Structure–property quantification of corrosion pitting under immersion and salt-spray environments on an extruded AZ61 magnesium alloy

Corrosion Science 53 (2011) 1348–1361

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Structure–property quantification of corrosion pitting under immersion and salt-spray environments on an extruded AZ61 magnesium alloy Holly J. Martin ⇑, M.F. Horstemeyer, Paul T. Wang Center for Advanced Vehicular Systems (CAVS), Mississippi State University (MSU), 200 Research Boulevard, Starkville, MS 39759, USA

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Article history: Received 23 August 2010 Accepted 21 December 2010 Available online 12 January 2011 Keywords: A. Magnesium A. Alloy A. Aluminium C. Intergranular corrosion C. Pitting corrosion

a b s t r a c t Extruded AZ61 magnesium coupons were exposed to immersion and cyclical salt spray environments over 60 h in order to characterize their corrosion rates. The characteristics of general corrosion, pitting corrosion, and intergranular corrosion were quantified at various intervals. General corrosion was more prevalent on the immersion surface. In addition, more pits formed on the immersion surface due the continuous exposure to water and chloride ions. However, the pits on the salt spray surface showed larger surface areas, larger volumes, and covered more area on the micrographs as compared to the pits on the immersion surface, due to the dried pit debris that trapped chloride ions within the pits. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction The aerospace and automotive industries have recently developed an interest in lightweight metallic alloys [1,2]. Because of its low density and high mechanical stiffness, magnesium alloys are one such material being investigated. A high corrosion rate as compared to steel or aluminium, though, relegates the use of magnesium to places that are unexposed to the environment [3–5]. One way to improve the corrosion resistance of magnesium is to include different elements, such as aluminium and zinc [4]. The addition of aluminium has been shown to affect corrosion resistance when the magnesium alloy contains up to 10% of aluminium [4]. The presence of aluminium in the b-phase precipitate, which appears as Mg17Al12, can lead to improved corrosion resistance when the b-phase was continuous and finely divided or reduced corrosion resistance when the b-phase was small [4,6–8]. When the b-phase is continuous, corrosion resistance is improved because corrosion is confined within the magnesium grains [6–8]. However, when the b-phase is small, corrosion resistance is decreased because the small particulate act as a micro-galvanic cathode, increasing the corrosion rate [8]. In addition to the distribution of the b-phase, the environment to which magnesium is exposed also plays a vital role in the corrosion properties. Salt spray testing and immersion testing are two of the main techniques for corrosion studies, which were employed in this research in an effort to expose the AZ61 magnesium alloy to an environment similar to that experienced by automotive engine ⇑ Corresponding author. Tel.: +1 662 325 9399; fax: +1 662 325 5433. E-mail address: [email protected] (H.J. Martin). 0010-938X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.12.025

blocks. The two testing environments are so commonly used that ASTM standards have been developed for both the salt spray environment and the immersion environment, labelled as ASTM B-117 and ASTM G-31, respectively [10,11]. However, the two ASTM standards require different concentrations of salt, meaning that a direct comparison between the results obtained from the two methods cannot be made [10,11]. In addition, the two methods do not translate well to corrosion field tests performed for the automotive industry [12,13]. This issue has led to the industrial development of cyclical tests, which contain a pollution phase and a wet or dry phase, in an effort to expose test alloys to the environmental factors associated with engine cradles, such as deicing salt, mud, and condensation [13,14]. However, even an accurate comparison between these tests cannot be made, as none of the variables are similar, with varying concentrations of salt, temperature, humidity, and exposure time [13]. In an effort to solve these issues, research into the most corrosive test matrix using an as-cast AE44 magnesium alloy compared corrosion mechanisms from exposure to one of four cyclical test combinations, including a humidity–drying cycle, a salt spray–drying cycle, and a salt spray– humidity cycle, and a salt spray–humidity–drying cycle [15]. By examining the corrosion mechanisms, the salt spray–humidity– drying test matrix proved to be the most corrosive [15], which was further used to compare with an immersion environment. In order to test the corrosion environments, one examines the different magnesium alloys individually. In the first comparison between the cyclical salt spray environment and the immersion environment used as-cast AE44 magnesium [16], which contains aluminium and rare earth elements. The addition of rare earth (RE) elements, such as cesium,

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lanthanum, neodymium, and praseodymium, improves the creep resistance of magnesium due to the formation of meta-stable REcontaining phases along the grain boundaries [1,2,17–19]. Along with improved creep resistance, corrosion resistance is also improved with the addition of RE elements, due to the formation of passive films that contain these RE elements [17–20]. In addition, the RE elements shift the corrosion location from along the aphase magnesium grain – b-phase precipitate boundary to the interior of the a-phase magnesium grain [18,21]. With respect to the individual pit characteristics, the immersion surface showed larger pit surface areas and pit volumes while both surfaces had equivalent pit depths [16]. With respect to the surface characteristics, the pit number density and pit area were larger on the immersion surface, indicating also that the nearest neighbour of pits was high [16]. The differences in pit characteristics and in surface characteristics were attributed to the environment to which the coupons were exposed. Since the coupons exposed to the immersion surface had water and chloride ions continuously surrounding them, the pits could form and grow due to the reaction with the chloride ions, while general corrosion eventually reduced the magnesium surrounding the pits, reducing the pits in number and in size [16]. The coupons exposed to the cyclical salt spray environment had a harder time forming pits and having the pits grow because water and chloride ions were not present continuously. In addition, because the as-cast skin was still present, corrosion resistance was improved more than 10-fold over the bulk, similar to AZ91 [6,9]. In fact, the as-cast skin can affect corrosion even after polishing, where a thin layer of the small grains characteristic of the as-cast skin may remain on the surface of lightly polished (2400 grit) magnesium [22]. Looking at semi-polished and polished as-cast AE44 exposed to the immersion environment, the greater polished surface showed more corrosion; however, this trend was opposite from that observed on polished copper and stainless steel [22]. The differences between the magnesium surface and the copper and stainless steel surfaces can be attributed to the possible presence of a small layer of as-cast skin that reduces the corrosion on the semi-polished surface that was not present on the polished surface [22]. The process of extrusion, however, would remove this as-cast skin, affecting the corrosion resistance of the magnesium alloy. The purpose of this study, then, was to quantify the corrosion mechanisms on extruded AZ61 exposed to either the cyclical salt spray environment or the immersion environment. This research, therefore, is an effort to determine how common exposure environments, similar to those experienced by automobiles, affect corrosion mechanisms. Optical microscopy, laser profilometry, weight, and thickness measurements were performed on an extruded AZ61 exposed to one of two environments over 60 h of exposure.

