![]() This type of corrosion is more common among alloys than with pure metals and is dependant on the specific chemical environment whereby only small concentrations of active chemicals are required for catastrophic cracking. Stress corrosion cracking refers to the growth of cracks due to a corrosive environment which can lead to the failure of ductile metals when subjected to tensile stress, particularly at high temperatures. This type of corrosion occurs along or adjacent to the grains, affecting the mechanical properties of the metal despite the bulk of the material being unaffected. It can also be caused by the enrichment or depletion of an alloying element at the grain boundaries. Intergranular corrosion occurs when impuraties are present at the grain boundaries which form during solidification of an alloy. Crevice corrosion can take place ar lower temperatures than pitting corrosion, but can be minimised by proper joint design. The imbalance between the crevice and the rest of the material contributes to the high rates of corrosion. The stagnant microenvironment prevents circulation of oxygen, which stops re-passivation and causes a buuild-up of stagnant solution moving the pH balance away from neutral. This localised corrosion usually results from a difference in the ion concentration between two areas of metal. This form of corrosion occurs in areas where oxygen is restricted such as under washers or bolt heads. This dangerous form of corrosion can cause a structure to fail despite a relatively low loss of metal. Pitting corrosion can be caused by damage or a break in the oxide film or a protective coating and can also be caused through non-uniformities in the structure of the metal. This pitt can create a hole or cavity which typically penetrates the material in a vertical direction down from the surface. This localised type of corrosion happens when a local anodic or cathodic point forms a corrosion cell with the surrounding surface. One of the most aggressive forms of corrosion, pitting can be hard to predict, detect or characterise. This is the most common form of corrosion which usually takes place evenly over large areas of a material's surface. The rusting of iron can also occur when iron reacts with chloride in an oxygen-deprived environment, while green rust, which is another type of corrosion, can be formed directly from metallic iron or iron hydroxide. Rust can also effect iron alloys such as steel. The most common type of iron corrosion occurs when it is exposed to oxygen and the presence of water, which creates a red iron oxide commonly called rust. Corrosion can also happen when metals like steel are placed under too much stress causing the material to crack. Metal corrodes when it reacts with another substance such as oxygen, hydrogen, an electrical current or even dirt and bacteria. National Structural Integrity Research Centre.Structural Integrity Research Foundation.
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