Diffusion coating of ferrous metals with aluminium - using a compsn. contg. particulate aluminium, a halide salt and an inert particulate cpd.

摘要

Process for applying an Al layer to ferrous metal articles comprises (a) forming a dry Al compsn. comprising =50 wt.% particulate Al, nearly equal 1/8 - 3/4 wt.% AlCl3, nearly equal 1/8-3/4 wt.% of a halid salt consisting of an ammonium halide or an anhydrous metal chloride as a transport-promoting aid, and a remainder of an inert particulate material; (b) conditioning the compsn. in an atmos. of is not 50% R.H., such that the total amt. of aid is not 1-0.5 wt.% of the compsn. (c) subjecting the dry compsn. to a first thermal treatment in a closed vessel at nearly equal 400-482 degrees C for =10 hrs.; (d) subjecting the pretreated material to =1 further thermal pretreatment by mixing it with the previously named aid and re-heating the compsn. at 400-482 degrees C for nearly equal 10 hrs.; (e) remixing the compsn. with the aid under the same dry conditions; nf) embedding the article to be treated in the compsn. and sealing the whole in a steel retort; and (g) heating the material at 400-482 degrees C esp. 413-454 degrees C in a diffusion coating cycle until the desired thickness of Al coating has been achieved. The process allows ferrous metal articles such as chrome steel to be Al coated by diffusion at moderate temps., which do not cause deterioration in the properties of the steel, while suppressing the formation of large amts of AlCl3 vapour.