Why does stainless steel rust?

14 Jul,2019

Why does stainless steel rust?

When brown rust spots appear on the surface of stainless steel, people often express surprise: “Stainless steel shouldn’t rust,” “If it rusts, it’s no longer stainless steel,” or “Perhaps there’s a problem with the steel itself.” In fact, these are mistaken assumptions stemming from a lack of understanding about stainless steel. Under certain conditions, stainless steel can indeed corrode. Stainless steel possesses the ability to resist atmospheric oxidation—its “stainlessness”—and also exhibits corrosion resistance in environments containing acids, alkalis, or salts—its “corrosion resistance.” However, the extent of this resistance varies depending on the steel’s chemical composition, its metallurgical condition, the service conditions, and the nature of the surrounding environment. For example, 304 stainless steel demonstrates excellent resistance to rust in dry, clean atmospheres; yet, when exposed to the salt-laden sea spray typical of coastal regions, it will quickly begin to rust. Therefore, no single grade of stainless steel is universally resistant to corrosion and immune to rust in every environment.

　　In addition, stainless steel derives its corrosion resistance from an extremely thin, robust, dense, and stable chromium-rich oxide film that forms on its surface, which acts as a protective barrier to prevent further penetration of oxygen atoms and oxidation. However, if this passive film is continuously damaged for any reason, oxygen atoms from the air or liquid will steadily penetrate, while iron atoms from the metal will continually detach, forming loose iron oxides and leading to ongoing surface corrosion. Such surface films can be compromised in many ways; the most common mechanisms are as follows:

　　1) When dust containing other metallic elements or foreign metal particles adheres to the surface of stainless steel, the condensed moisture between these deposits and the stainless steel can form a micro‑galvanic cell, triggering an electrochemical reaction that damages the protective passive film. This phenomenon is known as electrochemical corrosion.

　　2) Organic matter such as fruit juices, vegetable juices, or noodle soup adhering to the stainless steel surface can, in the presence of water and oxygen, form organic acids, which over time may corrode the metal surface.

　　3) Acid, alkali, or salt‑containing substances (such as alkaline or lime water used for wall decoration) adhering to the stainless steel surface can cause localized corrosion.

　　4) In polluted air—such as atmospheres containing high concentrations of sulfides, carbon oxides, and nitrogen oxides—condensation can form droplets of sulfuric, nitric, and acetic acids, leading to chemical corrosion.

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