October 4, 2008

Hot Dip Aluminizing

Aluminum can also be coated on to iron and steel articles by hot dipping, but the process is considerably more difficult than in the case of tin and zinc, for a number of reasons. Because of the higher melting point of aluminum the process must be carried out at about 700oC, at which temperature it is very difficult to preserve the steel in an oxide free condition. The aluminum melt itself cannot be kept without an oxide film on its surface, even in an inert atmosphere. The major difficulty in aluminizing, however, is the rapidity of the formation of an intensely brittle aluminum iron intermetallic compound at the interface. The growth of this layer can be restrained to a limited extent by additions of silicon or beryllium to the melt. A 6% silicon aluminum alloy is usually used.

The steel is freed from oxide by pickling, but must be dried before entry into the molten alloy. Simple and partially successful schemes of providing a thin replacement film of copper on the surface, and then coating it with glycerin, have been used, but for large scale aluminizing very complicated plant for deoxidizing the sheet and introducing it into the molten bath under a protective gas are employed. Aluminized coatings are bright and metallic in appearance, although usually marred by streaks and patches of oxide, drawn off the surface of the melt. They will not withstand even slight bending without spalling and cracking, because of the inevitable layer of intermetallic compound at the interface. They have excellent resistance to exposure to the whether, to waters and to damp conditions without much change of appearance. However, their major field of application is in resisting the effect of high temperatures which would oxidize and scale the steel base. Aluminized steel thus finds its chief application for such purposes as motor car silencers and exhaust systems, for gas heater heat exchangers and for similar applications where hot products of combustion are concerned. A usual thickness is 0.001 to 0.002 in.

No comments: