What is Phosphating | Application Of Phosphating
What is Phosphating
Phosphating is the process of converting a steel surface to iron phosphate. This is mostly used as a pretreatment method in conjunction with another method of corrosion protection. A layer of phosphate coating typically includes iron, zinc or manganese crystals.
Phosphate coatings are usually applied to carbon steel, low-alloy steel and cast iron. The coating is formed with a solution of iron, zinc or manganese phosphate salts in phosphoric acid, and is applied by either spraying the solution onto the substrate, or immersing the substrate into the solution. When steel or iron parts are placed in the phosphoric acid, this causes a metal reaction which locally depletes the hydronium (H3O+) ions, raises the pH, and causes the dissolved salt to fall out of the solution and precipitate on the surface. The acid and metal reaction also creates iron phosphate, which may be deposited.
For paint and powder coatings, a phosphate coating has two main functions. First, the coating provides improved paint and powder coating adhesion since the phosphate crystals act as organic coating anchoring sites. Second, the phosphate layer acts as a corrosion barrier should the organic coating get scratched. In rust creep testing, the rust creep is reduced when phosphate is present under the paint
layer or powder coat layer compared to no conversion layer under the organic coating.
Phosphate can be used as a stand-alone coating for other purposes such as lubricity in parts forming but the other functions are beyond the scope of this report.
The most common phosphating chemistries are iron phosphate, zinc phosphate, and manganese phosphate. There are also other phosphating chemistries such as Plaforizing™ which are non-traditional in their chemistry and application since they are single step and typically an organo-phosphate that react with both the organic contaminants and the metal surface.
The main thrust in recent years for improving the phosphate process is to reduce the temperature requirements for the phosphate bath. Some chemistries have been developed that work well at room temperature. In general, there has been a trend from high temperatures, 90 F to 200 F, to much lower temperatures, 70 F to 140 F, resulting in energy savings.
The following is a typical phosphating procedure:
- Cleaning the surface
- Surface activation
- Neutralizing rinse (optional)
The main uses of phosphating are:
- Corrosion protection in conjunction with organic coatings, such as paints and polymer films
- Facilitation of cold-forming processes, such as wire drawing and tube drawing, or deep drawing
- Corrosion protection in conjunction with oils and waxes protection with no subsequent treatment
- Improving anti-friction properties, such as break-in, wear resistance, anti-galling and coefficient of friction
- Providing strong adhesion bonding for subsequent painting or other organic coating
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