GB787383A - Process for producing superficial protective layers - Google Patents

Abstract

A continuous, dense adherent surface layer of a regular desired thickness consisting entirely of a homogenous definite phase AmBn is formed on a metal A by free diffusion of a donor element B under conditions such that (a) the donor supplying the element B is on an average richer in atoms B than the phase AmBn, (b) the donor is in direct, intimate contact with the surface of the metal A, such contact being nowhere interrupted and (c) the progress of diffusion in equilibrium is ensured by the absence of any chemical, mechanical or thermal perturbation. In modifications in which an unwanted layer of another phase forms on the outside (1) the unwanted layer is removed mechanically, chemically or electrically or (2) the diffusion treatment is continued in the absence of the donor, so that the unwanted layer itself becomes a donor. The donor may be a powder of B or of an alloy a mixture of B with A or with an inert substance or may be a preformed coating deposited electrolytically, by chemical deposition, by vapour deposition or by sputtering: the metal A may itself be a coating. If undesirable phases are liable to form below the layer of phase AmBn, a limited layer of A is superposed on a layer of an inert metal C and diffusion of B with A carried out until the layer A and the undesirable phases have been absorbed by the phase Am Bn. In examples, Mo is heated in Si powder in a purified H2 atmosphere for 30 minutes at 1100-1300 DEG C.; Mo is treated in MoSi2 powder at 1500 DEG for 1 hour: Mo is heated in Be or Al powder in H2 atmosphere at 1100 DEG C. for 1 hour: W, NC, Ta or alloys thereof are similarly treated; Mo or Fe is first coated with Cr and then heated with Si, Be or Al; Cr is itself so treated; Cu is heated in Be powder for 2 hours at 800 DEG C. in H2; Cu is treated with Al powder; Cu is heated in Zn powder for 20-30 minutes at 380 DEG C. in H2; Cu is heated in 80-20 Zu-Cu powder or in Cu Zn; Cu is coated with Sn, e.g. electrolytically, and heated at 450 DEG C. in a protecting atmosphere or at reduced pressure; Cu is heated with an alloy of 50-60 per cent Sn and 40-50 per cent Cn;: Ni is coated with Al and heated in a reducing atmosphere at 600-1200 DEG C.; Ni is chromized and a Crm Sin or Crm Ben layer formed thereon; Ni is molybdenized and a Mom Sin or Mom Ben layer formed thereon; Ni is coated with Su and heated at 450 DEG C. for 30 minutes.