolled aluminium 8015-alloy at R_a ≈ 0.83 μm. (a) Sample 1; (b) sample 2; (c) sample 3.

Figure 7.

Tafel analysis of cold-rolled aluminium 8015-alloy at R_a ≈ 0.18 μm. (a) Sample 1; (b) sample 2; (c) sample 3.

Figure 8.

Tafel analysis of cold-rolled aluminium 8015-alloy at R_a ≈ 0.04 μm. (a) Sample 1; (b) sample 2; (c) sample 3.

Corrosion parameters	Sample 1	Sample 2	Sample 3
E. corr (V)	−0.7746	−0.7289	−0.7324
i cor. (A)	9.78E-07	1.42E-06	1.16E-06
I cor. (A/cm^2)	9.78E-07	1.42E-06	1.16E-06
Rp (Ohm)	14,040	6051	7173
ba (V/dec)	0.036	0.021	0.02
bc (V/dec)	0.266	0.288	0.485
C. Rate (mm/y)	0.01063	0.01538	0.01266

Table 6.

Corrosion rate analysis of cold-rolled AA8015-alloy at R_a ≈ 1.54 μm.

Corrosion parameters	Sample 1	Sample 2	Sample 3
E. corr (V)	−0.735	−0.7639	−0.7708
i cor. (A)	1.17E-06	1.19E-06	4.23E-07
I cor. (A/cm^2)	1.17E-06	1.19E-06	4.23E-07
Rp (Ohm)	8021	10,650	28,580
ba (V/dec)	0.022	0.032	0.03
bc (V/dec)	0.813	0.379	0.443
C. Rate (mm/y)	0.01268	0.01297	0.004599*

Table 7.

Corrosion rate analysis of cold-rolled AA8015-alloy at R_a ≈ 0.83 μm.

Corrosion parameters	Sample 1	Sample 2	Sample 3
E. corr (V)	−0.7509	−0.7086	−0.729
i cor. (A)	2.53E-06	1.39E-06	8.81E-07
I cor. (A/cm^2)	2.53E-06	1.39E-06	8.81E-07
Rp (Ohm)	8236	8130	17,130
ba (V/dec)	0.054	0.028	0.039
bc (V/dec)	0.458	0.341	0.309
C. Rate (mm/y)	0.02749*	0.01516	0.009581

Table 8.

Corrosion rate analysis of cold-rolled AA8015-alloy at R_a ≈ 0.18 μm.

Corrosion parameters	Sample 1	Sample 2	Sample 3
E. corr (V)	−0.683	−0.731	−0.7398
i cor. (A)	3.89E-06	7.11E-07	4.38E-07
I cor. (A/cm^2)	3.89E-06	7.11E-07	4.38E-07
Rp (Ohm)	4089	23,520	38,160
ba (V/dec)	0.039	0.044	0.043
bc (V/dec)	0.721	0.286	0.407
C. Rate (mm/y)	0.04223*	0.00773	0.004764

Table 9.

Corrosion rate analysis of cold-rolled AA8015-alloy at R_a ≈ 0.04 μm.

The mean value calculated results in Table 10, reveals the effect of polarisation resistance (R_p) and corrosion rate on the surface roughness of cold-rolled AA8015-alloy in natural seawater solution at room temperature. Significant observation reveals increase in R_p values as the cold-rolled alloy surface roughness get smoother. Low R_p value of 9.088 kΩ recorded for surface roughness, R_a ≈ 1.54 μm shows low resistance to corrosion attack as compared to high R_p value of 30.84 kΩ recorded for surface roughness, R_a ≈ 0.04 μm. This is evidence in the visual inspection macrograph after electrochemical corrosion. See Figure 9. Similarly, the rate of corrosion decreases as the surface roughness reduces.

Surface roughness, Ra (μm)	Polarisation resistance, Rp (kΩ)	Corrosion rate, (mm/yr)
1.54	9.088	0.01289
0.83	9.336	0.012825
0.18	12.630	0.0123705
0.04	30.840	0.006247

Table 10.

Mean values of polarisation resistance and corrosion rate of cold-rolled AA8015-alloy at different surface roughness values.

Figure 9.

Macrographs showing localised corrosion attack on cold-rolled AA8015-alloy surface samples after electrochemical corrosion. (a) R_a ≈ 1.54 μm; (b) R_a ≈ 0.83 μm; (c) R_a ≈ 0.18 μm (d) R_a ≈ 0.04 μm.

4.3 Visual inspection and SEM analysis

The macrographs in Figure 9 were taken after electrochemical corrosion experiment. The presence of black spots on the surface reveals the extent of the attack in the form of localised corrosion. The magnitude of dissolution of the cold-rolled aluminium alloy in natural seawater solution revealed in the macrographs confirms increase in corrosion resistance as the surface roughness becomes smoother.

Further microstructural analysis using Scanning Electron Microscope (SEM) revealed corrosion by pitting in all the surface roughness conditions, given in Figures 10–13. In addition, substantial insoluble substrate complexes were observed, confirming evidence of corrosion. However, SEM images at surface roughness, R_a ≈ 1.54 μm is without the presence of insoluble substrate complex. This could be due to the high mechanical surface flaws because of the 320-grit SiC paper. Moreover, EDS analysis in Figure 14 shows sulphur and chlorine ions present in the insoluble substrate confirming the adsorption chloride and sulphur molecules in seawater at the defective spots were oxide layer is dissolved.

Figure 10.

SEM images of corroded cold-rolled AA8015 sample at R_a ≈ 1.54 μm. (a) Image before corrosion at 314-x magnification; (b-d) images after corrosion at increased magnifications.

Figure 11.

SEM images of corroded cold-rolled AA8015 sample at R_a ≈ 0.83 μm. (a) Image before corrosion at 209-x magnification; (b-d) images after corrosion at increased magnifications.

Figure 12.

SEM images of corroded cold-rolled AA8015 sample at R_a ≈ 0.18 μm. (a) Image before corrosion at 219-x magnification; (b-d) images after corrosion at increased magnifications.

Figure 13.

SEM images of corroded cold-rolled AA8015 sample at R_a ≈ 0.04 μm. (a) Image before corrosion at 246-x magnification; (b-d) images after corrosion at increased magnifications.

Figure 14.

EDS spectrum showing the elemental composition of the insoluble substrate on the surface.

5. Conclusions and future work

Electrochemical corrosion of the cold-rolled AA8015-alloy at varying surface roughness in natural seawater was investigated. This was shown in the open circuit potential and potentiodynamic polarisation. Outcome revealed:

1. Evidence of pitting corrosion

2. Subsequent surface roughness conditions examined shows that surface roughness affects the corrosion resistance of cold-rolled AA8015-alloy

3. Presence of insoluble substrate on AA8015-alloy surface in corrosion process

4. Corrosion resistance increases with high percent reduction of cold rolled AA8015-alloy

For in-depth analysis and to understand the active corrosion characteristics of cold rolled AA8015-alloy and in addition to the relationship between the microstructural evolution and corrosion behaviour of cold rolled AA8015 in natural sea water. Future investigation using X-ray diffraction (XRD), electron back scatter diffraction (EBSD) and X-ray photoelectron spectroscopy (XPS) are recommended.

Additional information

This paper is a revised and expanded version of a paper entitled [Corrosion behaviour of cold-rolled aluminium 8015-alloy in natural sea water at 0.18 μm surface roughness] accepted for presentation at [4th International Conference on Mechanical, Manufacturing and Plant Engineering, Melaka, Malaysia. November 14–15, 2018].