Painting, magnesium alloys

Epoxy PANI Magnesium alloy Painting EiS in 3.5% NaCl The corrosion protection of PAN coating on AZ91D magnesium alloy is better with respect to the varnish coating but decreases in the presence of O. [55]... 

By the use of many commercial abrasive processes, the corrosion resistance of magnesium alloys can be reduced to such an extent that samples of metal that may lie quiescent in salt water for many hours will, after shot blasting, evolve hydrogen vigorously, and the corrosion rate, as measured by loss of weight, will be found to have increased many hundred-fold. The effect in normal atmospheres is naturally much less, yet the activation of the surface is an added hazard and is the opposite of passivation which is essential if later-applied paint finishes are to have proper durability. 

Chromates are very effective inhibitors of the corrosion of magnesium alloys by saline and other waters, and many treatments have been developed by means of which substantial hlms containing slightly soluble chromate are formed in the metal surface. Except on parts which are to be exposed only to a rural atmosphere, chromate treatment must be supplemented by paint, for which it provides a good base. 

Etch priming is widely used on aluminium alloy, and is particularly effective on cadmium and zinc. The adhesion to stainless steel and titanium is good. It has also been used quite widely on bare steel and on magnesium alloy, but on these metals its performance is not, in the opinion of some investigators, always quite reliable. For best protection the etch primer coating is followed with a full paint scheme. 

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. 

Magnesium alloys do not retain a high lustre like the aluminium alloys and cannot be electroplated with the same ease as the zinc alloys. Finishing is usually achieved by painting or lacquering. Chromate treatments are used to protect against corrosion, but have little decorative value. 

CoiTosion prevention is achieved by correct choice of material of construction, by physical means (e.g. paints or metallic, porcelain, plastic or enamel linings or coatings) or by chemical means (e.g. alloying or coating). Some metals, e.g. aluminium, are rendered passive by the formation of an inert protective film. Alternatively a metal to be protected may be linked electrically to a more easily corroded metal, e.g. magnesium, to serve as a sacrificial anode. 

Non-oxidising and weak acids, in contrast to oxidising acids, can penetrate paint films without destroying them they then react with the metal base to form salts with resultant stresses which cause cracks. Magnesium-rich alloys are particularly prone to attack by acids their salts, having considerable volume, in severe cases effloresce through the broken paint films. 

Dry Copper, brass, aluminum, magnesium, magnesium-aluminum alloy, stainless steel, mild steel, mild steel coated with acid-proof black paint and mild steel plated with copper, cadmium, nickel or zinc are not affected... 

The uses of aluminum are similar to those of the light metals, some of which have previously been discussed under magnesium. Aluminum is also used in fabricating wire cables for electric-power transmission lines in the manufacture of kitchen utensils, furniture, and paint and in producing a wide variety of other useful articles. In connection with the general topic of alloys, attention is called to several important alloys containing aluminum. 

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. 

---