2. Materials and methods 2.1. Testing Twelve AZ61 coupons (2.54  2.54 cm  varying thicknesses) were cut from an extruded crash rail provided by Ford using a CNC Mill (Haas, Oxnard, CA). The coupon surfaces were left untreated, with no surface grinding or polishing, to test the corrosion effects on an extruded AZ61 magnesium alloy. Two different test environments were used in this study: salt spray testing and immersion. For salt spray testing, a Q-Fog CCT (Q-Panel Lab Products, Cleveland, OH) was used to cycle through three stages set at equal times, including a 3.5 wt.% NaCl spray at 35 °C, 100% humidity using distilled water at 35 °C, and a drying purge at 35 °C. For immersion testing, an aquarium with an aeration unit was filled

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with 3.5 wt.% NaCl at room temperature. For both tests, the six coupons per test environment were hung at 20° to the horizontal, as recommended by ASTM B-117 [19]. The coupons were exposed to the test environment for 1 h, removed, rinsed with distilled water to remove excess salt, and dried. Following the profilometer analysis, the coupons were then placed back into the test environment for an additional 3 h, an additional 8 h, an additional 24 h, and another 24 h. These times allowed for a longitudinal study to follow pit growth and surface changes over time, where t0 = 0, t1 = 1 h, t2 = 4 h, t3 = 12 h, t4 = 36 h, and t5 = 60 h. Between analyses and environmental exposures, the coupons were stored in a desiccator to ensure that no further surface reactions occurred. 2.2. Analysis Following each time exposure, the coupons were analyzed using optical microscopy and laser profilometry. The coupons were weighed prior to testing and following each exposure on two different scales and an average was taken. Four thickness measurements were taken on each sample prior to and following the test. Because the coupons were cut from an engine cradle, the thicknesses of the coupons varied from side to side, meaning an average was taken per coupon based on the four measurements. Measurements for all figures were averaged from the data with error bars based on one standard deviation. Optical microscopy with an inverted light was used to take multiple images of the resulting surface at 5 magnification and 10 magnification (Axiovert 200 M Mat, Carl Zeiss Imaging Solutions, Thornwood, NY). The 5 magnification images were combined and then analyzed using the ImageAnalyzer (v. 2.1-2) provided by Mississippi State University to determine the number of pits, the pit surface area, the nearest neighbour radius, and the intergranular corrosion area fraction necessary for the development of a corrosion model not detailed in this paper but previously outlined by Horstemeyer et al. [23]. The 10 magnification was used to pictorially show the changes over the six cycles. Laser profilometry was used to scan a 1 mm by 1 mm area on two coupons per environment following each test cycle (Talysurf CLI 2000, Taylor Hobson Precision Ltd., Leicester, England). The resulting 2-D and 3-D images were used to document the changes in the pit characteristics due to the different test environments over the six cycles (Talymap Universal, v. 3.18, Taylor Hobson Precision Ltd., Leicester, England). Data was collected based on 14 pits within each 1 mm by 1 mm area, for a total of 28 data points per environment per cycle. 3. Results Figs. 1 and 2 show the bulk changes in the coupons exposed to the two different corrosive environments. Because corrosion characteristics, such as pitting, may only slightly affect bulk changes, such as weight and thickness, other surface related phenomena were examined. Figs. 3–7 show the changes in individual pit characteristics of pit depth, pit surface area, and pit volume over the five exposure times for both the immersion environment and the salt spray environment, where Figs. 3–5 graphically compare the changes in pit depth, pit surface area, and pit volume, while Figs. 6 and 7 illustrate the changes in the pits over time. In addition to changes in individual pit characteristics, changes in the surface characteristics should also be scrutinized. Figs. 8–12 show the changes in surface characteristics, including pit number density, pit area, nearest neighbour distance, and intergranular corrosion area fraction (ICAF) as shown graphically in Figs. 8–11, while Figs. 12a and 12b show the changes in surface characteristics pictorially. Figs. 1 and 2 show the average weight loss and thickness loss, respectively, over the five exposure times for both the immersion

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Fig. 1. Average normalized weight loss of extruded AZ61 coupons based on test environment over 60 h (sample weight divided by 6.4516 cm2). The error bars were one standard deviation in each direction. Notice that the coupons exposed to the salt spray environment lost less weight, and actually gained weight due to the attachment of chloride ions. Note that both weight changing trends followed a linear behaviour.

