Corrosion control is one of the very important properties of metalworking fluids and metal treatment fluids used in industry. Fifteen years ago it was estimated that corrosion problems cost the US economy more than a quarter of a trillion dollars or 3.1% of the US Gross Domestic Product at that time.
Metalworking and metal treatment fluids must be able to control corrosion on both the metal part being manufactured, as well as the metals that comprise the machinery conducting the operation (turning center, grinder, stamping press, washer, etc.). Four commonly used metals in industry today are iron in the form or cast iron & steel, aluminum, magnesium, and titanium. In this article we will take a brief look at important aspects of each.
Factors that affect corrosion control, besides the composition of the fluids being used include moisture/humidity, temperature, air pollution, fluid contaminants (acids, bases, salts, micro-organisms), water quality in the plant, fluid concentration control, concentration differences across the metal surface, crevices in the surface, and contact between dissimilar metals (a galvanic couple).
Iron is by far the metal with highest usage due to its relatively low cost and high strength, but the main disadvantage is that it corrodes quickly. Even if a manufacturer is machining or forming aluminum parts, the machine tools will be mostly iron and steel. So rust control is a critical property of metalworking and metal treatment fluids. Within the laboratory, a commonly used method to evaluate rust control is the ASTM D4627 cast iron chip rust test. Typical corrosion inhibitors used include amines, amine-borates, and amine-carboxylates (amine salts of short chain fatty acids). As you can see from this short listing, amines (nitrogen containing organic compounds) are critical to obtaining good rust control. Among the in-plant conditions that can cause rust problems despite the metalworking fluid are poor quality water, high microbial counts, high humidity, and acidic air contaminants.
Figure 1. Cast iron chip corrosion testing in progress.
Aluminum is a light weight metal, about one-third the density of steel, and it is remarkable in its resistance to corrosion. None the less, it does corrode under certain conditions, such as attack by salts (for example, chlorides), high pH, and when in contact with other metals. A simple way to evaluate the potential for a fluid to stain aluminum is to place a clean metal strip of the alloy in a beaker of fluid so that it is about half immersed, and leave it in the fluid overnight. A more complex procedure is ASTM F1110, “Standard Test Method for Sandwich Corrosion Test”, or commonly referred to as the Boeing sandwich test. This involves putting a piece of filter paper wetted with the fluid in question between two aluminum panels and placing the “sandwich” at elevated temperatures and cycled humidity for seven days. Any stain worse than that caused by deionized water is a failure. The fluid must pass both in concentrate form and at the recommended dilution. Commonly used corrosion inhibitors for aluminum include silicates and phosphate esters.
Figure 2. Example of bimetallic or galvanic corrosion. A round steel specimen (left) and cast iron specimen (right) were placed on an aluminum strip wetted with metalworking fluid.
Figure 3. Two aluminum panels with wetted filter paper on left, and on the right is the completed “sandwich” taped together and ready to hang in humidity cabinet.
Magnesium is the third most commonly used structural metal, following steel and aluminum. Its density is two-thirds that of aluminum, but it has good strength. Its two main drawbacks, however, is that it will burn if it gets hot enough, and it reacts with water to generate hydrogen gas which is explosive. The latter property is particularly problematic if a water-based fluid is being considered. One test method is to place magnesium chips in the fluid, and then collect and measure the volume of hydrogen gas generated during a set period of time. MWF products can be ranked by the volume of hydrogen produced, as well as by degree of stain on the magnesium surface.
Titanium has the highest strength to weight ratio of any metal, and thus is being used increasingly to reduce weight and improve fuel economy in aerospace, military, and industrial applications. It is also used for medical and dental implants. Titanium is a poor conductor of heat, so heat does not dissipate quickly. Therefore, during cutting operations the heat of deformation is concentrated on the cutting edge of the tool and on the tool face. Titanium also has a low modulus of elasticity, making it more “springy” than steel. This means that the work-piece tends to move away from the cutting tool unless heavy cuts are maintained. Although titanium is relatively corrosion resistant, it is susceptible to stress corrosion cracking at elevated temperatures – a particularly important failure mode for aerospace applications. ASTM F945 is a procedure for evaluating stress-corrosion cracking. Pre-stressed titanium specimens are exposed to the test solution for one minute, allowed to dry, and then baked under one of two conditions. Method A is at 900ºF for 8 hours while Method B is at 500ºF for 168 hours. A 100 ppm sodium chloride solution serves as the positive control.
(Left) Figure 4. Titanium bars are stressed by bending into a U-shape.
(Right) Figure 5. Microphotograph of stress crack in titanium bar following ASTM F945.
In this brief article we examined the importance of corrosion control in metalworking and metal treating applications relative to four different metals commonly used in industry. Each metal has unique physical and chemical properties, as well as unique corrosion issues. Common corrosion testing methods have also been presented.
Jerry P. Byers is a Past President of STLE, and works at Cimcool Fluid Technologies. You can find his contact information in our member directory.
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