Phosphating

Published: | Updated: March 16, 2021

Definition - What does Phosphating mean?

Phosphating is a conversion coating that is applied to steel or iron components, and is mostly used as a pretreatment method in conjunction with another method of corrosion protection. The process involves immersing a component in a dilute solution, which then converts the surface of the metal into a layer of microscopic phosphate crystals.

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 either by spraying the solution onto the substrate or by immersing the substrate into the solution. When steel or iron parts are placed in the phosphoric acid, this causes the metal to react in a way that locally depletes the hydronium (H3O+) ions, raises the pH and causes the dissolved salt to fall out of the solution and precipitate onto the surface. The acid and metal reaction also creates iron phosphate, which may be deposited.

Phosphating is also known as phosphatizing and phosphate conversion coating.

Corrosionpedia explains Phosphating

The Phosphating Procedure

A typical phosphating procedure consists of the following steps:

  1. Cleaning the surface
  2. Rinsing
  3. Surface activation
  4. Phosphating
  5. Rinsing
  6. Neutralizing rinse (optional)
  7. Drying

Uses for Phosphating

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
  • Corrosion protection with no subsequent treatment
  • Improving anti-friction properties, such as break-in, wear resistance, anti-galling and reducing the coefficient of friction
  • Providing a strong adhesion bonding for subsequent painting or an organic coating

Types of Phosphating

There are three varieties of phosphating - iron, zinc and manganese - whose distinctions are indicative of the unique crystals that are formed in each. Manganese phosphating is generally seen as providing more corrosion resistance, while iron phosphating is more stable than zinc phosphating when under higher pressures and temperatures.

In iron phosphating there is an increase in paint adherence and resistance impact while providing protection against oxidation. Iron phosphating is known to offer the least corrosion resistance of the phosphating processes; although due to its low cost it is more often used on indoor equipment and parts that are usually not subjected to high levels of wear and corrosion. The color of its deposit ranges from a pale blue for a light deposit to a grey color for a heavy deposit.

Manganese phosphating's primary application is as an undercoat for lubricants or paint, as is the case with all phosphate processes. Manganese phosphating is the most absorbent of the three processes and is often used as a carrier for oils and waxes used for rust. It is primarily used to prevent metal on metal contact. Manganese phosphating is the most wear resistant of the three, but is not as corrosion resistant as zinc phosphating. It prevents galling without the coating flaking or peeling off, which makes it popular in the automotive industry. Manganese phosphate is dark grey, near black in color.

Zinc phosphating is the preferred choice for corrosion protection of ferrous metals. It is suitable for application in tough weather and is popular in the automotive industry. As is seen in the other processes, zinc phosphating increases the lubricity and acts as a carrier for paint, oils and waxes. In terms of color, zinc phosphating is a dark shade of grey, although it can be dyed to give it a matte black finish.


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