Hydrogen-water vapor pretreatment of Fe-Cr-Al alloys

A method for forming an aluminum oxide film on a substrate formed from a material selected from the group consisting of a steel comprising from about 1.5 weight percent to less than about five weight percent aluminum and an aluminum-coated steel having an average aluminum content of less than about 10 weight percent, said method comprising the steps of:

• exposing a surface of said substrate to an atmosphere having as its reactive portion a mixture consisting essentially of hydrogen and greater than about 11 ppm water vapor at a temperature of between about 800.degree. C. and 1000.degree. C., wherein said atmosphere has a H.sub.2 /H.sub.2 O ratio productive of an oxygen partial pressure sufficient to oxidize the aluminum constituent at said surface of said substrate so as to form a precursor aluminum oxide on said surface, wherein said H.sub.2 /H.sub.2 O ratio is less than or equal to about 1350 and said oxygen partial pressure is greater than or equal to about 2.7.times.10.sup.-23 atmospheres;
• and
• oxidizing said surface of said substrate at an elevated temperature to form said aluminum oxide film from said precursor aluminum oxide layer on said surface of said substrate;
• whereby said step of exposing is a pretreatment step to promote subsequent aluminum oxide whisker formation during said step of oxidizing said surface of said substrate.

A method for forming an aluminum oxide film on a monolithic support structure for treating the exhaust gases of an internal combustion engine, said method comprising the steps of:

• providing said monolithic support structure formed from a material selected from the group consisting of a steel comprising from about 1.5 weight percent to less than about five weight percent aluminum and an aluminum-coated steel having an average aluminum content of less than about 10 weight percent;
• exposing a surface of said monolithic support structure to an atmosphere having as its reactive portion a mixture consisting essentially of hydrogen and greater than about 11 ppm water vapor at a temperature of between about 800.degree. C. and 1000.degree. C., wherein said atmosphere has a H.sub.2 /H.sub.2 O ratio of less than or equal to about 1350 so as to be productive of an oxygen partial pressure sufficient to form a precursor aluminum oxide on said surface;
• and
• oxidizing said surface at an elevated temperature to form said aluminum oxide film from said precursor aluminum oxide layer on said surface;
• whereby said step of exposing is a pretreatment step to promote subsequent aluminum oxide whisker formation during said step of oxidizing said surface.