After machining parts, manufacturers have to clean them. Once clean, the parts may be painted, assembled, or stored with a rust inhibitor coating. That makes cleaning crucial. Poor cleaning could lead to rust, uneven painting or failure in assembly. Many manufacturers do not have the time or budget to rapidly change out (1-day to 1-week intervals) cleaning tanks; if you are fortunate to have short intervals, little maintenance is required as there is limited time for problems to arise. However, you should still perform basic tests to ensure your cleaner is working properly.
For manufacturers with longer tank life, there are 2 main properties to monitor to ensure the performance of your alkaline cleaner: pH and concentration. When the pH and concentration change, the efficacy of the cleaners can decline and lead to more frequent change-outs and greater cleaner consumption. Basic monitoring can prove to be highly cost effective and extend tank life.
pH Testing
The ability to remove soils declines as the cleaner’s pH drops. The recommended pH for cleaning non-aluminum parts is > 9.0 and > 9.5 for aluminum parts. Additionally, many corrosion inhibitors have a pH-dependent solubility curve. As the pH drops, less inhibitor is incorporated into the solution and the more likely corrosion on cleaned parts can occur. These inhibitors start to drop around 10.2 pH, but do not cause significant issues until the pH drops below 9.5.
Maintenance programs typically attempt to adjust the pH back into range after falling out rather than monitoring the pH for remaining in range. We recommend the latter method as this ensures the pH does not drop dangerously low and potentially renders the solution unusable. Trying to keep the pH between 9.5 (or higher) and 10.5 is ideal. Adding a caustic to boost pH could cause problems as you will get a false concentration reading. If you plan to adjust the pH, have a qualified on-site analytical chemist perform the adjustments.
To measure the pH, pull a sample of agitated solution and wait for it to cool to room temperature. Once cooled, use a narrow range pH strip to test the pH. Higher temperatures can render pH tests unreliable and measuring in the tank could capture the pH of contaminants on the strip.
Concentration Testing
The other key test involves testing the concentration using titration. Titration is used to find the tank strength (concentration). Titration involves adding an acidic reagent to a know volume of tank solution until a defined pH is reached. This can be done simply by adding drops of a reagent to the tank solution. This process involves a dye that changes colors at the defined pH endpoint. This is useful for finding the concentration with a +/- 2% tolerance.
This method is rather crude, but can be highly refined by having a trained chemist use lab equipment to run the test. Lab equipment is required for tests that require tighter tolerances than +/- 2%. Look for a constant concentration over time. If the concentration drops, some likely suspects are: cleaner carry-out, leaks, overly aggressive oil skimming, pH degradation by contaminants, and excessive raw material stripping. If the concentration increases, some likely suspects are: excessive add-back of cleaner or the presence of metalworking fluids. Sometimes soil could affect the strength reading and could indicate rising concentration or stable concentration when the strength is actually dropping.
Takeaways
The pH and concentration tests should be conducted each shift in the manner described above. Investing in lab equipment may prove worthwhile if you have stringent tolerances. If the concentration is too low, add some cleaner to put the solution in the proper range. If too high, add water to bring solution within the proper range. pH adjustment is not recommended unless you have a trained chemist on-site. For more information on cleaner management, contact Twin Specialties and ask about our “Coolant Management Guide” that includes tips and information for aqueous cleaners.