Doctor of Philosophy, University of Akron, 2024, Chemical Engineering

The aim of this work is to draw links between 1-D pitting corrosion in literature and crevice corrosion in Alloy 625 and SS 316L in chloride-containing environments. Crevice corrosion initiation, propagation, and repassivation were studied using real-time optical imaging and UV spectroscopy, and the crevice corrosion mechanism was investigated for different cases with the ultimate goal of finding a unifying mechanism for crevice corrosion that is in line with 1-D pitting corrosion. In the first part of this work, crevice corrosion was investigated for LPBF AM Alloy 625 specimens in comparison to the conventionally wrought condition. The AM specimens tested were of two different orientations with respect to the build platform. In addition, the tests were carried out for specimens of the as made or not treated condition as well as specimens that were subject to post manufacturing heat treatments including stress relieving, solution annealing, and solution plus stabilization annealing. Hence, the effect of heat treatment and build orientation on the susceptibility of LPBF additively manufactured Alloy 625 to crevice corrosion was investigated. There was not sufficient evidence that build orientation affects crevice corrosion susceptibility. Nevertheless, it has been shown that heat treatment affects crevice corrosion susceptibility. In addition, though the crevice corrosion susceptibility of as made AM Alloy 625 was not remarkably different from that of wrought 625, solution annealing improves crevice corrosion performance of the AM specimens beyond that of the wrought condition. Crevice corrosion performance differences could be explained with microstructure reflected in the corrosion morphology. Nevertheless, the different AM alloys studied followed the same kinetics/mechanism as the wrought alloy with similar trends in current density and repassivation potential. Such kinetics/mechanisms appear in literature for 1D pits supporting the applicability of the “1D (open full item for complete abstract)

Committee: Robert Lillard (Advisor); Qixin Zhou (Committee Member); Dmitry Golovaty (Committee Member); Gregory Morscher (Committee Member); Linxiao Chen (Committee Member) Subjects: Chemical Engineering; Materials Science

Master of Science, University of Akron, 2016, Applied Mathematics

A three-stage, unidirectional pit growth model, from initiation to stable growth and repassivation as the bulk potential is decreased, is developed. Stage I models metastable pit growth under ohmic control and a constant current density. Here it is assumed that the pit is covered by a semi-permeable oxide layer. Stage I is terminated when the metal concentration reaches its saturation limit at which time the pit cover instantaneously bursts. Stage II models the stable pit growth under diffusion control and the formation of a salt film at the bottom of the pit. During Stage II the bulk potential is decreased at a specified scan rate. When the bottom pit potential reaches the transition potential, Stage III begins. Here we model the pit growth under ohmic control, for a prescribed polarization curve, until the metal repassivates as the potential is decreased. The governing system of equations for each stage is solved numerically to determine the potential drop, and the concentrations of sodium, chloride, and metal ions within the pit. The pit depth as a function of time is determined from Faraday's Law in Stages I and III, and from a mass balance at the electrolyte/metal interface in Stage II. The cumulative pit depth is fit to a power law model that is used in existing Markov models for pit initiation and growth, and is compared with experimental pit depths for stainless steel in seawater.

Committee: Curtis Clemons Dr. (Advisor); Kevin Kreider Dr. (Committee Member); Gerald Young Dr. (Committee Member); Timothy Norfolk Dr. (Committee Chair) Subjects: Mathematics

Doctor of Philosophy, The Ohio State University, 2009, Materials Science and Engineering

Ni-Cr-Mo alloy are known as one of the most versatile Ni-based alloys in resisting corrosion. Additions of Cr and Mo have been proven to be the cause of the good corrosion resistance behavior. The role of Cr and Mo in resisting localized corrosion particularly, pitting corrosion was studied. Ni-Cr-Mo alloys were fabricated with twenty four unique compositions. Polarization experiments were conducted on these alloys in 0.5M NaCl and low pH chloride solution at varying temperatures from 45 to 90°C to obtain corrosion parameters namely, pitting and repassivation potentials. Multiple linear regression analysis was performed to construct a mathematical expression that correlate the pitting and repassivation potentials to the alloying content at each environment. This expression allows us to roughly predict the corrosion behavior of the Ni-Cr-Mo alloys through Cr and Mo contents only. It appears that Cr content is more dominant than Mo content in raising the pitting potential in neutral chloride condition. The effect of both Cr and Mo are quite uniform in affecting the repassivation potential values of the alloys in neutral chloride. The effect of Mo is greater than Cr in the low pH and higher temperature solutions in affecting both pitting and repassivation potentials.Pitting corrosion is commonly preceded by the occurrence of metastable pitting. Thus, one can better understand the pitting process of an alloy through its metastable pitting behavior. Rigorous metastable pitting study was conducted on Ni-Cr-Mo alloys with Cr content varying from 20 to 29 wt.% and Mo content varying from 12 to 25 wt.%. Potentiostatic experiments were performed in 90°C of 0.5M NaCl to promote the metastable pitting incidence. Analysis was done on the potentiostatic data to better characterize the metastable pitting behavior of the Ni-Cr-Mo alloys. It was observed that a higher Mo content in the Ni-based alloy is responsible for the lower peak current values and slowing down the growth rates (open full item for complete abstract)

Committee: Rudolph Buchheit Prof. (Advisor); Gerald Frankel Prof. (Committee Member); John Morral Prof. (Committee Member) Subjects: Materials Science

Doctor of Philosophy, The Ohio State University, 2005, Materials Science and Engineering

Arc welding of stainless steels generates welding fumes containing carcinogenic hexavalent chromium. To mitigate this problem, a Cr-free filler metal needs to be developed to reduce the health risks for welding stainless steel. Ni-Cu alloy, Monel K400, was selected initially based on its galvanic compatibility with types 304 and 316 SS in chloride environments Type 304L (UNS S30403) stainless steel plate was successfully welded with Monel filler wire, resulting in high quality welds with no cracks. The welds survived long term exposure in 0.1 M NaCl with no evidence of corrosion. However, segregated regions in weld that are rich in Cu are the weak spots for corrosion susceptibility. The effects of alloying elements on the corrosion properties of Ni-Cu alloy were investigated to optimize the composition of a Cr-free consumable for welding of stainless steels. Cyclic polarization experiments were performed in 0.1 M NaCl on Ni-base alloys containing different amounts of Cu, Pd, Mo, Fe, and Cr. The localized corrosion behavior improved as the Cu content decreased from 30 to 5 wt%. The addition of 1 wt% Pd ennobled the alloy and greatly improved the localized corrosion properties. The optimized composition is a Ni-based alloy containing 5-10 wt% Cu and 1 wt% Pd. Ni-10Cu-1Pd alloy across the entire dilution range exhibited higher repassivation potential than Type 304L stainless steel in 0.1 M NaCl solution. The repassivation potential was also higher in aerated solutions with chloride concentrations of 105 through 35000 ppm. The breakdown behavior of Bead-On-Plate weld was similar to that of Type 308L welds. After 31 days exposure of samples with crevice-formers in 500 and 1000 ppm chloride solutions, the Bead-On-Plate weld showed much shallower attack than the 308L weld. The passive film on Ni-10Cu-1Pd alloy mainly consisted of outer Ni-hydroxide and inner oxide, but the noble elements of Pd and Cu did not contribute to the formation of the passive film. However, Pd cata (open full item for complete abstract)

Committee: Gerald Frankel (Advisor) Subjects: Engineering, Materials Science