What is stainless steel ?

Stainless steel is the generic name for a number of different steels used primarily for their resistance to corrosion. The one key element they all share is a certain minimum percentage (by mass) of chromium: 10.5%. Although other elements, particularly nickel and molybdenum, are added to improve corrosion resistance, chromium is always the deciding factor. The vast majority of steel produced in the world is carbon and alloy steel, with the more expensive stainless steels representing a small, but valuable niche market.

What Causes Corrosion ?

Only metals such as gold and platinum are found naturally in a pure form – normal metals only exist in nature combined with other elements. Corrosion is therefore a natural phenomena, as nature seeks to combine together elements which man has produced in a pure form for his own use. Iron occurs naturally as iron ore. Pure iron is therefore unstable and wants to “rust”; that is, to combine with oxygen in the presence of water. Trains blown up in the Arabian desert in the First World War are still almost intact because of the dry rainless conditions. Iron ships sunk at very great depths rust at a very slow rate because of the low oxygen content of the sea water . The same ships wrecked on the beach, covered at high tide and exposed at low tide, would rust very rapidly. For most of the Iron Age, which began about 1000 BC, cast and wrought iron was used; iron with a high carbon content and various unrefined impurities. Steel did not begin to be produced in large quantities until the nineteenth century. Carbon steel can be defined as an alloy of a small content of carbon combined with well refined iron. Despite its various additions stainless steel still behaves as steel, it is not like the nickel alloys that are really alloys of a number of different metals, iron only being one. Even highly alloyed stainless steel grades such as 316 are a minimum of 62% iron.

Carbon steels without any protection will form a coating of rust which will in a sense protect the rest of the steel. So constantly removing the rust exposes a new fresh layer of steel to be attacked. This is called general corrosion . Various coatings will impede the rusting process, in particular painting, coating with zinc (galvanised steel), and epoxy resins. Another lateral way of reducing corrosion is to put corrosion inhibitors into the solutions that would otherwise cause iron to corrode. One of the most common examples of this is the corrosion inhibitors added to the coolant used in cars

Advantage of Stainless Steel

For a wide variety of applications, stainless steel competes with carbon steels supplied with protective coatings, as well as other metals such as aluminium, brass and bronze. The success of stainless steel is based on the fact that it has one unique advantage. The chromium in the stainless steel has a great affinity for oxygen, and will form on the surface of the steel at a molecular level a film of chromium oxide. The film itself is about 130 Angstroms in thickness, one Angstrom being one millionth of one centimetre . This is like a tall building being protected from the rain with a roof the thickness of one sheet of ordinary copy paper. This layer is described as passive, tenacious and self renewing. Passive means that it does not react or influence other materials; tenacious means that it clings to the layer of steel and is not transferred elsewhere; self renewing means that if damaged or forcibly removed more chromium from the steel will be exposed to the air and form more chromium oxide. This means that over a period of years a stainless steel knife can literally be worn away by daily use and by being re-sharpened on a sharpening stone and will still remain stainless. Silver plated cutlery will eventually wear through to the base alloy, but stainless steel cutlery cannot wear through. Manhole and access covers in the water treatment and chemical industry are widely made out of both galvanised steel and stainless steel. In normal use galvanised steel can last many years without corrosion occurring and in these cases there would be little advantage apart from aesthetic reasons to switch to stainless steel. Where stainless comes into its own is where the galvanised coating is constantly being worn away, for example by chains being dragged over it, or constantly being walked over, or where very corrosive chemicals are being randomly splashed onto it.

This leads on to the important point that the initial investment cost of producing a component or fabrication in stainless steel will always be more expensive that using ordinary steel, not just because of the higher cost of stainless steel, but also because it is more difficult to machine. However it is the better life cycle costs of stainless steel that make it attractive, both in terms of much longer service life, less maintenance costs, and high scrap value on de-commissioning.

Product characteristics

Stainless steel can be selected for use compared to other materials for a number of different reasons, not just its resistance to corrosion. These include:

• Its aesthetic qualities: it can be polished to a satin or mirror finish.
• “Dry Corrosion” occurs to steel at higher temperatures where it oxidises or scales up. Stainless steel is far more resistant to this than ordinary carbon steel and grades such as 310 (25% chromium 20% nickel) were specifically developed for use at high temperatures.
• Non-contamination of the liquids stainless comes into contact with, because there is no coating to break down and dissolve.
• Weight savings: as thinner sections can be used, more innovative design structures can be used, with cost savings on foundations and platform weights.
• Many anti-corrosion coatings are fire hazards or the materials themselves have a low melting point.