Fig. 2. Average thickness loss of AZ61 coupons based on test environment over 60 h. The error bars were one standard deviation in each direction. Notice that the coupons exposed to the immersion environment lost less magnesium than the coupons exposed to the salt spray environment, although the thickness loss followed the same logarithmic trend.

environment and the salt spray environment. As one can see, both environments followed a linear trend for weight loss (Fig. 1) and a logarithmic trend for thickness loss (Fig. 2). Interestingly, the salt spray samples saw a greater thickness loss but also ‘‘gained’’ weight through the experiments, due to salt collecting in the forming pits. The immersion samples lost less thickness than the salt spray samples but also lost more weight than the salt spray samples. Fig. 3 shows the changes in both the maximum pit depth and mean pit depth over 60 h. The mean pit depth and maximum pit

depth followed a second-order polynomial trend for the salt spray environment, where the pit depth decreased until approximately 29 h, before increasing to 60 h where the pit depth was higher than the initial pit depth. The mean pit depth and maximum pit depth followed a slight second-order polynomial trend for the immersion environment, where the final pit depth at 60 h was higher than the initial pit depth. Fig. 4 shows the changes in the individual pit surface area over 60 h for both the immersion environment and the salt spray envi-

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Fig. 3. Maximum and mean pit depths based on test environment over 60 h. The error bars were one standard deviation in each direction. Notice that the maximum pit depth and the mean pit depth for the salt spray environment followed a second-order polynomial trend, where pit depth decreased to approximately 30 h before increasing to 60 h. Also notice that the maximum pit depth and the mean pit depth for the immersion environment followed a slight second-order polynomial trend, where pit depth increased to 60 h. However, given the high uncertainty bands, the trends were unclear.

Fig. 4. Individual pit surface area based on test environment over 60 h. The error bars were one standard deviation in each direction. Notice that the initial pit area for both environments was approximately the same. However, after the initial exposure, the pit area on the salt spray surfaces was larger. The salt spray surface followed a secondorder polynomial trend, which increased to approximately 48 h before decreasing to 60 h, while the immersion surface seemed to reach a saturation value.

ronment. The individual pit surface area for the salt spray environment followed a second-order polynomial trend which increased to approximately 48 h before decreasing to 60 h, although the decrease was small. Notice the error bars increased at t4 and t5 as compared to t3, indicating that there was a large variation in the surface area of the pits. The individual pit surface area for the immersion environment followed a slight second-order polyno-

mial trend, but the pit surface area seemed to reach a saturation point between t2 and t3. In addition, notice there was not much variation in the pit surface area based on the error bars. Fig. 5 shows the changes in the individual pit volume over 60 h for both the immersion environment and the salt spray environment. The pit volume for the salt spray environment followed a second-order polynomial trend which increased until 60 h. Notice

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Fig. 5. Individual pit volume based on test environment over 60 h. The error bars were one standard deviation in each direction. Notice that the initial pit volume was higher for the salt spray environment as compared to the immersion environment. The salt spray surface followed a second-order polynomial trend, although the volume mainly increased, while the immersion surface reached a saturation value.

Fig. 6. Pit number density based on test environment over 60 h. The error bars were one standard deviation in each direction. Notice that the initial number of pits on the surface was higher for the immersion environment as compared to the salt spray environment. The immersion surface followed a second-order polynomial trend, with the pit number density increasing until approximately 30 h. The salt spray surface also followed a second-order polynomial trend, with the pit number density increasing until approximately 26 h.

the error bars increased following t3, indicating that there was a large variation in the area of the pits. As with the individual pit surface area, the individual pit volume for the immersion environment followed a slight second-order polynomial trend, although the pit volume seemed to reach a saturation point between t1 and t2. Also notice there was not much variation in the pit volume as shown be the slight changes in the error bars.

Fig. 6 shows the change in pit number density, which is the number of pits per unit area, for both the immersion environment and the salt spray environment. As one can see, the number of pits on both surfaces followed a second-order polynomial trend, where the pit number density increased until approximately 30 h for the immersion surface and 26 h for the salt spray surface. After those times, the pit number density decreased, resulting in a pit number

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Fig. 7. In-plane pit area based on test environment over 60 h. The error bars were one standard deviation in each direction. Notice that the initial pit area on the surface was approximately equal for the immersion environment as compared to the salt spray environment. Also note the large deviation in pit area for both environments at 36 h, indicating a large difference due to growth and coalescence of previously formed pits and pit nucleation. The immersion surface followed a second-order polynomial trend, with the pit area increasing until approximately 32 h. The salt spray surface followed a slight second-order polynomial trend, although the pit area never began decreasing during the 60 h experiment time.

Fig. 8. Nearest neighbour distance based on test environment over 60 h. The error bars were one standard deviation in each direction. Notice that the initial nearest neighbour distance on the surface was lower for the immersion environment as compared to the salt spray environment. Both the immersion surface and the salt spray surface followed second-order polynomial trends. The nearest neighbour distance decreased until approximately 29 h for the salt spray environment and approximately 19 h for the immersion environment before increasing.

density at 60 h that was still lower than the initial pit number density for the immersion surface and approximately equal on the salt spray surface. Fig. 7 shows the changes in pit area, which is the 2-D area covered by the pits as seen by the micrographs, for the immersion and

salt spray environments. As with the pit number density, the pit area on the immersion surface followed a second-order polynomial trend, while the pit area on the salt spray surface was not linear but was a slight second-order polynomial. In the immersion environment, the pit area increased until approximately 32 h, after

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Fig. 9. Intergranular corrosion area fraction based on test environment over 60 h. The error bars were one standard deviation in each direction. Notice that the initial intergranular corrosion area fraction on the surface was higher for the immersion environment as compared to the salt spray environment. Both the immersion surface and the salt spray surface followed second-order polynomial trends. The ICAF decreased until approximately 18 h for the salt spray environment and approximately 29 h for the immersion environment before increasing.

