COATING SYSTEM FOR HIGH TEMPERATURE STAINLESS STEEL

A method for protecting carbon steel and stainless steel, and particularly high temperature stainless steel, from coking and corrosion at elevated temperatures in corrosive environments, such as during ethylene production by pyrolysis of hydrocarbons or the reduction of oxide ores, by coating the steel with a coating of MCrAlX or MCrAlXT in which M is nickel, cobalt, iron or a mixture thereof, X is yttrium, hafnium, zirconium, lanthanum, scandium or combination thereof, and T is silicon, tantalum, titanium, platinum, palladium, rhenium, molybdenum, tungsten, niobium, or combination thereof. The coating and substrate preferably are heat- treated at about 1000 to 1200°C for at least about 10 minutes, preferably about 20 minutes to 24 hours, effective to metallurgically bond the overlay coating to the substrate and to form a multiphased microstructure. The coating preferably is aluminized by depositing a layer of aluminum thereon and subjecting the resulting coating to oxidation at a temperature above about 1000°C for a time effective to form an alumina surface layer. An intermediary aluminum-containing interlayer may be deposited directly onto the substrate prior to deposition of the overlay coating and is heat-treated with the coating to form a protective interlayer between the stainless steel substrate and coating to disperse nitride formation at the substrate/coating interface. Also, the coating may be deposited onto and metallurgically bonded to the substrate by plasma transferred arc deposition of atomized powder of MCrAlXT, obviating the need for a separate heat treatment. Alternatively, a blended powder composition to produce a desired MCrAlXT alloy may be applied to the substrate.