Introduction
Corrosion resistance is the primary feature that distinguishes stainless steel from other types of steel. According to modern understanding, stainless steel can only remain “rust-free” or “corrosion-resistant” in unpolluted atmospheric environments. In practical applications, especially in many industrial environments, stainless steel can still suffer from corrosion due to the erosion of corrosive media.
Forms of corrosion such as intergranular corrosion, pitting, and stress corrosion cracking are often the main manifestations. The corrosion resistance of stainless steel depends on its chemical composition, surface condition, and environmental factors. In specific application conditions, once the steel type and environmental conditions are determined, a clean and smooth surface that forms a uniform and dense passivation film is the premise for ensuring its corrosion resistance. The local damage of this passivation film is the fundamental cause of local corrosion.
Stainless steel is not absolutely rust-free; its corrosion resistance is relative and conditional. Maintaining its surface cleanliness, that is, in a passivated state, is particularly important. Let’s first discuss the formation and protection of the passivation film on stainless steel surfaces.
How Does the Passivation Film Form on Stainless Steel?
At high temperatures, stainless steel surfaces form highly adhesive oxide scales, mainly composed of Cr₂O₃. However, the passivation film formed on stainless steel surfaces at room temperature is not a “true” oxide film; its physical and chemical structure is not fully confirmed yet. It is often referred to as a mixed oxide or hydroxide film. A pure stainless steel surface in the atmosphere or other oxygen-containing environments will spontaneously form this passivation film, which has self-healing characteristics. Room temperature, purification, and oxygen-rich environments are three crucial factors for passivation film formation. Purification or passivation treatment aims to form a purified film on the surface, ensuring its corrosion resistance.
What Causes Contamination of the Passivation Film?
The passivation film on stainless steel surfaces is very thin, only 1-3 nm. During storage, transportation, and subsequent processing, particularly, the passivation film can easily get contaminated for various reasons, affecting its corrosion resistance. The main causes of passivation film contamination include:
- Collisions or sharp object impacts, scratching the stainless steel surface.
- Contamination from various liquids, solvents, billets, water, markings, grease, or protective materials causing localized denaturation of the passivation film.
- Deposition or embedding of dust containing Zn, Pb, Cu, Al, Fe, and brass, bronze, or galvanized products on the stainless steel surface, disrupting the uniformity of the passivation film.
- Local or overall oxidation color spots caused by welding, hot forming, heat treatment, and other thermal processes. These spots differ significantly in thickness, composition, and properties from the required passivation film and are detrimental to the corrosion resistance of stainless steel.
How Can the Passivation Film Be Protected?
Preventing passivation film contamination is crucial throughout the manufacturing, storage, transportation, use, and remanufacturing processes of stainless steel pipes. To ensure protection, the following measures should be taken:
- Avoid using carbon steel or galvanized wire ropes, chains, etc., during handling and lifting operations, or avoid direct contact with stainless steel. All handling equipment should be kept clean, and stainless steel or wooden materials should be used as isolation when possible.
- Keep stainless steel surfaces clean to prevent contamination by debris and other metals. When contamination is unavoidable, use clean kraft paper, burlap, cardboard, etc., as isolation.
- Tools such as saws, files, grinders, and stainless steel wire brushes should be dedicated for stainless steel and must not be used with other steel or non-ferrous metal materials.
- Depending on the pipe diameter, wall thickness, batch size, transportation distance, or method, use different packaging methods like single-piece wrapping, boxing, or bundling.
- Avoid direct ground storage. Prevent direct contact of the stainless steel surface with asphalt, galvanized or carbon steel, various non-ferrous metals, or materials containing these metal components. Also, prevent immersion in sewage, seawater, lake water, and river water.
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