Updated 1/21/2023 Published 7/1/2003

Zinc Phosphate: Questions and Answers

Our experts share specific questions about zinc phosphate and pretreatment

Edited by: PF Editorial Team

Zinc Phosphate Sludge

Q: We run a zinc phosphate spray coating line where mostly mild steel is treated. Since the startup of the line, we have experienced problems with excessive sludge buildup. We have tried the following to resolve the matter:

Increased TA/FA ratio to 20.
Run an alkaline cleaner with no free alkalinity.
Installed sludge filters to continuously remove the sludge.
Performed AA analysis on the sludge to determine Fe:Zn ratios; Fe=7% and Zn=2%.
Adjusted operating temperature to 55-60C.
Adjusted operating pressure to 1.5 bar.
Dosed phosphate and accelerator with a dosing pump to maintain concentration levels.

Could you please advise on further alternatives? We have considered changing to iron phosphate, but some of the final product is sent to the coast.

A: Zinc phosphates do build up sludge on the bottom of chemical solution tanks. The amount of sludge is related to the quantity of products processed and the amount of chemical solution in the system, which is somewhat dependent on the size of the chemical solution tank.

Owing to the increase in the price of fuel, many pretreatment systems were and still are built with small processing chemical tanks. This is because the smaller the tank, the lower the volume of processing chemicals that must be heated. This has reduced energy consumption; however, it has increased maintenance costs because the smaller the tank, the greater the sludge buildup.

Another alternative for you is to install larger chemical solution tanks in the phosphate stage.

(Follow-up to zinc sludge question).

I would like to present you with another cause for excessive zinc sludging and the solution.

I have found that continual, excessive sludging in phosphate coating tanks is caused by the reaction of the low pH phosphate coating liquid with calcium in the water. An analysis should show that the primary ingredient in sludge is calcium phosphate (Ca3PO4). Fortunately, calcium phosphate sludge is a non-hazardous solid waste and is easily disposed.

A temporary solution is to create a maintenance cycle where the phosphate coating liquid is periodically removed from the tank, the sludge chipped out and the phosphate coating liquid returned to the tank.

A permanent solution is to change water sources or to install water-softening equipment that will remove the calcium from the make-up water. Even though calcium is the most likely cause of the problem, before anything is done, an analysis must be made of the sludge to be absolutely certain that it is indeed a build-up of calcium phosphate.

Zinc Phosphate Application

Q: We have been asked to apply a zinc phosphate to steel surfaces in one of our products. For a number of reasons, we do not want to use a dipping process. Does zinc phosphate have an alternate application method other than dipping?

A: Zinc phosphates are applied on steel surfaces using solutions of zinc phosphate, phosphoric acid and activators. These proprietary chemical solutions deposit a crystalline coating of zinc phosphate during the application process. During the initial steps in the process, the metal is cleaned to remove oily soils. They can be applied cold or hot. Although viable, the cold processes are not widely used.

Cold zinc phosphates are applied using brush, spray, dip or flow coating on steel. Cold phosphates are used in maintenance and in-the-field painting. There have also been thixotropic zinc phosphate pastes used in maintenance painting pretreatment.

Most OEM manufacturing plants use a hot zinc phosphate process. Hot zinc phosphates are applied by dipping or spraying. It is also possible to apply them using a multi-stage steam spray applicator. The advantage of dipping and spray processes is control of crystalline structure and coating weight.

Chromating Vs. Phosphating

Q: In one of your columns, it was stated that chromate conversion coatings don’t work on steel. That confuses me. Is it that it works on galvanized but not bare steel?

Also, when does one choose chromating vs phosphating for galvanized steel? Can you imagine an organic coating that would not need chromating or phosphating to achieve a strong and corrosion inhibiting coating?

A: Chromates work on galvanized steel but not uncoated steel. The chromates are used extensively with galvanized steel or other parts that are zinc plated. That provides an aesthetically acceptable appearance and minimizes the onset of white corrosion product that is typical with zinc coatings.

Is chromating or phosphating necessary? It is all related to the final performance expectations of the coating. It is possible to paint clean steel without further pretreatment, but generally you would not expect to achieve as good a performance as you would with some sort of pretreatment. If high performance were required, very good corrosion resistance and adhesion, a phosphate coating of some type would be recommended for bare steel. Generally, zinc phosphate will provide enhanced performance over iron phosphate, but it does have the drawback of being more expensive to operate and maintain.

Moderate performance may not require anything more than a clean, dry surface free of rust and oil. Here, paint selection will be more critical. A solvent-borne paint usually will stand a better chance at providing a good coating where a very light amount of residue remains from a rust inhibitor. Some waterborne paints have closed the gap in the recent years, however, and they are better able to tolerate poor surface conditions than they were in the past.

Steel Pretreatment Prior to Powder Coat

Q:What pretreatments are recommended for hot-rolled, cold-rolled, and satin finish steel prior to powder coating?

