PISTON 3349883 - Caterpillar

Because of the subject matter, this article is not written in Caterpillar Fundamental English.
Caterpillar currently uses two types of phosphate coatings: Zinc-Iron Phosphate and Manganese-Iron Phosphate. Zinc phosphate coatings are used mainly where corrosion prevention is the principal objective. This coating is normally gray or dark gray, thin and fine grained with a dull finish.
Manganese phosphate coatings are used mostly for break-in protection, and are used where there are high contact loads. Manganese phosphate coatings can be identified by a black, coarse finish which appears to sparkle.
Some typical applications for these coatings are the following:
Zinc Phosphate
Transmission Housing
Oil Filter Bases
Strainers
Hardened Washers
Other internal or lightly loaded components
Manganese Phosphate
Bevel gear sets
Thrust Washers
Other Heavy Load Applications
Where a bolt is used to attach a manganese-phosphate-coated part, there should be no coating on the surfaces that contact the bolt. A phosphate coating in this area can relax the torque on the bolt and result in a loose part. If parts are received with phosphate coating on the contact area, the coating should be removed before installation. A case in point--many bevel gears have phosphate coating on the teeth. If this coating is found on the bolt face or mounting flange face of a bevel gear, it should be removed before assembly.
Some parts, such as gears, may give the impression that they are improperly fabricated because of the look and feel of the phosphate coating. These parts, however, are good parts and should be used. The coating will gradually wear off from the mating surfaces and improve them as they break in.
There is a third type of phosphate coating used by Caterpillar. This is an iron-phosphate coating. Iron phosphating is used in multiple stage parts cleaners to provide a good base for painting, and has a very coarse finish.