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Zinc Phospate in steel

Zinc Phospate in steel

(OP)
We have some carbon steel parts which need to be protected from corrosion. The part has internal threaded section about and inch in diameter. The parts are to be installed under the car and will be exposed to contaiminates. We are looking for the best and cost effective way to protect the part from corrosion. One way is to apply zinc phospate to prime the part than apply paint on top. The zinc phospate provides corrosion protection for everywhere(e.g. thread) where paint cannot reach. From internet search, some site mentioned this coating forms abrasive crystalline. My concerns is if zinc phospate is applicability to bolted joint. Would the friction and abrasive characteristic damage the threaded joint. Would structural integrity (e.g. preload) be maintained with the crystal structure damaged? If oil is needed to lub the joint, would it be compatible with thread locking compound, e.g. Loctite 243.

RE: Zinc Phospate in steel

Zinc phosphate forms crystals, but they are not abrasive to steel. This coating is used widely for threaded fasteners. It does not provide enough corrosion protection for steel parts underneath vehicles subjected to chloride ion from deicing salts. A better coating would be a non-electrolytically applied zinc coating, such as Magni 565 or similar competing ones (e.g. Delta Protekt, Zintek, and Geomet). If you do continue with zinc phosphate and need a lubricant and a thread locking compound, you need to coat the parts in this order:

phosphate
thread locker
lubricant

This is how we specify similar parts, although they are for use inside driveline housings, not exposed to the outside environment. For external applications, we use the zinc-based coatings.

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