State the metal and the residue you can accept. A film that protects cast iron can stain aluminium or copper if it was never screened.
Two tests, two chemicals
Cast-iron chip rust and aluminium stain are not the same failure. Coolant users often pass a steel test and then reject a batch of stained aluminium housings. The inhibitor package needs the alloy list.
Esteem’s corrosion-inhibitor pages sit next to metal complexing and lubricity. Complexing agents control ions in cleaners and water; corrosion inhibitors protect the part in a diluted fluid. Do not swap the names on the purchase order.
How inhibitors upset emulsions
High salt and some soaps shift HLB and throw cream or scum. If rust improves and the sump splits, the inhibitor level or the ion type is wrong for that emulsifier, not proof that emulsifiers “do not work.”
Log pH. Many ferrous inhibitors live in a narrow alkaline window. A biocide that drops pH can turn a passing fluid into a rusting one without any emulsifier change.
| Risk | Specify | Separate from |
|---|---|---|
| Cast-iron rust | Ferrous inhibitor and pH | HLB only |
| Aluminium stain | Alloy-safe package | A steel-only inhibitor |
| Copper stain | Yellow-metal screen | Amine level by habit |
| Paint failure | Light residue | A heavy oily film |
What the film has to stop
An aqueous metalworking dilution is mostly water, with a little oil and a lot of air, sitting on a freshly cut metal surface. Corrosion is the metal returning to an oxide while that water is present. Ferrous inhibitors, often carboxylic acid salts, borate-amine packages, or film-forming nitrogen compounds, adsorb and slow that reaction. They are consumed by fresh metal, by hard water, and by acid from microbial activity.
Aluminium and yellow metals need a different film. High free amine or high hydroxide that protects steel can darken or pit them. If the shop machines mixed metals, the inhibitor package has to be chosen for the most sensitive surface, not for the cast-iron coupon alone.
How the emulsion and the inhibitor interact
The same fatty acid can be an emulsifier soap and a corrosion inhibitor. If hard water precipitates it, both the emulsion and the film fail together. A tighter synthetic or semi-synthetic system may rely less on that soap and more on a dedicated inhibitor, which then has to be soluble at the use dilution.
Phosphate esters and imidazolines appear in this conversation as film formers or as co-emulsifiers. They change foam and they change which metal is protected. Adding one on top of a borate-amine package without a coupon test is how mixed-metal staining starts.
Coupons, dilution, and the sump
Test at the use dilution, in the customer’s water, on the alloys actually cut. A concentrate that looks inhibited at 10 percent can fail at 5 percent because the film-former is below its adsorption threshold. Record cast-iron chip or coupon results and, if relevant, aluminium stain.
Bacteria lower pH and eat the inhibitor. A Monday morning corrosion complaint is often a biocide and pH complaint. Put reserve alkalinity, pH, and a corrosion coupon on the same control sheet so the inhibitor is not blamed for a sump that has drifted.
How to Specify with Esteem
- List alloys in the shop, including fixtures and yellow-metal valves.
- State the dilution and whether the film must be paintable or weldable later.
- Give the humidity and storage time before the next operation.
- Keep the inhibitor dose separate from the emulsifier dose in the trial.
- Report staining and cast-iron chip rust as two results.
Open corrosion inhibitors and keep the emulsifier request on the lubricity article.