TAILIEUCHUNG - Volume 01 - Properties and Selection Irons, Steels, and High-Performance Alloys Part 6

Tham khảo tài liệu 'volume 01 - properties and selection irons, steels, and high-performance alloys part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Aluminum coatings are applied by hot dip methods at about 675 to 705 C 1250 to 1300 F . Aluminum alloy 1100 is usually used because of its general all-around corrosion resistance. As with any hot dip coating a metallurgical bond is formed that consists of an intermetallic alloy layer overlaid with a coating of pure bath material. Aluminum coatings do not corrode uniformly as do zinc and cadmium coatings but rather by pitting. In some cases these pits may extend entirely through the coating to the base metal in others only through the overlay to the intermetallic layer. Pits which may occur in a part soon after exposure sometimes discolor the coated surface but cause little damage. The complex aluminum and iron oxide corrosion product seals the pits and because the corrosion product is tightly adherent and impervious to attack corrosion is usually limited. There is little tendency for corrosion to continue into the ferrous base and there is none for undercutting and spalling of the coating. Aluminum coatings will protect steel from scaling at temperatures up to about 540 C 1000 F the aluminum coating remains substantially the same as when applied and its life is exceptionally long. Above 650 C 1200 F the aluminum coating diffuses into the steel to form a highly protective aluminum-iron alloy. This diffusing or alloying is timetemperature dependent the higher the temperature the faster the diffusion. However scaling will not take place until all the aluminum is used up which may take a thousand or more hours even at temperatures as high as 760 C 1400 F . The prevention of galling at elevated temperatures is another characteristic of aluminum coatings. Stainless steel fasteners for use at 650 C 1200 F have been aluminum coated just to prevent galling. Coated nuts can be removed with an ordinary wrench after many hours at these temperatures which is impossible with uncoated nuts. Fastener Performance at Elevated Temperatures Selection of fastener material is perhaps .

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