Abstract
The inhibition effect of three rare earth metals (REMs), Ce3+, La3+ and Pr3+, on the electrochemical properties of synthesized single-phase intermetallic compounds (IMCs) representing phases commonly found in AA2024-T3 were studied and compared with chromate inhibition. REM additions had little effect on corrosion potential, pitting potential and icorr in short-term tests. This is in contrast to effects observed with chromate inhibitor additions where the pitting potential was seen to increase dramatically. All three REMs decreased the oxygen reduction reaction (ORR) kinetics on each synthesized phase, but none as much as chromate. The results in these studies are consistent with the idea that inhibition by REMs results from hydroxide precipitation at IMCs due to a local pH increase associated with the ORR. As hydroxide precipitation is a comparatively slow process, REMs cannot inhibit the initial dealloying of S phase, leading to the formation of dealloyed layers under hydroxide layers. Once the hydroxide precipitate forms, the subsequent corrosion of S phase is strongly inhibited. The dependence of inhibition on the formation of hydroxide deposits demonstrates that REMs are “slow” inhibitors, which must be considered in their applications.
Highlights
Fontana Corrosion Center, Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210, USA
rare earth metals (REMs) inhibition was first demonstrated in experiments with AA7075, wherein the ability to inhibit corrosion was evaluated by mass loss and linear polarization measurements.[18]
Similar inhibition efficiencies and trends were observed for other lanthanide salts including YCl3, LaCl3, PrCl3 and NdCl3.24 In all cases, inhibition derives mainly from inhibition of oxygen reduction reaction (ORR) kinetics, and inhibition efficiency tends to increase with increasing immersion time.[25]
Summary
Fontana Corrosion Center, Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210, USA Alloying elements such as Cu and Mg are intentionally added to AA2024-T3 to improve mechanical properties such as strength and toughness.[1] their solubility in Al is low and these elements precipitate to form intermetallic compound (IMC) particles during solidification processing and heat-treatment.[1,2,3] These IMCs usually possess electrochemical properties different from the surrounding matrix, and can be selectively dissolved or induce localized galvanic corrosion.[4,5,6] Passive or dealloyed films on an IMC particle surface can protect the particle from corrosion, but they can rupture or deteriorate under aggressive environmental conditions or mechanical stress, leading to increased electrochemical reactivity and localized corrosion.[7,8,9] Inhibitors have been used to stifle the localized corrosion and improve corrosion resistance, thereby extending the service time of aluminum alloys.[10] Chromate is a noteworthy inhibitor with the ability to self-heal if chemical or mechanical damage occurs.[11,12,13,14,15,16,17] chromate is toxic and carcinogenic in nature and its application is limited. RCl3 solution (R represents Ce3+, La3+, or Pr3+) in this paper refers to solutions containing 0.1 M NaCl plus 4 mM REM salt, while the “chromate solution” refers to 0.1 M NaCl plus 1 mM K2CrO4
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