What causes rusting on machined parts?

2. Bacterial Growth

Presence of bacteria in coolants is usually indicated by the foul smell which machine operators are familiar with. Anti-rust additives are consumed by these bacteria resulting in the rusting of machined parts. To avoid this, coolants should be maintained at right concentrations so that the biocides can take effect. pH should also be maintained and chips at the bottom of the sump should be removed where they usually thrive to discourage bacterial growth.

3. pH

To prevent growth of bacteria, coolants should be maintained at pH levels of 8.9 and above. pH meters are normally used to quickly monitor the pH of the coolant in the sump.

4. Excess Tramp Oil

When slideway oils and hydraulic oils leak into the coolant sump, rust formation can occur. Since these oils are already used and may have been oxidized, contaminated workpieces might not be protected against rust. Skimming the oil manually using a dipper or installing an automatic oil skimmer usually does the job.