The fact that stainless steels do not undergo corrosion is due to the presence of a passive layer. But what exactly is it, how does it form, and what happens when it gets damaged? Today at INOX Polska, we take a closer look at this fascinating phenomenon.
What Is the Passive Layer?
The passive layer is responsible for protecting stainless steel against corrosive agents. It is a thin, transparent oxide film that forms on the surface of stainless steel and adheres tightly to it, covering the entire element.
If the environment contains oxygen, this layer can naturally regenerate itself — after about 24 hours, the material regains full stability. However, this self-healing ability may be disrupted in particularly aggressive environments, such as marine atmospheres with high chloride concentrations.
What else affects the durability of the passive layer? The key to maintaining its protective function is proper maintenance of the steel surface. Before starting any project, it’s also important to verify the properties of the specific grade of steel and choose a material suited to the environment in which it will be used.
Stainless Steel Passivation
Sometimes, the natural corrosion resistance of stainless steel is not sufficient. In such cases, the thin and delicate passive layer can be reinforced through a process called passivation — a chemical or electrochemical treatment that enhances the steel’s surface protection. Research shows that proper passivation can increase the thickness of the protective layer several times over. Importantly, the process does not alter the shape or structure of the surface — only its anti-corrosive properties are improved.
Passivation is often applied after industrial processes used in stainless steel treatment. A good example is welding, during which the material may lose its natural ability to regenerate the passive layer.
To ensure effective passivation, it is essential to thoroughly clean the surface after welding or any other invasive operation.
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