The Right Water Chemistry: Understanding The Aqueous Influence Upon Metalworking

The performance of coolants and other water-soluble metalworking fluids depends on the quality of the water in the solution. Many variables must be controlled to ensure that this water meets the grade.

Article From: 1/25/2004 Modern Machine Shop,

When is water not pure H2O? When it is contains any material that is not water. This condition is almost always the case in manufacturing plants that are using water to dilute metalworking fluids. Metalworking fluids need good quality water to perform at maximum effectiveness. When lubricant/chemical suppliers recommend a water-soluble product for a customer's plant, they must determine the water quality as well as the fluid's application requirements. Two customers with identical processes may have widely different experiences with the same metalworking fluid because their water quality is different. The effect of water quality can influence any application using water-soluble fluids, including drawing/forming compounds, coolants, quenching fluids, cleaners and corrosion inhibitors. When water evaporates from any of these fluids, negative effects may increase.

Analyzing water is critical for revealing and understanding how the water used at a customer's facility can affect the performance of a metalworking fluid. It is important to know water chemistry in depth. Water can constitute from 80 to 95 percent by volume of the diluted metalworking fluid mixture in individual sumps or central systems.

When developing any metalworking process, a person must understand all the variables that will influence it, including the water used for both charging and replenishing systems. Contaminants in this water can be as detrimental as other foreign fluids, such as tramp oil or grease.

Parameters

Aqueous chemical and physical properties such as pH, conductivity, alkalinity, total hardness (calcium and magnesium levels), other ion and elemental levels, surface tension, turbidity, ECA (electrokinetic charge), foam characteristics, and microbiological levels (bacteria, fungi, yeast and algae) may all influence metalworking fluid performance. These properties can affect corrosion protection of the metalworking fluid; residue properties; foam; emulsion stability for semi-synthetics and soluble oils; susceptibility to microbiological attack; charge density of both true solutions and emulsions; filtering properties; and wetting. Ion analysis (ICP) should be used to monitor process water regularly to establish a baseline because water quality can change dramatically during the year as seasonal precipitation patterns change. The parameters that should be evaluated continuously are pH and hardness.

The pH of water indicates whether it is acidic (pH of 0.0 to 7.0), neutral (pH of 7.0) or basic or alkaline (pH of 7.0 to 14.0). Most water used commercially in the United States exhibits a pH range from 6.4 to 8.5, depending upon the original source and type of pretreatment conducted by the local water authority. Water used for metalworking fluids should exhibit an optimal pH range from 7.0 to 8.5.

Total hardness indicates the presence of dissolved minerals and their salts in water. Predominant ions are calcium and magnesium. Other ions contributing to hardness include iron, zinc, aluminum, potassium and silicon. Total hardness is reported in parts per million (ppm) of calcium carbonate (CaCO3). It can also be reported in units called grains. One grain of hardness is equivalent to 17 ppm of calcium carbonate. Water hardness is typically defined using the following scale:

TOTAL HARDNESS (ppm)          EVALUATION
0 - 49                                             Very soft
50 - 124                                         Soft
125 - 249                                       Medium
250 - 369                                       Hard
370 and above                                Very hard 

Hardness can readily affect metalworking fluid performance. Soft water may degrade the performance of all fluids by promoting the formation of foam. This condition is especially likely when using synthetics in grinding operations and semi-synthetics and soluble oils in both machining and grinding applications. Foam can drastically impair fluid performance by contributing to poor wetting and coverage properties, which diminish lubricating, cooling and proper film coverage for in-process corrosion protection. Foam can also hamper fluid detergency, making it more difficult to handle and filter swarf. When dense foam forms, it can lessen the filtering capabilities of a system by interfering with indexing mechanisms and by creating poor filter beds. Foam can also suspend tramp oils, preventing skimmers and other mechanical devices from removing them effectively. Tramp oils can act as a matrix, becoming finely suspended on a dense bed of foam. This development further intensifies a dense foam layer. Excessive foam can also lead to housekeeping issues by causing system barges and return lines, such as floor troughs, to overflow. Foam can also cause pump cavitation, creating excessive wear and premature mechanical failure.

