GEORGIA INSTITUTE OF TECHNOLOGY

 Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Friday, February 25, 2022

3:00 PM

via

BlueJeans Video Conferencing
https://bluejeans.com/457455123/7345

will be held the

DISSERTATION DEFENSE

for

Jahnavi Desai

"Correlating Microstructure with Corrosion Properties of Aluminum and Stainless Steel Alloy"

Committee Members:

Prof. Josh Kacher, Advisor, MSE

Prof. Hamid Garmestani, MSE

Prof. Preet Singh, MSE

Prof. Mathew McDowell, ME/MSE

Prof. Chaitanya Deo, MSE/NRE

Abstract:

Aluminum and stainless steels are staples in the metals industry and are heavily used in manufacturing sector due to their  versatility, corrosion resistance and good mechanical properties. My primary research objective is to develop a relationship between microstructure and a material’s susceptibility to degradation in corrosive environments. Susceptibility to, and mechanisms of, corrosion are investigated using multiscale electron microscopy techniques. The specific material systems studied are Aluminum alloy 5456 and additively manufactured (AM) stainless steel 316L.

Al5456 alloy: These alloys undergo sensitization when exposed to high temperatures resulting in the formation of beta phase which dissolves preferentially in a corrosive environment resulting in intergranular corrosion or grain boundary corrosion. The main idea is to understand the microstructural parameters influencing this process to develop better alloys with enhanced corrosion resistant properties. Corrosion experiments combined with EBSD analysis were used to determine the microstructure influences on β phase precipitation and intergranular corrosion. This combined approach facilitates the rapid characterization of a large number of grain boundaries (~28,000 in this study), providing a statistical framework for understanding the results produced in the earlier studies, which focused on relatively very small number of grain boundaries. The influence of extrinsic characteristics of grain boundary such as local dislocation density were investigated and qualitative/quantitative observations are reported. Clear trends with GND were observed, with the fraction of uncorroded grain boundaries decreasing with the increase in GND density.

AM 316L stainless steel: Past studies have shown that the SLM stainless steel 316L results in a rich hierarchy of microstructural features, including non-equiaxed and refined grain structures, residual stresses, a dense network of dislocation structures that are often accompanied by Cr and Mo segregation, and internal porosity as well as the absence of MnS inclusions. The nature and structure of these microstructural features are dependent of processing conditions, including scan patterns, laser power, and scanning rates, as well as the geometry of the printed parts.  The influence of microstructural parameters such as porosity, inclusions, dislocation cell structure, grain boundaries and melt pool boundaries on general corrosion resistance and on pitting  have been studied in the recent years. However, these corrosion studies are still in the early stages and show some discrepancies in the observations made. In this study,  the influence of native oxide film on the passive film characteristics and localized corrosion of SLM 316L stainless steel are studied. The analysis showed that corrosive attack varied between initial attack of Cr and Mo-enriched dislocation cell boundaries to cell interior depending on the presence of a native oxide film on the initial sample surface. A corrosion mechanism has been proposed to explain this variation in corrosion attack behavior. The Pitting behavior of this samples is evaluated and the general corrosion performance is compared with the wrought counterparts.