Contamination Part 4
UNDERSTANDING CONTAMINATION IN METALWORKING FLUIDS
PART 4 - CAUSES AND EFFECTS OF WATER RELATED CONTAMINATION
by Joe Eppert
Previous articles in this series on contamination of metalworking fluid (MWF) systems have discussed tramp oil and machining particulate contamination. In the current discussion, the effects of water related contamination on MWFs will be discussed. As will be shown, the impurities in the water used for dilution or volume make up will greatly affect the performance of the MWF. It should be noted for this discussion in particular that our focus will be on metalworking fluids, and does not directly apply to the use of straight oils.
In general, the impurities of water we are mainly concerned with include hardness ions, chloride ions, and sulfate ions. Hardness ions include the calcium and magnesium content of the water and are generally expressed in a concentration measure of parts per million (ppm) “as calcium carbonate." The ideal water hardness for a MWF environment is 80-125ppm as calcium carbonate (3). We will discuss what problems are faced outside of this range. We would also like to keep chloride and sulfate levels below 150ppm, as above this range they begin to have an effect on the MWF functionality as well.
Typically MWFs are sold in concentrate form and are to be diluted with water prior to use. In the majority of cases, dilution ratios are such that the resulting mixture is anywhere from 85 to 95% water. The remaining portion contains numerous additives to facilitate lubrication, corrosion resistance, biological stability, etc. An advantage to using water as the main component in the fluid is that the MWF has excellent cooling abilities. However, water is prone to evaporation, and as MWF volume is lost, periodic additions will have to be made. So, we are left with two modes of water addition to a MWF system, that from dilution of the concentrate and that from volume make up. The effect of evaporation is quite significant if we are using a water source with impurities in it, as these impurities can increase 3 to 5 times the original amount in a 30-day period (3).
As has been the case in previous discussions, the effects of water impurities on MWF functionality can be broken up into chemical effects, interacting effects, machining performance effects, and health and safety effects. Each of these areas will be discussed in turn.
Chemical Effects of Water Related Contamination:
Chemical effects of high hardness levels include stability of the MWF (for emulsion type MWFs) due to emulsion splitting and causing a variation in emulsion droplet size (5, 6). In general, as the hardness increases, stability problems become more likely. As ions build up in the MWF and react with particular emulsifiers, insoluble calcium and magnesium soaps can be formed (1).
The ionic strength of the water may cause the emulsifier being used to become weakened, and therefore cause emulsion stability to decrease. As the stability of the emulsion decreases, the MWF performance will also tend to decrease (4). However, there has been a reported case that for a particular MWF hardness actually improved the lubricity of the fluid (8).
Interacting Effects of Water Related Contamination:
The primary interaction effect of water impurities with other components in the MWF concerns the growth of microorganisms. Very similar to humans, microorganisms require a number of minerals for optimal growth. Not only can the water itself act as a source of microorganisms to the MWF system, but also any impurities in the water can facilitate bacterial growth. Additionally, many of the organisms able to live in water can also live quite well in MWFs, making the effective control of water impurities quite important (2). The use of controlled water sources, for example deionized water, is one generally accepted method for controlling growth of bacteria in MWFs (5).
In experimental studies, it has been found that the amount of water hardness and salt addition greatly affect microorganism deterioration of MWFs (1, 2). In addition, it was found that the use of distilled water results in a low level of microbial growth, and that the low amounts of ions in the MWF were most likely the limiting factor (1).
High inorganic salt levels in the MWF will lead to the deterioration of the MWF due to sulfate reducing bacteria. In particular, it has been noted that the rate of MWF spoilage is related to the level of inorganic salts (1).
Machining Performance Effects of Water Related Contamination:
As far as machining performance is concerned, water impurities will lead to corrosion problems, foaming problems (if the water hardness to too low), decreased performance of recycling equipment, decreased tool life, and poorer workpiece surface finish.
High levels of dissolved ions can increase the corrosivity of the MWF by increasing the electrical conductivity of the fluid (3). Even at low levels, chlorides and sulfates (which are non-hardness ions) have the potential to cause corrosion problems (6, 7). “High” levels of chlorides or sulfates are generally greater than 150ppm (3).
If the water used to dilute the MWF is soft (less than 80ppm), foaming can become a concern (3). However, if the water is too hard, chemical and interaction effects are greater. So, we are left with a desired operating range for hardness as 80ppm to 125ppm. If outside of this range, we should expect to see some problems related to the hardness of the water.
If the hardness is at such a level that insoluble precipitates form, the performance of the equipment used to filter or clarify the MWF can be decreased (6). Most notably, this decreased performance of the recycling equipment will result in increased maintenance costs to operate the equipment (7).
For tool life and workpiece surface finish, many references note that the quality of the water will decrease tool life and give a poorer surface finish (6, 7). Although an exact reason for this relationship is not clear, it has been noted that the use of deionized water for MWF dilution does result in improved surface finish (2).
Health and Safety Effects of Water Related Contamination:
Although there may not be any significant health and safety effects of water related contamination, the cleanliness of the operation will be effected by the formation of insoluble soap scum and residue that occurs at high levels of hardness ion concentration (1, 3, 7). This scum and residue can be responsible for coating workpieces, coating the machine, and plugging filters.
Water related contamination:
decreases the stability of emulsions
facilitates microbial growth
can lead to corrosion problems
impacts tool life and surface finish
can cause problems for recycling equipment
is responsible for formation of scum and residue
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Bennett, E.O., 1974, “Water Quality and Coolant Life,” Lubrication Engineering, Vol. 30, No. 11, pp. 549-555.
Byers, J.P. ed., 1994, Metalworking Fluids, J.P.B., ed., Marcel Dekker, Inc, New York.
Organization Resources Counselors, Inc., Management of the Metal Removal Fluid Environment, 2nd Edition.
Rossmoore, H.W., 1975, “Extending Cutting Fluid Life,” Manufacturing Engineering, Vol. 75, No. 5, pp. 27-28.
Silliman, J.D. ed., 1992, Cutting and Grinding Fluids: Selection and Application – Second Edition, J.D.S., ed., Society of Manufacturing Engineers, Michigan.
Sluhan, W.A., “Water is Water... Or is it?,” Modern Machine Shop.
Yang, C.C., 1979, “Effects of Water Hardness in the Lubricity of a Semi-Synthetic Cutting Fluid,” Lubrication Engineering, Vol. 35, No. 3, pp. 133-136.