A: That partially depends on the condition of the incoming material and the requirements of the final product.

In general, cleaning hot-rolled material can be much more difficult and is subject to much more variability as it comes in the door. It may require a good acid or electrolytic cleaning for the removal of mill scale and/or smut prior to a conventional pretreatment and paint. This may cost more, but could be offset by changing the satin finish steel to cold-rolled. I am not sure of the benefit of a satin finish when it will be painted.

The typical pretreatment for steel prior to powder coating would be some sort of phosphating. The type and size of the phosphate coating depends on floor space, available capital, current and potential future customer requirements, etc. The minimum you would want to do would be a three-stage phosphate system where the first tank is a cleaner/coater. It contains surfactants to degrease the material, and, when clean, an iron phosphate coating is deposited. This system would not likely be capable of reliably coating the hot-rolled steel. In general, the steel cannot be very dirty as it enters this stage, since the first stage is meant to do two steps. Rinse and seal steps would follow it.

The more common phosphate line is a five-stage system: clean, rinse, iron phosphate, rinse and seal, providing fairly good paint adhesion and moderate corrosion resistance.

To obtain increased corrosion resistance, a zinc phosphate line would be necessary. Although there are variations, it typically consists of the following steps: clean, rinse, activating rinse, zinc phosphate, rinse, and seal. Some seals also may require one or more DI water rinses afterwards.

Minimizing Rust, Part I

Q:What are the best processes for minimizing rust of powder coated steel poles?

A: Rusting on steel can be tested in the laboratory using a salt spray test (ASTM B117). This accelerated test is used to determine corrosion limits (rusting) of a particular substrate (steel) using a specific pretreatment and coating. Normally, the test product is scribed (scratched to the base metal) before it is put into the salt spray test chamber. Periodically, the test part is removed from the chamber and the coating adjacent to the scribe is examined for “creep” (corrosion under the coating). When a given amount of creep is achieved, then the test is concluded, and the time in the chamber is noted in hours. This is how you determine which coating and pretreatment should be used on your particular substrate to obtain the corrosion resistance you desire. What is “best” for your fielded conditions may be different from what is best for someone else’s fielded conditions. Therefore, only you can determine what corrosion resistance is best suited to meet your objectives. You must discuss this corrosion requirement (in salt spray hours) with both your pretreatment and powder coating suppliers.

Following are some examples to guide you in determining what is best for your application (substrate = cold rolled steel and creep is rated at level 6 or 0.125 inch):

Mild corrosion resistance 250 salt spray hours; requires good cleaning and a single powder coating.
Good corrosion resistance = 250-500 salt spray hours; requires 70 mg/sq ft of iron phosphate and a single powder coating.
Better corrosion resistance = 500-750 salt spray hours; requires 70 mg/sq ft of zinc phosphate and a single powder coating.

Superior corrosion resistance = 750 to 1,000 salt spray hours; requires 70 mg/sq ft of iron or zinc phosphate and a zinc-rich powder primer or e-coat epoxy primer and a powder topcoat (two coats in total).

Minimizing Rust, Part 2

Q: I read your clinic about minimizing rust issue and would like to point out some errors. The pretreatment requirements listed to achieve a particular result in the salt spray test are part of the “urban legend” of the metal pretreatment business.

In general, there is no correlation between phosphate coating weight and corrosion resistance. To say that a part that has 60 mg/sq ft of iron phosphate coating is somehow worse than one that has 70 mg/sq ft and thus will not achieve 500 hr before failing in a salt spray test is unfounded. Corrosion resistance is directly related to coating quality, not the amount of coating. The protective quality of an iron phosphate coating will vary, according to the accelerator system used in the product.

It is far more important that a phosphate coating (iron or zinc phosphate, doesn’t matter) provide uniform and complete coverage on the metal substrate. Voids in the coating will be more likely to cause the early onset of corrosion than a coating weight that is slightly below some arbitrary standard. G.G.

A: The answer used ³ (greater than or equal to) when it should have used » (approximately) when referring to how much iron or zinc phosphate should be on a steel substrate.

An even phosphate coating is very important in corrosion resistance, and 60 mg/sq ft may provide sufficient corrosion resistance. “More is not better” when applying iron phosphate, especially when it goes much beyond 70 mg/sq ft. This is because too much iron phosphate will leave a powdery residue on the substrate, which will cause all sorts of finishing defects (poor adhesion, and surface contamination to name a few).

There is disagreement on one point: “There is no correlation between phosphate coating weights and corrosion resistance.” It is proven that to achieve the specific threshold of corrosion resistance listed in the examples, you need a minimum coating weight that is closer to 70 mg/sq ft than it is to 30 mg/sq ft. So coating weight does matter!

If you have a question relating to zinc phosphting or pretreatment prior to painting or powder coating, email our experts. They can help you with all your finishing questions.