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Corrosion protection coatings

Sacrificial coating

Sacrificial coating is a method of corrosion control by applying a thin layer of a metal having lower (than the protected metal) value of electrode potential (higher position in the table of Electrochemical series).

The most common example of sacrificial coating is galvanized steel (steel coated with zinc).
Less noble (more active) sacrificial coating oxidizes (dissolves)according to the anodic reaction. The electrons given up by the dissolved coating atoms (zinc) flow to the protected part (steel) making it to act as a cathode and preventing its corrosion.

Coating made of more noble metal (eg. tin or nickel plated steel) is able to provide corrosion protection as long as the coating is undamaged. Any localized coating defect causes intensive corrosion of the steel part, which acts as anode in the galvanic cell. The presence of cathodic tin enhances the steel corrosion.

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Polymer coatings

Polymer coatings applied on a metal surface prevent its corrosion due to separation of the metal from the electrolyte and from other metal parts.
Polymer coatings/paints are composed of binders (major polymer constituent of the coating), solvents (water or Hydrocarbon solvents) for viscosity control, pigments and additives (eg., Corrosion inhibitors).

Polymer coatings may be applied by following techniques: spraying, brushing, dipping, rotational molding, fluidized bed, plasma spray, electrostatic bed.

Common polymer coatings:

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Ceramic coatings

Ceramic coatings applied on a metal surface prevent its corrosion due to separation of the metal from the electrolyte and from other metal parts.
Ceramic coatings provide corrosion, erosion and thermal protection. They also possess good cosmetic appearance. Common ceramic coatings are produced from slurry applied onto the substrate with subsequent heating. The coating thickness is about 4 mils (100 µm).

Porcelain enamel (vitreous enamel) is a ceramic type coating produced by fusion of glass onto a metallic substrate. The coated part is then fired to produce high Adhesion strength.

Anodizing coating is an oxide film obtained in electrochemical process as a result of oxidation of an anodically connected aluminum part in an acidic electrolyte solution. Thickness of anodizing coating may reach 4 mils (100 µm), which is much higher than natural oxide film on aluminum surface.

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Corrosion/rust protection oils

Corrosion/Rust protection oils are lubricants providing temporary protection of metal parts from corrosion.
Rust protection oils contain Corrosion inhibitors, which form a barrier film on the substrate surface reducing the corrosion rate. The inhibitors used in oils consist of polar molecules possessing water-repellent properties.
Rust inhibiting additives contain also surfactants (wetting agents), which form strong bond with the metal part surface and provide efficient spreading the inhibitor molecules over the surface. Sodium sulphonate is commonly used as wetting agent.
Rust protection oils are used for protection finished and packed metal parts and also parts temporary stored between the fabrication stages.

Rust protection oils are commonly based on mineral oils (either paraffinic or naphtenic).

Suppliers of rust protection oils determine the oils performance in the Product Data: protection film thickness, storage period varying from short-term (two weeks) to long-term (12 months or even longer), indoor or outdoor storage, type of metal to be protected, finger prints protection etc.

Rust prevention oils are applied by dip, spray or brush-on. The protection film may be easily removed by either a detergent (alkaline cleaner) or a solvent.

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corrosion_protection_coatings.txt · Last modified: 2012/05/31 by dmitri_kopeliovich
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