<P> In the most common use of the word, this means electrochemical oxidation of metal in reaction with an oxidant such as oxygen or sulfur . 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> <P> Galvanic corrosion occurs when two different metals have physical or electrical contact with each other and are immersed in a common electrolyte, or when the same metal is exposed to electrolyte with different concentrations . In a galvanic couple, the more active metal (the anode) corrodes at an accelerated rate and the more noble metal (the cathode) corrodes at a slower rate . When immersed separately, each metal corrodes at its own rate . What type of metal (s) to use is readily determined by following the galvanic series . For example, zinc is often used as a sacrificial anode for steel structures . Galvanic corrosion is of major interest to the marine industry and also anywhere water (containing salts) contacts pipes or metal structures . </P> <P> Factors such as relative size of anode, types of metal, and operating conditions (temperature, humidity, salinity, etc .) affect galvanic corrosion . The surface area ratio of the anode and cathode directly affects the corrosion rates of the materials . Galvanic corrosion is often prevented by the use of sacrificial anodes . </P>

Why does corrosion usually occur at an anode