Rust is a potential problem whenever ferrous parts are being cleaned. There are several options available to prevent or eliminate rusting both during and after the parts cleaning process. Steel will rust if it is exposed to oxygen and moisture as discussed in the blog Some Things You Should Know About Rust.
Although iron oxide (rust) gets by far the most attention, many other metals are also subject to oxidation of one form or another. In some cases the oxidation is called “corrosion” or “tarnish.” The “recipe” for oxidation is metal in the presence of moisture and oxygen. Almost invariably, increased temperature accelerates the oxidation process. Since aqueous cleaning is all about water to both clean and rinse and since temperature enhances both cleaning and rinsing and since drying with heat is an integral part of many cleaning processes, it would seem that the cards are stacked in favor of oxidation no matter what. Fortunately, today’s cleaning processes are designed to not only prevent oxidation during but also after the cleaning process.
The simple and obvious solution, of course, is to prevent the “oxidation triangle” of metal, oxygen and water. During cleaning, this can be accomplished using chemicals which “tie up” the oxygen making it unavailable to oxidize the metal. A comprehensive discussion of the ways that this “chemical barrier” can be created would require a greater knowledge of chemistry than I possess. My simple explanation is that chemistry formulations designed for cleaning metals prone to oxidation are formulated in such a way that any oxygen present reacts with the cleaning chemistry more readily than it does with the metal being protected. Since there is no oxygen available, there is no “oxidation triangle.”
In some cases, oxidation preventing chemicals known as rust inhibitors (RI to those in the know) or corrosion inhibitors are part of the cleaning chemistry formulation. Rust or corrosion inhibiting formulations are often available to be added to cleaning chemistry formulations that do not already include them. Any additive should be checked for compatability with the cleaning chemistry before use. Chemical suppliers will be able to recommend appropriate additives for use with their cleaning chemistry formulations.
Preventing oxidation in the cleaning process is only the beginning of the battle against the “oxidation triangle.” The prevention of oxidation during rinsing is frequently more difficult than it is during the cleaning process. During the cleaning process, a chemistry of some sort is taken for granted. The goal of most rinsing is to use the cleanest water possible. There are a couple of ways to prevent oxidation during rinsing. Many cleaning chemistries formulated with oxidation inhibitors are designed so that a molecular film of the protective formulation remains on the parts after cleaning. This molecular film, although often not detectable, prevents oxidation through the rinsing and drying stages. Also, as the rinse is used, the carry-over of cleaning chemistry on the parts from the cleaning tank to the rinse adds a very small amount of cleaning chemistry containing the oxidation inhibitor to the rinse.
Useful Tip - Rust, corrosion or other oxidation is sometimes noted on the early runs of parts through a newly drained and refilled cleaning system or in a cleaning system with a rinse that is replenished at a rapid rate. To prevent this oxidation, a very small amount of the mixed cleaning solution can be added to the rinse (I have seen cases where 1 cup in a 200 gallon rinse did the trick) to provide the needed oxide inhibition in the rinse.
Finally, there are many oxidation inhibitors that are formulated to be added directly to the rinse. The concentrations are such that there is no residue of consequence left after rinsing and drying. In flowing rinses, a metering device is often used to maintain a consistent concentration of the oxidation inhibitor. These oxide inhibitors have varying degrees of effectiveness. Some provide protection for an hour or so while others prevent oxidation for several days to several weeks depending on conditions – protection is shorter-lived in hot and/or humid environments. The prevention of oxidation beyond several weeks usually requires additional measures which will be discussed in upcoming blogs.
- FJF -