A proprietary method of producing a protective phosphate coating on ferrous metals. Parker A treatment involves immersing in a bath of acid manganese phosphate. The Parker D is a modification using acid zinc phosphate with a nitrate iron as accelerator.

Parkerizing (also called phosphating and phosphatizing) is a method of protecting a steel surface from corrosion and increasing its resistance to wear through the application of an electrochemical conversion coating. Parkerizing is usually considered to be an improved zinc or manganese phosphating process, and not to be an improved iron phosphating process, although some use the term parkerizing as a generic term for applying phosphating (or phosphatizing) coatings that does include the iron phosphating process. Parkerizing is commonly used on firearms as a more effective alternative to bluing, which is another electrochemical conversion coating that was developed earlier.