Steel and other metals are susceptible to a variety of forms of corrosion, particularly in environments where oxygen levels are low and chlorides (salts) are high, such as exposure to seawater, road salt, or chemicals. Stress corrosion cracking occurs when mechanical stresses, the environment, and high temperatures combine to form small cracks in the metal that can quickly expand. Intergranular corrosion happens when stainless steels are exposed to prolonged, high heat.
Science Daily recently reported on an Arizona State University research team’s new insights about intergranular stress corrosion cracking (SCC), an environmental cause of premature failure in engineered structures, including bridges, aircraft and nuclear power generating plants.
The Arizona research team demonstrated that the roles of stress and corrosion can act independently.
“The finding is the culmination of about 30 years’ work on this kind of stress corrosion problem,” said lead researcher Karl Sieradzki, a professor of materials science and engineering at ASU. “We now have a view into how new alloys can be designed to avoid this form of stress corrosion-induced failure.”
The team examined the behavior of a laboratory model silver-gold alloy, which mimics the corrosion behavior of important engineering alloys, such as stainless steels and nickel-base alloys used in nuclear power plants.
Corrosion in these engineering alloys, as in the model silver-gold alloy, results in the formation of nanometer-sized holes within the corroded layer. According to Sieradzki, the key parameter determining the occurrence of rapid SCC is the adhesion between the corroded layer and the un-corroded alloy. Using the atomic scale techniques of high-resolution electron microscopy and atom probe tomography, together with statistical characterizations, the team determined that the apparent requirement for the simultaneous presence of stress and corrosion exists because of time-dependent morphology changes that affect adhesion.
As long as adequate adhesion between the layers is maintained, a crack that starts with the corroded layer may penetrate into the uncorroded alloy. This means that there can be a significant mechanical component to stress corrosion cracking that cannot be identified by any measurement of corrosion. The result is that a corrosion measurement can underestimate the rate of SCC by multiplicative factors of 10 or more.
“In nuclear plants, SCC maintenance and plant shut downs are based on previous experience with similarly designed reactors,” Sieradzki explained. “While we are not building new nuclear plants in the U.S., these findings should trigger the search for new, corrosion resistant alloys that can be used for replacement parts in existing plants and in other important structural applications.”
Steel and other metals derive properties such as strength, ductility, and corrosion resistance from alloying elements. The addition of certain alloying elements is one way to improve corrosion resistance and prevent failure of finished steel products. X-ray fluorescence provides fast, accurate elemental analysis of the composition of steel during the manufacturing process to help ensure that the correct alloying elements are added in the correct amounts. Handheld XRF analyzers are capable of distinguishing alloy grades that are nearly identical in composition to one another.