Whatever type of metalworking fluid (MWF) is used in a shop, its performance depends on chemical additives. Forgive the cute phrasing, but each added additive adds something essential to the qualities of the fluid, and helps the end user produce a better product at a lower cost. Each additive either makes the fluid itself better at lubricating or cooling, or adds something the lubricant won’t have by itself, such as when a rust inhibitor is added to a water-based MWF.
Some additives are easy to identify and define; others are more esoteric and need some explanation. Let’s look at substances commonly added to modern metalworking fluids and their benefits. There will be plenty of overlap in the descriptions, so we’ll list them alphabetically.
These prevent or retard the growth of bacteria and fungus in fluid and machinery.
Additives containing boron are common because they’re excellent at inhibiting both corrosion and biological contamination. However, compounds containing boric acid can be troublesome for operators and in terms of disposal, so many MWF producers offer boron-free alternatives.
These may be composed from a wide range of compounds (including esters and fatty acids), but all perform the same function: They provide an additional lubricant layer directly on the surface of the workpiece as they bind to it through molecular attraction.
While the function of these substances is obvious from the name, it’s worth noting that the anti-corrosion additives a shop uses should be a match for the types of metal they process. Various corrosion inhibitors will often share chemical properties because their main job is to prevent oxidation, but they can’t be used interchangeably because of the differing properties of the underlying metal. Something described as a rust preventive will be well suited to ferrous materials; if an additive is described simply as anti-corrosion or a corrosion inhibitor, users and distributors should make sure what metals it’s designed for before deploying it.
Water based MWF formulas depend on emulsifiers to keep the oily components of the fluid in suspension as small particles.
If you see that an ester has been added to an MWF, you can be certain it’s there to act as a boundary lubricant. These compounds are formed when an organic acid reacts with an alcohol.
Extreme pressure additives
In modern high-speed machining, workpieces are subjected to astounding pressures (up to 200,000 psi) and to temperatures as high as 2,000 degrees at the point of contact. These highly engineered boundary lubricants bond in a layer on the metal’s surface for extra lubricity and protection.
These compounds are named for the fact that many of them are found in animal fats. They’re found in a wide range of consumer products such as soaps and cosmetics, and are commonly added to straight oil MWF’s. They can act as corrosion inhibitors, emulsifiers or increase lubricity. Some act as boundary lubricants.
Sometimes oxidation is a good outcome. Stainless steel, for example, remains corrosion free because an ultra thin layer of chromium oxide isolates the rest of the metal from atmospheric oxygen. The process of creating this oxide layer is called passivation. Sodium silicates are an example of one type of passivation additive, though they can cause instability in semisynthetic fluids.
When choosing or recommending a fluid, it’s important to match the user’s requirements to the fluid category, but also determine what additives will help them produce the best product. The more advanced the engineering of the base fluid and its added components, the more confident you can be of a great result.