What Does Passive Film Mean?
Passive film refers to the spontaneous formation of an ultra-thin film of corrosion product on a metal's surface that acts as a barrier to additional chemical reaction. Passive film is extraordinarily helpful in mitigating corrosion damage. However, even a high-quality alloy corrodes if its ability to make a passivating film is hindered.
If breakdown happens within the passive film due to chemical or mechanical factors, types of corrosion including erosion corrosion, crevice corrosion and stress corrosion cracking may occur.
Passive film formation is also known as passivation.
Corrosionpedia Explains Passive Film
Passive film formation involves the formation of a shielding outer layer of base material, which can be applied as a micro-coating, or can occur naturally. As a corrosion prevention method, passive film is a lightweight coat of protecting material that forms a shell against corrosion.
The chemical composition and microstructure of a passive film are different from the metal on which it is formed. Generally, passive film thickness on aluminum, untainted steels and alloys is under ten nanometers. The passive film is wholly distinct from chemical compound layers that are shaped upon heating and are within the micrometer thickness vary — the passive film recovers if removed or broken, whereas a chemical compound layer does not. Passivation in natural environments like air, water and soil at moderate pH is seen in such materials as aluminum, stainless steel, titanium and silicon.
Passive film is primarily determined by metallurgic and atmospheric factors. There are some conditions that inhibit passivation, such as high pH for aluminum and zinc, low pH or the presence of chloride ions for stainless steel, extreme temperature for titanium and halide ions for silicon. Exposure to a liquid metal like mercury or hot solder usually circumvents passivation mechanisms.