Parkerizing
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Parkerizing (sometimes called phosphating) is a method of protecting steel surfaces from corrosion and thus increasing their durability. It is commonly used on weapons as an alternative to bluing, and is considered more effective than bluing. It is ineffective on non-ferrous surfaces such as aluminum, and on stainless steel.
The process involves dipping the metal into an acid solution whose active ingredient is zinc. The solution is brought to a temperature of about 190-210°F for a period ranging between five and 45 minutes, while the metal is submerged; this results in a non-reflective, gray finish. Another method uses manganese instead of zinc for a blacker finish. It is different from coating the metal—the steel reacts with the zinc and the treated finish is thus part of the metal.
Parkerizing is considerably less efficient at preventing corrosion unless the treated surface is kept covered with a light coating of oil (a heavy coating is unnecessary and undesirable).
The method was developed for the United States military during World War II by the Parker Company. Parkerized was originally a registered trademark of the Parker Company.