As a first line of protection against corrosion, structural components of automobiles are coated with hot molten wax, auto-deposition electro-coat, metallic, organic, or powder. Phosphate conversion coatings enhance paint adhesion and thereby aids in resisting corrosion. Hot molten wax coatings are usually used under body components to prevent corrosion. The e-coating process is a process in which electrically charged particles are used to coat a part.
Metallic coatings such as zinc, aluminium, zinc-iron are applied on steel parts to prevent corrosion. Powder coating, organic auto deposition also prevents corrosion. Paint is an organic coating which is a cost-effective method of preventing corrosion. Auto deposition is a waterborne procedure that is dependent on chemical reactions for achieving deposition. Powder coating is done by the application of dry powder which is then heated to give a smooth film like finish.
Let us discuss the various types coating for preventing corrosion:
- Conversion Coatings/Phosphate conversion coatings – these are employed to increase paint adhesion and indirectly prevent corrosion. There are a few varieties of phosphate coatings like iron, manganese and zinc. Post cleaning the surface, coating of phosphate is applied by the immersion of the metal in the processing solution for a few minutes. The thickness of conversion coating depends how well the metal has been cleaned, time of immersion, processing bath composition etc.
- Hot Melt Wax Coating is a thermoplastic corrosion prevention compound and a water based formulation. Hot melt coatings are being used since the 1970s and are known to increase durability and provide protection from corrosion. Waxes are applied through a dipping process, in which the wax is heated between 125 and 195 degrees. Post a hot water rinse and alkali cleaning, the parts are dipped in molten wax. The thickness of the wax deposited depends on the extent of heating that is done to the metal body itself. After the dipping process is complete, both are allowed to return to ambient temperature.
- E-coat or Electrocoat (E-coat) or Electrophoretic deposition is a procedure in which electrically charged particles are deposited for coating a conductive part. It is used for complex automobile bodies to simple stamps. Electrocoating requires the coating binder, additives and pigments to have electrical charge. Charged materials, influenced by an electric field, move through the water to the surface that has been dipped. Electrocoated parts generally are given zinc or iron phosphate treatment before deposition as this increases the application of the Electrocoat.
- Metallic coatings can be applied to non-ferrous or ferrous surfaces to not only prevent corrosion but also give a decorative finish. There are 4 common methods used for applying metallic coatings.These are: Electroplating- when the coating is put on the substrate metal by applying an electrical potential with the help of an electrolyte. Mechanical plating – Fine metal powder is welded cold to the substrate by tumbling all the parts like metal powder, glass beads in an aqueous solution. This process is used for applying cadmium or zinc to minute parts like fasteners. Electroless is a process by which nickel or cobalt is deposited on metal by a chemical reaction and a catalyst. Hot dipping is a process by which the substrate is immersed in a molten bath of metals like zinc, zinc-iron, aluminium, aluminium-zinc, zinc-nickel, tin and lead-tin etc.
- Organic Coating is generally paint that is applied for acting as a barrier against corrosion. This prevents or slows down the transfer of electrochemical charge from the corrosive solution to the metal below the organic coating. This type of coating is a complex amalgamation of carriers, binders, additives and pigments. Some of the common organic coatings are Alykd and epoxy ester coatings, two-part coatings like urethane coatings, radiation curable coatings like acrylic and epoxy, polymers, latex coatings like acrylic, vinyl, acrylic and styrene polymer combinations.
- Autodeposition coatings- Autodeposition is a waterborne process that depends on chemical reactions to achieve deposition. The process is accomplished with the help of an autodeposition bath having mildly acidic latex emulsion polymer, de-ionized water etc. The reaction consists of mildly acidic bath reacting with the steel that releases the iron ions. Autodeposition coats any metal touched by the liquid. Tubular shaped parts, parts having intricate designs can be coated by autodeposition.
- Powder Coatings- in this process, a dry powder is put on the clean surface, then heating t so that the powder melts into a continuous and smooth film. The common types of coating that are used are epoxy, urethane, nylon, acrylic, vinyl, and polyester. Modern methods of application techniques can be put into four main categories like fluidized bed process, electrostatic spray process electrostatic bed process, and plasma spray process. The electrostatic spray process is common where an object is sprayed with charged, non-conducting powder.
These processes are effective and are selected on the basis of the substrate metal that requires to be coated and made free from corrosion.