As hardness increases, it can adversely affect the stability of semi-synthetic and soluble oil emulsions. The formation of hard water soaps from calcium and magnesium ions and anionic components (typically fatty acid-based emulsifiers) can radically alter emulsion particle size. This development will rapidly lead to scum formation and lose emulsions in which cream and free oil are present. Both semi-synthetics and soluble oils are harmed by calcium soaps. Semi-synthetics are especially hampered by high magnesium levels. Hardness can build up in water stored for use as well in metalworking fluid systems as a result of evaporation. Many systems can loose from 5 to 25 percent of their water on a daily basis, depending upon system size, openness to plant environment, time of year, geographical location, plant conditions (air temperature and circulation patterns) and metalworking fluid temperature as the water circulates. Adding hard water to make up for loss resulting from evaporation will cause levels of hardness in the system to rise rapidly. As a result, emulsion instability in semi-synthetic and soluble oils will lead to corrosion problems, susceptibility to emulsification of tramp oils and microbiological attack, poor tool life, inferior surface finish, foam, and filtering problems. Optimal hardness levels must be maintained by using the appropriate blend of water types for proper metalworking fluid performance.

Water Terms

Alkalinity is another indication of the overall health of both the water and the metalworking fluid in use. Two types of aqueous alkalinity are measured.

Permanent alkalinity is referred to as P alkalinity. It is expressed as ppm of calcium carbonate and is a measure of the carbonate ion level (CO3- 2).

Total alkalinity is referred to as M alkalinity. It represents the combined alkalinity of P alkalinity (carbonate ion level) and the bicarbonate ion level (HCO3-). M alkalinity is also expressed as ppm of calcium carbonate.

Conductivity is a measure of the water's ability to conduct an electrical current. It indicates the total level of dissolved minerals and salts present in the water. As conductivity increases, water quality decreases. Conductivity measurements do not differentiate between minerals or salts and can be used to detect both the presence of contaminants in the system and the build up of hard water soaps. Conductivity is expressed in µmhos or microsiemens.

Several other parameters should also be monitored, and the results should be recorded in an aqueous database.

ECA (electrokinetic charge) is an indication of the charge strength of the water (degree of anionic or cationic charge).

Surface tension, expressed in dynes per centimeter, reveals how water and aqueous solutions "wet out" and cover the surface of the tooling and workpiece. Metalworking fluid chemistries can lower surface tension so that aqueous and fluid films offer better coverage and performance.

Turbidity measures the degree to which water is opaque or clear. Usually expressed in ntu (nephroletic turbidity units), it is determined on a turbidity unit using transmitted light. Turbidity indicates the level of suspended solids in the water or aqueous solution.

Total ion analyses (ICP) tracks the level of cations and anions (measured in ppm) in an aqueous solution. This test can be used to measure initial harmful ion levels in water, to detect the contaminants or formulation components, and to do selective depletion analyses. Important ions that should be tracked in the incoming water include chlorides, sulfates and phosphates. Chloride ions (Cl-) can contribute to corrosion problems, especially at levels above 25 to 40 ppm. Sulfate ions (SO4-2) can also contribute to corrosion problems and can support the growth of certain bacteria. Phosphates (PO4-2 and others) can lead to foam problems, alkalinity issues and bacterial growth.

Finally, bacteria and fungi can grow in a metalworking fluid, causing poor performance. Microbiological contamination often leads to lubrication, corrosion and fluid stability problems. Microbiological contamination can lead to health problems, such as respiratory irritation or dermatitis, among plant personnel exposed to the fluid. This growth can be monitored via a number of methods including biostrips, plate counts and dissolved gas methods such as HMB. It is very important to monitor this parameter and to respond quickly by controlling incoming water quality, adjusting coolant concentration or adding the appropriate biocide, thus preventing a serious crisis.

