Why Stainless Corrodes

Stainless steel is a costly alloy of iron and other metals - Most important among them chromium, which forms a stable oxide that halts the corrosion process. This is similar to anodization of aluminium, in which a stable oxide layer is formed on the surface to prevent further corrosion. There is no such luck for ordinary steel - rust is porous and water can always penetrate down to the nonrusted surface. It will go right through the steel if you give it a chance.Some things cause steel or iron to rust faster than others. Water will cause iron and steel to rust. Dissimilar metals rust faster than single metals because of electrochemical reactions, so steel rusts faster than iron, and joints between dissimilar metals rust very quickly. Salt water will cause rust faster than water because salt water is a better electrical conductor. Like most chemical reactions, heat also speeds rust.

Stainless steel remains stainless, or does not rust, because of the interaction between its alloying elements and the environment. Stainless steel contains iron, chromium, manganese, silicon, carbon and, in many cases, significant amounts of nickel and molybdenum. These elements react with oxygen from water and air to form a very thin, stable film that consists of such corrosion products as metal oxides and hydroxides. Chromium plays a dominant role in reacting with oxygen to form this corrosion product film. In fact, all stainless steels by definition contain at least 10 percent chromium.

The presence of the stable film prevents additional corrosion by acting as a barrier that limits oxygen and water access to the underlying metal surface. Because the film forms so readily and tightly, even only a few atomic layers reduce the rate of corrosion to very low levels. The fact that the film is much thinner than the wavelength of light makes it difficult to see without the aid of modern instruments. Thus, although the steel is corroded on the atomic level, it appears stainless.

So stainless steel does not rust, because it is sufficiently reactive to protect itself from further attack by forming a passive corrosion product layer – Provided it is treated correctly.

There are eight basic form of corrosion (Pitting, Crevice, ...) common in industry.

Detrimental effects can occur when corrosion accompanied synergistically by mechanical load (static, cyclic). More concern should be given due to stress corrosion cracking, pitting, and intergranular corrosion as these types of corrosion is the most cause of failure in gas pipeline and process industry.

Stainless steels are not indestructible materials, nor immune to all corrosive attack.

Common forms of corrosion on alloys

• General corrosion is a uniform attack of the entire metal surface. It is the least dangerous because rates of corrosion can be measured and predicted.
• Galvanic corrosion occurs when two different metals are in electrical contact and immersed in the same corrosive solution.
• Erosion corrosion is a combination of mechanical and corrosive attack. Abrasive particles, expose fresh metal surfaces which then suffer high rates of corrosion.
• Intergranular corrosion is due to the formation of chromium carbides at high temperatures (450°-859°C).These form preferentially at the grain boundaries thus reducing the chromium content and resulting in a path of lower corrosion resistance around the grains.
• Pitting corrosion is a dangerous, very localised form of corrosion which results in holes, but with little measurable general metal loss. Some corrosive environments have the ability to attack localised weak points in the passive film.
• Crevice/shielded corrosion occurs where the surface of stainless steel is shielded or occluded thus preventing the free access and availability of oxygen to the surface.
• Microbiologically induced corrosion results from the attraction and adherence of bacteria to the surface of the metal. A condition similar to a crevice is thereby produced.