Rinse Water Monitoring and Process Design

For high-quality surface finishing, you must have a top-notch critical cleaning process. It is essential to remove oils, metalworking fluids and fluids used to prevent corrosion or other surface damage during storage. In the euphoria associated with finding a cleaning agent/cleaning chemistry that actually removes the soil without damaging the metal, it is easy to forget that once that cleaning agent has done its job, you typically have to remove it. The cleaning process is only as good as the rinse step. Inadequate or inefficient rinsing can be costly.

In monitoring rinse water, one issue is what the rinse water “looks like” after use when it is ready to be removed from the process tank. Another issue is water conservation. You have to adhere to environmental regulations covering when and how water can be released from the system. If you monitor, it may be possible to reduce water added to the rinse system, which can lead to cost savings. However, the focus for manufacturing is the quality of the rinse water that comes into contact with the part being rinsed, and the goal is to understand how water quality impacts product quality and performance. Monitoring is a tool, not an end in itself. Getting the right quality rinse water in contact with the part involves starting with optimal process design, overall process control, maintenance and employee training.

Consider a small cleaning system consisting of one wash tank containing an alkaline cleaner and three rinse tanks. The cleaning process consists of someone manually submerging the parts in each tank. In such small systems, one simple approach to monitoring rinse water quality is to check the pH. Too often, we find that the cleaning solution is, say, pH 9 and the final rinse tank is also pH 9. It may be that the parts are effectively being cleaned in three wash steps and zero rinse steps. Before setting up a rinse water monitoring program, perhaps process design is needed to manage carryover into the rinse tank and to establish rinse water quality. It can be truly illuminating to measure the pH of the incoming rinse water. The pH is considered a secondary standard by the EPA, and the listed range of pH for drinking water is 6.5–8.5.

Let’s review the pH scale, which is logarithmic, much like the Richter scale for earthquake measurement. A one-point difference on the Richter scale can make the difference between a gentle wake-up call and a colossal event. A one-point difference in pH can shake up your yield, and not necessarily in a good way. Some municipal water that is considered acceptable for drinking may be unacceptable in metal cleaning and metal surface prep. If you are using untreated tap water to rinse “fussy” alloys like brass, rinse water that starts out above pH 8 could contribute to surface quality problems. These problems can be magnified by even small amounts of cleaning agent residue.

Monitoring pH. Sometimes, we are told the pH is being monitored, but there are still problems. Assuming that the wrong pH rinse water is a cause of surface quality issues, the question arises as to how pH is determined. pH paper is relatively inexpensive and rapid. However, relying on pH paper to monitor process baths, even rinse baths, can provide a false sense of security. Interpreting color is subjective; some people are better at discriminating subtle differences in color than others. Also, as a rule of thumb, the fewer things there are in a mixture, the less detective work it takes to characterize that mixture. Rinse tanks are not necessarily simple; they may contain residue of metals, salts and metalworking fluids. “Fully formulated” cleaning agents have lots of “stuff,” like organic and inorganic chemicals, in them that often interfere with the pH paper, giving misleading results. Using a pH meter is more likely to give a “true” reading of the level of alkalinity of acidity.

Monitor the right thing. When monitoring rinse water, some manufacturers automatically assume that monitoring rinse water pH is the “be-all and end-all.” But knowing the pH may not be enough. What’s important to monitor depends on what is acceptable relative to the part being cleaned. Many aspects of rinse water can be monitored. Examples include total dissolved solids, oil level, conductivity or even a specific chemical or class of chemicals. Choosing the right thing to monitor means understanding product requirements.

Barbara Kanegsberg and Ed Kanegsberg Ph.D. are industrial product cleaning consultants with BFK Solutions LLC Independent Consultants, and industry leaders in critical/precision and industrial product cleaning. For questions or to receive their newsletter, contact them at 310-349-3614 or info@bfksolutions.com.