Rust on ferrous (Iron-based, Steel-type) metals, after undergoing machining and grinding, can be troublesome. Some companies spend considerable man-hours and extra-processes to re-work the pieces, to de-rust, while other companies outright treat rusted parts as scrap. Bottom line is that rusted pieces cost a lot of money.
While there are too many possible causes for rust on ferrous metals, we will focus on those factors related to water-based machining and grinding fluids.
1.) Concentration – It may see terribly obvious, but a fair number of machine shops encounter rust occurrence from running the coolant at low concentrations. At low concentrations, the oil-level (oil inhibits rust) in the emulsion, as well as the anti-rust additives, in proportion, will also run low.
Solution – Ideally, emulsions and semi-synthetics should run at > 5%, while synthetics from grinding should be at > 3%.
2.) Low pH – as previously stated, water-based machining and grinding fluids should run at pH levels of 8.9 and above. Such pH levels deter rust, as well as bacterial activity.
Solution – Top-up fresh fluid regularly to boost pH, or if necessary, dose the sump with a pH booster
3.) Bacteria – not only does bacteria produce foul odors that machine operator have come to hate, bacteria causes rust on parts as it depletes the functional additives in an emulsion or solution, additives such as the anti-rust package found in coolants.
a. always run at the right concentrations to have sufficient biocides in the system
b. maintain the right pH to deter bacterial growth
c. remove chips at the bottom of the sump as bacteria thrives in that area
d. use clean containers and fluid handling devices
4.) Excessive Tramp Oil – tramp oil leaks from slideways and the hydraulic system, if excessive, can contaminate the workpiece and cause rust. While minerals do not cause rust per se, the leaked oils are ‘used’ already, possibly oxidized, and thus do not contain sufficient protection against rust anymore.
Solution – skim the tramp oil at regular intervals, manually with a dipper (tabo) or foam, or use an automatic belt skimmer
Other causes include an acidic environment (machining near pre-treatment processes with acids), high-humidity conditions, over-long storage after machining or grinding processes without the use of rust-preventive oils, and poor water quality with high dissolved solids.