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Phosphatizing, Phosphating and Phosphate Conversion Coating

Overview

Phosphatizing, also called phosphating or phosphate conversion coating, is a metal pretreatment primarily used to prepare steel for paint or coatings and to prevent corrosion. Phosphatizing is termed a conversion coating because, unlike paint or traditional coatings, it does not lie on the surface of the metal, but rather the surface of the metal is chemically changed to a new substance. The coating will, therefore, not chip- or scratch-off exposing the base material to corrosion and wear.

Super Shot Peening can provide phosphate coatings for very large (up to 35 feet in length on a double dip) and very small parts. We offer both zinc and manganese phosphate coatings. Both types produce components with black or blue-black, non-reflective, non-metallic, non-conductive, “velvety” finishes.

Although—as with all of our services—phosphating can be performed in the field, most of our phosphating jobs are performed on-site. Rush jobs are easily accommodated. In fact, many jobs have been completed within a period of only one or two hours while our client’s driver has waited on-site. Please contact us for availability of our tanks.

 

Technical Information

Benefits of Zinc and Manganese Phosphate Conversion Coatings
Both coatings are applied for the following benefits:

  • Paint and Coating Primer
  • Increased Oil and Lubricant Adhesion
  • Anti-Galling Properties
  • Corrosion and Rust Inhibition: Phosphatized parts can withstand up to 240 hours of salt spray without signs of corrosion.
  • Thin Coating/No Distortion: Screws and threaded parts can be phosphatized.
  • Cost Efficient: It’s cheaper to phosphatize hidden parts, springs, bearings etc. than to paint or plate them.
  • Aesthetic Appeal: The blue-gray coating is often more appealing than the unfinished metal.

Zinc phosphate is particularly useful during certain processes such as wire-drawing. The nature of the zinc phosphate coating allows the adhesion of lubricants even under extreme heat and pressure. Zinc phosphate is used often in the auto and appliance industries.

Manganese phosphate is the thicker of the two phosphate coatings. Its crystal structure retains more lubrication than zinc phosphate and is favored for its superior anti-galling properties.

How Phosphatizing Works
To apply the phosphate coating, parts can be either bathed in the phosphatizing chemicals or the chemicals can be sprayed onto the metal using a system similar to a conventional pressure-washer. In either case the solution applied to the metal is composed of four parts: detergents, metal phosphates (in our case zinc- or manganese-phosphate), phosphoric acid, and additional agents (such as reaction catalysts or chemicals to tie-up bi-products). The detergents help to clean and degrease the surface of the metal. The phosphoric acid provides for minor acid etching or picking allowing for better paint and lubrication adhesion. The metal phosphates react with the surface of the part to form the corrosion-resistant phosphate coating.

First, the iron comes into contact with the phosphoric acid causing acid pickling. Iron (Fe) is oxidized and hydrogen ions (H+) ions are separated from the phosphoric acid and reduced to form hydrogen gas (H2). Some of the oxidized iron sloughs off or dissolves leaving the surface acid-etched.

In the second reaction, the zinc phosphates are deposited onto the substrate to form the conversion coating.

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