Describe a chromate coatings.

Chromate coating is a type of conversion coating used to passivate steel, aluminum, zinc, cadmium, copper, silver, magnesium, and tin alloys. It is primarily used as a corrosion inhibitor, primer, decorative finish, or to retain electrical conductivity. The process is named after the chromate found in chromic acid, also known as hexavalent chromium, the chemical most widely used in the immersion bath process whereby the coating is applied. However, hexavalent chromium is toxic, thus, highly regulated, so new, non-hexavalent chromium-based processes are becoming more readily available at a commercial level. One alternative contains trivalent chromium. In Europe the RoHS (Restriction of Hazardous Substances) Directive is commonly referred to regarding elimination of hexavalent chromium in electrical and electronic equipment, and the REACH ("Registration, Evaluation, Authorisation and Restriction of Chemicals") Directive to wider applications including chromate conversion coating processes, paint primers and other preparations. Chromate conversion coatings are commonly applied to everyday items such as hardware and tools, and can usually be recognized by their distinctively iridescent, greenish-yellow color.

Chromate conversion coatings on an aluminium substrate are known by the following terms: chemical film, yellow iridite, and the brand names Iridite and Alodine (in the UK, Alocrom). It is also commonly used on aluminum alloy parts in the aircraft industry. Iridite 14-2, a chromate conversion coating for aluminum, contains chromium(IV) oxide, barium nitrate, sodium silicofluoride and ferricyanide. Alodine may also refer to chromate-coating magnesium alloys.

Chromate conversion coatings can be applied over phosphate conversion coatings used on ferrous substrates. The process is used to enhance the phosphate coating.

Steel and iron cannot be chromated directly. Steel plated with zinc may be chromated.

Chromating is often performed on galvanized parts to make them more durable. The chromate coating acts as paint does, protecting the zinc from white corrosion, thus making the part considerably more durable, depending on the chromate layer's thickness.[citation needed] Steel and iron cannot be chromated directly. Chromating zinc plated steel does not enhance zinc's cathodic protection of the underlying steel from rust.

The protective effect of chromate coatings on zinc is indicated by color, progressing from clear/blue to yellow, gold, olive drab and black. Darker coatings generally provide more corrosion resistance. However, the coating color can also be changed with dyes, so color is not a complete indicator of the process used. The composition of chromate conversion solutions varies greatly, depending on the material to be coated and the desired effect. Most solution formulae are proprietary. The widely used Cronak process for zinc and cadmium consists of 5–10 seconds of immersion in a room-temperature solution consisting of 182 g/l sodium dichromate crystals (Na2Cr2O7 · 2H2O) and 6 ml/l concentrated sulfuric acid. Chromate coatings are soft and gelatinous when first applied, but harden and become hydrophobic as they age. Curing can be accelerated by heating up to 70 °C, but higher temperatures will gradually damage the coating.

Oxidation is the process of formation of oxide films on the metal surface. Oxidation is used for the deposition of oxide layers, both for protection purposes and for imparting decorative properties to the metal product.

Chemical oxidation is carried out by processing the product in solutions (melts) of oxidants (chromates, nitrates, etc.). With this method, the surface of the product is passivated or applied to protective and decorative layers. For ferrous metals, chemical oxidation is carried out at a temperature of 30 to 100 ° C in alkaline or acidic formulations. For acid oxidation, a mixture of several acids is used, for example, nitric (or orthophosphoric) and hydrochloric acid with some additives (Ca (NO3) 2, Mn compounds). Alkaline oxidation is carried out at temperatures slightly higher, about 30 - 180 ° C. Oxides are added to the formulation. After the oxide layer is applied, the metal products are well washed and dried. Sometimes the finished coating is oiled or further processed in oxidative solutions.

Protective layers obtained with the use of chemical oxidation have less protective properties than films obtained by anodizing.

Thermal oxidation is the process of formation of an oxide film on a metal at elevated temperatures and in oxygen-containing (perhaps water vapor) atmospheres. Thermal oxidation is carried out in heating furnaces. When thermal oxidation of low-alloy steels or iron (the operation is called blasting), the temperature is raised to 300-350 ° C. For alloyed steels thermal oxidation is carried out at higher temperatures (up to 700 ° C). The duration of the process is about 60 minutes. Very often thermal oxidation is used to create an oxide layer on the surface of silicon products. Such a process is carried out at high temperatures (800 - 1200 ° C). Oxidized silicon products are used in electronics.

Anodization is one of the ways to produce an oxide film. Anodizing is carried out in liquid or solid electrolytes. When anodizing, the surface of the metal that is oxidized has a positive potential. Anodizing is used to obtain protective and decorative layers on the surfaces of various metals and alloys.

Anodizing is most often used to obtain a coating on aluminum and its alloys. On aluminum, layers are obtained with protective, insulating, wear-resistant, decorative properties.

Plasma oxidation is carried out at low temperatures in a plasma that contains oxygen. Plasma for this type of oxidation is formed by discharges of direct current, microwave, high-frequency discharges.

Plasma oxidation is used to produce oxide layers on various semiconductor compounds, silicon surfaces. Plasma oxidation can increase the photosensitivity of silver-cesium photocathodes.

Microarc oxidation (MDO) is a method of obtaining multifunctional oxide layers. Microarc oxidation - marching from anodizing. It allows to apply layers with high protective, corrosion, heat-resistant, insulating, decorative properties. In appearance, the coating obtained by the microarc method is very similar to ceramics.

Now this is one of the most promising and in-demand methods of applying oxide layers, because Allows to apply heavy-duty coatings with unique characteristics.

The process of microarc oxidation is, in most cases, carried out in weakly alkaline electrolytes with pulsed or alternating current. Prior to the coating, no special surface preparation is required. The peculiarity of the process is that. That energy is used from electrical microdischarges, which move randomly along the surface being treated. These microdischarges have a plasmachemical and thermal effect on the coating and electrolyte. The oxide layer is approximately 70% formed deep into the base metal. Only 30% of the coating is completely outside the product.

The thickness of the coatings obtained by the microarc method is about 200 - 250 μm (thick enough). The temperature of the electrolyte can vary from 15 to 400 ° C, and this does not have a special effect on the process.

The electrolytes used do not have a harmful effect on the environment and their service life is very long. The equipment is compact, does not take up much space and is simply in use.

The dissipation capacity of the electrolytes used is high, which makes it possible to obtain coatings even on difficult-to-relieve parts.

Microarc oxidation is used to form coatings mainly on magnesium and aluminum alloys.

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