Corrosion is the deterioration of a material by chemical or electro-chemical reaction; it is the material returning to its natural oxide state. Corrosion can be the result of different processes & can be avoided or minimized by use of an appropriate corrosion protection coating. Methods of corrosion:
- Chemical – reaction between the metal and a gas or liquid stage
- Electro-chemical – by creating an electro-chemical cell with oxidation of the anode
- Biochemcial – by bacterial attack
The chemical formula of RUST: Fe(OH)2, FeO, Fe(OH)3, Fe2O3
Corrosion can form uniformly or may follow the grain boundaries or may penetrate into the steel in the form of a localised pit. The speed of corrosion depends on the aggressiveness of the environment, the oxidation products structure and behavior of the steel. In the case of steel, the rust doesn’t adhere to the surface and will flake away leaving the steel unprotected and the corrosion propagates continuously.
Interior and Exterior environments may influence the steel in different ways.
Corrosion Protection Coating – Interior Factors
C1 – very low: heated buildings with clean atmospheres – offices/shops
C2 – low: unheated buildings where condensation may occur – sports halls
C3 – medium: production rooms, high humidity and some internal air pollution – food-processing, dairies
C4 – high: chemical plants, swimming pools, coastal boat-yards
C5I – very high industrial: buildings with almost permanent condensation and high levels of pollution
C5M – very high marine: buildings with permanent condensation and high levels of pollution
Corrosion Protection Coating – Exterior Factors
C2 – low: area of low levels of pollution – rural areas
C3 – medium: urban areas with moderate sulfur dioxide & coastal areas with low levels of salinity
C4 – high: industrial areas and coastal areas with moderate levels of salinity
C5I – very high industrial: industrial areas with high humidity and aggressive atmosphere
C5M – very high marine: coastal and off-shore areas with high salinity
CORROSION PROTECTION COATING – METHODS
Barrier corrosion protection coating generally works well provided there is no defect in the coating layer. In the case of a defect corrosion will often spread under the coating layer lifting the coating off allowing an even greater area to be exposed. Standard corrosion protection coating include, liquid paints and powder coatings offering a barrier protection layer.
To provide a very good level of corrosion protection coating the barrier layer should be impermeable to water. This requires a high level of binder to be in the coating mix. Phosphating pre-treatment before the coating is applied offers a further layer of corrosion protection coating. A thin layer of metal phosphates provides some protection and improves adhesion.
This is applying a layer of less noble metal over the steel which will oxidize preferentially. Zinc brings a cathodic protection to iron by electro-chemical effect. In addition to this cathodic protection zinc offers a shield as well.
In the presence of water and oxygen zinc converts to various zinc salts, insoluble in most natural environments. These salts build to a dense layer slowing down the rate of corrosion and provides in the case of scratches a sealing effect which also contributes to the slow down of corrosion.
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