Phosphating

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phosphating

[′fäs‚fād·iŋ]
(metallurgy)
Forming a phosphate coating on a metal. Also known as phosphatizing.

Phosphating

 

the creation of a conversion coating of insoluble phosphates (phosphate coating) on metal articles. When covered by paint or varnish, the conversion coating inhibits corrosion. Phosphating is used mainly on low-alloy and carbon steels and on cast iron. The phosphate coating (thickness, 2–5 micrometers) retains lubricant, a feature that acts to lower the coefficient of friction. Owing to their high electrical resistivity, the coatings can withstand potential differences of 300–500 volts, and they are stable at temperatures up to 400°–500°C.

Phosphating is carried out by immersing the article in a solution of iron, manganese, zinc, and cadmium phosphates that has been heated to 90°–100°C. Industry produces manganese ferric phosphate mixture in a form designed expressly for phosphating. The process usually takes approximately one hour. After being dried, the article is passivated in a weak chromate solution. Phosphating can also be carried out electrochemically with alternating or direct current; here, the process takes 15–20 min.

REFERENCES

Lainer, V. I. Zashchitnye pokrytiia metallov. Moscow, 1974.
V. I. LAINER
References in periodicals archive ?
Tegehall, PE, Vannerberg, NG, "Nucleation and Formation of Zinc Phosphate Conversion Coating on Cold Rolled Steel.
Until recently, Roper Corporation used an iron phosphate conversion coating that required a bath of 125 to 140oF (52 to 60oC) and a post-treatment rinse stage.
Bonderite[R] 952 is a zinc phosphate conversion coating formulated for a spray application on substrates such as steel, galvanized and electrogalvanized, zincalloy, and aluminum.
Phosphate conversion coating involves the formation of a surface layer of hydrated metal phosphate crystals whose coverage approaches 100%.
These processes are capable of both removing fabricating soils and developing an iron phosphate conversion coating in one operation.
First significant advantage of the process is that there is no effluent to dispose of after the degreasing stage because the oils and greases removed from the components being treated are absorbed and become part of the polymeric phosphate conversion coating on steel, aluminium or zin coated steel.
More recently, Zhai and coworkers [Paper 2008-01-1158, SAE World Congress, Detroit, Ml, April 2008, SAE International, Warrendale, PA] showed that painted modified FZ on cold-rolled steel gave corrosion resistance comparable to that of painted phosphate conversion coatings.
We also use a wide variety of aqueous chemical solutions such as in acid pickling, phosphate conversion coatings and degreasing.
It is compatible with all lubricants, cutting oils, and hydraulic oils and acts as a topcoat over black oxide and phosphate conversion coatings to seal out corrosion and provide break-in protection.
In the case of aqueous processes such as amorphous (iron) phosphate conversion coatings, separate cleaning and rinsing stages can be used before the phosphate stage or as is often the case, cleaning and phosphating can be combined to give a suitable conversion coating.
Nate Goodnow, managing director of PPG Coating Services, said, "The industry has been excited about the introduction of Alltech's level of coatings competence in Taiwan, with state-of-the-art equipment and technology for processing zinc phosphate conversion coatings and multiple colors of electrocoat.