Multi-layer, high-temperature corrosion protection coating

An article for use at high operating temperatures comprising a base material which is a metal, suitable for high temperature application, whose outer metallic surface forms a surface of the article when unprotected by a protective coating but whose outer surface is corrodible at high operating temperatures when unprotected, said metal being one into which nickel can diffuse at high operating temperatures and a multi-layer, high-temperature corrosion protection coat upon the corrodible metallic surface which comprises:

• (a) first protective layer adjacent to said metallic surface said layer having a nickel containing matrix and comprising 1-15% zirconium alone, or a mixture of zirconium and of up to 80% titanium replacing zirconium, present at least in part as fine particles disposed in said matrix, 10-30% chromium, optionally 0.05-2% yttrium, lanthanum, rare-earth and/or berylium, up to 4% silicon, up to 2% boron, and remainder essentially nickel;
• (b) a second protective layer adjacent to said first protective layer comprising at least 60% chromium and remainder selected from the group consisting of iron, iron plus nickel and mixtures thereof;
• (c) a diffusion zone of high nickel content extending from the base material into the first protective layer (a);
• (d) a diffusion zone which is a nickel chromium alloy of variable constitution within the first protective layer (a) and beneath the second protective layer (b) and;
• (e) diffusion regions of zirconium nickel alloy of variable zirconium content surrounding the zirconium particles;
• the proportions of the metals in the protective layers (a) and (b) being the proportions before the creation of the diffusion zones.