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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40 DECEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 12 COATINGS & LININGS C Al and Al-Mg coatings were depos- ited onto AISI 1015 mild steel (UNS G10150) substrates using ion vapor deposition (IVD) from pure Al and Al-5 wt% Mg wires, respectively. In each case, a shot peening process was carried out after deposition. Open circuit potential measurements and potentiodynamic polarization scans were conducted. The corrosion behavior of hexavalent chromate lay- ers formed on both IVD coatings was also investigated. C a d m i u m p l a t i n g h a s b e e n u s e d extensively within the aerospace industry for corrosion protection of diverse items made from high-strength steel. The high toxicity of the cadmium coating and its p l ati n g pro c e ss i n cre a s e d i nt ere st i n replacing cadmium coatings with other coating systems. More than 30 years ago, Mc D on n el l D o u g l a s Aero sp a c e b ega n re p l a c i n g c a d m i u m a n d z i n c p l a t i n g processes with the ion vapor deposition (IVD) process. IVD is used for applying a pure aluminum coating, an environmen- tally friendly replacement for cadmium p l a t i n g ( M I L - C - 8 3 4 8 8 ) , f o r c o r r o si o n protection. The Al coating provides good fatigue resistance because it is soft and less prone to serve as a crack initiation layer. The corrosion potential of a pure Al coating is relatively close to those of many struc- tural aerospace Al alloys; hence there is very little potential driving galvanic corro- sion between them . However, in many applications, the sacrificial corrosion and tribological properties of a pure Al coating are not sufficient. Therefore, post-deposi- t i o n c o nv e r s i o n t re a t m e n t s s u c h a s chromating are frequently used to enhance the surface properties of an Al coating. The most promising alloying element for improving wear, mechanical, and sacri- ficial corrosion properties of an Al coating is magnesium. 1 Physical vapor deposition (PVD) of Al-Mg alloy coatings was studied as one possible alternative to electrodepos- ited cadmium coatings and conventional hot-dip galvanized coatings. 1-5 It was found that coatings with Al-20 wt% Mg have cor- rosion properties similar to those of cad- mium coatings. 1 Alloying with Mg improved the mechanical and sacrificial properties of Al coatings. From the galvanic coating compatibil- ity standpoint, the ideal Mg composition would be one that electronegatively shifts the corrosion potential of the Al-Mg IVD coating and, at the same time, maintains a low susceptibility to galvanic corrosion with the structural Al alloys. To achieve the desired improvement, 5 wt% Mg was cho- sen to be alloyed with pure Al. Experimental Procedure IVD coatings were deposited onto an AISI 1015 (UNS G10150) mild steel sub- strate (25 by 50 by 6 mm) by Acorn Surface Infuence of Mg Addition on Corrosion Behavior of Ion Vapor Deposited Al Coatings nayef M. alanaZi, Saudi Aramco, Dhahran, Saudi Arabia a. leyland, a.l. yerokhin, and a. Matthews, University of Sheffeld, Sheffeld, United Kingdom

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