Stainless Steel Rust Malaysia | Stainless Steel Corrosion?

Stainless steels are steels that are alloyed with around 12-30% of Chromium. The chromium protects the steel by forming a protective layer of chromium oxide which protects the iron in the stainless steel from being oxidized and corroded. However, stainless steels are not entirely immune to corrosion. Under extreme or harsh conditions, even stainless steels can corrode. According to the British Stainless Steel Association, there are 6 types of corrosion that may occur to stainless steels.

The first is pitting corrosion. This occurs when the layer of chromium oxide is attacked by certain chemical molecules. An example of such is chloride which is commonly found in your everyday salt, bleach and sea water. This form of corrosion is not that severe and its effects are merely superficial. You can avoid this simply by making sure that the stainless steel does not come into contact with corrosive chemicals or exposing it to harsh environments.

The second type of corrosion that may occur is crevice corrosion. It is where the stainless steel is placed in a tight space or crevice in which little oxygen is available. This means that the chromium within the stainless steel cannot bond with oxygen to form the protective layer of chromium oxide. Crevice corrosion can range from less severe to highly severe depending on the size of the crevice. A narrow and deep crevice will result in severe corrosion of the stainless steel. Examples of crevices include the space between nuts and washers, the thread area of screws, the shank of a bolt, and also in welds.

General corrosion is another type of corrosion that may occur to stainless steel. This happens when chemicals such as strong acids attack the layer of chromium oxide that is protecting the steel. This usually occurs when the stainless steel is in a situation where the pH value is less than 1, or in other words, in a very acidic condition. Concentrated hydrochloric acid and sulphuric acid are some of the acidic compounds that are very aggressive towards stainless steel. This type of corrosion is less severe in stainless steel compared to conventional steel.

A rare form of corrosion in stainless steel is stress corrosion cracking. It is rare because it requires a specific set of parameters in order for it to occur. The stainless steel would need to be placed in an environment where there are tensile stress, temperature, and corrosive substances. An example is in hot water tanks that may be found in industrial plants. The stress may also occur during the manufacturing process of the stainless steel. Stainless steels with high residual stress are especially vulnerable in chlorine-rich environments such as the ocean and swimming pools.

Another type of corrosion is intergranular corrosion. This occurs when the level of carbon in the stainless steel is relatively high and causes the chromium to combine with carbon to form chromium carbide at high temperatures of about 450-850°C. Typically, this happens during welding. When the chromium combines with carbon, its concentration within the steel is lowered. Thus leaving stainless steel prone to corrosion as little chromium is available to from the layer of chromium oxide which is what makes the steel stainless.

The final type of corrosion is galvanic corrosion. This is where two different metals come into contact with each other in an electrolyte condition ( a condition that permits the flow of electrons). If current flows between the two metals, the metal that is less resistant to corrosion will corrode away. The rate of corrosion is affected by the contact area of the two metals, the temperature and the composition of the electrolyte.

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