Water Types And Treatment Regimens

Good water chemistry is essential for the long life and proper performance of metalworking fluids. Sufficient volumes of good quality water must be available on demand for charging systems and for restoring the makeup of a solution. It may be necessary to install storage tanks to maintain adequate water quantities to meet all production requirements. Types of water and treatment programs are summarized below.

Tap water is readily available, but it comes from a variety of sources. It is often used as the initial source for metalworking fluid systems. Tap water chemistry can vary widely depending upon its original source, geographical location and pretreatment conducted on site or by the local water authority.

Softened water has passed through an exchange resin system. Calcium and magnesium ions are exchanged for non-hardening sodium ions. This process results only in a change of ions and no change in the total quantity of dissolved minerals. The sodium ions do not promote the formation of insoluble soaps. However, softening agents do not remove corrosive ions such as chlorides. Softened water is NOT recommended for regular use in metalworking fluids.

Demineralized water processes represent the most efficient treatment of hard water. Demineralization actually removes the dissolved minerals. The two most common and cost-effective methods are deionization and reverse osmosis. Deionization removes minerals by passing the water through a mixed resin bed. Two ion exchange resins selectively remove both cations and anions. Cations are replaced by hydronium ions and anions are replaced by hydroxyl ions. Mixed bed units are extremely effective in reducing hardness levels to almost zero ppm. Such water exhibits a zero ECA value (neutral ionic charge). Resin beds need to be flushed or backwashed on a regular basis to remove all contaminants and to prevent microbiological interference. Beds also need to be regenerated on a regular basis. Reverse osmosis forces water through a semi-permeable membrane under high pressure and varying flow conditions. This process can remove as much as 95 percent of dissolved minerals. Reverse osmosis units are often used in conjunction with a water softener pretreatment stage. Filters need to be flushed and replaced as necessary. Microbiological contamination also needs to be regularly monitored.

New Developments

Water quality has always been important. However, two recent developments have made water quality even more urgent.

  1. Coolant formulators are often confronted with changes in raw materials because of economics and sources of supply. For example, a U.S. sulfonate plant closed recently. The chemicals produced there were used as emulsifiers and corrosion inhibitors. This closing has forced many coolant formulators to change their products. The new formulas may not be as effective in all water quality conditions.
  2. Regulatory compliance and the cost of waste treatment have driven users to make metalworking fluids last longer and extend times between replacement, giving water contaminants more time to build up and create problems.

Formulators must now enhance their products for higher levels of stability and use newer technology additives. They must communicate regularly with their customers to monitor performance and meet the demand for more affordable fluids.

Follow The Guidelines

Good water quality is essential for the proper performance, biostability and sump life of metalworking fluids. Aqueous databases should be maintained and major parameters should be monitored for all water sources. A variety of treatment methods are available for producing safe and effective water for both charging and replenishing a metalworking fluid system. Regardless of treatment methods or fluid system capacities, the following aqueous guidelines should be practiced and strictly enforced:

Appearance: Clear and water-white, free of all debris
Odor:  None
pH:  7.0 - 8.5
Hardness: 125 - 200 ppm
Total Alkalinity: 25 - 100 ppm
Turbidity: < 5 ntu
Bacteria: None
Fungi (both yeasts and molds): None
Chlorides: < 20 ppm
Sulfates: < 40 ppm 
Phosphates: < 40 ppm

A balanced treatment program is crucial for effective water use. Users should contact their fluid formulator for help in determining their water quality and for recommended treatment options. Tap water can often be used for charging systems and treated water (preferably by deionization or reverse osmosis) can be used to make up for evaporation. Aqueous databases should be maintained and reviewed on a regular basis. Water chemistry should always be reviewed and product compatibility should be evaluated if required.

When it comes to water-soluble metalworking fluids, water should not be taken for granted. Knowing and monitoring water quality can prevent problems and improve any plant's productivity.

About the author: Bob Trivett is a senior chemist at Pico Chemical Corporation. He is responsible for formulating various metalworking compounds and for servicing customers. Mr. Trivett has more than 20 years of experience in the chemical/lubricant industry for metalworking applications.

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