When stainless steel corrodes, it does not rust all over like non stainless steel but acquires small-localised pits.
British spoons and forks are made from stainless steels that have such excellent corrosion resistance that they are virtually immune to pitting. For knives, however, the blade steel is selected to give a compromise between corrosion resistance and cutting properties. In order to achieve a lasting edge, a steel that is much harder but of lower corrosion resistance than spoon and fork steel is used. The corrosion resistance of this steel is strongly influenced by care in manufacture (thorough hardening and avoidance of overheating when grinding etc.) but only marginal improvements are achieved by the selection of alternative steels, however expensive. Research is continually being carried out in the search for significantly better steels and method of treatment.
Prolonged contact with water is probably 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. Why then do blades in one establishment last for years without trouble, whilst in others pitting becomes apparent within a few months? The answer lies in the length of time the blades are wet; knives that are left undried or actually immersed in water overnight can receive, in this time, as much exposure to the corrosive effect of water as three or four months' ordinary use where they are in contact with water for only a few minutes each time they are washed.
Common salt and liquid bleaches or disinfectants are very corrosive and will greatly accelerate the corrosive action of water. Some water-softened water can have a high salt content. After adding salt to water softeners in a dishwasher, make certain that it is put through the rinse programme recommended by the suppliers before washing knives in the machine. Detergents, on the other hand, are normally harmless when fully dissolved - indeed they often inhibit corrosion by water. If, however, certain powder detergents are allowed to come into contact with blades in hot water before the detergent is fully dissolved, pits and/ or stains can from within a few minutes. Undissolved table salts can have similar effect.
If pitting does occur, a diligent enquiry into the washing procedure is recommended to find out whether the knives have ever been left in contact with water for a long time. If left un-wiped for a period of time the humidity will cause pitting.