Pretreatment magnesium alloys

Magnesium Alloy, Processesfor Pretreatment and Prevention of Corrosion On, MUitary Specification MIL-M-3171, Dept, of Navy, Naval Air Engineering Center, Philadelphia, Pa., July 11, 1966. 

MIEM-3171, Processes for Pretreatment and Prevention of Corrosion of Magnesium Alloys, U.S. Government Printing Office, Washington, D.C., 1974. 

J. Zhang, C. Wu, Corrosion protection behavior of AZ31 magnesium alloy with cathodic electrophoretic coating pretreated by silane. Prog. Org. Coat. 66 (2009) 387—392. 

Military Specification, M1L-M-3171C (super-ceded by SAE-AMS-M-3171, May 1998), Magnesium Alloy, Processes for Pretreatment and Prevention of Corrosion, 1998. 

Chromate conversion coatings are used widely on aluminum alloys as a pretreatment for painting, though in some applications, where noncondensing atmospheric exposure is expected, they may be used as the primary means of corrosion protection. Chromate conversion coatings are used on magnesium, cadmium, and zinc, and on galvanized steel to suppress the formation of white rust. 

Palladium black was prepared from palladium nitrate and formaldehyde solution by dropwise addition of potassium hydroxide solution (50 wt. %) at about 10°. The solution and precipitate were warmed at about 60° and the precipitate washed several times by decantation. It was then placed in a Soxhlet extractor and washed for 48 hr. (about 100 times). The precipitate was then dryed at 110°. The palladium-silver system is known to be one of complete miscibility (3). Alloys of silver-palladium were prepared following a procedure discussed elsewhere 4). Their preparation involved a low-temperature coprecipitation of both metals from a solution containing proper amounts of their nitrates. Alloy formation was checked by means of x-ray diffraction patterns which were obtained with Cu-Ka radiation. The computed lattice constants are shown in Fig. 1 to be a linear function of the alloy composition. Hydrogen, used for pretreatment of all samples, was obtained from a commercial tank and purified by passage through a Deoxo unit, magnesium perchlorate, and a charcoal trap immersed in liquid nitrogen. 

Magnesium CCCs are usually formed in acidic solutions in which Mg surfaces are easily activated. Nitric acid-and chromic acid-based mixtures are common. Because of the high corrosion susceptibility of Mg alloys, CCCs are more often used as a surface pretreatment for paint than as stand-alone corrosion-resistant coatings. 

FIGURE 32. Magnesium concentration as a function of argon ion bombardment dose for the various pretreated alloys (a) degreased (note no magnesium detected in the EIC oxide layer) (b) phosphoric acid anodized. Reprinted with permission from Reference 36. Copyright 1982 Gordon and Breach Science Publishers. 

Organosilane-based coatings have been successfully tested as anti-corrosion pretreatments for aluminium alloys, steel, copper, magnesium and galvanised steel and the results generally demonstrate improved corrosion resistance [26-66]. 

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