Anodizing chemicals Home >> Metal plating chemicals articles >> Different Process of IL Chromate Conversion

Chromate conversion coating is a method of coating applied to aluminium, zinc, cadmium, copper, silver, magnesium, tin and their alloys to either prevent or prolong the occurance of corrosion. Hexavalent chrome has been used over the last few decades as a corrosion preventative for zinc and aluminum surfaces. This coating has been a very successful corrosion preventative for both painted and unpainted aluminum, zinc and other metal surfaces. Commonly referred to by the trade name Alocrom, Chromate Conversion Coatings provide good levels of corrosion resistance combined with low electrical resistance. Often referred as a base coat prior to the application of paint or powder coating, this chemical treatment is applied by dip or brush. Alocrom is a cold chemical process and is suitable for treating most modern day aluminum alloys including cast, drawn, billet and sheet material.

Chromate conversion offers a cost effective non-electrolytic process for coating aluminum resulting in excellent corrosion resistance, a good pre-paint finish and good adhesion qualities in the electronics industry, chromate is used extensively to coat equipment shelves and other zinc plated steel and aluminum surfaces. These conversion coatings form an ideal substrate for paints and precious metals, providing a clean, essentially inert surface, which gives optimum adhesion. The chromate coatings are formed by the reaction of water solutions of chromic acid or chromium salts. The coatings usually have good atmospheric corrosion resistance. Chromate coatings are widely used in protecting common household products, such as screws, hinges, and many hardware items with the yellow-brown appearance. Chromate conversion is either carried out by immersion or by brush application. When aluminum and aluminum alloys are immersed in a controlled chromic acid bath with special additives, a chemical conversion reaction occurs at the metal’s surface. This reaction produces a complex, non-crystalline, non-porous coating consisting of a thin aluminum oxide film with entrapped chromates.

Chromating is commonly used on zinc-plated parts to protect the zinc from white corrosion, which is primarily a cosmetic issue. It cannot be applied directly to steel or iron, and does not enhance zinc's anodic protection of the underlying steel from brown corrosion. It is also commonly used on aluminum alloy parts particularly in the aircraft industry. Chromated parts retain their electrical conductivity to varying degrees, depending on coating thickness. The process may be used to add color for decorative or identification purposes. The successful application of this conversion process requires the Aluminum to be clean and free from organic soils, oxides and corrosion products. Chromate conversion coatings are common on everyday items such as hardware and tools and usually have a distinctive yellow color.

Steel parts must be plated with zinc or cadmium prior to chromating. Cadmium is a metal and one of the major constituents of II-VI compound semiconductors. The most remarkable characteristics of cadmium are its great resistance to corrosion, its low melting-point and excellent electrical conduction. Cadmium compounds exhibit excellent resistance to chemicals and to high temperatures. Finally, cadmium pigments produce intense colourings such as yellow, orange and red, and are well known pigments in artists' colours, plastics, glasses, ceramics and enamels.

Reducing the risk of cadmium problems

• Use fertilisers low in cadmium.
• Do not overfertilise.
• Add lime to acid soils (pH (CaCl2) less than 6.0).
• Keep your soil’s zinc levels up.
• Choose plant varieties that take up less cadmium.
• Increase soil organic matter levels.
• Make sure irrigation water is of good quality.

 

The coatings are particularly useful in protecting against oxidation even under humid conditions. The coatings are very thin and can be used for threaded parts without having to make allowance for dimensional changes. Most organic coatings applied directly to Aluminum surfaces will not adhere well, and if subjected to any deformation, they will tend to flake off exposing the bare Aluminum. Chromate conversion is helpful in promoting good initial adhesion useful for painting and powder coatings. The application of Chromated Aluminum can cover a wide range of functions - to provide mild wear resistance, better drawing or forming characteristics and to provide as a decorative finish and is also ideal for pre-treatment prior to Powder Coating. The key features of Chromate conversion are the thin chromate film on aluminum, good corrosion resistance and excellent base for paint or rubber bonding. It also has little or no abrasion resistance.

Chromate Benefits

• Low-price option for corrosion and wear protection
• A bonding mediator for paint and adhesives
• A lubrication carrier
• Final finish
• least electrical resistance
• No result on critical dimensions, in most cases