<Li> Stress corrosion cracking </Li> <P> Corrosion is a natural process, which converts a refined metal to a more chemically - stable form, such as its oxide, hydroxide, or sulfide . It is the gradual destruction of materials (usually metals) by chemical and / or electrochemical reaction with their environment . Corrosion engineering is the field dedicated to controlling and stopping corrosion . </P> <P> In the most common use of the word, this means electrochemical oxidation of metal in reaction with an oxidant such as oxygen or sulfates . Rusting, the formation of iron oxides, is a well - known example of electrochemical corrosion . This type of damage typically produces oxide (s) or salt (s) of the original metal, and results in a distinctive orange colouration . Corrosion can also occur in materials other than metals, such as ceramics or polymers, although in this context, the term "degradation" is more common . Corrosion degrades the useful properties of materials and structures including strength, appearance and permeability to liquids and gases . </P> <P> Many structural alloys corrode merely from exposure to moisture in air, but the process can be strongly affected by exposure to certain substances . Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface . Because corrosion is a diffusion - controlled process, it occurs on exposed surfaces . As a result, methods to reduce the activity of the exposed surface, such as passivation and chromate conversion, can increase a material's corrosion resistance . However, some corrosion mechanisms are less visible and less predictable . </P>

Where does corrosion take place give two example
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