Parkerizing is a technique used to add a phosphate coating to steel to provide it with protection against wear and corrosion. This coating is typically composed of a zinc phosphate, though it can also use iron or manganese instead. It cannot be applied to nonferrous metals such as copper and aluminum, and even steel that has a high content of nickel can be unsuitable. The techniques is most commonly used on guns, and has been a popular alternative to bluing since the middle of the 20th century. Most parkerized metal takes on a matte gray finish, though some techniques can result in a darker black color.