Clean Steel Using COR Rinse 404

Q. We have been using cor rinse 404. Could you tell me if there are any issues with adhesion with different paints systems? How do you check if the metal is clean? T.M.

A. To fully answer your first question, I would need to have more information on the Cor Rinse 404. I used to be involved with a zinc phosphating pretreatment process in one of our manufacturing plants and the chemical supplier for that line used similar terminology their product names (Cor*** followed by a number). I went to their web site to find out if they listed a Cor Rinse 404, but the site did not have a full product listing.

From the name of the product, I will assume that it is a final step in a multistage pretreatment process, such as phosphating. Most of the final rinse coatings used in phosphating lines fall into two general categories, chrome and non-chrome seals. Technology has advanced today to the point where many of the non-chrome seals provide similar corrosion resistance and overall performance as the chrome-containing products. Provided it is a final rinse or seal on a pretreatment or phosphate system, I would have to assume that the product was formulated with a variety of different paints in mind. Generally, most of the final rinse and seal products are meant to “plug” any microscopic porosity of the phosphate coating and provide a slightly acidic residue on the surface (slight is the keyword).

Based on your adhesion problems, I would make a few quick verifications on your process. First, check all aspects of the process line to make sure it is running per specification for time, temperature and chemical concentration. Ideally you would have a work instruction at the line where the department supervisor would routinely be making these inspections and (hopefully) recording the results. If no record is available, ask the operators on the floor about how the line has been running, anything unusual with it, maintenance problems, etc. Additionally, you should consult the technical data sheets provided from your chemical supplier. They should list ranges for all the pertinent control parameters including time, temperature and concentration. They may also list incompatibilities with other process chemicals and the nature of the chemicals regarding where and how they are used.

With this information in hand, all the process tanks should be checked to see how well they fall within the suppliers recommendations. Time is generally the broadest of the variables with more time usually (but not always) better for the individual process steps. Additional time can help get a dirty part cleaner and extra rinsing will help to remove more of the process chemical. Generally, the phosphating process is somewhat self-limiting, though, and additional time may not produce an appreciable increase in coating weight. That would generally have to be accomplished with a change in the bath chemistry (pH, accelerator content, etc.).

The process tank temperatures should be checked to make sure they comply with the supplier’s recommendations since the chemistry can be effected in one of a couple ways. Having the tank temperature too low may not produce the desired results since chemical reactions usually go faster with higher temperatures. A very general (and somewhat overused) rule of thumb says you can reduce time in half for every increase of about 10°C you make in your process tank. In some cases, you may get little or no reaction below a minimum threshold temperature. That said, too high a temperature can sometimes be as bad as too low a temperature. If the temperature exceeds the recommended guidelines for the cleaning stage, it is possible that some of the surfactants could start to come out of solution when they reach the cloud point of the cleaner. This is due to the fact that some nonionic surfactants will display an inverse solubility with temperature. If this were to happen, a significant amount of the surfactants could be removed with an oil/water separator present on some cleaning systems.

Another important (arguably the most important) aspect to check is the tank concentration. The supplier should have provided a titration and control procedure for use with the chemicals used for the pretreatment process. Too little of the chemical and it will not perform its intended purpose. Too much and it is possible that besides wasting money, you could be causing rejects since the cleaner could have reached a saturation point for a given temperature and could be depositing out on the part you are supposed to be getting clean. This chemical residue may be all that is needed to cause the paint adhesion problems you are experiencing. Ultimately, I would contact you chemical supplier to discuss the problem with them, but not until you arm yourself with the appropriate information concerning the current state of your manufacturing process.

To answer your second question could be even lengthier. However, in the interest of saving some trees, I will keep it to a minimum. The short, evasive answer to your question, “How to check if a metal is clean?” is that it depends. There are several ways to do this, none of which is a perfect, instrumented, referred method to evaluate and discern cleanliness. Ultimately cleanliness (like beauty) is in the eye of the beholder. Your part cleanliness is subject to the subsequent user or manufacturing steps the part will see. For instance, part cleanliness for a steel part that will have a structural weld applied to it are probably less demanding than the requirements for a cold rolled steel sheet metal piece that will be receiving a conversion coating pretreatment and powder coating.

Unfortunately, the production floor cleanliness testing methodologies are relatively crude, but are often better than nothing. Something simple like a water break test will tell you if your part is producing a surface with a low interfacial tension. In short, if the water beads like a freshly waxed car, it has a significant amount of organic residue remaining on the surface. The more the water sheets and wets the entire surface, it is usually cleaner. It is important to use clean water for doing this test, since water from the process line with surfactant or other chemicals can lower the surface tension of the water such that it looks like it is producing a water-break free surface. It is possibly to make this into a semi-quantitative test by assigning a percentage to the amount of the surface that is wetted by the water. The higher the percentage, the cleaner your part is.

Another alternative is something called the white glove test. This is simply wiping the surface of the part with a clean, white cloth and inspecting for residue afterwards. This can be compared to some boundary samples (pictures for example) of known good and bad results so the test can be used for comparison. This test and the previous water break test may not produce the same directional results. For instance, it may be possible to have a good result in the water break test, but do poorly with the white glove test since some of the residue remaining that shows up in the white glove test may be oxide and smut residue. Unfortunately, there still is no such thing as an inexpensive clean-o-meter that can be used on the production manufacturing floor for in-process checks to verify surface cleanliness.