Understanding the basic elements that cause rust is the first step in preventing rust
October 7, 2013
Applying a corrosion prevention product to steel isn’t enough. If the workpiece is contaminated with something as minor as fingerprints or steel fines, corrosion can get a foothold. Understanding the components that make up a corrosion cell, and how a corrosion cell works, is necessary in learning how to prevent corrosion from getting a start.
As a metal fabricator, most of your focus is on changing the shape of a metal workpiece, whether you’re cutting, bending, end forming, piercing, notching, machining, or some other process. Most of these processes require an oil-, solvent-, or water-based fluid to prevent friction, which in turn prevents overheating or premature wear.
Another consideration, one no less important than fabricating the workpiece, is preventing corrosion. Some fabricators rely on the metalworking fluid to provide both in-process and final corrosion protection; others use a final process to apply a short- to long-term corrosion preventive. Either way, corrosion prevention agents provide a necessary function. Without protection, the iron (Fe) in the steel interacts with oxygen (O) in the atmosphere, causing the steel to corrode.
Whether the corrosion takes the form of red rust (ferric oxide, Fe2O3) or black stain (ferrous oxide, Fe3O4), the process is similar: Oxidation of the metal is linked to reduction of other constituents in the process, including the metalworking fluids.
Ferrous metal corrosion is the oxidation of iron metal from Fe to Fe+2, further to Fe+3, caused by electrons flowing from an anode (a point of positive polarity) to a cathode (a point of negative polarity). A common battery uses a similar process to carry electrical current from one terminal to the other. Corrosion control processes stop the flow of electrons or disrupt the chemical reaction at the cathode or anode.
Rust Requirements. Three components or constituents are necessary for rust to form:
Six common conditions can turn any piece of steel into a corrosion cell (see Figure 1).
A few key points to keep in mind:
Water-soluble machining and grinding fluids provide temporary corrosion protection. However, fabricators can’t rely on these to prevent corrosion because the duration of protection needed varies from fabricator to fabricator; some need just a few hours of protection until parts go to the next process, while others store the parts for weeks. The storage and coolant conditions are critical factors in determining how long the fluids provide corrosion protection.
The factors that affect the duration of corrosion protection include:
Knowing about upstream manufacturing processes and fluids used in those processes is a help in understanding how to handle the second factor, surface cleanness. What sorts of metalworking fluids have been used on the part? Has the part been stored between manufacturing steps? How has it been handled? If the part has metalworking fluid residues or if it has been stored in an area with fines and dust, surface cleanness is an issue that must be addressed.
Third, after coating but before packaging, good handling practices are necessary to maintain the protective film’s integrity. Gloves are necessary to prevent the oils in workers’ skin from coming into contact with the steel.
Fourth, the fluid delivery system must have adequate capacity to wet the parts thoroughly and must be maintained to deliver a consistent quantity of corrosion preventive to the parts. A good filtration system—one that minimizes the sizes and quantities of fines and the levels of tramp oils, chlorides, and sulfates—extends the fluid’s ability to prevent corrosion. Also, the fluid’s concentration must be maintained at the correct level, which should be measured with an instrument more precise than a refractometer.
Fifth, the packaging to enclose the parts must be of sufficient quality and in good condition, not torn or damaged, to prevent direct access to the coated parts.
Finally, the storage environment must be controlled to prevent gross fluctuations of temperature and humidity (less than 15 degrees F and less than 10 percent change in relative humidity in a 24-hour period).
A number of short-term and long-term tests can measure corrosion protection. All of these tests are designed to mimic real-life applications under accelerated conditions. Be aware that the interpretation of the test results can be just as important as setting up and controlling the conditions of the tests.
Metal Removal Fluids. The chip test is used to assess the interaction of the metal removal lubricant and metal chips that are generated. If the chips are not consistent relative to alloy type, chip size, and cleanness (for example, no fines on the chips), the results will be inconsistent. Most chip tests involve a fixed amount of chips covered by a measured amount of coolant. The wet chips are then set on filter paper or metal blocks to determine the rust potential. Most chip tests last a few hours.
Corrosion Prevention Fluids. Corrosion preventives have more stringent requirements and better-defined test methods than metal processing fluids. Some of the more common tests involve cabinets that control the temperature and humidity. In addition, the handling of coated test panels must be controlled relative to the amount of corrosion preventive that is applied to the surface. Most testing is conducted with duplicate or triplicate panels and the water source must be pure to eliminate any contaminants (chlorides or sulfates) that could affect the results.
When you first investigate a rust issue, it is important to learn the metal surface’s exposure history; you need to trace back through all of the processes to determine where the corrosion began. The investigation should involve every process and fluid that contacts the parts. Only by tracing the entire process do you have a chance at determining the application that has the greatest impact on the corrosion problem. In addition, the fluids involved in the process should be evaluated for fitness for use relative to fresh fluid.
Cause-and-effect diagrams can help you find the root cause (see Figure 2 and Figure 3).
Analyzing the fluid provides an indication of the corrosion preventive’s effectiveness. Seven tests measure the fluid’s inherent characteristics—acidity, moisture, dirt, percent solids, calcium, viscosity, and specific gravity. An eighth evaluation, the copper corrosion test, is a subjective measure of how the fluid stains copper.
According to the “Corrosion Costs and Preventive Strategies in the United States,” a 2002 study commissioned by the Federal Highway Administration, undertaken by CC Technologies Laboratories Inc. and sponsored by NACE International, the direct costs of metal corrosion in the U.S. total $276 billion annually. To put this into perspective, it amounted to more than 3 percent of U.S. gross domestic product.
Corrosion comes with another cost. A manufactured component or assembly that fails or requires a remedial corrosion prevention treatment results in customer dissatisfaction. Using comprehensive corrosion control methods is critical in minimizing both types of costs.