The pH scale measures how acidic or basic a water diluted metalworking fluid is. The pH scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic. A pH greater than 7 is basic. Most metalworking fluids are formulated to be in the basic or alkaline range. A new, unused metalworking fluid at a concentration of 5% would typically have a pH between 8.5 to 9.5. Most modern metalworking fluids buffer out to a pH of 8.8 – 9.2. So why do metalworking formulators design their fluid to operate in this general range? There are many positive benefits for controlling pH of a metalworking fluid within this relatively narrow range.
If the metalworking fluid is a semi-synthetic or emulsified oil, the emulsifiers are generally performing better at this pH range (or even higher) than at a lower pH. Another benefit of high pH is that bacteria that commonly grow in water diluted metalworking fluids do not grow as well at pH of 9.0 versus pH of 8.0. Finally, some steel alloys can corrode at pH levels below 8.0, so keeping the pH near 9.0 can lessen corrosion on steel alloys in some cases.
However going above the pH of 9.2 can cause several issues. Dermatitis or skin irritation can occur as the fluid approaches pH 9.5 or higher. Also eye, nose or throat irritation can occur from mist generated during the machining operations if the pH is above 9.5. Certain metals do not tolerate high pH such as some aluminum alloys or yellow metals (brass, copper, bronze.) Aluminum or yellow metals can stain at highly alkaline pH levels, or even dissolve.
pH alone does not tell the whole story of the acid or basic nature of your fluid. Reserve alkalinity is an important concept when attempting to control the pH of the fluid. For example, consider raising the pH of distilled water to 9.0 with sodium hydroxide. This solution would have essentially no reserve alkalinity. That is, as easy as it is to raise the pH to 9.0 with sodium hydroxide, only a very small amount of an acid solution is required to lower the pH to 3.0. If you adjust the pH of distilled water to 9.0 with triethanolamine, this solution will have more “staying power” at that pH, thus reserve alkalinity. In the manufacturing environment, good reserve alkalinity means less frequent adjustment of pH is required versus using sodium hydroxide and adjusting the pH possibly every day.
What chemicals can I use to control pH after the metalworking fluid is in the machining environment? Many shops still use sodium hydroxide because it is readily available and cost effective, but as mentioned before, offers no reserve alkalinity. Sodium borate or salts of boric acid are simple to use and offer some buffering. And if you add too much sodium borate to a solution, it will not raise the pH much higher than 9.2. However boron chemistry is under scrutiny in Europe due in part of the REACH Directive. Certain European countries have regulations on boric acid compounds and human exposure. Amines are another good choice for pH control and include such chemicals as amino-2-methyl-1-propanol, monoethanolamine and triethanolamine.
Please take special note that if the pH of your metalworking fluid falls to 8.0 or lower, and you rapidly add any alkaline compound such as sodium hydroxide or a common amine to adjust the pH upwards to 9.0, you run the risk of releasing ammonia gas (also called an ammonia blush). This "ammonia blush" can be quite irritating and in some cases essentially intolerable to the workers in the immediate metalworking environment. To be safe, always consult with your metalworking fluid supplier before setting up a pH control program or adding other chemicals to your fluids. Remember to always wear the correct personal protective equipment (PPE) and be properly trained when handling any of the chemicals that we have discussed above.
So how do I measure pH? There are pH paper “strips” that can read to narrow pH levels (+/- 0.4.) pH strips can only be used once and can give a general indication of the pH of the metalworking fluid. pH meters can range in cost from $ 100.00 to well over $ 1,000.00 depending on the accuracy of the meter. When choosing a pH meter, it should read to at least two places past the decimal point, such as 9.15. pH meters should be calibrated daily and at a minimum, once per week. pH meters do not measure reserve alkalinity. In order to measure reserve alkalinity, you will have to have a small lab set up and be able to perform colorimetric titrations. Again this is a good time to work with your metalworking fluid supplier for the proper equipment and titration procedures.
Keeping logs on chemical additions and pH trends can tell you a lot about the stability of your metalworking fluid. pH of a metalworking fluid should be relative stable over time and slow to change. The change will generally be from a high alkaline value (9.0) to a lower value (8.5 or lower.) Rapid changes in pH are a clear danger signal that something (chemical, mechanical, biological, thermal) is acting on the fluid. This rapid change in pH should be investigated as soon as possible.
In summary, controlling pH will produce obvious benefits as mentioned above. Once set up with the proper equipment and training, controlling pH can be readily straightforward and cost effective.
John Burke is Director of Engineering Services at Houghton International. He is an STLE member, and you can find his contact information in our membership database.
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