Why temporary anti-corrosion protection exists
Machined metal parts, semi-finished assemblies, and components in transit are particularly vulnerable to corrosion. The clean, active metal surface produced by machining is more reactive than an oxidised surface. Without protection, a relative humidity of 60–70% is sufficient to initiate oxidation in just a few hours on carbon steel parts.
Temporary anti-corrosion protection is, by definition, removable: it must not interfere with subsequent processes (assembly, surface treatment, welding). This requirement differentiates temporary protection products from permanent coatings.
VCI — Volatile Corrosion Inhibitors
VCI (Volatile Corrosion Inhibitors) are organic compounds that vaporise at ambient temperature and form a monomolecular protective layer on metal surfaces. They are marketed in paper, film, bags, solid emitters, and water emulsions.
The advantage of VCI is that it protects complex geometries without direct contact: the vapour penetrates cavities, threads, and interior surfaces inaccessible with oil or wax. In many cases it leaves no visible residue and does not require cleaning before assembly.
- Typical duration: 1–3 years in sealed packaging, 3–6 months in open packaging.
- Effective for: steel, cast iron, aluminium, copper and its alloys (verify compatibility by metal).
- Limitation: requires that the packaging space is sealed for the vapour concentration to be effective.
VCI does not work well in very large or very open packaging without the appropriate emitter. The ratio between emitter area and space volume is critical.
Protective oils
Protective oils are mineral or synthetic oils formulated with corrosion inhibitors, generally calcium or barium sulphonates at high concentrations. They are applied by immersion, spraying, or brushing and form an oil film that acts as a physical barrier against humidity.
They are the most widely used solution for machined parts, stamped components, and metal assemblies. Their durability depends on film thickness, ambient humidity, and storage temperature.
- Low-viscosity oils (10–32 cSt): easy application, thin film, duration 1–6 months.
- High-viscosity oils (100–460 cSt): thick film, greater protection, up to 12–24 months indoors.
- Water-displacing oils: applied to wet parts, actively displacing water. Especially useful in operations where parts emerge wet from machining.
Waxes and preservation fluids
Anti-corrosion waxes form a solid or semi-solid film that offers greater durability than oils in aggressive environments. They are common in the automotive industry for body protection and in maritime export of machinery.
Water-based preservation fluids with inhibitors (rust preventive water-based type) are suitable for environments where a non-flammable or low-environmental-impact product is required. Their durability is generally lower than oil-based products.
Most common mistakes
- Applying anti-corrosion protection over dirty parts or parts with traces of unrinsed coolant: the mixture can generate emulsion and drastically reduce effectiveness.
- Using VCI in open or high-volume packaging without the appropriate emitter.
- Not verifying VCI compatibility with the metal: some ferrous VCIs can attack copper or aluminium.
- Storing protected parts outdoors without UV protection: ultraviolet radiation degrades many protective oils within weeks.
- Removing the anti-corrosion protector with aggressive solvents before assembly when the product is directly compatible with the subsequent process.
Temporary anti-corrosion protection is not the area to save on materials. The cost of a rejected part due to corrosion — including the prior manufacturing process — far exceeds that of any adequate protection. The correct choice depends on the metal, the intended duration, and the next process. If you have recurring corrosion situations in storage or during transport, we can analyse the case and recommend the right product.
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