THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING
GEORGIA INSTITUTE OF TECHNOLOGY
Under the provisions of the regulations for the degree
DOCTOR OF PHILOSOPHY
on Thursday, January 31, 2019
in Love 295
will be held the
"Effect of Strain on General and Localized Corrosion Behavior of Steels"
Prof. Preet Singh, Advisor, MSE
Prof. Arun Gokhale, MSE
Prof. Faisal Alamgir, MSE
Prof. Matthew McDowell, ME/MSE
Prof. Thomas Fuller, ChBE
Metallic structures in various chemical-processing industries as well as in other structural applications have shown accelerated corrosion in areas with strain/deformation. During manufacturing, assembly and even during service, strain is inevitably introduced to metallic materials. Although pertinent prior research has concentrated on the stress corrosion cracking (SCC) and corrosion fatigue, but the effect of strain on other modes of corrosion in different material/environment systems is not very well understood.
Present study mainly focuses on the influence of strain on general corrosion behavior of steels. Effect of strain on corrosion reactions also depends on whether the metal is under active state or has a passive film formed at the surface. When passivity is unstable, both elastic strain and plastic strain were found to shift the open circuit potential (OCP) of carbon steel A569 to a more negative value in an acidic environment. Thermodynamically, this shift indicates a stronger tendency of general corrosion due to the presence of strain. Consequentially, both deformed tensile A569 specimens and cold-rolled A569 specimens exhibited an accelerated general corrosion rate to different degrees. However, cold-rolling was found to accelerate the re-formation of the passive film on the A569 surface when passivity is stable.
Applied strain also participates in localized corrosion by causing local damage to the protective passive film. When passivity is broken down locally, tensile strain, both elastic and plastic was found to promote pitting corrosion of stainless steel (SS) 304 in a chloride-containing aqueous environment. It was found that elastic strain increased the pitting susceptibility of SS304, by stabilizing the pit-growth process regardless of different surface treatments. Increase in the pitting susceptibility with strain reached a limit at the yield stress of SS304. An increase in the tensile strain into the plastic range did not lead to further damage in the pitting potential of SS304. However, the plastically deformed SS304 experienced different extents of recovery in the pitting potential after the applied stress was released. This recovery of pitting resistance ceased at certain plastic stain level, which was found to be ~9% for SS304. Localized areas associated with inhomogeneity and surface defects such as slip-steps were found to be more prone to pitting attacks. After re-polishing and removal of surface defects, the pitting potential of SS304 was fully recovered for plastically strained specimens. Based on these results, mechanisms for the effect of strain on general and pitting corrosion are proposed. Results from the present study also provides the possible ways to mitigate the strain-induced pitting and general corrosion in steels.