Why Your Cutting Fluid Stains Aluminium and How to Prevent It

Aluminium staining from cutting fluid is one of the most frustrating problems in machining: the dark, grey or black marks on parts are almost always irreversible, meaning quality rejections and scrapped material.

This problem has different causes to steel corrosion. Here we explain what is really happening and how to prevent it.

Aluminium does not rust: it corrodes through alkaline attack

Unlike steel, aluminium does not form iron oxide (rust). What occurs is an alkaline attack: aluminium reacts with alkaline media (high pH) forming aluminate, which manifests as dark, black or greyish stains on the surface.

Specific causes of staining

1. Cutting fluid pH too high

This is the main cause. Most cutting fluids formulated for steel have a working pH between 9.0 and 9.5. For aluminium, that pH is already aggressive.

2. Cutting fluid not formulated for aluminium

Many workshops use the same fluid for all materials. Aluminium requires specific formulations: no chlorine or active sulphur, with non-ferrous metal inhibitors, and controlled pH below 9.0.

3. Excessive concentration

Higher concentration means higher alkalinity. With aluminium, excess concentration is worse than too little.

4. High pH mains water

If your mains water already has pH 8.0+, the final emulsion pH will be even higher.

5. Extended contact time

Parts left submerged in cutting fluid for hours are much more vulnerable than parts removed immediately after machining.

How to prevent aluminium staining

1. Use aluminium-specific cutting fluids: vegetable-based fluids like Eqo-max 711 are an excellent choice
2. Measure pH weekly: maintain strictly between 8.3 and 8.8
3. Don't exceed concentration: stay in the lower half of the recommended range
4. Remove parts promptly: don't leave aluminium parts submerged during stops
5. Check top-up water pH: consider using osmotic water if mains pH is high

Already stained parts?

Bad news: alkaline attack stains on aluminium are generally irreversible. The chemical attack has dissolved a surface layer of the metal. Prevention is the only reliable solution.