Zinc Phosphating Questions

Q. Please do me a favor by answering these questions: 1. What is the specific or accurate range of result expected to have a good phosphate coat after carrying out a coupon test using manganese phosphating or zinc phosphating procedure. 2. What is the expected result for the free acid test during titration of the phosphate solution (Mn and Zinc). 3. Is there an alternative iron content improver apart from iron wool and what is the accurate result of iron content result after titration of the phosphate solution (for both zinc and manganese phosphating). Thank you. —A.J.

A. Zinc and manganese phosphate have different chemistries and purposes. The zinc phosphate coating is most often used as a conversion coating to steel to act as a paint base to provide improvement in performance and corrosion resistance. The second typical application for zinc phosphate is to act as a base that is then coated with zinc stearate soap for use in severe cold forming operations. Manganese phosphate is also a conversion coating, but typically applied to steel, then usually oiled. The purpose of this combination is to act as a base for moving parts to allow a break-in period. Many years ago, automotive cylinders were coated in this way and it was typically recommended that new cars be driven more slowly initially during this break-in period.

1. Naturally, they will also have differences in coating weights. The coating weight range for zinc phosphate is about 150-1,000 mg/ft2 (1.6-0.8 mg/m2). The coating weight for manganese phosphate is higher at about 500-3,000 mg/ft2 (5.4-32.2 mg/m2).

2. The amount of free acid will vary by type of bath, but is not the only attribute to track in a phosphate system. Typically, a total to free acid ratio is used to track the quality of the phosphate tank. Total acid is a titration that goes to a lower pH endpoint using a different indicator than the free acid titration. Generally, the ratio will be in the range of 10:1, but will vary by the specific supplier’s chemistry. I would recommend contacting your current pretreatment chemical supplier and ask them to provide you with complete quality control procedures and training.

3. During start-up of a zinc or iron phosphate tank, there will generally not be sufficient iron to effectively create the phosphate coating quickly enough, therefore, in the past steel wool was often added to the tank in an effort to quickly increase the dissolved iron content of the tank. Again, I would suggest contact with your chemical supplier who will be of more help in identifying a specific starter chemistry that may not require the use of steel wool. However, that is still an effective means.

A different type of titration or analysis needs to be performed in order to determine the amount of dissolved iron in the tank.