Aluminium coating for steel screws or bolts - obtd. using paste contg. aluminium and alumina powders, and double heat treatment

摘要

The steel parts are freed from surface oxide and scale, then covered with a paste contg. (a) Al powder with size 5 mu m; (b) alumina powder, size 10 mu m; (c) a paste vehicle based on an organic polymer. The coated parts are heated to 600-700 degrees C to fix the coating, then heated at 1000-1100 degrees C. The pref. paste has compsn. by wt. 15-25% Al (a), 20-40% alumina, size 50 mu m, 1-2% AlF3, remainder a polymer decomposing at 300-350 degrees C without carburising the steel nor leaving any trace, plus a solvent for the polymer. The pref. vehicle is 15-20% polyisobutylene with 85-80% white spirit. The paste may also contain an alkali metal- or ammonium-halide, esp. max. 3% NH4Cl. The surface oxide etc.is pref. removed by (a) heating at 300-400 degrees C in an oxidising atmos., then at 800-900 degrees C in a reducing atmos.; or (b) pickling in dil. HCl. Esp. for coating of screws, bolts, nuts or other fixing elements. The process improves the corrosion resistance of the steel, and coated parts can be used in contact with Al or Al alloy parts without creating potentials promoting corrosion.