300 series stainless steels are widely used in the fluid industry due to their exceptional corrosion resistance and rust-proof properties. Common products include quick connectors, metal hoses, and corrugated pipes. However, choosing the right grade can be confusing. Let’s explore the corrosion resistance of 304, 304L, 316, and 316L stainless steels.
Chemical Corrosion
Chemical corrosion occurs when metal undergoes direct oxidation-reduction reactions with its environment. For example, chloride ions in chemical raw materials react with iron to form ferrous chloride: Cl₂ + Fe → FeCl₂. This process strips metal atoms from the surface, weakening the material.
Major Forms of Metal Corrosion Damage
1. Surface Corrosion (Uniform Corrosion)
Surface corrosion affects most or all of a metal’s surface uniformly. It’s the most common type of corrosion, caused by widespread chemical reactions with the environment.
2. Intergranular Corrosion
This corrosion progresses along the boundaries between metal grains. It arises from chemical composition differences between grain surfaces and interiors, grain boundary impurities, or internal stress.
3. Pitting Corrosion
Pitting corrosion targets localized areas on metal surfaces, often in passivated metals exposed to solutions with corrosive anions (e.g., Cl⁻) and oxidants. Small pits can deepen rapidly, leading to structural failure.
4. Stress Corrosion
Stress corrosion occurs when tensile stress combines with a corrosive medium, causing cracks that propagate over time. Even low stress levels can lead to sudden failure in susceptible materials.
5. Corrosion Fatigue
Repeated alternating loads in a corrosive environment degrade a material’s fatigue strength, reducing its lifespan compared to corrosion-free conditions.