Stainless steel is an iron alloy made up with 12-30% chromium. The chromium present on the surface when exposed to oxygen creates a protective coating. This coating, commonly called a “passive layer’, works to slow or prevent corrosion from entering into the substrate, thus making it to appear as a “rust less” surface. But, like everything else, even this magical material has its limitations.
The Layman’s Explanation -Science of Corrosion
1. Biofilms- Both fresh and salt water contain microorganisms that interact with each other to form layers of slime or biofilm. This is more prevalent in waste or water recirculation systems that create an anoxic environment where Nitric Acid (HNO3) is present. The slime creates acidic enzymes and when covered by a film that prevents oxygen from reaching the surface of the steel, the steel cannot rebuild its protective layer, and it corrodes.
2. Chemicals- The passive layer on stainless steel can be attacked by certain electro chemical reactions. Aggressive anions like Chloride (Cl−), present in Chlorine, forms the negatively charged side of the electrolysis process when electrolytes like salt (seawater) are present. Because ions are absorbed into the surface of the steel its corrosion protection becomes unstable and drastically degraded. Hydrochloric and Sulphuric acid can also degrade the passive layer and depending on concentration and temperature can be particular aggressive towards stainless steel.
3. Galvanic Corrosion- If two dissimilar metals are in contact with each other and with an electrolyte like water, especially sea water, it is possible for a galvanic cell to be set up. This is like a battery and accelerates the corrosion of the less ‘noble’ metal.
4. Stress Corrosion Cracking- A rare form of corrosion that requires very specific combination of tensile stress, temperature and presents of a corrosive environment like Hydrogen Sulphide or even sea water. Cracking would be more prevalent in applications with excessive thermal cycling of the material.
5. Intergranular Corrosion- If the Carbon level in the steel is too high, Chromium can combine with the Carbon to form Chromium Carbide. This occurs at temperatures between about 450-850 deg C. This process is also called sensitization and typically occurs during welding. The Chromium available to form the passive layer is effectively reduced and corrosion can occur.
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The Federal Group USA