Phosphating of metals is one of the most important surface treatment methods for steel. Treatments are used for corrosion protection together with paints, to assist in cold-forming processes, for corrosion protection together with oils and waxes, and for stand-alone corrosion protection. H&W offers zinc phosphate applications, which are often specified by the military and defense sectors, especially for equipment that might be exposed to severe environmental conditions. Cleaning is paramount prior to coating and it can have a significant effect on the growth rate of the phosphate coating, its thickness, and the crystal size.
The color of the zinc phosphate coating lies between light and dark gray. Standards that we follow yield either a fine-grain coating or heavy-grain, determined by the application. Heavy-grain is usually chosen for its ability to retain rust preventive oils and waxes since it acts as an absorbent substrate. Fine-grain has a finer crystal structure and is typically used as a base for paint or other organic coatings.
Fine-grain is an excellent pretreatment for steel prior to painting. It offers moderate bare corrosion protection. This fine-grain conversion coating method is for the pretreatment of ferrous metals and zinc/zinc alloy coated metals before paint is applied, as per the requirements of Federal Specification TT-C-490 and Military Standard MIL-DTL-16232.
The bath concentration, temperature, and immersion time for parts assure that the phosphate coatings meet all the requirements of the specification. The equipment used for the fine-grain bath is constructed of materials resistant to the action of the phosphating solutions. Phosphatized parts are not allowed to dry between the phosphatizing solution treatment and the subsequent cold running water rinse. Fog sprayers or other means are used for prevention of drying until the final rinse.
Control of the chemical content of the phosphate coating solutions consists of free acid, total acid, and ferrous iron. Unless otherwise specified, the frequency of testing for free acid, total acid, and ferrous iron is made prior to starting production. High production rates may require more frequent analysis to assure process control within established limits.
Heavy-grain is a pretreatment for steel often required in military applications. In general, heavy-grain coating in this application is thicker and coarser. It places more sacrificial zinc on the surface, creating more corrosion resistance.
Prior to beginning the zinc phosphate coating process, the parts are thoroughly cleaned of oil, grease, wax, dirt, scale, and other foreign matter, and will not show visible signs of red corrosion products.
Adequate rinsing is performed following any chemical process to remove residual material. Special care is exercised in rinsing complex shapes to prevent contamination of any subsequent process. Care is also exercised to ensure complete drying in crevices, seams, or other difficult-to-dry places prior to painting.