Selecting the proper water quality for your metalworking fluid can be a challenge.
Wait a minute, how can this be? I can certainly understand that salt water from the ocean would not be good for mixing with metalworking fluids, but there is plenty of fresh water around so why be concerned?
Some scientists describe water as the universal solvent. These scientists claim that more chemical compounds dissolve into water than any other liquid. Some of these dissolved chemical compounds have a detrimental impact on metalworking fluids. Hardness is a very common contaminant in water. Hardness is the amount of calcium, magnesium and iron in water. The level of hardness is expressed as mg/L as calcium carbonate (CaCO3) or in grains per gallon (GPG.) It is generally agreed that water above 100 mg/L as CaCO3 is considered “hard”. So why is this issue of hardness a problem?
Some of the chemical compounds used in water miscible metalworking fluids have anionic or negative surface charges. Calcium and magnesium have positive surface charges. So the calcium or magnesium in the water combines with these negatively charged chemical compounds resulting in a decline in the performance of the metalworking fluids. Some metalworking fluids when mixed into water with hardness levels even as low as 100 mg/L can have a detrimental effect on that fluid. Since metalworking fluids typically evaporate water during the manufacturing process, the hardness builds, since only pure water evaporates. The levels of calcium and magnesium build up over time and can actually split the metalworking fluid into two phases, rendering it useless.
Another problem is chlorides in water. We need to distinguish between chlorine used for sterilization of potable water sources, and chloride (such as in sodium chloride, i.e. table salt.) The amount of chlorine used to sterilize potable water is generally so low (dose rate of 1 to 2 mg/L) that the residual chlorine levels are well below the danger level. However, chlorides are naturally occurring in many water supplies. Levels of 10 mg/L are generally tolerable but levels about 50 mg/L and above can cause serious problems. The same way hardness builds in a metalworking fluid through evaporation is the same way chlorides build, that is, by evaporating the pure water and leaving the residual chlorides behind. When chlorides build to 350 mg/L and if you are processing mild steel or cast iron parts, rust will be an ongoing battle. As chloride levels approach 600 mg/L, rust on steel and cast iron surfaces can appear in seconds and is commonly referred to as flash rust.
Using purified water would seem to be the obvious direction, and in most cases purified water is the right choice. Two common methods for purifying water in metalworking plants is by deionization (DI) or reverse osmosis (RO). Both of these processes can produce water that is less than 10 mg/l hardness, and reduce chloride content to 1 mg/L or less.
Make up water from DI or RO has drawbacks. The first drawback is cost. It could cost as much as $ 0.01 - $ 0.02 USD per gallon to treat the water to high purity levels. While this does not sound like much, some plants consume millions of gallons of water per year. Also, depending on the level of hardness and the type of treatment, these DI or RO systems waste upward of 40% of the feed volume as rejected salt and contaminant brine.
The second drawback is storage and distribution. Purified water can easily support biological growth, so continued sterilization with ultraviolet light or slug doses of hydrogen peroxide may be necessary. Purified water that is not mixed with a metalworking fluid will corrode steel surfaces, so storage tanks should be made from high density polyethylene (HDPE) and piping from chlorinated polyvinyl chloride (CPVC.)
The third drawback is increased foam. If the supplier of metalworking fluid formulates his product to be stable in hard water and you switch to DI or RO water, then foam can be a serious issue. In some cases the foam is so difficult to control that end users artificially add “hardness” back into a system as a foam control measure. This can be just a chemical addition such as a calcium compound or mixing hard water with DI or RO water.
A fourth drawback of adding DI or RO water is the potential of short term poor tool life. Although calcium and magnesium challenge the stability of metalworking fluids by negative reaction, the by-product of this negative reaction is the formation of calcium or magnesium insoluble soaps. These insoluble soaps are essentially identical to soap scum in a shower or bathtub. They have some lubrication value and therefore can have a positive impact on tooling performance. Adding only DI or RO water eliminates this lubricating soap reaction, and the tooling performance can diminish. In cases with aluminum, magnesium and steel machining, these metals can react to form insoluble soaps, and the tooling performance slowly improves over time after the first fill with DI or RO water.
Note well that some water sources such as deep wells can vary in hardness and chloride levels from day to day or season to season. So if you are trying to evaluate the contaminants in your incoming water, it may take a series of samples over an extended period of time to identify the peak levels of hardness and chlorides.
Remember that it only takes a few seconds for undesirable consequences to occur to your manufacturing processes and fluid stability when you add low quality water to your metalworking fluid.
This article explains only a small part of the effect on water on your metalworking processes. The subject of water contamination is discussed in more detail in the STLE Metalworking Education courses, found at the Annual Meeting and offered throughout the year.
John M. Burke is Director of Engineering Services at Houghton International, Inc. He can be reached at firstname.lastname@example.org.
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