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Inorganic coatings can be produced by chemical action, with or without electrical assistance. The treatments change the immediate surface layer of metal into a film of metallic oxide or compound which has better corrosion resistance than the natural oxide film and provides an effective base or key for supplementary protection such as paints. In some instances, these treatments can also be a preparatory step prior to painting. (reference)
Anodizing involves the electrolytic oxidation of a surface to produce a tightly adherent oxide scale which is thicker than the naturally occurring film. Anodizing is an electrochemical process during which aluminum is the anode. The electric current passing through an electrolyte converts the metal surface to a durable aluminum oxide. The difference between plating and anodizing is that the oxide coating is integral with the metal substrate as opposed to being a metallic coating deposition. The oxidized surface is hard and abrasion resistant, and it provides some degree of corrosion resistance.
However anodizing cannot be relied upon to provide corrosion resistance to corrosion prone alloys, and further protection by painting is usually required. Fortunately the anodic coating provides an excellent surface both for painting and for adhesive bonding. Anodic coatings break down chemically in highly alkaline solutions (pH > 8.5) and highly acid solutions (pH < 4.0). They are also relatively brittle and may crack under stress, and therefore supplementary protection, such as painting, is particularly important with stress corrosion prone alloys.
Anodic coatings can be formed in chromic acid, sulfuric acid, phosphoric acid or oxalic acid solutions. Chromic acid anodizing is widely used with 7000 series alloys to improve corrosion resistance and paint adhesion, and unsealed coatings provide a good base for structural adhesives. However these coatings are often discolored and where cosmetic appearance is important sulfuric acid anodizing may be preferred.
The Al2O3 coating produced by anodizing is typically 2mm to 25mm thick, and consists of a thin non‑porous barrier layer next to the metal with a porous outer layer that can be sealed by hydrothermal treatment in steam or hot water for several minutes. This produces a hydrated oxide layer with improved protective properties. Improved corrosion resistance is obtained if the sealing is done in a hot metal salt solution such as a chromate or dichromate solution. The oxide coatings may also be dyed to provide surface coloration for decorative purposes, and this can be performed either in the anodizing bath or afterwards. International standards for anodic treatment of aluminum alloys have been published by the International Standards Organization and cover dyed and undyed coatings. There are many reasons to anodize a part. Following are a few considerations and the industries that employ them:
Further, anodizing will not rub off, is an excellent paint base, removes minor scuffs, and is sanitary and tasteless. There are many variations in the anodization process. The following examples are given to illustrate some of the processes used in the industry:
Anodizing treatments are also available for magnesium and titanium alloys. The treatments commonly used with magnesium alloys involve several processing options to produce either thin coatings of about 5 mm thickness for flexibility and surfaces suitable for paint adhesion, or thick coatings, up to about 30 mm for maximum corrosion and abrasion resistance. When anodizing is used for the treatment of titanium and titanium alloys it can provide limited protection of the less noble metals against galvanic corrosion, and when used together with solid film lubricants it helps to prevent galling. The process produces a smooth coating with a uniform texture and appearance, and a uniform blue to violet color.
A number of proprietary chromate filming treatments are available for aluminum, magnesium, cadmium and zinc alloys. The treatments usually involve short time immersion in strongly acid chromate solutions, but spraying or application by brushing or swabbing can also be used for touch-up of parts. The resulting films are usually about 5 mm thick, and are colored depending on the base alloy, being golden yellow on aluminum, dull gold on cadmium and zinc, and brown or black on magnesium. The films contain soluble chromates which act as corrosion inhibitors, and they provide a modest improvement in corrosion resistance of the base metal. However their main purpose is to provide a suitable surface for sealing resins or paints. Epoxy primer, for example, which does not adhere well to bare aluminum, adheres very well to chemical conversion coatings. Among the best known coatings used with aluminum alloys are those produced by the Alodine 1200 or Alocrom 1200 processes.
A process for zinc alloys has been described to consist of immersion for a few seconds in a sodium dichromate solution at a concentration of 200 g/liter and acidified with sulfuric acid at 8 ml/litter. The treatment is performed at room temperature and is followed by rinsing and drying to produce a dull yellow zinc chromate coating.
A number of proprietary treatments such as 'Parkerizing' or 'Bonderizing' are available for use on steel. They are applied by brushing, spraying or prolonged immersion in an acid orthophosphate solution containing iron, zinc or manganese. For example a solution might contain Zn(H2PO4)2.2H2O with added H3PO4. The coatings consist of a thick porous layer of fine phosphate crystals, tightly bonded to the steel. When forming steel sheet, the parts are often phosphatized in order to improve the surface properties of the sheet. The acid recirculation stream from the phosphatizing bath must be cleaned after contact with the metal.
The coatings do not provide significant corrosion resistance when used alone, but they provide an excellent base for oils, waxes or paints, and they help to prevent the spreading of rust under layers of paint. Phosphating should not be applied to nitrided or finish‑machined steel, and steel parts containing aluminum, magnesium or zinc are subject to pitting in the bath. Some restrictions apply also to heat treated stainless and high‑strength steels.
Steels containing nitride forming elements such as chromium, molybdenum, aluminum and vanadium can be treated to produce hard surface layers providing improved wear resistance. Many of the processes employed are proprietary, but typically they involve exposure of cleaned surfaces to anhydrous ammonia at elevated temperatures. The nitrides formed are not only hard, but also more voluminous than the original steel and therefore they create compressive residual surface stresses. Because of this nitrided steels usually exhibit improved fatigue and corrosion fatigue resistance. Similar beneficial effects can be achieved by shot peening.
Austenitic stainless steels and hardenable stainless steels such as martensitic, precipitation hardening and maraging stainless steels are seldom coated but their corrosion resistance depends on the formation of naturally occurring transparent oxide films. These films may be impaired by surface contaminants such as organic compounds, metallic or inorganic materials. Treatments are available for these materials to clean and degrease surfaces and produce uniform protective oxide films under controlled conditions. These usually involve immersion in an aqueous solution of nitric acid and a dichromate solution.
See also: Cladding, Electroplating, Pack cementation, Electroless plating, Physical vapor deposition, Hot dip galvanizing, Thermal spraying
