Conversion Coating Questions

Q. I’m a newcomer to pfonline.com, and I have really enjoyed reading your articles. I’m trying to gain a better holistic understanding of product finishing, and I’m particularly interested in pretreatment. I am struggling to understand some of the intricacies related to conversion coat suppliers, why someone would choose a specific conversion coat and where exactly in the supply chain that coat would be applied.—B.D.

A. Product finishing may be best understood by starting at the end, meaning understanding the customer, application and specifications. Once you understand the operating environment and expectations, you have a better idea how to develop a system that is capable of performing at the required cost targets. 

Factors related to this are the base material and its quality, cleaning, pretreatment and finishing (typically plating or painting). Some parts may have both aesthetic requirements and a functional life expectation. In that case, the overall life expectation may be significant, and it is simply not practical to have a finish requirement of a decade or more. In those cases, your customer may have a finish requirement in the range of 1–5 years with a functional requirement of 5–10 years. It will be important to choose the correct base material as well as coating (i.e., galvanized, aluminumized, galvannealed or galvalume) and coating thickness to insure that, as the finish degrades, the function requirement of the component remains.

It is then important to understand how to clean the substrate to remove both mill and process lubricants. The base material will have lubricants from the mill that are part of its processing there and are typically used for their corrosion resistance until the material is intended for use. Additionally, in the manufacturing process, there are usually lubricants applied to form, bend, draw, shear, etc., the material into a usable shape. When these steps are completed, it will be important to remove all the residues prior to the conversion coating, otherwise that step will be spotty and incomplete. Several options exist for the type of cleaning. The choice of cleaner will usually depend on the type and amount of lubricant, base material and cleaning method.

Following the cleaning step would be the conversion coating process. In this case, the choices are somewhat more limited due to compatibility with the underlying substrate. One traditional conversion coating is phosphating (iron, zinc and, to a lesser extent, manganese) that is generally performed on a ferrous surface. Activators in the formulation can be added to allow the conversion coating to also form on the surface of a zinc-coated product (galvanized or galvannealed) or an aluminum surface. The zinc phosphating process will produce a higher coating weight and is considered to be good for both paint adhesion and corrosion resistance. The iron phosphating process will generally require fewer steps. It will generate a lighter coating weight and is intended primarily for paint adhesion and less for corrosion resistance. 

The other primary category of conversion coating is chromates that can be applied to aluminum- and zinc-coated surfaces. In this case, the conversion coating is amorphous, but can still have a measurable, but lower coating weight than the phosphate coatings. Due to environmental, health and safety requirements, there have been numerous non-chrome or trivalent-chrome alternatives that have been developed to replace the hexavalent-chromium-based conversion coating system. These are applied similarly and have similar limitations.

Additionally, several newer alternative conversion coatings have been developed that have unique properties. Several suppliers now offer some coatings that are cerium-oxide-based or are based on one of the transition metals, such as titanium, zirconium and hafnium. These are generally acidic in nature, so apply a relatively thin coating that is multi-metal compatible. They can have similar or improved performance properties and can be more expensive (for the chemical), but also have less process waste to dispose of that can lower overall cost to operate the process.