Key corrosion mechanisms explained
Corrosion is the natural degradation of a material due to atmospheric conditions or other factors. Corrosion is generally defined as dissolution of a material due to a reaction with the surrounding environment. Only metals such as gold and platinum are found in a pure form, normal metals only exist combined with other elements. Corrosion is therefore a natural phenomenon, as nature seeks to combine elements which man has produced in a pure form. More specific: iron occurs naturally as iron ore, so pure iron is unstable, as it wants to "rust", that is, to combine with oxygen in the presence of water. Carbon steels without any protection will therefore form a coating of rust, which will in a sense protect the rest of the steel and constantly removing the rust exposes a new fresh layer of steel to be attacked. The corrosion resistance of a stainless steel is dependent on a thin invisible film on the steel surface, the passive film. There are, however, environments that cause permanent breakdown of the passive layer. Under circumstances where the passive layer cannot be rebuilt, corrosion occurs on the unprotected surface. Different media can cause different types of corrosion attack that may vary in nature and appearance, and several forms of corrosion can occur on stainless steels.
In this case the passive layer on a stainless steel surface breaks down partly or completely. The corrosion then propagates at a rate determined by a combination of the corrosive environment and the alloy composition. Uniform corrosion or general corrosion occurs on stainless steel in acid environments or hot alkaline solutions. Severe environments from a corrosive point of view are high concentrations of hydrochloric or hydrofluoric acid in which the corrosion may propagate at a rate that can be detrimental to a construction.
Pitting is a form of localised corrosion and is characterised by attacks at small discrete spots on the steel surface. Pitting occurs mainly in the presence of neutral or acidic solutions containing chlorides or other halides. Chloride ions facilitate a local breakdown of the passive layer, especially if there are imperfections in the metal surface.
Crevice corrosion is a form of localised corrosion and occurs under the same conditions as pitting, i.e. in neutral or acidic chloride solutions. However, attack starts more easily in a narrow crevice than on an unshielded surface. Crevices, such as those found at flange joints or at threaded connections, are thus often the most critical sites for corrosion.
Stress corrosion cracking
A material failure may be accelerated by the combined effect of corrosion and mechanical stress. The most common type is transgranular stress-corrosion cracking, SCC, that may develop in concentrated chloride-containing environments. Previously, it was generally considered that an elevated temperature was necessary for SCC to occur. In recent years, however, SCC has been experienced at ambient temperature on standard grade steels like 304(L) or 316(L) that were exposed to high tensile stresses. In these cases the steel surface was contaminated with solid salt deposits and the humidity of the atmosphere was rather high. These two factors resulted in a thin liquid film saturated with chloride. Other contaminants, such as H2S, may increase the risk of SCC in chloride containing environments. Other environments that may give rise to SCC, particularly on low alloy steels, include very alkaline solutions at high temperatures. A typically SCC attack takes the form of thin, branched cracks.
When two different metals are immersed in a corrosive solution, each will develop a corrosion potential. If the corrosion potential of the two metals is significantly different, and they are in direct contact and immersed in an electrolyte, the more noble metal will become the cathode and the more active metal will become the anode. A measurable current may flow between the anode and the cathode. The corrosion rate of the anode will be increased and the cathode decreased. The increased corrosion of the anode is called "galvanic corrosion".
This type of corrosion may occur if the area around the grain boundaries is less corrosion resistant than the matrix in the medium in question. The classical case is when chromium carbide is precipitated at the grain boundaries. The adjacent matrix will be depleted in chromium and a narrow region around the grain boundary may, therefore, be less corrosion resistant than the rest of the material.
It is well known that a material subjected to a cyclic load far below the ultimate tensile stress can fail, a process called fatigue. If the metal is simultaneously exposed to a corrosive environment, the failure can take place at even lower loads and after shorter time. Contrary to a pure mechanical fatigue, there is no fatigue limit load in corrosion-assisted fatigue.
Atmospheric corrosion occurs on a steel surface in a thin wet film created by the humidity in the air in combination with impurities. Corrosive conditions for stainless steel can be the chloride depositions from a marine atmosphere on the steel surface in a humid atmosphere these, conditions may alter the choice of steel grade to a more corrosion resistant alloy. The type of corrosion is most commonly local in the presence of chlorides, i.e. pitting or crevice corrosion.
Atmospheric environments are most commonly divided into four categories: rural, urban, industrial and marine. The environments vary depending on the severity from a corrosive point of view. The importance of keeping the surface clean by regular washing to avoid staining and dust cannot be stressed enough.