Surface Coatings for Corrosion

The main method of protecting metal components from attack by the local environment is by use of surface coatings. There are a number of surface coating systems available:

Grease / Oil
Various machines require grease and oil for lubrication. These products also provide a degree of corrosion protection of the lubricated surfaces and adjacent surfaces. Grease can be used to provide corrosion protection for tools and equipment used in workshops which are stored for short times when not in use.

Wax
There are a number of wax based products available which can be sprayed on surfaces at risk and provide a ignificant level of corrosion resistance at minimum cost. Depending on the operating condition these product can last from 1 to 10 year or more.

Paint
Probably the most widely used system of corrosion protection is by painting the metal surfaces at risk. Notes on painting are provided elsewhere on this site Painting Notes

The advantages of painting is that the protection is relatively convenient to apply and the paint can be tailored to suit the duty. Paints can be engineered for convenience of decontamination and provide a degree of surface protection against the environment in addition to the required corrosion protection. Paint is also used for decoration and equipment identification.

The main disadvantages of painting are that damaged painted surfaces or low quality painted surfaces will fail rapidly resulting in enhanced local corrosion of the base metal.

Zinc Coating (Galvanising)
Galvanised steel which is steel electroplated with zinc, uses the zinc layer as a protection to the steel. Zinc is anodic and sacrificially removed. The steel is cathodic and thus is protected. The important advantage of this process is that if the zinc coating is scratched the steel surface beneath is still galvanically protected. Notes on this process are provide elsewhere on this site Painting Notes

Plastic Coating
Virtually all plastics can be applied as metal surface coatings by spraying, fluidized-bed, electrostatic, rotational moulding, flock or 'slush' coating or dipping   The coating system has the benefits of the strength of the base metal with the relevant properties of the plastic coating. This option has similar disadvantages as the painting option. The resulting surface must be sound and continuous and the thermal and mechanical properties of the coating are generally inferior to the base metal.

Vitreous Enamel
Enamel (Vitreous Enamel) is a thin layer of glass fused by heat on to the surface of a metal being protected. The process involves dipping or spraying the glass coatings onto metallic substrates and subsequent fusion operations. Vitreous enamelled components may have single or multiple coatings and they may be fired after each application or they may be fired as a single operation. The firing process uses a high temperature furnace to chemically bond the enamel to the metal substrate.

Items that are typically vitreous enamelled include;

Enamelled steel surfaces have excellent corrosion protection. Enamelled surfaces are protected, smooth and very decorative.

Ceramic
Ceramic coatings are used to provide corrosion resistance against numerous chemicals as the ceramic materials are inert. They also provide erosion resistance as the ceramics are very hard. Butterfly disc valves, plug valves and valve bodies can be coated by this process to overcome corrosion and erosion. Ceramic coatings can also withstand high temperature conditions. A typical coating thickness is 50 to 100 microns and can be done selectively. These coatings provide excellent finish and are deposited as slurry on metallic substrates, like cast iron, steel, stainless steel and aluminium. They are subsequently chemically treated to attain hard, impervious and corrosion resistant layers of excellent bond strength.

Anodising
Aluminium has a high natural resistance to corrosion because an oxide film Al2O3forms on the surface providing a protective layer. Anodising is a technical electrolytic process to form a thicker Al2O3 film than forms naturally. Better protection can be obtained in more aggressive environments such as sea water. 

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