Considerations When Choosing a Conversion Coating
Posted on Tuesday, April 25, 2023
By Brian Korecky
There are a variety of factors to consider when choosing the best pretreatment conversion coating for your specific application and process. The application system, substrate or substrates to be pretreated, and specifications for the final product all impact this decision. Let’s take a closer look at the benefits and challenges of various conversion coatings.
Cost, while very important, is intentionally left out of this discussion as the system and specifications will be the main drivers here. The discussion of cleaning is also limited as for the purposes of this article we will assume the substrate has been cleaned and is ready for the conversion coating, with the exception of a 3-stage automatic spray system.
The application system is a major factor in determining which conversion coating chemistry can be used. Basic guidelines are:
- Spray wand systems: Iron phosphate and thin film conversion coatings.
- 3-stage automatic spray systems: Iron phosphate and thin film conversion coatings.
- 5-stage and greater: All listed conversion coatings.
The substrate or combination of substrates also impact conversion coating options (see Table 1):
- Hot or cold rolled steel: All conversion coatings.
- Aluminum extrusions: Chrome and chromate, and thin film conversion coatings.
- Aluminum castings: Chrome and chromate, and thin film conversion coatings.
- Stainless steel: Thin film conversion coatings.
- Zinc coated (galvanized, galvanneal, galvalume, etc.): Zinc phosphate, chrome and chromate, and thin film conversion coatings.
Iron phosphates also provide some cleaning benefits and can be used in a system where it is necessary to clean and coat in one stage, making it very efficient. While it is a tried-and- true conversion coating, there are some downsides to using iron phosphate. It must be applied with heat, there will be sludge buildup in the tank, and as a standalone product it cannot coat non-ferrous metals. Removal of iron phosphates is fairly straightforward as most municipalities only require a pH adjustment before discharge.
Another traditional type of conversion coating is zinc phosphate. Zinc phosphate provides excellent corrosion protection as well as excellent powder coating adhesion. However, the increased performance comes with additional labor and maintenance costs. Unlike an iron phosphate, this conversion coating requires both a conditioner as well as an activator which will put additional stress on any system. A continuous sludge will be created and must be removed by filters or settling tanks. Wastewater requires proper use of coagulants and flocculants to remove any heavy metals prior to discharge. This conversion coating must also be applied with heat.
When properly applied, this crystal coating will look dark gray in color. Typically zinc phosphates are used in the automotive, appliance, wire, and military industries.
Chrome and Chromate Coatings
Chrome conversion coatings have been around since post-WWII but have seen a decline in use due to environmental and safety concerns. Most notably, hexavalent chromium is a known carcinogen and prolonged exposure can increase the risk of various cancers. Waste disposal also comes with a high cost to remove heavy metals, which requires pH adjustments and proper use of coagulants and flocculants. In addition, this chemistry must operate at a warm temperature above ambient. The conversion coating, however, provides excellent corrosion and adhesion and can be applied with a variety of equipment systems. Primary industries that use this type of coating include aerospace and architecture.
Chromate coatings differ from chrome phosphate coatings by removing phosphates and adding accelerators which increase coating weights and lower process or contact times within a system. They also provide increased corrosion protection due to the higher coating weights.
Thin Film or Next Generation Coatings
As the industry has evolved over the years, so have conversion coatings. The push towards an eco-friendly and sustainable environment has led the charge on greener technology. These conversion coatings are rapidly replacing traditional conversion coatings previously mentioned. These newer coatings contain a variety of zirconium, titanium, or vanadium as well as silanes. Their benefits greatly outweigh the negatives expressed in the previous talking points.
Thin film/next generation coatings provide both good adhesion and corrosion protection. They also only require ambient heat, which is an important factor in today’s manufacturing environment. The combined rising costs of inflation and energy have many companies evaluating a variety of solutions that can save overhead costs.
Another cost savings associated with these coating types is reduced wastewater treatment. Unlike zinc phosphates there are no heavy metals to treat, such as zinc and nickel, before discharge. For municipalities that do not allow for discharge of phosphates, this would be the superior alternative.
These coatings are also compatible with mixed metals and can be applied with all application types. In addition, thin film/next generation conversion coatings do not require a final seal be applied. This allows for fewer stages in a system with equivalent or better performance specifications. There is also little to no sludge buildup in the tank, which reduces cleaning requirements. Requiring RO (reverse osmosis) or DI (deionization) water to be used in both charging the system or in either a halo or final rinse further prevents additional contamination of hard water salts that can be created from such minerals such as calcium, chlorides, or sulfates.
If using a stainless-steel tank, it is important to note that iron and zinc phosphates will actually coat the inside of a mild steel tank, preventing it from rusting out. Conversely, zirconium conversion coatings will actually deteriorate mild steel over time. If using a zirconium coating and changing out the tank is not a viable option, adding a tank liner is sufficient. Zirconium coatings can show a gold to a blue hue after application, while dry-in-place (DIP) chrome-free technology does not have a color change.
There are plenty of conversion coating solutions to consider. Be sure to contact your pretreatment vendor to help you make the best choice for your application. They will look at your system, substrates, required specifications, and employee safety, and based on that information provide options for your application. Regardless of which chemistry is chosen, corrosion protection and better powder adhesion is the goal.
Brian Korecky is a technical sales and service representative with Bulk Chemicals, Inc.