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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.
This process is done by putting zinc or a zinc plated metal in a solution of sodium dichromate and sulfuric acid for a few seconds. [6] Zinc chromate can also be synthesized by using neutral potassium chromate (K 2 CrO 4) and zinc sulfate (ZnSO 4), which forms a precipitate. [7] K 2 CrO 4 + ZnSO 4 → ZnCrO 4 + K 2 SO 4
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.
Chromate conversion coating (chemical film) per MIL-DTL-5541F for aluminium and aluminium alloy parts; A standard overview on black oxide coatings is provided in MIL-HDBK-205, Phosphate & Black Oxide Coating of Ferrous Metals. Many of the specifics of Black Oxide coatings may be found in MIL-DTL-13924 (formerly MIL-C-13924).
Zinc is rarely anodized, but a process was developed by the International Lead Zinc Research Organization and covered by MIL-A-81801. [17] A solution of ammonium phosphate, chromate and fluoride with voltages of up to 200 V can produce olive green coatings up to 80 μm thick. [17] The coatings are hard and corrosion resistant.
Primer reduces the number of paint coats needed for good coverage and even color. A thin layer of paint may still be permeable to water. Water can permeate into the wood and cause warping, mildew, or dry rot. Primer improves the waterproofing of the finish. [4] Primers are not used as part of a wood stain treatment because they obscure the wood ...