Q. Some of our parts show water spots through the coating after it is cured. You can see the outline of the spot through the film, especially with some of the lighter colors. The film thickness is around 2-4 mils. The powder supplier assures us there is nothing wrong with the powder. What can cause water-spots to show through the coating like this? N.H.
A. Water spots are a film defect that usually results when residual salts leave white rings on the surface of the part after it exits the washing and drying process. These residues are left behind when the part is not thoroughly cleaned or not properly rinsed. The moisture evaporates and leaves a small deposit of salt, much like salt that can show on a car when water beads up after a rain or after being washed. The salt residues can show through the powder coating as a visible line. The root cause could be poor cleaning, no rinsing, poor rinsing, a contaminated final rinse stage or poor incoming city water quality.
The chemicals used to clean and phosphate metal parts prior to powder coating are inorganic compounds that are dissolved in water to create a chemical solution. The amount of solids in a solution can be measured in terms of total dissolved solids (TDS) or it can be measured by conductivity. The unit of measure for TDS is parts per million (ppm). The unit of measurement for conductivity is a micromho or microsiemen. Handheld meters for monitoring TDS can measure ppm or micromhos or both. To convert ppm to micromhos, multiply by a factor of two. Dirty water that is high in solids content such as iron, calcium or other minerals will be high in ppm or conductivity. Filtered water such as demineralized, deionized (DI) or reverse osmosis (RO) water, measures less than 25 ppm, often less than 10 ppm.
Tap water that is considered to be “hard-water” may measure more than 300 ppm. Hard water that has evaporated could cause a water-spot effect. Hard water sometimes is softened by replacing the calcium in it with sodium. Softening will help the plumbing, but it does not make better rinse water because the solids are still in it and still can be left behind after evaporation. The use of either RO or DI water purification systems, with or without softening, removes all or most of these salts or ions from the water. RO removes the solids by pushing water under very high pressures through a membrane filtration system. Deionization chemically removes the solids (ions) from the water through an ion-exchange process.
Final rinse tanks need to be monitored and TDS levels should be kept lower than 300 ppm. The actual water quality is somewhat dependent on the product and the environment in which it will be used. A high-quality product that will be used in a harsh environment needs exceptional rinsewater quality. If incoming water quality is poor, the use of a DI or RO halo rinse is sometimes added at the end of the final rinse. Nozzles that deliver just a fine mist, wet the part with “solids free” water, and the rinse stage is replenished with clean water. The halo is installed in the exit vestibule of the washer, and it can be added to an existing system.
Make sure the parts are thoroughly clean and water-break-free (no beading of water) when they exit the cleaner stage. Make sure you also are using clean water and getting a complete rinse. That should end the spotting problem.