How do we control the slime build up in the vicinity of anode cells? S.A.
More than likely, the slime that you refer to is the result of bacteria in your system. Bacteria contamination in electrocoat systems has been a problem almost since the beginning of electrocoating. The older anodic paint technology was even more susceptible to bacteria contamination than the cathodic systems. However, as the cathodic systems have continued to reduce VOC’s and HAP’s, their susceptibility to bacteria contamination has increased. The solvents and hazardous components in the paint and bath have acted as natural bactericides.
Your electrocoat system, being organic in nature, is an excellent “host” for bacteria growth. Sources of the bacteria are the paint feed, dragout from the pretreatment system or the water make-up to the bath, post rinse and/or pretreatment. Airborne bacteria are another source that is more difficult to identify.
Water supply has been the most common source of bacteria and deionized (DI) water is much more susceptible than reverse osmosis (RO) water. While RO systems will not generate any bacteria, contaminated water, storage tanks and piping systems, if previously used for DI water, are places that bacteria can get into the RO water. If you have (or are considering) replacing a DI water system with a RO system, any common tanks, valves and/or piping should be replaced or sanitized.
As with any quality or contamination problem, identifying the problem and locating the source(s) is the first step. Your paint and/or water equipment suppliers should be able to supply you with test equipment (kits) or do the testing for you. All locations of bacteria need to be identified before treatment begins to keep the contamination from spreading back to areas already treated. For example, if the bacteria is “killed” in the paint tank but is still present in the post rinse or pretreatment system, it will migrate back to the paint tank.
Several methods of treatment are available when bacteria contamination is identified. Treating (or eliminating) the source is always best. Then treating the actual contamination should follow. If you have a DI water system, UV light treatment has been fairly successful. If you have a storage tank (DI or RO), it should be tested and/or treated. A second UV light may be needed between the storage tank and the pretreatment and paint systems.
Bactericides are the best method for treating the paint (bath and post rinse) or pretreatment (final rinses) systems. The use of bactericide has to be done with care for several reasons. Many of them are hazardous materials and may require waste treatment after use. The idea that “if a little is good, a lot is better” is definitely not the right approach. Bacteria are very tenacious and can develop immunity to some bactericides.
As always, getting your suppliers (paint, pretreatment and water equipment) together and working with you and each other to solve the problem is best. A joint meeting can often reduce “finger pointing” that may result if you consult with them one at a time.
Once the bacteria has been destroyed, continued testing is important. Monitoring regular bath (and post rinse) parameters (conductivity, pH, temperature, etc.) and keeping them within the desired limits are important, as one or more of them being out of spec may be an indication of bacteria contamination. Treating bacteria as soon as it is identified is important, as growth can be quite swift. Testing and bath and post rinse parameter monitoring is critical for all systems, even if there is no evidence of bacteria growth. By the time you see it, it is usually a serious problem.