[This article first appeared in the November 2013 issue of Clean Source]
Yellow, Orange, Gold – Rust! Beautiful as autumn colors, desirable as a patina on some sculptures, terrible on manufactured parts.
Corrosion compromises product quality, both by direct impact on surface quality and by contributing to contamination. In critical cleaning, cleaning agents and cleaning equipment can be sources of corrosion.
Corrosion is like entropy; it’s like water flowing downhill. The oxide of a metal is at a lower energy state than either the pure metal or an alloy. You can’t avoid corrosion, but you can minimize rust by controlling the environment.
Break the Square
Outsmarting corrosion is like outsmarting flammability. Picture a fire triangle. A fire needs a fuel, an oxidizer, and an ignition source. Think of corrosion as a square. You need an anodic component that supplies electrons, a cathodic component that consumes electrons, a conducting path that connects them, and an oxidizer.
To prevent a fire, remove one component from the “fire triangle.” To stop corrosion, remove one component from the corrosion square.
Breaking the square, and more important, keeping the square broken is sometimes easier said than done. It needs a little detective work and a bit of preventive action.
Cleaning equipment can itself be a source of corrosion. It should go without saying that using carbon steel is false economy for equipment used for aqueous processes and also for equipment used for solvent processes. The use of carbon steel is a periodic problem, so, I guess it doesn’t “go without saying.” Designers of cleaning equipment sometimes decide to economize. If you are purchasing cleaning equipment, new or used, check the specifications carefully. Even with well-enclosed systems for solvent cleaning, in our experience, use of carbon steel is unwise.
Process equipment can entrap sources of corrosion. This applies to spray nozzles and immersion baths. One aspect of “buying quality” in cleaning equipment is assuring rugged construction. For precision cleaning processes, process baths should have coved corners, not welded corners. It is too easy to blame the cleaning chemistry or the cleaning process; and of course both can be sources of cleaning equipment degradation. Even corroded tools used during equipment fabrication can contribute corrosion. Sometimes you can identify tooling introduced corrosion by the pattern.
Fixturing can trap corrosion products. Fixtures like baskets may degrade because they are repeatedly subjected to the cleaning process. Because corrosion of fixtures is often gradual, it is too easy to let the problem creep up on you. Therefore, it’s a good idea to inspect fixtures on a regular basis and perhaps to have a set schedule for replacing them.
Care taken at the final assembly won’t undo corrosion introduced earlier in the process. Review industrial cleaning processes to make sure the process is not performed in corroded equipment. Corrosion prevention should become part of GMP. If you use contract metal fabricators, again, it is probably less expensive to select your partners with quality in mind.
More about corrosion: Barbara Kanegsberg and Ed Kanegsberg, “Must it Rust? Causes, Consequences, and Control of Corrosion,” Controlled Environments Magazine, May, 2007. (For a PDF copy, contact us.)