What are the phosphating and its process?

Phosphating is a process of chemical and electrochemical reaction to form a phosphate chemical conversion film , and the formed phosphate conversion film is called a phosphating film .

The main purposes of phosphating:

  • Protect the base metal and prevent the metal from being corroded to a certain extent;
  • Used as a primer before painting to improve the adhesion and anti-corrosion ability of the paint film;
  • Used to reduce friction and lubricate in the metal cold working process.

The phosphating process has been used in the industry for more than 90 years, and can be roughly divided into three periods:

  • The period of laying the foundation of phosphating technology,
  • The period of rapid development of phosphating technology
  • The period of wide application.

Phosphating History


Phosphate film is used as an anti-corrosion protective film for steel , and the earliest reliable record is the patent obtained by Charles Ross in England in 1869 (BP No. 3119).

Since then, the phosphating process has been used in industrial production. In the long years of nearly a century, phosphating technology has accumulated rich experience and made many important discoveries.

During  World War I , the development center of phosphating technology was transferred from the United Kingdom to the United States.

In 1909, TWCoslet in the United States dissolved zinc, zinc oxide or zinc phosphate in phosphoric acid to make the first zinc-based phosphating solution .

This research achievement has greatly promoted the development of phosphating process and broadened the development prospect of phosphating process.

The Parco Power phosphating solution developed by Parker Antirust Company overcomes many shortcomings and increases the phosphating treatment time to 1h (1 hour).

In 1929, the Bonderizing phosphating process shortened the phosphating time to 10 minutes.

In 1934, the phosphating treatment technology achieved a revolutionary development in the industry, that is, the method of spraying phosphating liquid onto the workpiece was adopted .

After the end of World War II, phosphating technology rarely made breakthroughs, but only steadily developed and improved.

Phosphating is widely used in anti-corrosion technology, metal cold deformation processing industry.

The important improvements of phosphating treatment technology in this period mainly include: low-temperature phosphating, various methods to control the weight of phosphating film, and high-speed phosphating of continuous steel strip.

At present, the research direction in the field of phosphating technology mainly focuses on improving quality, reducing environmental pollution and saving energy.




Phosphating process:

A typical phosphating procedure includes the following steps:

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

The main uses of phosphating are:

  • Combines corrosion protection with organic coatings such as paints and polymer films
  • Convenient cold forming processes such as wire drawing, tube drawing or deep drawing
  • Corrosion protection combined with oils and waxes
  • Corrosion protection without subsequent treatment
  • Improve anti-friction properties such as fracture, wear, anti-galling and reduce the coefficient of friction
  • Provides strong cohesive bonding for subsequent paint or organic coatings


Types of Phosphating

There are three phosphides – iron, zinc and manganese – whose distinctions indicate the unique crystals that form in each.

Manganese phosphating is generally seen as providing more corrosion resistance while iron phosphating is more stable than zinc phosphating at higher pressures and temperatures.


The presence of phosphates increases paint compliance and impact resistance while providing antioxidant protection.

Of the phosphating processes, iron phosphating is known to provide the least corrosion resistance; although due to its low cost, it is more commonly used in indoor equipment and components that are generally free from high wear and corrosion.

The color of its sediments varies from pale blue for light sediments to gray for heavy sediments.


The main application for manganese phosphating is as a lubricant or primer for paints, which is the case for all phosphating processes.

Manganese phosphating is the most absorbent of the three processes and is often used as a carrier for rust preventive oils and waxes.

It is mainly used to prevent metal-to-metal contact. Manganese is the three most phosphated wear resistant , but not the corrosion resistant zinc phosphide.

It prevents the gall from peeling off the coating, which makes it popular in the automotive industry. Manganese phosphate is dark gray, close to black.


Zinc phosphating is the preferred choice for corrosion protection of ferrous metals. It is very popular in the automotive industry for severe weather applications .

As seen in other methods, zinc phosphating increases lubricity and serves as a carrier for paints, oils and waxes. In terms of color, zinc phosphide is a dark shade of gray, although it can be stained to give it a matte black finish.


(source: internet, reference only)