Phosphating Process

Q. We are in the process of setting up a pretreatment plant, and would like to know which phosphating process will be best, considering quality, investment, and maintenance for: 1) room temperature dicationic process, 2) tricationic process, and 3)high-temperature dicatonic process. K.D.

 

A. Given your list above, it is not clear which options you are considering for your phosphating line. There are two primary, traditional phosphating processes - iron and zinc. Either of those can have several variations that can be suited to a variety of different end-use requirements.

The first considerations should be the materials you intend to pretreat and the final specification requirements they will be expected to meet. Some alloy of steel makes up the majority of the metal pretreated by either iron or zinc phosphating. The next step following the phosphating is usually a paint or powder coating application, since pretreatments alone are generally not considered an adequate coating to survive even a mildly corrosive environment. Here are some other considerations:

Performance
The performance characteristics that the coating process is expected to meet will usually drive the selection of pretreatment and paint. A requirement for a long-term, high-corrosion-resistance and paint performance will usually require a zinc phosphate with a primer and top-coat.

For instance, in the automotive industry, it is standard practice to use two-side coated electrogalvanized material for the car body. That will then receive a multi-stage zinc phosphate pretreatment process, followed by a cationic electrodeposition primer, color coat and clear top coat. Zinc phosphate is specified where corrosion protection is an important requirement—this is the primary benefit of zinc over iron phosphate.

Capital
The investment required for the process outlined in the previous paragraph would be millions of dollars. I assume your investment is not that significant. Pretreatment systems can be specified for much less than this; however, the typical zinc phosphate line will usually need, at a minimum:

1. Alkaline cleaning stage
2. Rinsing (one or more stages - more counterflowed stages = lower flow and less waste treatment)
3. Conditioning rinse (generally a titanium salt intended to nucleate crystal growth in the phosphate stage)
4. Zinc phosphate
5. Rinsing (one or more stages)
6. Seal
7. DI water rinse.

For a zinc phosphate system, you will have to plan for a minimum of seven stages. In some cases where additional rinsing is desired in order to maintain a lower overflow rate and insure minimal carryover between stages, the lines will stretch to ten stages.

This list contains more stages than are required for a good iron phosphate system. The most common iron phosphate systems are either three or five stages. The process layout for a five-stage system is:

1. Alkaline cleaning stage
2. Rinsing
3. Iron phosphate
4. Rinsing
5. Seal.

The three-stage system combines the cleaning and conversion coating in the first stage, followed by a rinse and seal. This type of system would not be suitable for parts that are heavily soiled, though, since the formulation of a combined detergent and phosphate makes for a detergent less effective for removing heavy soils.

There is also a niche product that combines everything into one. This is called spray wand phosphatizing. It uses a chemical similar to the first stage of a three-stage phosphate line in that it cleans and coats, but does not require rinsing. It generally would be considered a further compromise in regards to cleaning and phosphating. As you can see, the zinc phosphate process would be a more capital-intensive line both to set up initially and to run over time.

Maintenance
Due to the additional stages, the zinc phosphate process will require more maintenance of equipment and waste. With additional stages, there will be more tank dumps, waste and waste disposal to consider. Also, the zinc phosphate stage will generate a considerable amount of sludge that will need to be removed and disposed of. Chemical maintenance of the zinc phosphate stage is also more difficult since it requires you to monitor the extent of oxidizer present, and then add back separately from the zinc phosphate.

Summary
From the brief review covered here, you can see that zinc phosphate can offer a performance enhancement over iron phosphate. However, this will come at the cost of higher capital outlay and higher operational maintenance.