THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Tuesday, February 6th, 2018

9:30 AM
in Love 295

will be held the

DISSERTATION PROPOSAL DEFENSE

for

Liang He

"Pitting Corrosion Behavior of Lean Duplex Stainless Steels in Chloride and Thiosulfate Containing Environments"

Committee Members:

Prof. Preet Singh, Advisor, MSE

Prof. Faisal Alamgir, MSE

Prof. Hamid Garmestani, MSE

Prof. Arun Gokhale, MSE

Prof. Matthew McDowell, ME

Abstract:

A number of chemical process industry streams contain chlorides along with thiosulfate or sulfate ions. These industries also include pulp and paper industry, mineral process industry, textile industry, and nuclear industry. Focus of this study is white water environment in paper machines, which is neutral to acidic environment that mainly contains Cl-, S2O32- and SO42- ions. Surfaces of paper machine equipment, like drying or pressing rolls, are very smooth. Any surface imperfection on the surface due to corrosion can potentially cause problems with the paper making process. Because of the occurrence of problems like pitting corrosion, crevice corrosion and stress corrosion cracking, pulp and paper industry tends to select austenitic stainless steels and duplex stainless steels (DSSs) as construction materials for paper machine.

Duplex stainless steels (DSSs) are dual-phase Cr-Mo-Ni-Fe steels systems that possess both high mechanical strength and high corrosion resistance. The most common grade is the standard DSS 2205 that possesses the yield strength twice as that of austenitic stainless steels and superior corrosion resistance than 304 and 316 austenitic stainless steels. Compared with standard DSSs like DSS 2205, lean DSSs (LDSSs) such as UNS S32101 and UNS S32003 with lower Cr and Ni content, are more cost-effective in less aggressive environments such as paper machine white water. However, as newly developed grades, their corrosion behavior has not been widely investigated.

Among different corrosion problems, pitting corrosion is a very important issue since it is an autocatalytic process and is particularly dangerous once it penetrates the thickness of the materials. In addition, it is often the initiation sites for stress corrosion cracking. The small size of pits makes it even more difficult to detect the attacked sites. During the past few decades, pitting corrosion of austenitic stainless steels in environments containing Cl-, S2O32-, NO3- and other anions has been studied extensively.  Because of the dual-phase microstructure of DSSs, pit initiation and pit growth mechanisms of pitting corrosion of DSSs are more complicated. Preferential dissolution of ferrite phase within pits has been observed in near-neutral S2O32--containing Cl- solution and alkaline carbonated Cl- solution. Pit initiation sites could also be altered due to the microstructure evolution of DSSs during heat-treatment like annealing and aging.

In this research, pitting corrosion behavior of LDSSs is studied in near-neutral environments that contain Cl-, S2O32- and SO42-. Effects of annealing and aging heat-treatment on the microstructure evolution, passive film properties and pitting corrosion are studied. Effects of Cl- and S2O32- concentrations on the stable pitting events, metastable pitting and repassivation behavior are studied. To study the mechanisms of pitting corrosion of DSSs in terms of pit initiation, pit propagation and any galvanic interactions between the two phases, the single-phase microstructures are obtained and their passive film properties, active dissolution and the galvanic corrosion in the simulated pit solution are studied. The comparison between different grades of LDSSs in simulated white water would provide insights into materials invention and selection for paper machines in a cost-effective way. The study on the effects of annealing and aging will provide information about how corrosion behavior of LDSSs would be affected by different heat-treatment procedures. The study on the properties of ferrite and austenite single phase and the coupling effects of the two phases in both simulated white water and simulated pit solution will provide insights into the pitting corrosion mechanisms of LDSSs and help explain phenomena of pitting corrosion in LDSSs.