End users are buying metalworking fluids with longer service life and recycling or reusing them over longer periods of time. Naturally, this is influencing how fluid suppliers are formulating their products.
But metalworking shop operators may be unaware of some unintended consequences of using longer-life fluids when contaminants build up over time. Accumulation of chloride ions can cause corrosion of steel alloys. These ions are present in water supplies and even in some types of equipment used to soften or purify water. New laboratory studies reveal at what levels chloride can be problematic and strategies for combating chloride-induced corrosion.
Profits and Pitfalls
According to Alan Cross, senior project engineer with metalworking giant Quaker Houghton, The trend to extend the service life of metalworking fluids and most industrial fluids is across the board in most industries and geographic areas. Companies are moving in this direction to save money, reduce waste, decrease environmental impact and lessen system downtime when fluids need to be replaced.
There are several underlying reasons for these shifts, from the perspective of Buffalo Grove, Illinois-based Angus Chemical. With increasing consumer and societal focus on sustainability, its no surprise that industrial trends are following suit, said Nicole Clarkson, global segment lead for metalworking fluids.
Where we see perhaps the biggest opportunity for extended fluid life is in processes where fluids may be recycled multiple times, such as metal removal processes. The longer a MWF can be effectively used, the less material will need to be disposed of as waste, she continued.
Curbing waste production is an important part of sustainability, Clarkson pointed out. Reducing waste in this case not only decreases the potential environmental impact, but also decreases waste disposal costs, minimizes unnecessary worker exposure and reduces manufacturing downtime.
As consumer and regulatory trends increasingly focus on a products total lifecycle, which encompasses worker safety and environmental sustainability, there will be an even deeper focus on these trends in the future.
Raw material availability is another factor in the drive toward longer-life, recyclable fluids. Certain additives are no longer produced due to environmental risk, and new products are being offered as replacements. Clarkson noted that limited availability of certain biocides can be a problem.
Developments in metalworking techniques are also influencing end user priorities, she noted. Technological advancements in machining equipment, both in performance and in process, are utilizing less coolant or MWF overall, as well as recycling MWF where possible. Formulators must be responsive to the needs of the industry, and fluids must be developed that can meet performance demands with smaller fluid volumes.
The trends of customers wanting longer fluid life and recycling MWFs is changing the way formulators develop products and choose raw materials, Cross told LubesnGreases. The formulas are becoming more complex, and new raw materials are being tested constantly to achieve products that work in the application and last as long as possible in the sump. With formulations that last longer in the sump, new products can be run for longer times and recycled.
Switching to an extended-life fluid would seem to be a very easy decision for MWF end users, cautioned Clarkson. However, not every fluid is suitable to be recycled, and often fluids that possess this performance attribute come at a much higher premium. As a result, MWF end users must determine the overall cost-in-use of long-lived fluids and, in cases where those costs are higher, balance that with their sustainability objectives.
Clarkson noted that long-life fluids present new opportunities for some chemistries that have not been as popular in the past. Different treat rates with these chemistries could create further positive impact on the performance and environmental profile of recyclable fluids for long service life applications.
On the other hand, there are also negative impacts of these trends. For example, MWF volumes are smaller, and some previous additive chemistries are not suitable for these new formulations. Cutting-edge fluids require multifunctional additives with even higher performance characteristics. Developing these formulations takes time as well as extensive field testing.
End users must learn to prevent the accumulation of contaminants and depletion of additives that happen over time with use of long service life MWFs, Clarkson said. Good plant hygiene, proper run rates and consistent make-up fluid schedules can help end users maintain fluid quality. Higher performing fluids and raw materials, as well as changes in equipment and machining processes, also can help remedy these issues.
Cross agreed: As fluid life is extended, various materials and contaminants accumulate in sumps. This is something that end users should monitor.There are no new test methods, but existing test methods are being used more frequently to monitor fluid health in sumps.
Chloride Corrosion
One problem of particular concern is the corrosion of steel alloys caused by chloride ions that accumulate in coolants, especially those with long service life.
Cross explained that coolant is the general term used to refer to a metalworking fluid, typically a metal removal fluid. Coolant comes in a concentrated formulation that end users blend with water. These products may contain oils, emulsifiers, corrosion inhibitors, anti-foaming agents and other components.
