This brown discolouration, or ‘tea staining’ of stainless steel, is corrosion due mainly to exposure to salt and sea air. Tea staining occurs predominantly within about five kilometres of the surf. It also occurs around bays and rivers and becomes progressively worse closer to the marine source. Depending on the level of exposure to, wind, pollution and high temperatures, ‘tea staining’ can occur up to 40 kilometres from the surf.

Stainless steel is the name given to a class of steels (alloys) containing the common element of chromium, at a minimum of 10.5%. Other elements such as nickel and molybdenum may be present, but it is the chromium that gives stainless steel its excellent corrosion resistance properties. The chromium reacts positively with oxygen to create a renewable protective layer of chromium oxide.

Stainless steel is used in many everyday applications in the home, in industry, in hospitals, in food processing, in farming, in aerospace and in many other areas. There are over 150 different stainless steel grades, but about 15 or so are the most commonly used. The three major classes of stainless steel are – 1: Austenitic (304 and 316 grade)   2: Ferritic    3: Martensitic.

The most common grades of stainless steel are 304 and 316 (marine grade). Exterior architectural fittings such as railings, balustrades and light fittings are commonly made from 316 grade stainless steel. Molybdenum is added to give316 grade added corrosion protection to fittings located close to marine environments. 304 grade stainless steel, without molybdenum, is common in sinks, work surfaces, kitchen preparation areas and barbeques.

In a word, yes. Although stainless steel is much more resistant to corrosion than ordinary carbon or alloy steels, in some circumstances it can corrode. It is ‘stain-less’ not ‘stain-free’. It does not corrode the way unprotected carbon steel readily corrodes, forming layers of red flaking rust when exposed to air and moisture.  The iron oxide film (the rust) is active and accelerates corrosion by forming more iron oxide. Stainless steels contain sufficient chromium to form a passive film of chromium oxide, which prevents further surface corrosion and blocks corrosion from spreading into the metal’s internal structure. Corrosion in stainless steel is caused through damage to the chromium layer and through contaminants becoming trapped on the surface of the stainless steel.