which the pit area decreased, resulting in a pit area at 60 h that was higher than the initial pit area. The salt spray environment saw an increase in the pit area throughout the experiment, with no decrease seen. In addition, notice the high standard deviation at 36 h (t4), indicating a large difference in pit area between the various pits present. Fig. 8 shows the changes in the nearest neighbour distance, which is the distance between two pits, for the two environments being tested. As with the pit number density and pit area, the nearest neighbour distance on both surfaces followed a second-order polynomial trend; however, the nearest neighbour distance is inversely related to the pit number density and pit area. In the immersion environment, the nearest neighbour distance decreased until approximately 19 h, after which the nearest neighbour distance increased, resulting in a nearest neighbour distance at 60 h that was higher than the initial nearest neighbour distance. The salt spray environment saw an initial decrease in the nearest neighbour distance until approximately 29 h, after which the nearest neighbour distance increased until 60 h, resulting in a final nearest neighbour distance that was higher than the initial nearest neighbour distance. Fig. 9 shows the changes in intergranular corrosion area fraction (ICAF) for the salt spray and immersion environments. Intergranular corrosion is the corrosion that occurs between the grains in the b-phase precipitate phase of the alloy, while the ICAF is the fraction of the surface that displays intergranular corrosion. As one can see, the ICAF on both surfaces followed a second-order polynomial trend. On the immersion surface, the ICAF initially decreased until approximately 29 h before increasing to a value higher than the initial ICAF. On the salt spray surface, a very small decrease was observed from the initial exposure until approximately 18 h before increasing to a value much higher than the initial ICAF. Figs. 10a and 10b show the changes in individual pit characteristics in 2-D over 60 h for both the salt spray environment (Fig. 10a) and the immersion environment (Fig. 10b) using laser profilometry, while Figs. 11a and 11b show the changes in individ-

ual pit characteristics in 3-D. Both figures are arranged to show the changes between individual pits covering an area of 30 lm by 30 lm for each of the times examined, where t0 = 0, t1 = 1 h, t2 = 4 h, t3 = 12 h, t4 = 36 h, and t5 = 60 h. The colours displayed in the figure indicated distance from the lowest point within the pits, as shown by shades of blue and listed as zero in the legend, to the highest point, as shown by the pinks and reds. When looking at the salt spray surface, an initial increase in the number of pits was seen between t0 and t1. Between t1 and t2, the pits grew slightly while the surrounding surface remained. Following t3, the surrounding surface was gradually reduced while the pits began to connect, growing in area. For both t4 and t5, the pits grew in area, connecting with nearby pits, while the surrounding surface was left uncorroded. When looking at the immersion surface, one can see that the number of pits increased between t0 and t1, while the pits began to connect following t2. The pits then began to shrink in size and number following t3 with almost no difference in pit number or size appearing between t4 and t5. Figs. 12a and 12b show the changes in surface characteristics using micrographs over 60 h for both the salt spray environment (Fig. 12a) and the immersion environment (Fig. 12b). The figure is arranged to show the changes between the surfaces covering an area of 1 mm by 1 mm with scale bars of 500 lm for each of the times examined, where t0 = 0, t1 = 1 h, t2 = 4 h, t3 = 12 h, t4 = 36 h, and t5 = 60 h, and to show the differences in the pits between the salt spray (Fig. 12a) and the immersion (Fig. 12b) environments. The lines seen on both samples were the result of the extrusion process and were not added by grinding or polishing. Examining the salt spray surface, one can see a large number of pits present appearing between t0 and t1. The pits present following t1 then increase in size following t2 and t3 while more pits also appear during those exposures. Following t4, pit growth and coalescence is seen, as the pit corrosion expands across the surface. Examining the immersion surface, one can see some pit formation appearing following each exposure (t1, t2, t3, and t4). Following t5, one can see the growth in pit area, although this does not follow the same trend as Fig. 9.

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Fig. 10a. 2-D Laser Ppofilometry of the salt spray surface over 60 h. All spots were examined over a 30  30 lm area. Height measurements were made from the lowest point, shown as dark blue and listed as zero in the legend, to pink and red areas that indicate height. Notice that the number of pits initially grew for both surfaces from t0 to t1 (1 h), while the pits grew in area following t2 (4 h), increasing in size and connecting with neighbours, while the surrounding area was generally left uncorroded (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

4. Discussion Figs. 1 and 2 show the overall changes in the coupons based on the environment to which each was exposed. The change in weight, as shown in Fig. 1, decreased for the immersion environment, while it slightly increased for the salt spray environment. The decrease in weight for the immersion environment is the effect of general corrosion removing magnesium from the coupons. The increase in weight for the salt spray environment is suspected to be caused by evaporation of water during the drying phase, leaving behind salt residuals which added weight. ‘‘Channels’’ were observed on the surface of the extruded material in which small

amounts of salt were trapped that were not removed during the cleaning process, thus resulting in the weight increase. As one can observe from Fig. 1, the overall increase in weight possibly caused by trapped salt only resulted in an increase of 0.001 mg, meaning that there was some removal of magnesium that was not observed due to the trapped salt, meaning that general corrosion was occurring on the salt spray samples even though there was no weight loss observed. Fig. 2 further confirms that general corrosion took place on surfaces exposed to both environments, as the overall thickness of the coupons decreased for both the salt spray and immersion environments. Essentially, the coupons lost magnesium atoms because of