Coolant products are classified according to their oil content as supplied. Emulsified oils have at least 50 percent oil, semi-synthetics are approximately 10 percent oil and synthetics have none. End users dilute products to make solutions that contain between 80 and 97 percent water.
The quality of the water used for dilution is a critical factor, especially when fluids are reused and more water is added to replace losses due to evaporation as well as drag-out on parts. Good water quality is the purity or absence of contaminants in water.
According to Cross, chloride corrosion-the oxidation of metal by chloride ions present in water-can take a widespread toll on metalworking equipment. Corrosion can crop up in makeup water supplies, water tanks, water piping, MWF sumps and any place where steel or other iron-containing alloys are wetted by water or MWFs that contain chloride ions, he said. Metal parts that are processed or rinsed with fluids containing high enough levels of such ions are also at risk of corrosion and staining.
Chloride ions can attack metal and form iron-chlorine compounds that can dissolve in water or cause the naturally occurring iron oxide surface layer to peel away from metal. In either case, fresh metal is exposed, more chloride ions attack and pits form.
In a study he presented at the Society of Tribologists and Lubrication Engineers annual meeting earlier this year, Cross performed three groups of laboratory experiments to observe the effects of chloride ion concentrations and metalworking fluid formulations on chloride-induced corrosion.
At the Quaker Houghton laboratory in Norristown, Pennsylvania, tap water was purified by using a commercial ion exchange unit to soften it, or remove calcium and magnesium ions, and a reverse osmosis membrane was used to reduce sodium chloride content to less than 1.0 milligram per liter. Then, sodium chloride was added to the purified water to prepare test solutions of 50, 100, 200 and 400 mg/L chloride.
Steel test panels (alloy 1018) were dipped in the test solutions and hung to dry. Rust was observed within one hour for panels that had been dipped in 200 and 400 mg/L chloride solutions.
A small amount of each test solution (1 milliliter) was placed on other panels, which were then stacked to model parts stacked in metalworking shops. Rust formed within one hour for all four test solutions.
Next, Cross prepared three coolant test solutions by blending 5 percent of emulsified oil, semi-synthetic and synthetic MWF products with each of the four aqueous test solutions. Test panels were dipped and hung to dry or stacked while slightly wet. No rust was observed for the emulsified oil, and corrosion was less severe for the semi-synthetic than the synthetic product.
Finally, Cross used each coolant test solution as he ground one side of a bearing race on a grinding disk for five minutes to simulate metalworking operations and aged the bearings in a non-condensing humidity cabinet at 95 percent relative humidity and 37.8 degrees Celsius for five days. Rust increased with chloride concentration in the coolant test solutions. The emulsified oil provided the best protection against corrosion, followed by the semi-synthetic and synthetic products.
From these tests, it is evident that higher concentrations of chloride ions in coolants can lead to increased corrosion and staining. However, the type of metalworking fluid used can influence or inhibit chloride-induced corrosion of steel. Emulsified oils, which contain relatively high levels of base oil, generally provide more effective protection against chloride-induced corrosion, especially under conditions where chloride ions contaminate coolant solutions. Coolant formulations fortified with rust and oxidation inhibitors can also be effective.
Cross recommended that end users of MWFs only use good quality water that is low in chlorides, as well as low in total hardness. He explained that the chloride level in any fresh water supply can become elevated by run-off that contains road de-icing salts, contaminants from coastal flooding, minerals from underground aquifers and other factors. Municipal water treatment facilities may monitor the level of chloride, but would not necessarily remove it. Kits that measure chloride ion levels are commercially available, he noted.
Furthermore, ion exchange resins used to soften hard water can malfunction and release chloride ions into the softened water.
Reverse osmosis systems are the most effective means to reliably purify water for diluting metalworking fluid concentrates and avoiding accumulation of chloride ions in sumps, Cross emphasized.
Mary Moon, Ph.D., is a professional chemist, consultant and technical writer and is technical editor of The NLGI Spokesman. Contact her at mmmoon@ix.netcom.com or 267-567-7234.