There are a number of steps that can be taken to maintain stainless steel knives.
One thing to look out for on knives is staining; it must be emphasised that staining is a rare phenomenon and that in most cases it is due to something that becomes firmly deposited on the steel, rather than to any attack of the steel itself. The most common cause of staining is attack by one of the proprietary dip solutions used for removing tarnish from silver. They contain acids that etch the steel, first giving it an iridescent rainbow stain and ultimately etching it a dull grey. Most stains that resist ordinary rubbing with a soapy cloth can be removed with stainless steel cleaners.
Dissolved Mineral Salts
All tap water contains dissolved mineral salts that would leave an extremely thin film on any article on which it was allowed to dry out without wiping. In most cases, the resultant stain will wipe off, but occasionally more vigorous treatment is needed, using a polishing preparation, such as stainless steel cleaners.
Strong detergents can leave an indelible rainbow stain on stainless steel if they are not rinsed off and are allowed to dry on its surface.
Very hot grease, fat or meat juices sometimes leave rainbow coloured stains on stainless steel, but this is more likely to occur on meat dishes than knives.
Corroding on Knives
When stainless steel corrodes, it does not rust all over like non stainless steel but acquires small localised pits or holes. Prolonged contact with water is most likely responsible for more pitting trouble than anything else. Knives have been made from stainless steel for such a long time that the highly corrosive effect of tap water, caused by the traces of mineral salts it contains is not appreciated. If pitting does occur, an enquiry into the washing procedure is recommended to find out whether the knives are ever left in contact with water for a long item.