Whether coatings are applied to steel by spray or immersion, a rule of thumb is that lightweight, amorphous phosphate coatings adhere better, while heavier, crystalline zinc phosphate coatings are more corrosion resistant (Ref 4).

Most phosphate-coated steel is low-carbon, flat-rolled material used for applications such as sheet metal parts for automobiles and household appliances, and phosphating processes have been designed for coating such material. Steels with carbon contents in the range specified for 1025 to 1060 inclusive are suitable for phosphating if the silicon content is held to normal limits. Steels with higher carbon contents, in the range from 1064 to 1095, may require the following modifications of phosphating processes to produce satisfactory results: increasing time; increasing temperature; or increasing solution strength. Copper content up to 0.3% in low-carbon steel, the normal limit for copper-bearing steel, is not a deterrent to phosphate coating. The addition of copper, by itself, at about 0.5% causes surface checking of steel during hot rolling. This acts as a restriction on the amount of copper that may be present to serve as a deterrent to phosphating.

Low-alloy, high-strength steel, provided nickel or chromium does not exceed 1%, can be successfully phosphated. Generally, with some modification, chromium content of up to 9% can be tolerated while still depositing a phosphate coating. Nickel-chromium and chromium stainless steels are not recommended for phosphate coating. For some applications, however, oxalate coating processes are used.

Because electrical steels used in motor laminations and electrical transformers have a silicon content in the range of 1.2 to 4.5%, they are not recommended for phosphate coating by normal phosphating processes. These require processes accelerated by the use of fluoride compounds or special dried-in place salts of phosphates.

Low-carbon steels annealed in a properly controlled atmosphere to provide a clean, oxide-free surface are readily phosphated. Temper-rolled, annealed, low-carbon steels are the most readily phosphated of all steels. Cold-reduced or cold-rolled, full-hard, low-carbon steels readily accept phosphate coatings.

Low-carbon, hot-rolled steel, and normalized and pickled steel, if thoroughly rinsed after pickling, phosphate well. Excessive amounts of residual pickling salts (sulfates) can interfere with normal phosphating. Pickling residue on cold-reduced or cold-rolled steels seldom presents problems, because of the extensive processing that follows the pickling operation. Cold reduction of 30 to 70% spreads the residue over large areas. Cleaning and scrubbing of a cold-reduced strip, followed by annealing and temper rolling, remove or dilute surface contaminants. Phosphating processes that provide for relatively long-time and high-temperature treatments are the least sensitive to small variations in alloy composition and surface conditions.

Stainless Steels (Ref 5). Although phosphate coating of stainless steel is difficult, it is sometimes attempted to protect against pitting in chloride atmospheres. Pretreatment is generally required if an organic coating will be applied.

One study achieved good zinc phosphate coatings using the following bath composition:




lb/gal X 10"2

Zinc oxide



Phosphoric acid



Calcium chloride



Ferric chloride



The researchers tested temperatures from 40 to 80 °C (105 to 175 °F) and immersion times of 15 to 50 min. Various stainless steel pretreatments were tested for their effects on coating adhesion and quality: solvent degreasing, immersion in hydrochloric acid or sulfuric acid solution, and sand blasting.

The results showed that the most effective operating conditions were 60 to 70 °C (140 to 160 °F) with immersion times of 15 to 30 min. Sandblasting with 16-mesh sand proved to be the best pretreatment. The zinc phosphate coatings on stainless steel panels prepared under these conditions were uniform and well adherent. The corrosion resistance of the panels was tested by immersing them in 0.5% NaCl solution and exposing them to a salt-spray chamber, and no rust spots were observed within 15 days for either test. Paint adhered as well to phosphated stainless steel panels as to nonphosphated panels.

Galvanized Steel. Many parts produced from galvanized sheet steel, such as certain automotive stampings and some appliances, require a phosphate coating as a base for a subsequent paint film. Phosphating imparts superior resistance to corrosion and greater ability to retain paint to galvanized sheet and strip steel by converting the surface to an insoluble phosphate coating. Galvanized steel can be readily phosphated provided the surface of the plate has not been passivated by a chromate-based solution. The passivated surface of the chromate-treated material resists the action of a phosphating solution. Treatment of such passivated surfaces requires the use of an alkali-permanganate solution or, depending on the age and degree of passivation, removal with strong alkaline cleaners.

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