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Fig. 10b. 2-D Laser profilometry of the immersion surface over 60 h. All spots were examined over a 30  30 lm area. Height measurements were made from the lowest point, shown as dark blue and listed as zero in the legend, to pink and red areas that indicate height. Notice that the number of pits initially grew in number from t0 to t1 (1 h), before connecting with neighbours following t2 (4 h). Following t2, the pits began shrinking in size before reaching an appearance that was similar for t4 (36 h) and t5 (60 h). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

the interaction with water at the surface, thereby reducing the thickness and the weight of the coupons. However, if general corrosion was solely the cause of thickness loss, then one would expect that the continuous presence of water in the immersion environment would cause more magnesium loss. In this study though, qualitative observations showed more pitting and intergranular corrosion along the edges of the salt spray coupons when compared to the immersion coupons. For the salt spray coupons, the thickness loss was not solely contingent on general corrosion, as some pits and intergranular corrosion occurred at the coupon edges, meaning the thickness would be affected by general corrosion, pitting corrosion, and intergranular corrosion. The presence

of pitting and intergranular corrosion on the salt spray coupons was believed to be related to our test setup. Because the coupons hung at a 20° angle to the horizontal, the salt-containing water would want to run off the edges and down to the bottom of the coupon, meaning more chloride ions would remain at the edges as the water dripped off. The increased amount of chloride ions at the edge of the coupons increased the pitting and intergranular corrosion during the drying phase, thereby reducing the thickness at the point of measurement. When one looks at the change in thickness, one would expect to see a much higher amount of weight loss, even if the salt residuals caused a change in the weight. In order to see a complete offset,

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Fig. 11a. 3-D Laser profilometry of the salt spray surface over 60 h. All spots were examined over a 30  30 lm area. Height measurements were made from the lowest point, shown as dark blue and listed as zero in the legend, to pink and red areas that indicate height. Notice that the number of pits initially grew for both surfaces from t0 to t1 (1 h), while the pits grew in area following t2 (4 h), increasing in size and connecting with neighbours, while the surrounding area was generally left uncorroded (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

Fig. 11b. 3-D Laser profilometry of the immersion surface over 60 h. All spots were examined over a 30  30 lm area. Height measurements were made from the lowest point, shown as dark blue and listed as zero in the legend, to pink and red areas that indicate height. Notice that the number of pits initially grew in number from t0 to t1 (1 h), before connecting with neighbours following t2 (4 h). Following t2, the pits began shrinking in size before reaching an appearance that was similar for t4 (36 h) and t5 (60 h) through t5. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

and even an increase in the weight of the coupons, there would need to be an almost exact one to one ‘‘replacement’’ of Mg(OH)2 and NaCl, as a mole of each weighs 58.32 g and 58.44 g, respectively. One would think that the chances of this happening in a salt spray environment, with water causing the removal of some of the chloride ions, would be very low. Therefore, the actual weight loss would be much less than the weight loss of the immersion coupons

and that the thickness loss was caused by the concentration of chloride ions at the edges of the coupons. Therefore, more general corrosion occurred on the immersion coupons as compared to the salt spray coupons, as demonstrated on as-cast AE44 coupons [16]. Figs. 3–5 show changes in individual pit characteristics. These individual characteristics were obtained to show how the pits, and the surface, changed based on exposure time. When looking

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Fig. 12a. Micrographs of the salt spray surface over 60 h. All spots were examined over a 1  1 mm area, with scale bars of 500 lm each. The darker areas indicate pit formation and coalescence, while the lighter areas indicate surfaces closer to the light and those affected by general corrosion. Notice that the pits grew in number through t3 (12 h), decreasing in number following t3. Also notice the pits grew in area through t5 (60 h). Also, the lines seen on the samples were the result of the extrusion process and were not a result of grinding or polishing.

at Fig. 3 for the salt spray surfaces, one can see that the pit depth decreased until t4, after which it increased. However, looking at the error bars show that the pit depth varied greatly among the pits examined, indicating that the decrease in pit depth might not actually be occurring. Instead, because more pits would appear following each cycle, until approximately 26 h as shown in Fig. 6, the pit depth of the ‘‘new’’ pits would be less than the pit depth of the established pits, reducing the overall pit depth. The pit surface area and volume for the salt spray surface grew unabated until t4 for the pit surface area and until t5 for the pit volume (Figs. 4 and 5). Like the pit depth, though, large variations in both pit surface area and pit volume exist on the salt spray surface, as the error bars show. These variations again are due to the new pits, which have smaller areas and volumes as compared to the established, growing pits. However, even with the reduction to the average caused by the small pit surface area and pit volume of these new pits, one can see that pit surface area and volume grew throughout most of the exposure time. Major differences are observed between the salt spray surfaces and the immersion surfaces for Figs. 3–5. When looking at the immersion surfaces, the pit depth gradually increases throughout the exposure time (Fig. 3). Minimal differences in pit depths are observed as demonstrated by the error bars. The pit surface area and the pit volume both reach a saturation value following t2, with

minimal changes in either the surface area or the volume as demonstrated by the error bars (Figs. 4 and 5). The small variations in pit depth, pit surface area, and pit volume indicate that pits which formed during the exposure time were then affected by something in the environment. Both the changes in pit characteristics on the salt spray surfaces and the lack of change in pit characteristics on the immersion surfaces can be explained by the exposure environment. Because the salt spray environment was cyclical, chloride ions were only exposed to the surface for a brief time, which would reduce how often new pits could form. The immersion surfaces were continuously exposed to the dissolved chloride ions, meaning new pits could form at any point. The ability of the pits to form whenever ‘‘desired’’ on the immersion surfaces means that depth would not be affected. However, because pits could only form during the chloride ion exposure period on the salt spray surfaces, the time separating the exposure would be a major determining force in pit depth. Following the ‘‘pollution’’ cycle, a wet phase (humidity) followed, where salt-free water was introduced to the surface. During this time, water was present to help remove some of the chloride ions and the corrosion by-products. Unlike the salt spray environment though, the immersion environment allowed water to be present against the coupons throughout the exposure time. On

