Zinc Phosphate vs. Manganese Phosphate

Question:

For many years we have used a heavy-zinc phosphate and oil treatment for our Tees that have a high-interference API 8RD thread that is installed on oil well tubing. We use this process primarily for thread galling resistance and prevention of rust on the supply store shelf is a secondary requirement. We have a new parts vendor that claims “Manganese phosphate is just as good as zinc phosphate.” Do you agree? D.S.

Answer:

I am not familiar with the use of it for thread cutting, but in general, it sounds like your vendor is correct. Zinc phosphate is primarily used for paint pretreatment. I would think the newer, microcrystalline zinc phosphates used for paint pretreatment would not hold as much oil as an older technology zinc phosphate or manganese phosphate.

Unlike zinc phosphate, the only purpose of manganese phosphate is for use with an oil coating for storage protection and lubricity/wear resistance. I believe a manganese phosphate with an oil coating was used in the past on automobile cylinders to minimize galling of components during the “break in” period. This was when the owner’s manual told to you to drive the first couple hundred miles at less then approximately 50 miles per hour. I may be dating myself here, but it is a little fuzzy since I don’t think I ever had enough money to own a new car during this time. I think I remember reading about it in manuals of older used cars.