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First, 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 on the steel itself. Probably the most common cause of staining is attack by one of the proprietary dip solutions used for removing tarnish from silver. Although excellent for cleaning silver and E.P.N.S., these solutions should never be allowed to come into contact with stainless steel; they contain acids that etch the steel, first giving it an iridescent rainbow stain and ultimately etching it to a dull grey. Even if care is taken to dip only the silver handle of a knife, it is so easy, when lifting it from the solution, to let drips fall onto the stainless steel blades of other cutlery that happen to be lying around.

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 Solvol's 'Autosol' (available from many motor car accessory shops and some other retail outlets).
Detergents, especially unnecessarily strong solutions of detergent, can leave an indelible rainbow stain on stainless steel if they are not rinsed off and are allowed to dry out on its surface.
Very hard water can deposit a chalky film on stainless steel, but this is only likely to occur in dishwashers that use un-softened or incompletely softened water.

Very hot grease, fat or meat juices sometimes leave stubborn rainbow coloured stains on stainless steel, but this is more likely to occur on meat dishes than cutlery. Again this does not mean there is anything wrong with the stainless steel, and the articles will be as good as new after the stain has been removed.
Heat by itself will impart a rainbow coloured heat tint to stainless steel, but this is only likely to occur if the cutlery is accidentally left on a hot plate or gas burner, when the cause would be immediately obvious.
Prolonged immersion in synthetic 'vinegar' (condiment) can stain stainless steel knives if left on for several hours, but more rapidly if the 'vinegar' also contains salt.

Sometimes rust coloured stains occur. Wet fragments of steel wool that find their way onto cutlery may go rusty and leave indelible rust stains on the stainless steel. Other rust coloured stains may come from corrosion pits in the cutlery, although the pits themselves may be so small as to be barely visible.
Most stains that resist ordinary rubbing with a soapy cloth can be removed with 'Autosol'. Finally, it must be pointed out that there may be other causes of staining that have not yet been identified.

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 the spoon and fork 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 us 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 un-dried 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 softeners are regenerated with salt. If they are not functioning correctly, the 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 supplier before washing knives in the machine. Detergents, on the other hand, are normally harmless when fully dissolved - indeed they can 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 form within a few minutes. Un-dissolved table salt can have a similar effect.
If pitting does occur, a diligent enquiry into the washing procedure is recommended to find out whether the cutlery is left in contact with water for a long time.