Patent ID: 7255893

Claim:
A method of forming a hematite-containing protective layer on a metal-based substrate for use in a high temperature oxidising and/or corrosive environment, said method comprising: applying onto the substrate a particle mixture consisting of: (a) 60 to 99 weight %, in particular 70 to 95 weight % such as 75 to 85 weight %, of: hematite (Fe 2 O 3 ), or hematite and: (1) iron metal (Fe) with a weight ratio Fe/Fe 2 O 3 of preferably no more than 2, in particular in the range from 0.6 to 1.3; (2) ferrous oxide (FeO) with a weight ratio FeO/Fe 2 O 3 of preferably no more than 2.5, in particular in the range from 0.7 to 1.7; or (3) iron metal (Fe) and ferrous oxide (FeO), with weight ratios Fe/Fe 2 O 3 and FeO/Fe 2 O 3 that are in pro rata with the ratios of (1) and (2); (b) 1 to 25 weight %, in particular 5 to 20 weight % such as 8 to 15 weight %, of nitride and/or carbide particles; and (c) 0 to 15 weight %, in particular 5 to 15 weight %, of one or more further constituents that consist of at least one metal or metal oxide or a heat-convertible precursor thereof; and consolidating the hematite by heat treating the particle mixture to: (1) oxidise iron metal (Fe) when present into ferrous oxide (FeO); (2) sinter the hematite (Fe 2 O 3 ) to form a porous sintered hematite matrix; and (3) oxidise the ferrous oxide (FeO), when present in the particle mixture as such and/or upon oxidation of said iron metal (Fe), into hematite (Fe 2 O 3 ) so as to fill the sintered hematite matrix, and form the protective layer that is made of a microporous sintered hematite matrix in which the nitride and/or carbide particles are embedded and which contains, when present, said one or more further constituents.