Corrosion is the natural degradation of materials due to atmospheric conditions or other factors. It is usually defined as the dissolution of a material due to a reaction with the surrounding environment. Only metals such as gold or platinum exist in their pure form; ordinary metals exist only in combination with other elements. Corrosion is therefore a natural phenomenon, as nature tries to combine the elements produced by man in their pure form. More specifically: iron exists naturally only as a mineral, so pure iron is unstable and seeks to rust, i.e. to combine with oxygen in the presence of water.
Unprotected carbon steel forms an oxide layer that tries to protect the rest of the steel, and constant removal of the oxide layer only leads to new layers of steel being attacked. The corrosion resistance of stainless steel depends on the presence of a thin, invisible layer on the surface of the steel, the passive layer. Where the passive layer cannot be reconstructed, corrosion occurs in the unprotected parts. Different media can cause different types of corrosion attack, which can vary in pattern and appearance; various forms of corrosion can occur on stainless steel.
Homogeneous corrosion Homogeneous corrosion：
In this case, the passive layer of the stainless steel is partially or completely destroyed. The rate of corrosion expansion is determined by both the corrosive environment and the alloy composition. Uniform corrosion or general corrosion of stainless steel occurs in acidic environments or in hot alkaline solutions. The worst environments from a corrosion point of view are those containing high concentrations of hydrochloric or hydrofluoric acid, where corrosion propagates at a corrosive and destructive rate.
Pitting corrosionPitting corrosion：
Pitting corrosion is a type of localised corrosion characterised by attack at small, discontinuous points on the surface of the steel. Pitting corrosion occurs mainly in neutral or acidic solutions containing chlorides and other hydrides. The chloride ions promote localised rupture of the passive layer, especially in the presence of defects on the metal surface.
Crevice corrosion Angular crevice corrosion：
Crevice corrosion is another type of localised corrosion that occurs under the same conditions as pitting, neutral or acidic chloride solutions. However, attack is more likely to start in hairline cracks than on bare surfaces. Cracks such as those found in flange joints or threaded joints are key points for this type of corrosion.
Stress corrosion Stress corrosion Corrosion：
Material failure is accelerated by the combined effect of corrosion and mechanical stress. The most common type is trans-lattice stress corrosion cracking (SCC), which can develop in environments with high chlorine concentrations. Previously it was thought that high temperatures were necessary for SCC to occur. However, in recent years it has been shown that SCC can also occur in standard grades of steel such as 304 stainless steel plate (L) or 316 stainless steel plate (L) when exposed to high stresses at room temperature.In these cases the steel surface is contaminated with solid salt deposits and the ambient humidity is high. Both of these factors result in a thin liquid layer saturated with chloride. Other contaminants, such as hydrogen sulphide, can increase the risk of SCC in environments where chlorides are already present. Other environments that may increase the risk of SCC, particularly in low alloy steels, include very alkaline solutions at high temperatures. A typical SCC attack looks like thin, branching cracks.
Galvanic corrosionElectrochemical corrosion：
When two different metals are immersed in a corrosive solution, each metal will develop a corrosion potential. If the corrosion potentials are significantly different and they are immersedfa in contact with the electrolyte, the nobler metal will become the cathode and the more active metal will become the anode. A measurable current can flow between the anode and the cathode. The corrosion rate of the anode will increase and that of the cathode will decrease. The increased corrosion of the anode is known as “electrochemical corrosion”.
Intergranular corrosion Intergranular corrosion：
This type of corrosion occurs if the area around a grain boundary is less resistant to corrosion than the substrate of the medium in question. A typical situation is when chromium carbide precipitates on a grain boundary. The adjacent substrate will be depleted of chromium and therefore the thin area around the grain boundary may be less resistant to corrosion than the rest of the material.
Fatigue corrosionFatigue corrosion：
It is well known that materials subjected to cyclic loads well below their maximum tensile stress will fail, a process known as fatigue. If metals are also exposed to corrosive environments, failure can occur in a relatively short period of time, even at lower loads. In contrast to purely mechanical fatigue, there is no limit to the fatigue load in fatigue corrosion.
Atmospheric corrosionAtmospheric corrosion：
Atmospheric corrosion occurs on the surface of steel in a thin wet layer formed by moisture in the air together with impurities. Corrosive conditions for stainless steel can be chloride deposits in the coastal atmosphere and these conditions can change the grade of steel chosen to make it a more resistant alloy to corrosive environments. The most common types of corrosion caused by atmospheric conditions are pitting and crevice corrosion.
Atmospheric conditions in a place can be divided into four categories: rural, urban, industrial and marine. The environment varies depending on how severe they are from a corrosion point of view. Keeping steel surfaces clean by regular cleaning to avoid getting dusty should be the first priority in preventing corrosion.