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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44 DECEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 12 TABLE 1. RESULTS OF THE POTENTIODYNAMIC POLARIZATION SCANS OF SHOT-PEENED Al AND Al-Mg IVD COATINGS WITH AND WITHOUT HEXAVALENT CHROMATE CONVERSION TREATMENT IN AERATED 0.6 M NaCl SOLUTION Coating Code β A mV/Decade β C mV/Decade i corr μA/cm 2 E corr mV R P kΩ/cm 2 E pitting mV I passivation μA/cm 2 Al IVD (A) 93.42 62.44 12 –1,119 1.4 –710 68.8 Al-Mg IVD (A) 77.37 53.77 26 –1,169 0.5 –776 161.9 Chromated Al IVD 73.51 50.71 11 –1,105 1.2 –715 57.4 Chromated Al-Mg IVD 81.83 57.93 24 –1,119 0.5 –611 129.3 Mild steel substrate 131.94 153.95 5 –647 6.7 — — (A) The IVD coating has been shot-peened but not treated with hexavalent chromate conversion treatment. COATINGS & LININGS IVD coatings. The addition of 5 wt% Mg to the Al IVD coating decreases the polariza- tion resistance and increases the corrosion CD, indicating that the corrosion resis- tance of the Al IVD coating will be dimin- ished if the concentration of Mg in the Al IVD coating is raised. Baldwin, et al. 1 pro- posed that the addition of excessive con- centrations of Mg to Al disrupts the protec- tive alumina surface oxide film formed on sputtered Al coatings and weakens its bar- rier protection capability. Furthermore, the pitting potential of the Al IVD decreased from –710 to –776 mV after the addition of 5 wt% Mg, which reduces the coating 's capability to resist pitting. The pitting potential corresponds to the polarization voltage at which pits start to appear on the coating, or the current starts to rapidly increase, beyond the passive region on the anodic branch. The passiv- ation current of Al IVD coatings increased with the addition of Mg, indicating that the di ssolution rat e was increased . Th ese results concur with previous studies on the effect of Mg on activation of Al in pitting corrosion. 10 When the applied polarization voltage reached 0 V, the IVD coatings were dissolved completely, and the passive region of mild steel started to appear. A l t h o u g h t h e A l - Mg I V D c o a t i n g responded less effectively to shot peening, a further effect of adding 5 wt% Mg to the Al IVD coating was to improve the sacrificial corrosion properties by shifting its corro- sion potential to more negative values than an Al IVD coating. The electrochemical potential of binary alloys is related to the thermodynamic equilibrium potentials of the two components. 11 The electrochemical potential adopted by the binary alloy is located between the potential values of the parent metals. The potential of pure Mg is ~–1.9 V (vs. SCE) in 0.6 M (pH 6.5) NaCl solution, 12 compared with a value of –1 V (v s. S C E) for pure Al . 13 A compari son b e tw e e n h e x av a l e n t c h r o m a t e l ay e r s formed on both shot-peened IVD coatings shows that both layers improve the corro- sion resistance of shot peened coatings, which reached –1,105 to –1,119 mV. A small, but significant corrosion potential decrease (≈14 mV) of the hexavalent chromate con- version layer formed on the shot-peened Al-Mg IVD coating can be seen compared to the Al IVD coating. The chromated Al IVD coating experienced a brief period of disso- lution compared to the chromated Al-Mg IVD coating before the passive region appeared. Moreover, the polarization resis- tance of the chromated Al-Mg IVD is lower than that obtained from the chromated Al IVD coating. This suggests that the effect of adding 5 wt% Mg on the corrosion behavior of an Al IVD coating was also observed after hexavalent chromate conversion treat- ments, where the corrosion CD of the chro- mated Al-Mg IVD coating increased. Conclusions Pure Al and Al-Mg coatings were depos- ited onto an AISI 1015 mild steel substrate by the IVD technique. The effect of adding 5 wt% Mg to a pure Al IVD coating was investigated, and the coating structure, microhardness, and corrosion behavior of the IVD coatings were analyzed. It was con- cluded that the addition of 5 wt% Mg can enhance the sacrificial protection proper- ties and hardness of Al IVD coatings. This enhancement would be beneficial for aero- space, particularly for fastener applica- tions, which would achieve the desirable improvement while maintaining galvanic compatibility with the aluminum alloy structures of aircraft. Acknowledgments The work presented here was funded in part by the U.K. Engineering and Physical Sciences Research Council. Financial sup- port from Saudi Aramco is acknowledged with thanks. The provision of IVD coatings by Acorn Surface Technology is also grate- fully acknowledged. References 1 K . R . B a l dw i n , R . I . B a t e s , R . D. Ar n e l l , C.J.E. Smith, Corros. Sci. 38, 1 (1996): pp. 155- 170. 2 B. Enders, S. Krauß, G.K. Wolf, Surf. Coat. Technol. 65, 1-3 (1994): pp. 203-207. 3 R. Arnell, R. Bates, Vacuum 43, 1-2 (1992): pp. 105-109. 4 B. Enders, H. Martin, G.K. Wolf, Surf. Coat. Technol. 60, 1-3 (1993): pp. 556-560. 5 B.A. Shedden, M. Samandi, B. Window, Surf. Coat. Technol. 97, 1-3 (1997): pp. 557-563. 6 D.W. Shoesmith, "Kinetics of Aqueous Corro- sion," ASM Metals Corrosion Handbook 13 (Materials Park, OH: ASM, 1987), pp. 49-62.

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