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Fig. 12b. Micrographs of the immersion surface over 60 h. All spots were examined over a 1  1 mm area, with scale bars of 500 lm each. The darker areas indicate pit formation and coalescence, while the lighter areas indicate surfaces closer to the light and those affected by general corrosion. Notice that the pits grew in number and area through t3 (12 h), while decreasing in number and size following t3. Also, the lines seen on the samples were the result of the extrusion process and were not a result of grinding or polishing.

the immersion environment, this continuous presence would remove pit debris, as well as surrounding magnesium. General corrosion as evidenced by the removal of the surrounding magnesium, would reduce both pit surface area and pit volume, since the ability to observe the pits growing would be reduced due to the reduction in surrounding magnesium. The removal of the pit debris would also serve as a way to reduce pit growth, since some water could diffuse into the pit and remove some of the chloride ions, thereby serving to slow pit corrosion. With respect to the salt spray surfaces though, the humidity would only serve to increase the ability of the pits to grow. Because the humid environment would only cause condensation not complete immersion, some chloride ions could be removed, but the condensed water could also diffuse into the pits without being able to diffuse out. In addition, the condensed water could remove some pit debris but would not be able to remove a great deal of pit debris. The presence of corrosion byproducts outside the pits becomes an ‘‘issue’’ during the drying phase. Following the wet phase, a drying phase was introduced for the salt spray cycle that dried the surface. This drying phase stopped any general corrosion that might be occurring, while also drying the corrosion by-products in place, thereby trapping chloride ions and water beneath the pit debris. The trap formed beneath the pit debris allowed the chloride ions and the water to keep reacting

with the magnesium, thereby allowing the pits to grow in both surface area and volume. Because there was continuous exposure to water in the immersion environment, the pit debris was unable to remain on the surface, which meant that the chloride ions could not become trapped beneath the surface and would not allow individual pit growth. Figs. 6–9 show the changes in the surface characteristics. These surface characteristics will be used to examine the evolution of corrosion damage in a similar manner to the model developed for mechanical energy dissipation [23]. Briefly, the corrosion model will be developed as follows, although the mathematical framework will be left for another paper:

/ ¼ /gc þ /IC

ð1Þ

/IC ¼ gp v p c

ð2Þ

where / is the total damage from corrosion arising from any type of corrosion mechanism, /gc is the damage from general corrosion (loss of thickness), /IC is the damage from pitting, gp is the pit number density related to nucleation of pits (number per unit area or volume), vp is the area of pit growth related to growth of the pits,

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and c is a function of the nearest neighbour distance related to the coalescence of the pits. Pit number density (pit nucleation) and pit area (pit growth and coalescence) are related in two ways, although both ways make the trends similar. First, as more pits form in a finite area, those pits cover a certain area, meaning that both the pit number density and the pit area would increase approximately the same. Second, as the pits grow due to the corrosive by-products, the pit area would increase, while the pit number density remains the same, meaning that the pit area would grow larger than the pit number density. Pit number density and pit area would both decrease for the same reason: general corrosion. As general corrosion removes the magnesium surface, any newly formed pits would be removed, reducing the pit number density. In addition, as general corrosion removes the surrounding magnesium surface and corrosion byproducts, the pit area would be decreased. Lastly, however, is an opposite trend between pit number density and pit area. As the pits grow in size, they approach nearby pits. Eventually, these pits would ‘‘consume’’ each other, decreasing the pit number density but increasing the pit area. Pit number density (pit nucleation) and the nearest neighbour distance (pit nucleation, growth, and coalescence) are related in a single way – inversely. As more pits become present on the surface, the distance between the pits decreases. As with pit number density, the nearest neighbour distance and pit area are also related inversely. As the pits grow larger, the distance between the pits decreases, causing the nearest neighbour distance to decrease. As with the effect coalescence has on pit number density and pit area, once the closest pits coalesced, forming one large pit, the nearest neighbour distance increased, as the distance between the newly formed, large pits increased. While not directly related to pit number density (nucleation), pit area (pit growth and coalescence), and nearest neighbour distance (pit nucleation, growth, and coalescence), intergranular corrosion area fraction (ICAF) is related to the coalescence of the pits, because pits typically form along the a-phase magnesium grains and the b-phase precipitate boundaries [8]. While the majority of the pit coalescence occurs via pit growth, ICAF also allows the pits to ‘‘link’’ along the precipitate boundaries. As the ICAF increased, the pit coalescence also should increase. When comparing Figs. 6–9 on the salt spray surface, one sees a second-order polynomial trend for pit number density and nearest neighbour distance, although the two are inversely related (Figs. 6 and 8). However, the trend for both the pit area and ICAF are between second-order polynomial and linear, as there is no decrease in pit area (Fig. 7) or ICAF (Fig. 9). As expected, as the pit number density increases due to pitting corrosion, the pit area increases while the nearest neighbour distance decreases. In addition, the ICAF increased at similar rates to the pit area, indicating that the pits grew both out and along the a-phase magnesium grain and the b-phase precipitate boundary. At approximately 26 h, the pit number density begins decreasing, while the nearest neighbour distance begins increasing at approximately 29 h. However, general corrosion did not remove much during the exposure time, as shown in Fig. 1, meaning that pit area was uninhibited by general corrosion removing magnesium surrounding the pit. When looking at Figs. 10a and 11a, one can see the changes in the appearance of the pits over time, as the pits form and grow larger (t1–t2) before coalescing (t3–t5). As one can see from Fig. 12a, the pit area and ICAF increased because the pits previously formed began coalescing, which decreased the pit number density and increased the nearest neighbour distance. When comparing Figs. 6–9 on the immersion surface, one sees a second-order polynomial trend for all four figures. As previously explained, when the pit number density decreased, the pit area

