Phosphate coatings consist of a crystalline conversion coating for steel substrates for break-in lubrication or as a pretreatment for other supplemental coatings, like paint, dry film lubricants, oils and waxes. Although it is a fairly simplistic process there are some engineering points that should be addressed when working with a metal finisher. The key items to review are stress relieving, hydrogen embrittlement relief, pretreatment, post treatment and the most obvious which phosphate system to use. Let’s address these one by one:
Zinc phosphate has modest corrosion protection and is a standard pretreatment for subsequent paint, dry film or oil based systems. In essence the phosphate provides a profile for improved adhesion of the designated coating. Zinc phosphate coating is used in a number of industries including heavy industrial, automotive and aerospace. Our process tanks can handle parts up to 14 feet in length and up to 2000# in weight.
MIL-DTL-16232, Type M
Manganese phosphate is typically called out when the application requires break-in lubrication either as a stand alone or with an oil or dry film lubricant system. Manganese phosphates are denser than zinc systems which enhances its lubricous nature. To ensure a proper phosphate coating we offer mechanical, as well as chemical pretreatments to meet a number of commercial or federal specifications. We can accommodate parts up to 7 feet in length and 2000# in weight.
During the manufacturing process, the alloy is pushed and pulled in such a way that the part is “cold worked” or “work hardened”. The result is a part that has residual stress which needs to be relieved. A bake prior to the phosphate process will equalize it and possibly prevent a premature failure in the field. We offer stress relief baking as an option.
A standard cycle for zinc phosphate may be nothing more than a standard alkaline clean and rinse. Depending on the specification additional steps may be added to enhance the specific type of crystal the end customer desires. Chemical “pickling or descaling” may be required to prepare a part that has excessive heat treat scale. Abrasive blasting may be desired, in lieu of pickling to minimize stock removal and to prevent the potential for hydrogen embrittlement. The important thing to note is that how you handle the part prior to metal finishing will affect the performance of the part, the coating and the price. The more steps that are required to prepare the part for finishing will ultimately affect the piece part price. Pre-planning is always recommended, which includes talking to your supplier.
After the part is phosphated there are a number of different post treatments to consider. Typically, a dilute chromic acid seal is applied to enhance the corrosion protection. The parts could receive a non-chromated seal such as sodium stearate or an oil dip, either solvent based or water soluble. Ultimately, the best post treatment is determined by the parts function.
Hydrogen Embrittlement Relief
During the coating process hydrogen evolves in the bath and on the parts. If the part has a Rockwell hardness over 39 Rc, the alloy is prone to absorb this residual hydrogen promoting a stress riser. If this occurs the component could experience premature failure when it is in service. To relief this additional stress, a post bake is recommended, which is another service we offer.