Phosphate conversion coating
[5]: 393 A phosphate coating is usually obtained by applying to the steel part a dilute solution of phosphoric acid, possibly with soluble iron, zinc, and/or manganese salts.
The bath is a solution of phosphoric acid (H3PO4), containing the desired iron, zinc or manganese cations and other additives.
The bath often includes an oxidizer, such as sodium nitrite (NaNO2), to consume the hydrogen gas (H2) — which otherwise would form a layer of tiny bubbles over the surface, slowing down the reaction.
[10] The main phosphating step can be preceded by an "activation" bath that creates tiny particles of titanium compounds on the surface.
[6] Parkerizing is a method of protecting a steel surface from corrosion and increasing its resistance to wear through the application of a chemical phosphate conversion coating.
The Parkerizing process cannot be used to protect non-ferrous metals such as aluminium, brass, or copper but can be used for chemical polishing or etching instead.
The process was first used on a large scale in the manufacture of firearms for the United States military during World War II.
Clark W. Parker acquired the rights to Coslett's and Richards' U.S. patents, and experimented in the family kitchen with these and other rust-resisting formulations.
Similarly, Baker and Dingman of the Parker Rust-Proof Company filed an improved manganese phosphating (Parkerizing) process patent in 1928 that reduced the processing time to 1⁄3 of the original time that had been required through heating the solution to a temperature in the precisely controlled range of 500 to 550 °F (260 to 288 °C).
The patent for this zinc phosphating process (using strategic compounds that would remain available in America during a war) was granted to inventor Romig of the American Chemical Paint Company in 1938 as U.S. patent 2,132,883, just prior to the loss of easy access to manganese compounds that occurred during World War II.
The result is a tightly adhering dielectric (electrically insulating) layer that can protect the part from electrochemical and under-paint corrosion.
[6] While a zinc phosphate coating by itself is somewhat abrasive, it can be turned into a lubricating layer for cold forming operations by treatment with sodium stearate (soap).
