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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. [8] [9] [10] The protective effect of chromate coatings on zinc is indicated by color, progressing from clear/blue to yellow, gold, olive drab and black.
Trivalent chromium plating, also known as tri-chrome, Cr 3+, and chrome(III) plating, uses chromium sulfate or chromium chloride as the main ingredient. Trivalent chromium plating is an alternative to hexavalent chromium in certain applications and thicknesses (e.g. decorative plating). [2]
Chromate conversion coating converts the surface aluminium to an aluminium chromate coating in the range of 0.00001–0.00004 inches (250–1,000 nm) in thickness. Aluminium chromate conversion coatings are amorphous in structure with a gel-like composition hydrated with water. [ 18 ]
Coatings of moderate thickness 1.8 μm to 25 μm (0.00007" to 0.001") [16] are known as Type II in North America, as named by MIL-A-8625, while coatings thicker than 25 μm (0.001") are known as Type III, hard-coat, hard anodizing, or engineered anodizing. Very thin coatings similar to those produced by chromic anodizing are known as Type IIB.
This plating is a combination of a high-nickel zinc-nickel alloy (10–15% nickel) and some variation of chromate. The most common mixed chromates include hexavalent iridescent, trivalent or black trivalent chromate. Used to protect steel, cast iron, brass, copper, and other materials, this acidic plating is an environmentally safe option. [6]
A conversion coating is a chemical or electro-chemical treatment applied to manufactured parts that superficially converts the material into a thin adhering coating of an insoluble compound.
In the 1970s, a new coating system was developed in the United States: zinc flake coating (patent number 1376067). By virtue of a thin coat thickness of typically 8-12 μm this system produced a high level of protection against corrosion and made it possible to avoid hydrogen embrittlement.
Partial predominance diagram for chromate. Molecular chromic acid, H 2 CrO 4, in principle, resembles sulfuric acid, H 2 SO 4. It would ionize accordingly: H 2 CrO 4 ⇌ [HCrO 4] − + H + The pK a for the equilibrium is not well characterized. Reported values vary between about −0.8 to 1.6. [4]