Magnesium alloys salt spray

Magnesium and its alloys Salt spray test ASTM... 

In considering the corrosion of magnesium and its alloys it is important to examine the methods available for assessing corrosion tendencies and particularly those known as accelerated tests. Tests carried out by immersion in salt water or by spraying specimens regularly with sea-water are worthless as a means of determining the resistance of magnesium alloys under any other than the particular test conditions. Extrapolation to less corrosive conditions is not valid and even the assessment of the value of protective measures by such means is hardly possible. The reason is to be found in the fact that corrosion behaviour is directly related to the formation of insoluble... 

Post-Treatments. Although many post-treatments have been used over plated metals, chromate conversion coatings remain as the most popular. Chromates are used to improve corrosion resistance, provide good paint and adhesive base properties, or to produce brighter or colored finishes. Formulations are usually proprietary, and variations are marketed for use on zinc, zinc alloys, cadmium, copper and copper alloys, and silver (157). Chromates are also used on aluminum and magnesium alloys (158,159). More recently, chromate passivation has been used to extend salt spray resistance of autocatalytic nickel plated parts. 

In principle, the aluminum-containing alloys of magnesium should all be capable of providing corrosion peiform-ance competitive with that established for high-purity alloys AZ91D and AZ91E in die cast and sand cast applications (i.e., <0.25 mm/yr (10 mpy) in ASTM B 117 salt spray), if the critical contaminants are controlled at appropriate levels [9,17]. 

If the atmosphere is clean but not dry and the humidity approaches 100%, a scattered pattern of corrosion spots eventually appears, but considerable areas of unaffected surface remain for a very long time. If, however, the surface becomes contaminated by corrosive dust or cathodic particles, the whole surface rapidly becomes covered with a grayish layer of corrosion product (Tawil, 1987). A clean, unprotected magnesium alloy surface exposed to indoor or outdoor atmospheres free from salt spray develops a gray Him that protects the metal from corrosion (Froats et al., 1987). The rates of corrosion and resistance to corrosion of magnesium alloys vary depending on alloy composition (Loose, 1946) as discussed previously. 

To evaluate magnesium alloys both immersion and salt spray tests are often used. Table 7.6 shows corrosion rates obtained after different types of accelerated corrosion tests taken from the literature (52, 62, 63). The corrosion rates show a wide scatter. For some of the accelerated corrosion tests such as the immersion in NaCl solution the corrosion rates are extremely high. 

For this reason, corrosion inhibitors such as cerium salts or organic biocides are successfully added to the silane sol-gel layer. Akid and coworkers, for example, have developed a broad range of chromate-free xerogel coatings based on doped Ormosil formulations with very high corrosion resistance and with little evidence of change after extensive salt spray tests for a variety of different metal substrates (aluminum alloys, steel, magnesium, etc.). 

Fig. 4.17 Samples of high-purity AZ31 (upper photographs) and ZW3 (lower photographs) magnesium-base alloys, fitted with mild-steel nuts and bolts and exposed to a variety of corrosion conditions, (a) 4-S hours immersion in 37 salt soln., (b) 180 days immersion in distilled water, (c) 4 days immersion in borehole water, (d) 180 days in humidity cabinet sea-water spray and (e) 180 days atmospheric exposure...

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