I have what is an increasingly prevalent problem. Our company is purchasing powder coated mild steel automotive piece parts from a factory in China. This factory has a zinc phosphate pretreatment process. We are using a high-quality paint manufactured by a major paint supplier in China, allegedly the same paint we use successfully at our own facility in California. The paint adhesion test results are very good; the salt spray tests are horrible, less than 200 hours. The zinc pretreatment process is a series of dipping tanks. The rinse water comes from a shallow well. The last rinse stage is spraying down the parts with a garden hose fed with well water. The pretreatment chemicals are supplied by a local supplier/blender that offers no technical service.
I need help determining what is necessary to upgrade the system and/or process controls to achieve a nominal 750 hour salt spray quality of coating. Any ideas? J.H.
If a garden hose consists of the final rinse stage, there is a long ways to go to upgrading that phosphate system. A well designed, high-quality zinc phosphate system consists of the following steps:
- Alkaline cleaning. This step is required to remove residual oils and contami- nants from surface of the steel. Sources may be from the rolling mills, metal work-ing, handling, "shop dirt", etc. Residual oils and greases that remain on the surface will not allow the zinc phosphate conversion coating to form since it acts as a barrier between the phosphating chemicals and the base metal causing some spotty adhesion problems and poor corrosion protection.
- Overflowing rinses. Neces- sary to remove the alkaline cleaner from the surface. While a slight amount of alkaline residue can be carried over to the next rinse and/ or the zinc phos- phate tank, excessive alka- line cleaner carryover will tend to neu- tralize the free acid in there. This will require addi- tional mainte- nance and upkeep to the system through titrations and additions.
- Nucleating rinse. This step is not always followed by all shops, so it may be considered optional. From my experience, the lines that have more stringent quality requirements usually use this step. This is a static “rinse” tank that contains a titanium salt at a very low concentration. The salt adheres to the surface of the steel and initiates more sites for the start of the zinc phos- phate process. This in turn produces a finer zinc phosphate crystal that is desirable for paint adhesion and corro- sion protection. Too low a concentra- tion does not produce the desired boost in performance and too high a concentra- tion is just throwing money down the drain.
- Zinc phosphate. Obviously, this is the tank where the conversion coating takes place. The free acid dissolves steel at the surface. This, in turn, creates an area of locally higher pH. The local increase in pH precipitates the zinc phosphate crystal that becomes an inte- gral part of the surface. In order to continue this reaction, the tank has to be maintained with the correct level of free acid, reserve acid and any accel- erators present. The accelerators are used to increase the reaction rate and are usually in the form of an oxidizer such as a chlorate, nitrate and nitrite.
- One or more overflowing rinses. This is required to remove the residual acid and other chemicals from the surface in order to ensure the reaction has stopped. Technically, this rinse and the one following the alkaline cleaner can each be done in one step/ tank. However, to maintain rinse water quality, they need to be overflowed. Since rinse water dilution is a logarith- mic function, addition of another coun- terflowed rinse tank at each process step can dramatically reduce rinse water use.
- Seal. This is a step that helps to “seal” any residual porosity in the zinc phos- phate coating itself. In former times, this was often a fairly simple chrome seal. For environmental reasons, non- chrome seals have been developed over the years that provide similar perfor- mance. They generally fall into two categories: organic and inorganic.
- Depending on the requirements of the seal step, it is often necessary to provide a final high quality rinse to the zinc phosphated parts. This will rinse away residual weak acid from the surface that may be present from the seal step. It is imperative to note that this rinse water needs to be very high quality since this is the last fluid the part is exposed to until the painting phase. The water in this stage almost always is run through a mixed bed ion exchange system or a reverse osmosis filtration stage to ensure it is of the proper quality.
The rinse water you are referring to will not be of sufficient quality. Even if all previous steps are performed with the highest quality, the performance of the coating will only be as good as the weakest step of the pretreatment process allows it to be.
Due to the source of this water, it will be very high in mineral content and moderately to highly conductive.
Surface water that settles through sedimentary rock and eventually is pumped back to the surface has most of the organic contaminants removed because of the filtration effect of the layers of soil and rock. However, along the way, the water very gradually dissolves the mineral content of the rocks. Well water has especially high levels of calcium and magnesium because of this effect.
As a result, using water like this in the final rinse essentially contaminates the surface with conductive minerals prior to paint. These minerals may not influence the paint adhesion, but due to the oxygen and water permeability of the paint film, it will eventually allow some amount of moisture to the surface. The conductive minerals are soluble in the water and will initiate corrosion under the paint film much more quickly than if the surface had been rinsed with high purity water from a deionizing or reverse osmosis system.