Why Your Cutting Fluid is Rusting Your Parts and How to Prevent It

script type="application/ld+json">{"@context": "https://schema.org", "@type": "FAQPage", "mainEntity": [{"@type": "Question", "name": "¿Cuáles son las causas principales de oxidación por taladrina?", "acceptedAnswer": {"@type": "Answer", "text": "Las 5 causas principales son: concentración demasiado baja (por debajo del mínimo recomendado), pH bajo (inferior a 8.5 favorece corrosión), contaminación por aceites parásitos que desestabilizan la emulsión, agua de mezcla inadecuada (agua dura con alto contenido en cloruros), y tipo de taladrina no apto para el material mecanizado."}}, {"@type": "Question", "name": "¿Cómo actuar ante oxidación de piezas por taladrina?", "acceptedAnswer": {"@type": "Answer", "text": "El protocolo de actuación incluye: medir concentración con refractómetro y ajustar al rango recomendado, medir pH (debe estar entre 8.5-9.2), retirar aceites parásitos con skimmer, aplicar aditivo anticorrosión si el problema persiste, y considerar cambio de taladrina si la formulación no es adecuada para el material."}}]}It is one of the most frequent complaints in machining workshops: "the coolant is rusting my parts". Corrosion and staining on machined parts is a costly problem that can lead to quality rejections, customer complaints and material waste.

In this article we analyse the real causes and give you practical solutions.

The 5 main causes of cutting fluid corrosion

1. Concentration too low

This is the most common cause. When the cutting fluid concentration drops below the manufacturer's recommended minimum, the corrosion inhibitors lose effectiveness. The water in the emulsion is exposed and oxidises metal surfaces.

2. Bacterial contamination

Anaerobic bacteria that thrive in cutting fluid (especially during stops like weekends) consume corrosion inhibitors and lower the pH. A cutting fluid with pH below 8.0 has lost its protective capacity.

3. Inadequate water hardness

Mains water with excessive hardness (>30°dH) or very soft (<5°dH) affects emulsion stability. Hard water generates calcium soaps that deplete corrosion inhibitors. Very soft water produces unstable emulsions and foam.

The ideal hardness for most cutting fluids is between 10 and 25°dH.

4. Tramp oil

Slideway oils, hydraulic and lubrication oils that fall into the cutting fluid sump form a surface layer that prevents oxygenation. Under this layer, bacteria proliferate and degrade the emulsion rapidly.

5. Parts left wet

Parts left with cutting fluid residue without drying or protection are the most prone to corrosion. Especially in humid environments or when temperature changes cause condensation.

Action protocol for corrosion

1. Measure concentration immediately with a refractometer and adjust to the recommended value
2. Measure pH: if below 8.5, you have bacterial degradation
3. Remove tramp oil from the surface
4. Check water hardness used for top-ups
5. If the sump is heavily degraded (pH < 7.5, strong odour), drain, clean and prepare fresh emulsion

Special case: aluminium staining

Aluminium does not "rust" like steel, but it stains from alkaline attack. A cutting fluid with pH too high (>9.5) attacks the aluminium surface creating irreversible dark stains. For aluminium, use cutting fluids specifically formulated for non-ferrous metals, with pH controlled between 8.5 and 9.0.