Water is essential to the use of many metalworking fluids—in most cases, the fluid in a machine is 90% water or more—in part because nothing beats water for easy cooling. But hard water can compromise the chemistry of a metalworking fluid (MWF) and ruin its effectiveness.
Consider this post a primer on hard water, what it can do, and how to prevent it from harming your bottom line.
What Makes It “Hard Water?”
Many aquifers contain water that has made its way through limestone or chalk formations underground. This water picks up minerals from those rocks (mainly calcium and magnesium carbonates) and keeps them in solution. Hardness in water is measured in terms of calcium carbonate equivalence. Another term used is “grains per gallon.” These terms sometimes confuse people, but they are related, as one “grain” of hardness is equivalent to 17.1 mg/L as calcium carbonate. If a customer has “20 grain” hardness water coming from their city water source, it is the same as 342 mg/L hardness – very hard water. By definition, hardness is caused by the total divalent ions in solution, most often calcium and magnesium. The USGS defines water above 180 mg/L hardness as very hard, water between 121 and 180 mg/L hardness as hard, water between 61 and 120 mg/L as moderately hard and water with 0 to 60 mg/L hardness as soft.
What Effect Does It Have?
Hard water’s most obvious effect is leaving limescale or salt like residues when it evaporates or is heated, bringing it out of solution and letting it deposit on the surface from which the water is evaporating.
In a machining environment, this can lead to clogged piping or other problems as parts in the machine are coated by scale or chalky precipitates. It can also lead to rancidity, as some bacteria prefer a mix that contains these salty residues.
Hard water, and/or water with high mineral content, including chlorides, contribute a number of problems to the long-term performance and stability of metalworking fluids. Hard water salts tend to destabilize conventional oil in water emulsions, leading to emulsion splitting, the presence of layers of tramp oil, and a decrease in metalworking performance. Minerals tend to build up in metalworking sumps over time, leading to deposits. Water with high chloride or sulfate concentration contributes to increased corrosion in metalworking sumps. Many of these problems can be solved economically by removing minerals with ion exchange or reverse osmosis water treatment.
Can Soft Water Cause Problems?
While soft water is not as problematic as hard water, it can greatly increase foaming, which can obscure the workpiece and interfere with pumps. Soft water is more corrosive than hard water and is not recommended for metalworking fluid makeup.
What Can Be Done About Hard Water?
Hard water may just be an inevitable problem given your shop’s location, especially if you’re located in the Midwest. So, what’s the best way for shops to deal with it? Here are a few options:
- Use an MWF that tolerates hard water—The newest generation of MWFs, such as Chemtool’s NuSol® line, are formulated with emulsifiers that are more resistant to hard water. These formulations are also biostable, so hard water salts won’t cause rancidity. While these qualities will protect the MWF itself, it is a good idea to deal with hard water so that you avoid the problems scale and other residues can cause in machinery.
- Monitor your fluid carefully—Untreated “city water” will often be of sufficient quality when charging a machine with an initial mix. However, hardness can increase in your fluid as evaporation and carryout occur and makeup water is added. Checking fluid regularly will let you head off hardness in individual machines. In fact, a robust monitoring program will create a baseline for each individual machine, allow you to see trends, and take proactive steps.
- Treat your water—Using a treatment setup in your shop will ensure that your water is not only the right hardness but is free of any other troublesome impurities. Chemtool does not recommend softened water for metalworking fluid use. Water softeners add sodium to the water to replace calcium and magnesium, but do not remove sulfate or chlorides which contribute to corrosion. The two most popular options are deionization, which passes water through a resin bed and removes virtually all hard water ions, and reverse osmosis, which removes about 95% of dissolved minerals.