Dr E. Ramanathan PhD
Black tarnishing of aluminium after exposure to cutting oil happens mainly due to sulphur or chlorine additives in the oil, combined with moisture and heat. These react with the aluminium surface, leaving a black or grey film. To remove and prevent it, you can use the following methods:
Removal of Black Tarnish
- Mild Acid Cleaning
- Use a dilute solution of citric acid (5–10%) or phosphoric acid (5–10%).
- Dip or wipe the aluminium part for a few minutes until the tarnish loosens.
- Rinse thoroughly with clean water and dry immediately.
- Alkaline Cleaning
- Use a weak sodium carbonate (soda ash) or trisodium phosphate solution (2–5%).
- Scrub lightly with a soft brush or non-abrasive pad.
- Rinse and dry.
- Commercial Aluminium Cleaners
- Proprietary aluminium brighteners (usually phosphoric acid + surfactant blends) can remove the black film effectively.
- Avoid strong caustic soda (NaOH) unless controlled, since it attacks aluminium rapidly.
- Manual Polishing
- For small parts, use a fine alumina polishing paste or scouring pad.
- Finish with a protective wax or light oil film.
Prevention During Machining
- Switch to non-staining cutting oils specifically recommended for aluminium (free from active sulfur or chlorinated EP additives).
- Maintain oil cleanliness and avoid water contamination.
- After machining, promptly wipe down the part and rinse with a mild alkaline cleaner or water-soluble detergent.
- Apply a temporary rust inhibitor or protective oil film if storage is needed.
Sulphur in cutting oils is usually added as an EP (Extreme Pressure) additive. Its role and effect on aluminium are important to understand:
Types of Sulphur in Oils
- Active Sulphur
- Chemically reactive form (sulfurized olefins, sulfur–chlorine compounds).
- At elevated temperature, it reacts with metals like copper and aluminium.
- Leaves black tarnish or staining on aluminium, brass, bronze, and sometimes mild steel.
- Commonly used in heavy-duty machining oils for steels.
- Inactive Sulphur
- Chemically bound form (sulfurized fatty oils, polysulfides).
- Less reactive; does not attack non-ferrous metals under normal machining conditions.
- Safer for aluminium and yellow metals.
Why Aluminium Turns Black
- When aluminium contacts active sulphur oils, the sulphur reacts with the oxide film on aluminium, forming aluminium sulfide or dark complex films.
- Heat and pressure during machining accelerate this tarnishing.
- Moisture and chloride in the oil can worsen the effect.
Recommendations
- For aluminium machining, always specify oils “non-staining, inactive sulphur” or sulphur-free oils.
- Water-miscible coolants with corrosion inhibitors are usually preferred.
- If you must use sulphur-containing oils, ensure they are inactive sulphur grades.
Here are reputable cutting oils safe for aluminium, explicitly formulated to avoid active sulphur—and thus prevent black tarnishing:
Selection of Cutting Oil
| Product | Active Sulphur? | Suitable for Aluminium? | Key Benefits |
|---|---|---|---|
| CINDOL 3101 | No | Yes | Clean finish, high lubricity |
| Bio-Mist EP | No | Yes | Eco-friendly, non-staining |
| DURALIFE Inactive Sulphur | Inactive | Yes | Non-corrosive, transparent |
| Goldenstone Frezol | No | Yes | Excellent EP/lubricity for non-ferrous |
| Huskey Clear Cutting Oil | No | Yes | Smooth finishes, non-staining |
| Mahathol NC (Inactive Type) | Inactive | Yes | Local, non-staining, cost-effective |
Final Tip
If you’re machining aluminium at scale or in a professional environment, opt for inactive-sulphur or sulphur-free cutting oils tailored for non-ferrous metals. This helps avoid staining and ensures better surface finish, tool life, and worker safety.
References :
- CINDOL 3101 Non-Staining Cutting Oil – Quaker Houghton Product Page, Distributor Technical Notes, Quaker Houghton Metal Forming Brochure.
- DURALIFE® Inactive Sulphur Neat Cutting Oils – Technical Data Sheet (TDS) PDF, Safety Data Sheet (SDS) Revision January 9, 2024.
- Goldenstone® Frezol Cutting Oil – Technical Data Sheet (TDS), published March 2018.
- Bio-Mist EP Cutting Oil – Renewable Lubricants Product Page (contains performance and safety data).
- Mahathol NC (Inactive Type) – Mahatha Petroleum Non-Staining Oil Product Page (technical description).
- Huskey Clear Cutting Oil – Huskey Specialty Lubricants Product Description (includes handling and compatibility notes).
- Keller-Heartt Guide – “Choosing the Right Cutting Oil” (background reference on active vs inactive sulphur oils).
- Tap Magic Cutting Fluid – Raptor Supplies Product Technical Listing.
- WD-40 Specialist Cutting Oil – ACORN Industrial Product Technical Listing.
