Corrosion Fundamentals

• Corrosion Fundamentals
• Why Metals Corrode
• Electrochemistry
• The Nature of Matter
• Electrochemical Cells
• Forms of Corrosion
• Uniform Corrosion
• Galvanic Corrosion
• Concentration Cell Corrosion
• Pitting Corrosion
• Crevice Corrosion
• Filiform Corrosion
• Intergranular Corrosion
• Stress Corrosion Cracking
• Corrosion Fatigue
• Fretting Corrosion
• Erosion Corrosion
• Dealloying
• Hydrogen Damage
• Corrosion in Concrete
• Microbial Corrosion
• Corrosion Control
• Design
• Materials Selection
• Protective Coatings
• Inhibitors and Other Means of Environmental Alteration
• Corrosion Allowances
• Cathodic Protection
• Sources of Additional Information

Passive metals, such as stainless steel, resist corrosive media and can perform well over long periods of time. However, if corrosion does occur, it forms at random in pits. Pitting is most likely to occur in the presence of chloride ions, combined with such depolarizers as oxygen or oxidizing salts. Methods that can be used to control pitting include maintaining clean surfaces, application of a protective coating, and use of inhibitors or cathodic protection for immersion service. Molybdenum additions to stainless steel (e.g. in 316 stainless) are intended to reduce pitting corrosion.

The rust bubbles or tubercules on the cast iron above indicate that pitting is occurring. Researchers have found that the environment inside the rust bubbles is almost always higher in chlorides and lower in pH (more acidic) than the overall external environment. This leads to concentrated attack inside the pits.

Similar changes in environment occur inside crevices, stress corrosion cracks, and corrosion fatigue cracks. All of these forms of corrosion are sometimes included in the term "occluded cell corrosion."

Pitting corrosion can lead to unexpected catastrophic system failure. The split tubing above left was caused by pitting corrosion of stainless steel. A typical pit on this tubing is shown above right.

Sometimes pitting corrosion can be quite small on the surface and very large below the surface. The figure below left shows this effect, which is common on stainless steels and other film-protected metals. The pitting shown below right (white arrow) led to the stress corrosion fracture shown by the black arrows.

A complete discussion of this corrosion is contained in Steven J. McDanels, "Failure Analysis Of Launch Pad Tubing From The Kennedy Space Center," Microstructural Science, Vol. 25, 1998, ASM International, Materials Park, OH, pp. 125-129.