Why Does Rust Occur?

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In General
Rust damage on products costs large sums every year, and can also cause breakdowns through various kinds of corrosion weakening sup- porting structures. There are therefore strong reasons to think about
what type of surface treatment a fastener should be given. Below, an account is given of the mechanism of corrosion, with some general advice which can help in choosing corrosion protection for fasteners. Also, several types of surface treatment on special parts are described.

What Is Corrosion? 

Steel rusts, copper oxidises and other metals, except those which are most inert, are broken down in a similar way. This type of material destruction is given the general name of corrosion. It occurs when the material reacts with its surroundings and is converted into other substances – to corrosion products. Almost all corrosion which occurs in normal working environments is of electrochemical type. It occurs in galvanic cells, corrosion cells, which function roughly in the same way as a flashlight battery. The battery has a carbon rod in its centre, and a casing of zinc plate. The carbon rod is called the cathode, the
zinc plate the anode. Inside the battery, there is also a paste or sludge which is called electrolyte. When you switch on a flashlight, current runs from the carbon rod through the bulb to the zinc plate. From there, the current goes through the electrolyte back to the carbon rod. The current takes with it zinc particles from the plate, which becomes corroded and, in due course, begins to leak. The cathode is inert (- pole), the anode is reactive (+ pole). Galvanic cells which cause the corrosion can form when two different metals (or one metal and another substance which conducts electricity in the same way as a metal, for example, graphite) come into contact with an electrolyte. Corrosion caused by such material combinations is called by the common name of galvanic corrosion. Galvanic cells, often extremely small, also occur on individual metal surfaces. This is connected to the fact that industrial metals consist of microscopic granules of varying composition, and can also be due to various impurities on the surface, such as oxide scale, slag residue, etc. The granules and particles are, as with various metals, of varying degrees of inertness in relation to one another.