Why Phosphate?

A bare metal surface does not provide a good base for paint films. At the microscopic level, the substrate surface has anodic and cathodic sites and even under a paint film, currents can flow unimpeded between those sites. This leads to corrosion. A phosphate coating, while only a few tenths of a micron thick, provides an insulating barrier that prevents the flow of those currents. By itself, the level of corrosion protection a phosphate provides is not significant, but it is greatly increased when another treatment—paint film, oil, or otherwise—is applied. In addition to the corrosion protection the phosphate layer provides, it also creates a porous surface to which the next coating can adhere. In terms of corrosion protection, the whole is greater than the sum of its parts.


What Happens in the Phosphating Process?

At the interface of the part and the working solution, a pH change is created by dissolving the substrate metal and creating hydrogen gas. This increases the salt content in the area immediately around the part-solution interface, which initiates a crystallization process. This crystallization is modified by the chemistries of the solution, and the phosphate coating is formed.


What Kind of Phosphate to Use?

The three main phosphate coatings are iron, zinc and manganese. The microcrystalline structure of iron phosphates provides a minimal amount of corrosion protection but an excellent base for paints. The larger crystal structure of zinc phosphates offers better corrosion protection and a superior paint-base but is more costly to generate. Zinc phosphates are also used as lubricants in drawing operations. The manganese phosphate has a large crystal structure that readily absorbs oil and is very hard. It is used for wear-resistance purposes rather than as a paint base.


What Indicates a Good Phosphate Coating?

The best way to determine the quality of a phosphate coating is by performing a coating weight analysis. A phosphated panel of known area is cleaned and weighed, then the phosphate coating is stripped and the panel is weighed again. The difference indicates the weight of the coating, which in turn is related to the area.

The physical appearance of a phosphate coating is not a reliable measure...

“Visual examination of an iron phosphate coating is meaningless, because the appearance of the iron phosphate has no relationship to its quality.”
Frank Altmayer and Edward A. Rodzewich Phosphating of Metals, AESF Technical Guide ©2000


How Much Should Phosphating Cost?

A phosphate coating should comprise 3-5% of the total cost of a painted finish. There are many factors to consider beyond just the dollar cost of chemistries, the most significant being energy costs. Ever-increasing fuel prices have made the cost of heating the solution and drying parts a bigger part of the equation than the cost of chemistries. This means that more efficient chemistries can actually generate net savings if they require less heat.


How is a Phosphate Controlled?

With Iron Phosphates, such as TD IRON PHOSPHATE, the quality/weight of the coating is controlled by the strength of the bath and how well the substrate has been cleaned. Sometimes water hardness is an issue or a little extra wetting action is needed. Is such cases TD DETERGENT ADDITIVE can improve the final result. Other additives can be custom designed for specific applications.

With zinc phosphates, such as HG HEAVY ZINC PHOSPHATE or HEAVY ZINC PHOSPHATE KE, the type and quality of the cleaning process of the substrate is what determines the type (size, density) of the crystal. Crystal formation occurs at the cathodic sites on the substrate. Few cathodic sites lead to a large crystals and loose structure. Many cathodic sites generate small crystals and a tight structure. Accelerants incorporated into the chemistry speed up the formation rate, which also leads to smaller crystals.

What Kind of Maintenance Does a Phosphate Require?

In running an iron phosphate solution, sludge buildup may be encountered. This results from the efficiency of an iron phosphate solution being <100% --the oxidation of the primary ferrous phophate forms the very insoluble ferric phosphate (FePO4) which precipitates out as sludge.

A phosphating operation, like any finishing installation, should be kept as clean as possible. Issues like sludge buildup are mostly dependent on throughput. Filtration may be an option to extend bath life and maintain performance. If oil can be removed regularly it should be.