can decrease or increase due to either general corrosion or pit growth, respectively. In the case of the immersion environment, general corrosion caused the decrease in pit number density, pit area, and ICAF, following 30 h, 32 h, and 18 h, respectively, as general corrosion began removing the surrounding magnesium (Fig. 1). The removal of magnesium from around the pits, as well as the removal of surface magnesium, which removed the newly formed pits, reduced both the pit area and the pit number density, while preventing intergranular corrosion between the pits. With the reduction in both size and number, the distance between the pits was able to increase following approximately 19 h, which would indicate that a few pits did coalesce prior to a switch from pitting corrosion to general corrosion, which is further demonstrated by the increase in ICAF after 18 h. When looking at Figs. 10b and 11b, one can see that the pits initially form and attempt to grow (t1–t2) before general corrosion begins to ‘‘even’’ the surface, thereby reducing the number and size of the pits (t3). Following t4 and t5, the number of pits and the size of the pits has been reduced to those present at t0. As one can see from Fig. 12b, the number of small pits present following t3 (the grey ‘‘shaded’’ areas) are mostly gone by t4, with several larger pits having formed. By t5, several large pits are present (pit area) that are far apart (nearest neighbour distance) with very few smaller pits present (pit number density), all of which follow the trends seen in Figs. 6–9. As with the individual pit characteristics, the differences in pit nucleation, pit growth, and pit coalescence can be attributed to the differences in the exposure environments. The higher pit number density on the immersion surfaces can be attributed to the continuous presence of chloride ions surrounding the magnesium surface due to the continuous presence of water, unlike the salt spray surfaces that experienced cyclical exposure to the salt-containing water. As shown in Figs. 6 and 10b, more pits formed on the immersion surface as compared to the salt spray surface. Because the presence of chloride ions is known to be responsible for the formation of pits [3], the continuous presence of chloride ions was enough to initiate more pit formations on the immersion surface. However, the continuous presence of water in the immersion environment resulted in the removal of corrosion by-products and the removal of the surrounding magnesium, meaning that the pit area would be decreased on the immersion surface, as shown in Figs. 7 and 10b. First, the removal of the corrosion byproducts on the immersion surface would reduce the ability of the pits to continue growing, as the corrosion by-products trap chloride ions within the pits. In the salt-spray environment, where the salt water is not present continuously, the corrosion by-products can build up, trapping chloride ions, and allowing the pits to grow even when there is no water present. Second, the removal of the surrounding magnesium is caused by the presence of water, meaning that the continuous presence of water would remove magnesium and prevent pit growth. In the case of the salt spray environment, water is only present during two of the three cycles meaning that during the drying phase, any general corrosion is stopped. With general corrosion stopped and the corrosion byproducts left in place, the pits on the salt spray surfaces are able to grow unabated, as shown in Figs. 7, 10a, 11a, 12a. It is the differences in exposure environments that cause more general corrosion to occur on the immersion surfaces and more pitting corrosion to occur on the salt spray surfaces. When comparing the results from the extruded AZ61 with the results from the as-cast AE44, one can see a distinct difference between the two types of magnesium. Because of the effect as-cast skin has on the corrosion resistance, one would think that the removal of the as-cast skin due to the extrusion process, which pushes magnesium through a billet to form the desired shape, would greatly influence the pit nucleation, growth, and coales-

H.J. Martin et al. / Corrosion Science 53 (2011) 1348–1361

cence. With respect to pit nucleation, more pits were formed on both of the extruded AZ61 surfaces as compared to the AE44 surfaces [16]. The smallest pit area existed on the AZ61 immersion surface, while the largest pits grew on the AZ61 salt spray surface [16]. Before the AZ61 salt spray surface reached t5, however, the pits growing on the AE44 surfaces were larger than either AZ61 surfaces [16]. The differences in pit number density and in pit area can be contributed to both the surface and the environment. The higher pit number density on both AZ61 surfaces would occur because the as-cast skin was removed, which ultimately prevented pit nucleation on the as-cast AE44 surfaces. Because the as-cast skin was removed, more general corrosion could occur on the AZ61 immersion surface, which reduced the pit area, making it smaller than the AE44 coupons. Also, because the as-cast skin was lacking, the pits could grow much larger over time on the AZ61 salt spray surface, as the as-cast skin was not preventing the pit growth while the as-cast skin was preventing the pit growth on the AE44 coupons. While there is a chance that the different elements in AE44 and AZ61 could affect the corrosion resistance, the removal of the as-cast skin does greatly affect the surface characteristics. 5. Conclusions An extruded AZ61 magnesium alloy was studied to understand its corrosion rates under two different environments: cyclical salt spray and immersion. The coupons were examined at various times to determine the effect of the two environments on general corrosion, pitting corrosion, and intergranular corrosion. The general trends were observed: 1. The general corrosion rate was higher for the immersion surfaces as compared to the salt spray surfaces, based on both thickness loss and weight loss. 2. Individual pitting characteristics, including pit surface area and pit volume, were greater for the salt spray surfaces as compared to the immersion surfaces. 3. The pitting nucleation (number density) rate was greater on the immersion surfaces as compared to the salt spray surfaces. 4. The nearest neighbour distance of pits was less for the immersion environment as compared to the salt spray environment, due to the higher pit number density on the immersion surfaces. 5. More surface growth and coalescence was seen on the salt spray surfaces as compared to the immersion surfaces.

