Corrosion

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Common forms of corrosion

Surface corrosion : caused by abrasion as a result of the surrounding environment eg.sand, salt in atmosphere etc. which exposes the base metal to oxygen in the air. This is generally called rust.

Localized corrosion : caused as a result of break down or damage of protective coating plating and is restricted to specific area.

Contact (galvanic) corrosion:  caused by the electro-chemical process resulting from dissimilar metals coming into contact.

Stress corrosion :  often referred to as environmental hydrogen embrittlement  is caused by simultaneous effects from a corrosive medium and mechanical stress in the fastener join area.

The  4 main ways to prevent, if not slow down the effects of corrosion.

1. The easiest way is to choose the right fastener material in keeping with the environmental surrounding of the fastener joint. Eg. Stainless steel, brass ,etc.

2. Chose the best & must suitable surface coating eg.zinc coating, galvanizing, etc. coating protect & fight corrosion in 3 basic ways:

  • They provide a barrier between the corrosive environment and the fastener base metal.
  • They provide ‘’ galvanic’’ or sacrificial protection.
  • They fight corrosion by ‘’ passivation’’ or ‘’inhibition’’. This is particularly true with stainless steels whereby by the clear original oxide film is restored, thus slowing down corrosion.

3. Electro chemical measures. Eg.cathodic protection (by adding a sacrificial element to the corrosion circuit).

4.Structural / design measures. Eg. Use insulating material, avoid crevices where corrosive agents can accumulate etc.

Sacrificial Protection v/s Barrier Protection

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Sacrificial coatings are those deposits that give themselves up to the corrosive media, thereby protection the base metal. The life of the protection is extended by increasing the coating thickness. Barrier coatings on the other hand, are deposits that reduce or eliminate the chances of moisture, oxygen and atmospheric gases from contacting the base metal by forming a barrier between the coating and the base metal. Unlike sacrificial protection, any damage to the barrier coating will immediately lead to corrosive attack on the base metal.

Dimensional effects of coatings

Coatings have the effect of enlarging the pitch diameter of fastener by four times the coating thickness. By virtue of the fact that coating thickness is measured perpendicular to the coated surface, while the pitch thread axis is a standard 60 degree metric/Unified series thread, results in a change of four times the coating thickness on the flank.

 

This can be a major problem / danger when specifying coating thicknesses and is one of the first points to consider when contemplating thicker than standard coatings.

 

Type of corrosion

– Direct chemical attack occurs when the fastener metal is in direct contact with eroding chemical e.g.: acids, oxides, chlorides etc. This is the severest form of corrosion. The chemical reaction that takes place between the two results in the fastener metal being eaten away, eventually resulting in the fastened joint to destruct or fall apart

– Electro-chemical reaction (Galvanic corrosion) take place when two dissimilar metals are in contact with each other in the presence of some electrolyte such as seawater, dilute acids etc. the less active metal will act as the cathode and attract electrons from anode and as result, the anode will erode. There are three conditions that must exist for galvanic corrosion to occur.

  1. There needs to be two electrochemically dissimilar metals present.
  2. There must be an electrically conductive path between the metals
  3. There must be a conductive part for the metal ions to move from the more anodic metal to the more cathodic metal.

If any one of these three conditions does not exist, galvanic corrosion will not occur. Galvanic corrosion can be minimized with the right choice of platings and finishes.

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What is Rust? It’s the result of the outer surface of Iron / Steel combining with oxygen from the air to form a new reddish compound called ferric oxide. Ferric oxide is often referred to as ‘’Red Rust’’.

Underneath this outer layer of ‘’red Rust’’ lies another harder & darker layer called ‘’black Oxide’’ or ‘’Black Rust’’ (Magnetite) which remains strongly bonded to the base steel ans acts as a partial protection for the steel from further oxidization.

‘’White Rust’’ is a powdery coating of fine zinc oxide that forms on zinc plated fasteners when exposed to the atmosphere over the time. This can be prevented by providing a chromate coating to the fastener after it has been zinc plated.

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