Tankside additives, when used in a properly managed coolant system, can help to hold the line on costs associated with the purchase and disposal of metalworking fluids. They can increase the capabilities of an inexpensive product and stretch the life of an aging system.

The best way to ensure the longevity of coolants, of course, is to eliminate sources of contaminants. Even the most carefully designed formulations eventually go bad if a system is abused. Excessive tramp oil, hydraulic leaks, cleaning chemicals, the dirt, gum, sludge, varnishes and bacteria from dirty machines, or foreign matters such as coffee, floor sweepings, solvents, food, paper, cigarette butts and human waste all contribute to a coolant's demise.

In the field, additive technology can enable a fluid to cope with unique industrial environments. This is true both for oil-based and synthetic products. Unless basic mechanical problems are first corrected, however, the remedy is only short term at best.

Tankside additives include: biocides, corrosion inhibitors, defoamers, emulsifiers, pH adjusters, deodorizers, dyes, emulsion breakers, and flocculents, plus other additives for smut reduction and lubricity. Understanding their capabilities can improve fluid management.

Left uncontrolled, microbial contamination will destroy both emulsified oils and synthetic coolants. When fluids leave the supplier, they are usually biostatic. In the field, however, they become contaminated and support biological growth. As a result, biocides have become the most frequently used of the tankside additives. They include both bacteriocides to kill bacteria and fungicides to kill fungi.

Bacteria are the most frequent source of rancidity which leads to so-called "Monday morning odors." Anaerobic bacteria, for example, are sulfate reducers. A by-product of their metabolism is hydrogen sulfide, the rotten egg smell. In extreme cases, this has led people to walk off the job. Bacteria may also be a factor in operator dermatitis.

Fungi, on the other hand, create large sheets of matter that can plug up filters, hoses and lines. As industry turns increasingly to synthetics and less to oil-based fluids, fungus problems are becoming more widespread.

Bacteria and fungus spores are everywhere. Since there is really no good way to keep them out entirely, control begins by starving to death their microbial growths. In other words, nutrients should be limited by fixing leaks and reducing the amount of petroleum or tramp oil as quickly as possible. Usually this is done with a skimmer, a coalescent filter, or centrifuge.

Only those biocides which are registered and approved by the U.S. Environmental Protection Agency should be used. They range in price from inexpensive little tablets to products costing up to $150 a gallon. Concentrates should be handled only by trained people, in conformance with the manufacturer's directions. Once in the system, however, these products are extremely safe.

Corrosion Inhibitors
Rusty parts and machinery continuously plague manufacturers. Closely related to this problem is the buildup of "white rust," or aluminum oxide. Other metals also dissolve in coolant and redeposit on parts and tools. When these conditions occur, it is over a period of time, as a synthetic coolant breaks down and some of its additives become depleted or deactivated.

Coolants can also attack high copper alloys such as brass. Certain diamond wheels that are used for cutting and grinding, for example, are weakened by the effects of coolants on their bronze binders.

Closely related is the problem of cobalt leaching from tools. Removal of cobalt from the matrix of cemented carbide, for example, leads to lower mechanical strength of the cutting tool insert and less appealing surface finishes on the workpiece. It can also contribute to hazardous levels of cobalt ions in the metalworking fluid which must be removed prior to disposal.

A variety of corrosion inhibitors, rust preventatives and metal deactivators are available. Their use as tankside additives can greatly reduce these problems.

Foam presents a number of problems, because it will not stay in a machine, and it can shorten tool life.

Usually foaming occurs in oil-based products on the initial charge when the coolant is clean. As the system ages a bit and accumulates tramp oil, the problem often goes away.

Certain operations, such as grinding, generate a great deal of foam. Therefore, soluble oil products are not recommended for them. Mechanical problems, such as an aspirating seal, can also produce foam.

Foam shortens tool life because the bubbles present alternating liquid and air. As a result, the part being machined is actually heat treated.

Two types of defoamers are available. Those having a silicon base are the most effective. However, they are unsuitable for many industrial environments, particularly those near paint finishing systems. For these applications, a non-silicon base is recommended.

