When ground parts start to rust, the first reaction is to increase the coolant strength. This usually starts a vicious cycle of increased abrasive wear and parts out of tolerance, with the subsequent changes in abrasive specification and new trials.
Rust is caused by low pH, (refer to the technical report of pH) poor coolant control and the depletion of rust inhibitors from the coolant.
Rust can also be caused by:
1. A Slow Drying Rate.
Anything that can be done to speed the rate of drying will help. Rack the parts, reduce the tote box loads or move parts to a warm, dry area.
2. Bacterial growth of acids in coolant.
Bacteria cause the formation of acids in the coolant solution, which will lower the pH. When coolant becomes foul, it must be changed completely.
3. Suspension of fine metallic particles in the coolant.
As an example, fine iron oxide is introduced when grinding rusted parts. When these particles are deposited on a freshly ground surface, a galvanic cell is formed, which will start electrolytic corrosion.
With proper care and control, rust can be eliminated without upsetting the operation.