Materials Performance

DEC 2014

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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43 NACE INTERNATIONAL: VOL. 53, NO. 12 MATERIALS PERFORMANCE DECEMBER 2014 AlMg IVD coating shows that the Al [111] peak is decreased. According to the phase diagram of Al-Mg alloy systems, alloying Al with 5 wt% Mg produces a single phase solid solution (α Al), which was observed in the XRD pattern of the Al-Mg IVD coating. These results are in agreement with previ- ous studies on phase analysis of Al-Mg alloy coatings performed by Enders, et al. 8 and Arnell, et al. 3 They found that highly super- saturated single phase solid solutions were formed over the ranges 0 to 20% Mg in Al (and 0 to 30% Al in Mg). The Knoop microhardness data of the uncoated substrate (AISI 1015 mild steel) and shot-peened Al and Al-Mg IVD coat- ings are shown in Figure 3. It is reported in the literature that alloying Al with Mg increases the hardness of Al PVD coatings. 8 Figure 3 shows that the addition of 5 wt% Mg to an Al IVD coating increases the hard- ness of the IVD coating from 104 to 129 Hk (in the shot-peened condition 0.25 N load). Open Circuit Potential Measurements The variation in O CP with time for shot-peened Al and Al-Mg IVD coatings (with and without hexavalent chromate conversion treatment), recorded during immersion in an aerated 0.6 M NaCl solu- tion, is shown in Figure 4. The Al IVD coat- ing initially adopted a potential of –1.03 V (vs. SCE) in the chloride solution . The potential then gradually increased to stabi- lize at –0.92 V, which is attributed to the formation of a stable oxide film. The initial OCP of the Al-Mg IVD coating was –0.78 V, indicating that a film might have formed during sample preparation. After immer- sion, the OCP of the Al-Mg IVD coating decreased sharply to –0.98 V. Then, the cor- rosion behavior of the Al-Mg IVD coating was characterized by potential fluctuations of up to 0.15 V, which is usually attributed to the effects of localized corrosion such as pitting attack. 9 This suggests that the addi- tion of 5 wt% Mg to the Al IVD coating (and the resulting porous surface) makes the coating more susceptible to pitting attack. The observed pores on the surface of the shot-peened Al-Mg IVD coating would tend to enhance transport of corrosive agents into the Al-Mg IVD coating, increasing the corrosion reaction at the interface. O C P m e a su re m e n t s of h e x av a l e n t chromate layers form ed on both shot- peened Al and Al-Mg IVD coatings were included. It was found that both IVD coat- ings treated with hexavalent chromate ini- tially adopted similar OCPs; but the OCP of the treated Al IVD coating was generally more stable than the OCP for the treated Al-Mg IVD coating, which was character- ized by frequent potential f luctuations of up to 0.05 V. While the onset of pitting for th e tre a t e d Al I V D c o a t i n g w a s o n ly observed after 40 h of immersion, pitting for the treated Al-Mg IVD coating was observed after <30 h of immersion. This indicates that the addition of 5 wt% Mg to the Al IVD coating affected not only the corrosion behavior of the IVD coating but also the corrosion behavior of the hexava- lent chromate conversion layer. Potentiodynamic Polarization Scans Potentiodynamic polarization scans were made for shot-peened Al and Al-Mg IVD coatings exposed to aerated 0.6 M (3.5 wt%) NaCl solution at room temperature. The scans for uncoated AISI 1015 mild steel were included, and the polarization curves are plotted in Figure 5(a). The potentiody- n ami c m ea surem ent s for c omm erc i al hexavalent chromate layers formed on shot-peened Al and Al-Mg IVD coatings were also included for comparison, and the polarization curves are plotted in Figure 5(b) with the results presented in Table 1. Figures 5(a) and ( b) show that th e cathodic polarization curves of both shot- p een ed IVD coatings, with or without hexavalent chromate treatment, are simi- lar. However, they have different polariza- tion behavior in the anodic branch of the curve, which indicates different pitting cor- rosion behavior. It is apparent that the cor- rosion potential of the Al IVD coating decreases by 50 mV after alloying with 5 wt% Mg. Previous work has shown that the corrosion potential of sputtered Al coatings al loyed w ith th e sam e amount of Mg d e c re a s e d b y 1 0 0 m V. 1 T h e re d u c e d decrease in corrosion potential of th e Al-Mg IVD coating (over the Al IVD coat- ing) compared to sputtered coatings can be interpreted as being due to the surface morphology and coating density of Al-Mg FIGURE 5 Polarization curves for (a) shot-peened Al and Al-Mg IVD coatings (compared to AISI 1015 mild steel) and (b) shot-peened Al and Al-Mg IVD coatings with hexavalent chromate treatment in an aerated 0.6 M NaCl solution. Influence of Mg Addition on Corrosion Behavior of Ion Vapor Deposited Al Coatings

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