Acknowledgements Financial support from the Center for Advanced Vehicular Systems (CAVS) at Mississippi State University is gratefully acknowledged. Support from Meridian Technologies, which provided the AE44 Mg engine cradle, is also deeply appreciated. This work was also supported by the Department of Energy and the National Energy Technology Laboratory under Award No. DEFC26-02OR22910. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, com-

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pleteness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favouring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Such support does not constitute an endorsement by the Department of Energy of the work or the views expressed herein. References [1] J.D. Majumdar, R. Galun, B. Mordike, I. Manna, Effect of laser surface melting on corrosion and water resistance of a commercial magnesium alloy, Mater. Sci. Eng. A 361 (2003) 119–129. [2] C. Blawert, E.D. Morales, W. Dietzel, K.U. Kainer, Comparison of corrosion properties of squeeze cast and thixocast MgZnRE alloys, Mater. Sci. Forum 488–489 (2005) 697–700. [3] M.G. Fontana, Corrosion principles, in: M.G. Fontana (Ed.), Corrosion Engineering, McGraw-Hill, Boston, 1986, pp. 12–38. [4] G. Song, A. Atrens, Understanding magnesium corrosion – a framework for improved alloy performance, Adv. Eng. Mater. 5 (2003) 837–858. [5] B.A. Shaw, Corrosion Resistance of Magnesium Alloys, in: L.J. Korb, ASM (Eds.), ASM Handbook, vol. 13A: Corrosion, ninth ed., ASM International Handbook Committee, Metals Park, 2003, pp. 692. [6] G. Song, Recent progress in corrosion and protection of magnesium alloys, Adv. Eng. Mater. 7 (2005) 563–586. [7] M.C. Zhao, M. Liu, G. Song, A. Atrens, Influence of the b-phase morphology on the corrosion of Mg Alloy AZ91, Corros. Sci. 50 (2008) 1939–1953. [8] G. Song, A. Atrens, Z. Wu, B. Zhang, Corrosion behavior of AZ21, AZ501, and AZ91 in sodium chloride, Corros. Sci. 40 (1998) 1769–1791. [9] G. Song, A. Atrens, M. Dargusch, Influence of microstructure on the corrosion of diecast AZ91D, Corros. Sci. 41 (1999) 249–273. [10] ASTM B117 – 07a (2007) Standard Practice for Operating Salt Spray (Fog) Apparatus, vol. 03.02, 2007. [11] ASTM G31 – 72 (2004) Standard Practice for Laboratory Immersion Corrosion Testing of Metals, vol. 03.02, 2004. [12] K.R. Baldwin, C.J.E. Smith, Accelerated corrosion tests for aerospace materials: current limitations and future trends, Aircr. Eng. Aerosp. Technol. 71 (1999) 239–244. [13] N. LeBozec, N. Blandin, D. Thierry, Accelerated corrosion tests in the automotive industry: a comparison of the performance towards cosmetic corrosion, Mater. Corros. 59 (2008) 889–894. [14] G. Song, D. St. John, C. Bettles, G. Dunlop, The corrosion performance of magnesium alloy AM-SC1 in automotive engine block applications, JOM 57 (2005) 54–56. [15] H.J. Martin, M.F. Horstemeyer, P.T. Wang, Effects of salt-spray, humidity, and drying on an as-cast AE44 magnesium alloy, Int. J. Corros. 2010 (2010) 1–10. doi: 10.1155/2010/602342. [16] H.J. Martin, M.F. Horstemeyer, P.T. Wang, Comparison of corrosion pitting under immersion and salt-spray environments on an as-cast AE44 magnesium alloy, Corros. Sci. 52 (2010) 3624–3638. [17] W. Liu, F. Cao, L. Chang, Z. Zhang, J. Zhang, Effect of rare earth element Ce and La on corrosion behavior of AM60 magnesium alloy, Corros. Sci. 51 (2009) 1334–1343. [18] W. Liu, F. Cao, L. Zhong, L. Zheng, B. Jia, Z. Zhang, J. Zhang, Influence of rare earth element Ce and La addition on corrosion behavior of AZ91 magnesium alloy, Mater. Corros. 60 (2009) 795–803. [19] Y.L. Song, Y.H. Liu, S.R. Yu, X.Y. Zhu, S.H. Wang, Effect of neodymium on microstructure and corrosion resistance of AZ91 magnesium alloy, J. Mater. Sci. 42 (2007) 4435–4440. [20] Y.L. Song, Y.H. Liu, S.H. Wang, S.R. Yu, X.Y. Zhu, Effect of cerium addition on microstructure and corrosion resistance of die cast AZ91 magnesium alloy, Mater. Corros. 58 (2007) 189–192. [21] N. Birbilis, M.A. Easton, A.D. Sudholz, S.M. Zhu, M.A. Gibson, On the corrosion of binary magnesium-rare earth alloys, Corros. Sci. 51 (2009) 683–689. [22] R.B. Alvarez, H.J. Martin, M.F. Horstemeyer, M.Q. Chandler, N. Williams, P.T. Wang, A. Ruiz, Structure–property corrosion relationships of a structural AE44 magnesium alloy, Corros. Sci. 52 (2010) 1635–1648. [23] M.F. Horstemeyer, J. Lathrop, A.M. Gokhale, M. Dighe, Modeling stress state dependent damage evolution in a cast Al–Si–Mg aluminum alloy, Theor. Appl. Fract. Mech. 33 (2000) 31–47.