Chrom conversion coating is a type of conversion layer used for passive steel, aluminum, zinc, cadmium, copper, silver, magnesium, and lead alloys. It is mainly used as a corrosion inhibitor, primer, decorative finishing, or to maintain electrical conductivity. This process is named after the chromate found in chromic acid, also known as hexavalent chromium, the most widely used chemical in the immersion process in which the coating is applied. However, hexavalent chromium is toxic, therefore, a very novel process, non-hexavalent chromosomes are becoming increasingly available at the commercial level. One alternative contains trivalent chromium. In Europe, the RoHS Rules (Restrictions on Hazardous Substances) are referred to the removal of hexavalent chromium in electrical and electronic equipment, and REACH ("Registration, Evaluation, Authorization and Restriction of Chemicals") Guidelines for broader applications include process chromate conversion coatings, primary paints and other preparations.
Chromate conversion coatings are generally applied to everyday items such as hardware and equipment, and are usually recognizable by their distinctive, yellow-green color.
Video Chromate conversion coating
Substrate
Aluminum
Chromate conversion coatings on aluminum substrates are known by the following terms: chemical film , yellow iridite , and brand names Iridite , Bonderite (formerly known as Alodine , in English, Alocrom ). It is also commonly used in aluminum alloy parts in the aircraft industry.
The most commonly called standard for applying chromate conversion layers to aluminum is MIL-DTL-5541 in the US, in England is Def booth 03/18.
Magnesium
Alodine may also refer to magnesium chromate-coating alloys.
Phosphate coating
Chromate conversion coatings can be applied on top of the phosphate conversion coating used on iron substrates. This process is used to enhance the phosphate layer.
Steel
Steel and iron can not be chromated directly. Steel plated with zinc can be chromed (see Zinc below).
Zinc
Chromating is often done on the galvanized part to make it more durable. The chromate layer acts as a paint, protecting the zinc from white corrosion, thus making the portions much more durable, depending on the thickness of the chromate layer. Steel and iron can not be chromated directly. The zinc chromium steel does not increase the protection of the zinc cathode from the underlying steel from rust.
The protective effect of chromate coatings on zinc is shown by color, evolving from clear/blue to yellow, gold, drab and black olives. Dark layers generally provide more resistance to corrosion. However, coating colors can also be changed with dye, so color is not a complete indicator of the process used.
ISO 4520 establishes a chromate conversion layer on coated zinc and cadmium coatings. ASTM B633 Type II and III specify zinc plating plus chromate conversion on iron and steel parts.
Maps Chromate conversion coating
Composition
The composition of chromate conversion solutions varies considerably, depending on the material to be coated and the desired effect. Most of the solution formulas are proprietary.
The most commonly used Cronak process for zinc and cadmium consists of 5-10 seconds of immersion in a room temperature solution consisting of 182 g/l of sodium dichromate crystals (Na 2 Cr 2 O 7 Ã, Â · 2H 2 O) and 6 ml/l of concentrated sulfuric acid.
Iridite 14-2, the chromate conversion layer for aluminum, contains chromium (IV) oxide, barium nitrate, sodium silicofluoride and ferricyanide.
Chromate coatings are soft and gelatinous when first applied, but harden and become hydrophobic due to their age.
Drying can be accelerated by heating up to 70 ° C (158 ° F), but higher temperatures will gradually damage the coating on the steel.
References
External links
- Chromating yellow and green chemistry on aluminum
- MIL-C-17711B, Chromate conversion coatings specifications for zinc and zinc molding and hot dipped galvanized parts
Source of the article : Wikipedia
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