CORROSION PROTECTION FOR NITRIDED STAINLESS STEELS IN WATER

The corrosion resistance of nitrided stainless steels in water depends on the chemical composition of the steel and on the temperature and degree of dissociation of the ammonia in which nitriding was done. in all cases there is a variation of degree of corrosion resistance with depth below the nitrided surface. There are four regions of corrosion resistance: the surface, having lowered resistance and a lowered negative electrode potential; a layer of high corrosion resistance having high electrcde potentials; a corrosively unstable layer with lowered electrode potentials; and a core region having high corrosion resistance and high electrode potentials. The thickness of the unstable layer and its depth below the surface varies but is always in the range where polishing, machining, or water may expose it and lead to rapid corrosion in water. Several corrosion inhibitors were tested on nitrided samples of 25Crl8N8V2 steel with the surfaces polished down to the unstable layer. The best results were obtained with samples treated for one hour in boiling 10% K/sub 2/Cr/sub 2/O/sub 7/ solution. After six months no corrosion had appeared on the surface of the samples. In addition, a supplementary treatment in a silicon bearing organic liquid, GKZh-94, after treatment inmore » a 10% solution of K/sub 2/Cr/sub 2/O/sub 7/ at 70 tained C, further increased corrosion resistance. The corrosion of the GKZh-94 treated samples in water, after exposure to kerosene at 250 tained C and to gasoline at room temperature, was also shown to be significantly innibited. (TTT)« less