Stainless Steel Rust

It can. Stainless steel resists corrosion because of a thin, self-repairing oxide film on its surface. If there are more corrosives on the surface, such as salt, than the grade can resist, or not enough oxygen to keep the film healthy, it corrodes. The usual reasons for rust on stainless are carbon steel contamination or the wrong grade for the setting.

Look at when it started. According to the Australian Stainless Steel Development Association (ASSDA), rust within a few days or weeks is almost certainly carbon steel contamination from fabrication or the local environment. Rust that takes longer usually comes from surfaces that are too rough, an environment too aggressive for the grade, or a lack of washing.

It can. 316 resists salt better than 304, but ASSDA says it will rust in seawater, especially in crevices such as a nut and bolt, and that in severe coastal settings where salty spray builds up over time, 316 can visibly corrode. Regular washing keeps it bright.

ASSDA's cleaning advice: try a cream cleanser on a soft damp cloth, rubbing with the grain. If that fails, a proprietary stainless steel cleaner may be needed; these are usually based on phosphoric, oxalic or sulphamic acid, so follow the maker's directions, then neutralise with a 1% ammonia or baking powder solution, rinse with clean water and wipe dry. Where rust has worn into the surface, get expert advice on professional polishing.

No. ASSDA says never to use steel wool on stainless: it is usually carbon steel, and any fragments left behind will rust onto the surface. The same goes for any scourer that has been used on ordinary steel. Use a clean nylon scourer or a soft cloth.

Some is. Grades 304 and 316 are virtually non-magnetic when annealed, but both can become attracted to a magnet after cold working, 316 less so because of its higher nickel. Ferritic, martensitic and duplex stainless steels are strongly magnetic. ASSDA is clear that magnetism is not related to corrosion resistance, so a magnet cannot tell you whether stainless will rust.

Tea staining is a brown discolouration of the surface caused by corrosion. ASSDA describes it as a cosmetic issue that does not affect the structural integrity or life of the material. It is most common within about 5km of a surf beach and 1km of still marine waters, and worse on rough, rain-sheltered surfaces.

ASSDA's guide is to wash the stainless whenever the surrounding area is cleaned, for example when a window needs washing. Where that does not happen, its table for 304 in seaside, industrial and severe urban areas is once a year for surfaces washed by rain, 3 to 4 times a year for eaves, soffits and other spots rain never reaches, and 4 to 12 times a year where deposits build up there. Separately, the manufacturer's self-drilling screw warranty requires those screws and the surfaces around them to be washed at least every six months in categories C2 to C3 and every three months in C4 to C5, as a condition of the warranty.