Corrosion problems and testing of metalworking fluids

Machining oils and fluids are frequently used in metal processing and treatment processes. Preventing metal corrosion and rust is a crucial performance indicator for metalworking fluids.

For ease of explanation, Gami technical engineers note that the term "metalworking oil" used below includes both oils and fluids used in metal machining and those used in metal treatment processes. The term "corrosion" here includes both rusting of iron and steel, and "corrosion" of other metals.

Machine tool "rusting" problem:

Corrosion is a chemical reaction that causes partial or complete peeling, dissolution, or softening of the metal surface. "Rusting" refers specifically to the corrosion of iron and steel. "Corrosion" includes not only ferrous metals but also other non-ferrous metals. Therefore, this article discusses not only the corrosion (rusting) of iron and steel.

Metalworking oils need to address two aspects of corrosion: preventing corrosion of the processed metal objects and preventing corrosion of machine parts (metal parts that perform the processing task). Four types of metals are most common in industry: steel, aluminum, magnesium, and titanium. We will discuss them one by one below.

What factors cause metal rust and corrosion?

Factors affecting corrosion include not only the composition of the metalworking oil itself, but also moisture/humidity, temperature, air quality, whether the oil is contaminated (acids, salts, other liquids such as chemical liquids, other oils, etc.), bacteria, water quality, improper concentration of the processing fluid, or uneven concentration of the processing fluid, leading to uneven concentration contact in different areas of the metal surface, cracks or notches on the metal surface, or galvanic corrosion between dissimilar metals.

Iron:

Iron and steel are the most common metals, inexpensive and strong, but their disadvantage is that they are easily corroded (rusted). In the laboratory, ASTM D4627 is used to test rust prevention performance. Common rust inhibitors in metalworking oils include amine compounds, which are crucial for rust prevention.  When testing, pay attention to checking the amine content. In actual use, factors affecting this performance include poor water quality, high bacterial content, high humidity, and acid content in the air. Aluminum:

Aluminum is lighter than iron, with a density of only about 1/3 that of iron.  Its advantage is that it is not easily corroded. However, under certain circumstances, aluminum can corrode, such as when in contact with certain metal salts (such as chlorides), at high pH values, or when in contact with other metals, leading to galvanic corrosion.

How to test whether metalworking fluid will corrode aluminum metal? The method is as follows:

Take a piece of aluminum, pour some metalworking fluid into a beaker, and immerse half of the aluminum piece in the liquid. Check the corrosion of the aluminum piece by the fluid the next day. Another more complex method is the ASTM F1110 corrosion test (Standard Test Method for Sandwich Corrosion Test), also known as the Boeing sandwich test, which uses a filter paper soaked in metalworking fluid, then sandwiches the paper between two pieces of aluminum, and subjects it to heating and humid conditions for 7 days.

Under the same experimental conditions, the difference is that deionized water is used to soak the filter paper, and the corrosion of the aluminum piece is checked after 7 days. By comparing these two sets of experiments, if the corrosion in the metalworking fluid experiment group is more severe than that in the deionized water experiment group, the metalworking fluid is considered to have unsatisfactory corrosion resistance. During testing, both the corrosion of the concentrated metalworking fluid (mother liquid) and the corrosion after dilution (diluted to the recommended concentration) should be tested. Common corrosion inhibitors for aluminum metalworking fluids include silicates and phosphates.

Magnesium:

Magnesium metal has an even lower density than aluminum (2/3 of aluminum), but it has good strength. Magnesium has two disadvantages: it is flammable and chemically reactive, reacting with water to produce hydrogen gas, which is flammable and explosive. This is particularly important when using water-based processing fluids. The detection method can involve reacting the metalworking fluid with a magnesium piece, collecting and measuring the volume of hydrogen gas produced, thereby determining the strength of the reaction between the metalworking fluid and magnesium.  The degree of corrosion of the magnesium piece should also be noted.

Titanium:

The advantages of titanium are its low density, high strength, and the highest strength-to-weight ratio among metals, as well as its corrosion resistance. However, titanium has poor thermal conductivity, resulting in poor heat dissipation. During processing and cutting, heat tends to concentrate on the cutting surface and contact surface. Titanium metal has excellent elasticity, meaning that during processing, the titanium material is prone to spring back and deviate unless subjected to significant force. Titanium metal is corrosion-resistant, but under high-temperature conditions, it is susceptible to stress corrosion and cracking. The test method for this problem is ASTM F945.

Conclusion: This article provides a brief analysis of the properties of four common metals, discusses the metal corrosion problems caused by the use of metalworking fluids, and the test methods for the corrosiveness of metalworking fluids on metals. Each metal has different physical and chemical properties; therefore, corrosion problems should be addressed based on the physical and chemical properties of the metal during use.