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

Stress corrosion cracking (SCC) is caused by the simultaneous effects of tensile stress and a specific corrosive environment. Stresses may be due to applied loads, residual stresses from the manufacturing process, or a combination of both.

Cross sections of SCC frequently show branched cracks. This river branching pattern is unique to SCC and is used in failure analysis to identify when this form of corrosion has occurred.

The photo below shows SCC of an insulated stainless-steel condensate line. Water wetted the insulation and caused chlorides to leach from the insulation onto the hot metal surface. This is a common problem on steam and condensate lines. Control is by maintaining the jackets around the lines so that moisture doesn't enter the insulation or is quickly drained off.

The next two photos show intergranular SCC of an aluminum aerospace part. The intergranular nature of the corrosion can be seen in the scanning electron microscope image on the left and in the microscopic cross section on the right. The arrows indicate the primary crack shown in both pictures. Note that secondary cracks are also apparent. These secondary cracks are common in stress corrosion cracking.

The failure above occurred on an aluminum alloy subjected to residual stresses and salt water. Changes in alloy heat treatment recommended by KSC Materials Laboratory eliminated this problem. McDanels, S. J., "An Overview of Fatigue and Other Metallurgical Failure Modes and Analysis at the Kennedy Space Center", Flammability and Sensitivity of Materials in Oxygen-Enriched Atmospheres: Eight Volume, ASTM STP 1319, W. T. Royals, T. C. Chou, and T.A. Steinberg, Eds., American Society for Testing and Materials, 1997.

Several years ago, wide spread use of plastic tubing was started in new house construction and for repair of old systems. Flexible tubing was used to connect faucets to the fixed metal piping. The picture below shows stress corrosion cracking after eight years in this service. The tubing was bent and stress cracks started at the outside tensile side of the tube. Flexible plastic piping is now used less often in this service-especially for hot water service.

(Courtesy of www.skyinc.net/~kpr)

Additional photos of stress corrosion cracking are shown in the section on microbial corrosion.