We all know, basically, what rust is and what causes it. What many don’t know, however, is that rust is unique in its properties. Knowing how to deal with and/or prevent rust is very important in many cleaning applications.
The above picture is actually worthy of hanging on a wall as art. In fact, I wouldn’t mind having a 20″ by 30″ print of it framed and hanging on the wall of my den (hint to my wife?). But, when encountered in a cleaning scenario, rust is about as far as you can get from pretty.
Very simply, rust is oxidized iron. It is interesting, however, that just exposing iron to oxygen (like that found in the air, for example) will not result in the formation of rust. Rusting also requires the presence of moisture which, as it happens, is almost invariably also present in the air around us. Rusting, therefore, can occur without the notable presence of liquid water. It is also interesting that iron exposed to ONLY pure water will not rust. There are a lot of things that are going on when iron or an alloy of iron rusts. Far too many to cover in this blog and, since I am not a chemist, it would be best if I left that to someone who knows a lot more about chemistry than I do. What IS important however are some of the properties of rust as we see it in the cleaning world.
- Rusting of iron is NOT a reversible process! Once something rusts, the iron consumed in the rusting process is gone forever from the rusted surface. Try as you might, a part that has suffered dimensional change as a result of rust formation can never be restored to its starting dimension – at least not by any chemical process that I know.
- The volume of rust produced by the rusting process is many many times that of the metal consumed. Thick layers of rust don’t necessarily indicate that the metal beneath has completely disappeared. Although metal CAN be totally consumed by the rusting process, it takes a long time. Even what appears to be a heavy layer of rust, therefore, may not compromise the structural integrity of the remaining metal.
- Rusting, unlike many other forms of metal oxidation, is not a self-limiting process. Oxidation of other metals including copper and certain alloys of iron comprising materials called “stainless steel” are protected from further oxidation once an impenetrable layer of oxidation is formed. Rusting, however, continues to completion.
Whenever we talk about cleaning parts made of iron or an iron alloy that will rust, there is concern about rusting at some point in the cleaning process. This is because the metal is exposed to the very elements (water and oxygen) that allow rusting to occur. Prior to the days of aqueous – based cleaning when parts cleaning was largely accomplished using solvents and vapor degreasing, rusting was not as prevalent or as much of a problem as it is today. Solvents, once used, contained at least a small component of oil left by the previously cleaned parts. This oil provided a thin residual film that prevented the cleaned metal surface from both oxygen and water. Even vapor degreasing left a molecular layer of hydrocarbons on the cleaned surface. This is because hydrocarbon compounds (oils) that vaporized at temperatures lower than that of the cleaning solvent were present in the vapor cleaning chamber along with vapors of the primary cleaning solvent. These low temperature boiling oils were deposited on the surface of the cleaned parts and remained even after the more volatile cleaning solvent evaporated. In short, oils do not evaporate as quickly as solvents and, in many cases, don’t evaporate at all, at least at room temperatures. This is probably a topic for another blog.
Today’s cleaning processes are designed to chemically prevent or inhibit rusting of clean, active surfaces both during and after cleaning. If surfaces DO rust, the result may be parts that are scrap. Upcoming blogs will explore ways to prevent and, maybe, remove thin layers of rust using today’s cleaning technologies.
– FJF –