What Is Phosphating? Iron Phosphate Conversion Coating for Powder Coating
Phosphating is a conversion coating process: an acidic phosphate solution reacts with clean steel and turns its surface into a thin, tightly bonded layer of insoluble phosphate. Before powder coating, that layer gives the powder more surface to grip and slows corrosion under the film. Iron phosphate is the most common choice for powder coaters because it can clean and coat steel in a single stage.
Below: how the reaction works, how iron phosphate compares with zinc phosphate and zirconium, what an iron phosphate line needs, and what to ask before you choose. Sources: Wikipedia, Products Finishing, Powder Coated Tough.
What is a conversion coating?
A conversion coating is a chemical treatment that "superficially converts the material into a thin adhering coating of an insoluble compound" (Wikipedia). Nothing is sprayed on top the way paint is: the base metal itself becomes part of the coating. Phosphate, zirconium (zirconate), chromate and anodizing are all conversion coatings.
How does phosphating work?
The bath is a dilute phosphoric acid solution. The acid reacts with the steel and uses up hydrogen ions, which raises the pH right at the metal surface. There the phosphate can no longer stay dissolved, so it deposits on the part as a fine, insoluble layer (Wikipedia). The coating grows outward from the surface, and "once the surface is sealed from the chemical solution, the reaction stops" (Products Finishing).
On steel, iron phosphate forms an amorphous coating that ranges from iridescent blue to gray. It adds surface area for the powder to grip and changes the surface chemistry, which improves both adhesion and corrosion resistance (Products Finishing).
What are the types of phosphate coating?
Type | Typical use | What to know |
|---|---|---|
Iron phosphate | Base for paint and powder coat | Light coating; can clean and coat in a single stage; spray or immersion |
Zinc phosphate | Heavier corrosion protection (automotive, appliance); galvanized steel | Needs separate cleaning plus a conditioner or activator stage |
Manganese phosphate | Wear resistance and oil-holding (lubricity) | Immersion only; not used as a powder coating base |
Sources: Wikipedia, Products Finishing.
Iron phosphate, zinc phosphate or zirconium: which fits a powder line?
Most powder coaters choose between iron phosphate and zirconium. Zinc phosphate is mostly used where the end customer requires it.
Iron phosphate | Zinc phosphate | Zirconium | |
|---|---|---|---|
Substrates | Steel | Steel and zinc-coated steel | Mixed metals: steel, aluminum, zinc-coated, stainless |
Stages | Can clean and coat in a single stage | Separate cleaner, conditioner and activator | No final seal needed, so fewer stages |
Bath temperature | Heated | Heated | Ambient |
Sludge | Builds up in the tank | Continuous; filters or settling tanks | Little to none |
Wastewater before discharge | Usually a pH adjustment | Heavy metals removed first | No heavy metals to treat |
Your substrates, customer requirements and internal spec determine which chemistry fits. Most CPR Systems customers run a single step clean and coat system, and we'll help guide you through that choice. For a deeper comparison, see Iron Phosphate vs. Zirconium Pretreatment.
What stages does an iron phosphate line need?
Iron phosphate systems "can involve two to six stages" (Products Finishing). The simplest is a cleaner-coater followed by a rinse. More demanding parts add a separate alkaline cleaning stage, more rinses, a post-treatment seal and a DI rinse. In a three-stage washer, cleaning and phosphating happen in the same stage; a five-stage washer cleans in stage one and phosphates in stage three (Products Finishing). See 3, 5 and 7-stage pretreatment washers compared.
A single step clean and coat system does the cleaning and the conversion coating in the same pass. A single step, variable pressure hot water BondTite washer cleans steel and aluminum products with either an iron phosphate or zirconium coating. Run at the recommended titration, you don't need to rinse. Rinse and seal coats are optional and increase performance; see Rinse and Seal After Iron Phosphate and our iron phosphate pretreatment page.
What affects phosphate coating quality?
Cleaning. Oil and soil left on the part keep the coating from forming evenly (Products Finishing).
Bath control. Concentration, free acid and iron content are checked by titration, and iron buildup often limits bath life (Products Finishing). See pretreatment bath maintenance.
Temperature and contact time. Crystal structure and coating thickness depend on bath concentration, composition, temperature and time (Wikipedia).
Rinsing. A rinse stops the reaction and removes unreacted chemistry (Products Finishing).
Does phosphating affect your wastewater permit?
It can. Iron phosphate wastewater usually needs a pH adjustment before it goes to the sewer, while zinc phosphate wastewater needs its heavy metals removed first (Powder Coated Tough). Your local sewer authority sets the limits; see Powder Coating Wash Water Disposal: EPA Rules and Compliance Options.
CPR Systems pretreatment equipment recycles its wash water, so there's no process discharge. To see what that could save your shop, try the savings calculator.
Working to an OEM spec?
Most CPR Systems customers aren't working to an OEM spec, so start with the standard single step clean and coat. If you supply Caterpillar or Applied Materials, start from their spec and work backward: see Pretreatment for Caterpillar Suppliers and AMAT Spec Pretreatment for Applied Materials Suppliers.
Phosphating FAQ
Is phosphating the same as a conversion coating?
Phosphating is one kind of conversion coating. Zirconium, chromate and anodizing are others. All of them convert the metal surface instead of adding a separate layer on top.
Is iron phosphate good enough for powder coating?
For many general industrial parts, yes: iron phosphate is widely used as a base for powder coat. Products Finishing notes it doesn't give the corrosion protection of zinc phosphate, so parts headed for severe environments may need more. Start from the spec your parts must meet.
Can you phosphate aluminum?
Iron phosphate is made for steel; Powder Coated Tough notes it can't coat non-ferrous metals as a standalone product. Shops running a lot of aluminum or mixed metals often choose zirconium. Ask us which fits your parts.
Do I need a rinse and seal after iron phosphate?
It's optional. Run at the recommended titration, you don't need to rinse. Rinse and seal coats increase performance, and some customer specs call for them. More in Rinse and Seal After Iron Phosphate.
What color is an iron phosphate coating?
Iridescent blue to gray on steel (Products Finishing). Zirconium coatings may show a gold to blue hue (Powder Coated Tough).
Send us your spec
Tell us what your parts need to meet and we'll work backward from it. We'd be glad to review your pretreatment guidelines and clean sample parts or panels for your own testing. Get a quote.
Comments