The type of metal being machined and how that metal is removed from the parent part can greatly affect the stability of the metalworking fluid. This reaction is both electrical and physical and can be explained in simple terms.
First let’s discuss some metalworking fluid basics. A common similarity of emulsified oils (also known as soluble oil), semisynthetic oils or synthetic oils is that these fluids have some portion of chemical compounds that have an inherent negative surface charge. These negatively charged chemical compounds are generally referred to as “anionic” and are identical to the negative electrical charges associated with direct current. Chemical formulators know that a certain amount of anionic chemistry is an important building block in the overall success of the final metalworking fluid formula.
The metal being machined, such as cast and ductile iron, steel, copper, aluminum and nickel alloys in their true metallic form do not carry a significant surface charge. The freshly machined surfaces now release positive charges. The positive charges as a result of the machining process are referred to as cationic and are highly reactive. These active surfaces will eventually go to equilibrium over some period of time (seconds, minutes or days). If no metalworking fluid is present, and in the case of a mild carbon steel part, the result is iron and oxygen coming together, manifested as rust. However in a water-diluted metalworking fluid, these reactive metal surfaces can chemically bind to the anionic chemical in the metalworking fluid. When this reaction occurs, the stability and performance of the fluid diminishes.
Some metal groups are more reactive than other groups. Iron (steel alloys) and aluminum alloys are more reactive than stainless alloys. A simple experiment is to take two identical mild carbon steel plates. Rub one with fine grit sand paper and leave the metal surfaces exposed to a humid environment. The sanded areas on the one steel plate will rust much faster than the surface that was not sanded. This is the same reaction that occurs when a metalworking fluid is exposed to a freshly machined metal surface.
How the metal is machined also plays an important part. Single point turning creates a relatively small surface area as compared to grinding for the volume of the metal (expressed as cubic inches). Polishing metal is more severe than grinding. So, fine grinding of cast iron would create the most active surfaces of metal for the volume of metal removed. Conversely, single point turning of 316 stainless would create less reactive surfaces and thus have less negative effect on the metalworking fluid.
The liquid volume of the metalworking fluid in relation to the total surface area generated in the machining process will determine the failure rate of the reaction. For example, in a large central system with a large volume of fluid and a small surface area being machined will result in a negligible effect on the metalworking fluid.
Formulating the fluid to be stable in this machining environment to minimize the cation/anion reaction has been a challenge for metalworking fluid chemists for years. An end user can minimize the cation/anion reaction somewhat by removing the chips and metal fines and the machined part from the fluid as soon as possible. That is partially why the fluid in well-filtered systems lasts longer than fluid in single sumps with no filtration.
How the fresh metal surface reacts with a metalworking fluid is discussed in greater detail in the STLE Metalworking Fluid Education Courses offered at various points throughout the year. For more details on STLE educational programming, visit STLE University. Courses include the Metalworking Fluid Management Certificate Course which will be offered in early 2013, and courses offered in early May at the 2013 Annual Meeting in Detroit, MI.
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