Conversion Coatings
A conversion coating is a thin layer formed by chemically reacting a metal surface, such as a phosphate or chromate film, that improves paint adhesion and corrosion resistance.
A conversion coating is a thin, adherent layer created by a chemical or electrochemical reaction between a metal surface and a treatment solution. Rather than being applied on top of the metal like paint, the layer is formed partly from the metal itself. Conversion coatings are used as pretreatments under paint and powder coatings, and sometimes as standalone corrosion protection for parts that will be oiled or waxed.
How conversion coatings work
When a clean metal surface contacts an acidic treatment bath, the metal dissolves slightly at the surface. This raises the local pH, and dissolved species in the bath precipitate as an insoluble film bonded to the metal. The resulting layer is usually measured in fractions of a micrometer to a few micrometers.
The layer helps a paint system in three ways:
- Adhesion. It provides a chemically receptive, often microcrystalline surface for paint to wet and anchor to.
- Underfilm corrosion resistance. It slows the spread of corrosion from scratches and edges, the process described in undercutting corrosion.
- Uniformity. It converts a variable, partly oxidized metal surface into a consistent one.
Main types
Iron phosphate
Iron phosphate is a light, amorphous coating formed on steel. It is common in multi-stage washers on paint and powder lines because it combines cleaning and conversion in fewer stages. It improves adhesion but offers moderate corrosion resistance, so it is often chosen for indoor or mild-service products.
Zinc phosphate
Zinc phosphate forms a heavier crystalline layer and gives better corrosion resistance under paint. It is widely used for automotive bodies, appliances and parts that see outdoor exposure. Zinc phosphate lines normally include an activation (conditioning) stage to promote fine crystal growth, and a sealing rinse. Manganese phosphate is a related heavy coating used mostly for wear resistance and oil retention on fasteners and gears.
Chromate conversion
Hexavalent chromate treatments have a long record on aluminum, zinc and cadmium surfaces and offer good corrosion resistance with some “self-healing” behavior. Because hexavalent chromium is a recognized carcinogen and is tightly regulated, many industries have moved away from it. Its use continues in some aerospace and defense applications where specifications still call for it.
Trivalent chromium (TCP)
Trivalent chromium processes were developed as replacements for hexavalent chromates, especially on aluminum. They are widely used under aerospace and military primers, subject to the governing specification.
Zirconium and titanium “thin film” pretreatments
These newer treatments deposit a very thin oxide layer from fluorozirconic or fluorotitanic acid baths. They often run at lower temperatures, produce less sludge than phosphating, and can treat mixed-metal loads. Many general industrial finishing lines have converted to them.
Other treatments
- Anodizing on aluminum is an electrochemical oxide-growth process. It is covered in its own entry: anodizing.
- Black oxide on steel is a conversion finish used mainly for appearance and mild corrosion resistance with oil.
- Passivation of stainless steel removes free iron and strengthens the natural chromium oxide layer. It is related but usually treated separately (see coating stainless steel).
Process sequence
A typical spray or immersion pretreatment line includes:
- Alkaline cleaning to remove oils and soils.
- Rinse.
- Activation or conditioning (for zinc phosphate).
- Conversion stage.
- Rinse.
- Final seal or deionized water rinse.
- Dry-off oven before painting.
Control of bath concentration, temperature, pH and contact time determines coating quality. Line operators typically titrate baths regularly and check coating weight on test panels.
Field alternatives
Conversion coatings are mainly a factory process. In the field, the closest equivalents are wash primers and etch primers, and phosphoric-acid metal preparations that leave a light phosphate film. These are not a substitute for abrasive blasting where a specification requires a blast profile.
Testing and quality control
- Coating weight is measured by weigh-strip-weigh methods on test panels.
- Appearance checks look for uniform color and the absence of powdery or loose deposits.
- Performance is verified on painted panels with adhesion testing and salt spray and cyclic corrosion testing.
Common problems
- Powdery or coarse phosphate. Often caused by poor activation, contaminated baths or excessive coating weight. Loose deposits weaken paint adhesion.
- Flash rust after the line. Inadequate final rinse or slow drying can leave rust spots before painting (see flash rust).
- Residual contamination. Poor cleaning upstream prevents a uniform conversion layer.
Environmental and safety considerations
Conversion baths contain acids and, depending on type, metals such as zinc, nickel, manganese or chromium. Wastewater must be treated before discharge, and sludge disposal is regulated. Safety data sheets for each chemical set the PPE and handling requirements (see safety data sheets).
Key terms
- Pretreatment. Any chemical step that prepares metal for painting, including cleaning, etching, conversion and sealing.
- Coating weight. The mass of conversion layer per unit area, often expressed in milligrams per square foot or grams per square meter and used for process control.
- Hexavalent chromium. The chromium form used in traditional chromate conversion coatings. It is a regulated carcinogen, which has driven a shift to trivalent and chrome-free products.
- Zirconium and titanium pretreatments. Thin-film conversion chemistries used as alternatives to phosphating, usually at lower temperatures with less sludge.
- Rinse quality. Contaminated rinse water can leave salts on the part that later cause blistering, so many lines finish with a deionized water rinse.
Frequently asked questions
Is a conversion coating a primer?
No. It is a pretreatment that improves the performance of the primer or paint applied over it.
Can conversion coatings be applied over rust?
No. The metal must be clean and free of rust, scale and oil for the reaction to occur evenly.
Do conversion coatings work on galvanized steel?
Yes, phosphate and some thin-film treatments are formulated for zinc surfaces. See galvanized steel surface preparation.
Related entries
- Powder coatings
- Coating aluminum
- Galvanized steel surface preparation
- Undercutting corrosion
- Anodizing (new)
- Filiform corrosion (new)
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.
