The first commercially available baths for producing corrosion resistant phosphate coatings were developed by Thomas Coslett during the turn of the century. Iron and steel parts were immersed in a phosphoric acid bath containing iron filings. After approximately 2 hours, a conversion coating was produced that helped in retarding corrosion.

Since that time, there have been numerous refinements to Coslett’s iron phosphate bath. Among these improvements was the development of heavy zinc phosphate coatings for corrosion resistant applications.

The coatings are widely used for one or a combination of the following reasons:

1. To prolong the life of the organic coatings.
2. To provide excellent paint or solid film lubricant resin bonding (e.g. MoS2 Films, PTFE Coatings).
3. To improve corrosion protection by providing a good base for absorbing and retaining rust preventing materials (e.g. oils, waxes, etc.).
4. To provide an excellent base for lubricants and drawing compounds.

Phosphate coatings are utilized in a variety of industrial markets to include: automotive, aerospace, military, and nuclear energy. At Imagineering, we consult with our customers and design an appropriate application. Barrel and rack processing techniques are tailored to meet particular customer specifications of order requirements.

Heavy zinc phosphates are used extensively to improve corrosion resistance. Some proprietary oil and wax formulations now on the market are capable of withstanding up to 240 hours neutral salt spray (ASTM B117). In addition to their corrosion resistant properties, phosphates also improve the overall appearance of finished goods by imparting a uniform, light to dark gray finish (the exact shade of gray is primarily dependent on the alloy and prior heat treatment). Uniform black coatings can also be obtained by adding a proprietary pre-dip step before the phosphate stage.

Cost is another reason to consider phosphates. Hidden metal parts, fasteners, hinges, pulleys, springs, slides, etc., can be zinc phosphated at a lower cost when compared to alternative finishes like plating or painting.

Phosphates are normally applied by immersion and often-utilized bulk processing techniques. Coating weights usually range between 1,500-4,000 mg/ft2 and depend on the surface condition of the steel, type of alloy used, and prior heat treatment. Varying the phosphate bath parameters can also have a dramatic effect.

Grain refiners can be incorporated into the phosphating cycle to assist in the formation of smoother, more uniform coatings. This ultimately results in an improved corrosion protector.

Manganese phosphating is a process used to produce a crystalline, oil absorptive coating with excellent wear resistance. These coatings are applied to facilitate initial break in of new parts such as piston rings, camshafts, cylinder liners, differential gears and transmission gears. The etch pattern produced by manganese phosphate on the surface of parts greatly improves oil retention and prevents galling of contact surfaces.

In addition to their anti-friction characteristics, manganese phosphate coatings exhibit excellent corrosion resistant properties when used with a suitable post rinse and rust preventative oil. For decades, the Defense Industry has specified manganese phosphate as the corrosion resistant coating of choice for a large percentage of steel and iron ordnance components.

Manganese phosphates are applied by immersion and, depending on the type of parts being coated, they may be processed on racks, in baskets or in tumbling barrels. Coating weights usually range from 1,500-4,000 mg/ft2 and depend on a number of factors including surface finish, type of alloy and prior heat treatment.

Grain refiners can be incorporated into the phosphating cycle, and are used prior to immersion in the manganese phosphate solution. This is done to influence the deposition of small crystals and control microfinish. In general, grain refiners tend to lower coating weights slightly.

Imagineering provides coating weight tests, which measure the amount and density of coating deposited.

DOD-P-16232
MIL-STD-171
MIL-DTL-16232
TT-C-490

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