Over a period of time, bacteria degrade the chemical emulsifier systems that are using tap water. The result is a buildup of hardness in the system and water soluble salts that crack an emulsifier. A buildup of metal can also crack an emulsifier. This causes curdling, the separation of oil and water.

Prevention through good system maintenance is the best way to avoid curdling. However, tankside emulsifiers can provide a temporary solution.

There is no substitute for putting in a new charge. When emulsifiers are added in the field, they seldom produce a good, tight emulsion. These tankside additives also emulsify tramp oil. However, they can bring back a coolant and extend the life of a system until it is given a new charge, such as following a holiday shutdown.

pH Adjusters
Maintaining pH goes a long way toward controlling rust and bacteria. It should be between 8 and 9.5. Occasionally, however, the pH of a coolant will drop too low.

Sometimes, a pH adjuster can be as simple as adding a caustic potassium hydroxide or caustic sodium hydroxide. This will raise the pH, but will not maintain it. Most likely, a buffering agent will be used to bring it up and maintain it.

To prevent dermatitis, pH should be under 9.5. Higher concentrations defat the skin. When 9.5 is exceeded, it is most likely the result of an alkaline cleaner entering the system.

Sometimes deodorizers are added to coolants to make them smell better. A better solution is to correct the underlying cause.

Like deodorizers, dyes can mask biological growth. Unless a dye has a functional value in a coolant, it is usually best to leave it out. Dyes can create disposal problems unless they are removed from the water before it goes to a treatment plant. Only those approved by the EPA should be used.

Emulsion Breakers
Even though most synthetics will reject oil, tramp oil can sometimes emulsify mechanically. To expedite its removal from an active system, additives can be put into the coolant either during operation or later on, when the user wants to crack the oil and coolant apart.

Flocculents provide a means of removing physical contaminates other than oil. When magnesium is machined with a water based coolant, for example, it forms magnesium oxide which cannot be removed by filtration.
A flocculent allows this and other metal wastes to be pulled together in a mass. Depending on the specific gravity, they either sink and are filtered out of solution, or they rise and are removed with a skimmer.

Antismut Clarifiers
Machining residues, often referred to as smut, not only form a messy layer on machine and tool surfaces, they also reduce tool life and mar surface finish. Basically, smut is made up of tramp oil and metal fines.

As the coolant carrying the tramp oil floods the tool and workpiece in the cutting zone, it splashes and collects on the tools, toolholders and surrounding machine surfaces. Eventually the water in the coolant evaporates leaving behind the smut which can dry and become insoluble.

Use of an antismut clarifier permits the oil to rise to the surface. Once the oil is removed, the metal fines fall to the bottom much more quickly, and they can be removed.

Lubricity Additives
Just as corrosion inhibitors are often depleted or deactivated as a system ages, so too are lubricity additives. A number of tankside additives are available to restore the lubricity of a product, or to increase the lubricity of a general purpose product.

Chlorinated paraffins can be added to a soluble oil to dramatically improve tool life and surface finishes. Lubricity additives are also available for semi-synthetic and synthetic fluids.

Unique applications involve alloys that have been heat treated or are extremely difficult to machine. For example, a screw machine job shop may normally use a light sulfurized straight oil for most applications, but periodically this will not suffice. For example, certain jobs running stainless steel or other alloys may break tools, unless lubricity is increased. Additives are mixed with the screw machine oil. When the job is finished, the oil gradually returns to normal.

When you add can be as important as what you add. If tankside additives are going to chemically react, this is a critical consideration. On the one hand, for example, a biocide and a pH adjuster will normally cancel each other out if they are added at the same time. On the other hand, certain combinations of biocides have a synergistic effect. They work better together than they do by themselves. As a rule of thumb, however, anything that involves reactive chemistry should be added separately.

By keeping a coolant system properly managed and in good mechanical condition, tankside additives can solve a variety of metalworking problems. While their effectiveness is usually temporary, they can be more cost effective than alternative solutions.


This "how to" article, published in the Jan. 1990 issue of Manufacturing Engineering, is typical of articles that Carl Parks writes for a readership of engineers -- in this case -- process engineers. It appeared under the byline of the client's technical service manager and was based on an article originally written by an Australian. It was assigned by Earl Lapp of Lapp Associates in Birmingham, Mich.