Thesis (Ph.D.)--Mechanical Engineering, Georgia Institute of Technology, 2008. / Committee Co-Chair: Preet M. Singh; Committee Co-Chair: Richard W. Neu; Committee Member: Hamid Garmestani; Committee Member: Timothy Patterson; Committee Member: W. Steven Johnson.

This study investigated localized corrosion behavior of austenitic stainless steels under stressed and unstressed conditions, as well as corrosion of metallic thin films. While austenitic stainless steels are widely used in corrosive environments, they are vulnerable to pitting and stress corrosion cracking (SCC), particularly in chloride-containing environments. The corrosion resistance of austenitic stainless steels is closely tied to the alloying elements chromium, nickel, and molybdenum. Polarization curves were measured for five commercially available austenitic stainless steels of varying chromium, nickel, and molybdenum content in 3.5 wt.% and 25 wt.% NaCl solutions. The alloys were also tested in tension at slow strain rates in air and in a chloride environment under different polarization conditions to explore the relationship between the extent of pitting corrosion and SCC over a range of alloy content and environment. The influence of alloy composition on corrosion resistance was found to be consistent with the pitting resistance equivalent number (PREN) under some conditions, but there were also conditions under which the model did not hold for certain commercial alloy compositions. Monotonic loading was used to generate SCC in in 300 series stainless steels, and it was possible to control the failure mode through adjusting environmental and polarization conditions. Metallic thin film systems of thickness 10-200 nm are being investigated for use as corrosion sensors and protective coatings, however the corrosion properties of ferrous thin films have not been widely studied. The effects of film thickness and substrate conductivity were examined using potentiodynamic polarization and scanning vibrating electrode technique (SVET) on iron thin films. Thicker films undergo more corrosion than thinner films in the same environment, though the corrosion mechanism is the same. Conductive substrates encourage general corrosion, similar to that of bulk iron, while insulating substrates supported only localized corrosion.

Stress corrosion cracking (SCC) of alloy 600 is regarded as one of the most important challenges to nuclear power plant operation worldwide. This study investigates two heats of alloy 600 (forged control rod drive mechanismnozzle and rolled divider plate) in order to obtain a better understanding of the effects of the material parameter on the SCC phenomenon. The experimental approach was designed to determine the effect of the manufacturing process (forged vs. rolled), the cold-work (as-received vs. cold-worked) and the strain path (monotonic vs. complex) on SCC of alloy 600. Specimens with different strain paths have been produced from two materials representative of plant components and tested in high temperature (360°C) high pressure primary water environment. The manufacturing process has been proven to have a great effect on the stress corrosion cracking behaviour of alloy 600. Indeed, the SCC susceptibility assessment has demonstrated that the rolled materialis resistant to SCC even after cold work, whereas the forged material is susceptible in the as-received state. Microstructural characterisations have been undertaken to explain these differences in SCC behaviour. The carbide distribution is the main microstructural parameter influencing SCC but the misorientation, in synergy with the carbide distribution, has been proven to give a better representation of the materials SCC susceptibilities.

This thesis evaluates the ability of laser surface melting to mitigate chloride stress corrosion cracking (See) of type 304 stainless steel. The effects of laser surface melting on microstructure, mechanical state, and corrosion behavior were examined. The major effect of laser surface melting of 304 stainless steel was found to be the introduction of tensile residual stresses on the order of the yield strength in the surface of the laser-melted regions. Exposure of laser-melted coupons to boiling magnesium chloride at 154°C revealed that the residual stresses were sufficient to cause failure by see processes in the absence of an external load. It was concluded that unless measures could be found to eliminate or reverse the residual stresses introduced by the laser melting process, the technique is not viable for mitigating chloride see in these alloys. / Master of Science

The effect of the residual stresses developed during simulated weld heat affected zone in austenitic stainless steel specimen on the stress corrosion cracking susceptibility was studied. Residual stresses was measured using X-ray diffraction technique. Boiling Magnesium Chloride was used as corrosive environment. Compressive stresses developed in the HAZ of the specimen and in regions away from the HAZ stress free values were obtained. The magnitude of the stress gradient decreased as the peak temperature attained during simulated welding decreased. Transgranular cracks were observed in the compressive stress gradient region and time to cracking decreased with increasing stress gradient. Higher nickel content alloys took longer to crack as opposed to lower nickel content alloys at approximately the same stress gradient. / Master of Science

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This thesis presents the work performed to evaluate the susceptibility of AISI 304 to stress-corrosion cracking in terms of time to crack nucleation and rate of crack propagation. U-bend specimens were exposed to magnesium chloride solutions boiling at atmospheric pressure for some predetermined time. The concentrations of magnesium chloride employed were 40, 42, and 44 per cent by weight. After exposure, specimens were microscopically examined and crack depths were measured and recorded. It was found that a straight-line relationship existed between maximum crack depth and exposure time which may be expressed by the empirical equation Log t = D/M+ Log C. The constants M and C are characteristic of the conditions of exposure and increase with a decrease in chloride ion concentration. The rate of crack propagation was found to be inversely proportional to time. Microscopic examination revealed that cracking was both transgranular and intergranular. There are indications that intergranular cracking was more pronounced at the low concentrations of the chloride ion. The results obtained justify the continuation of the work to determine factors affecting susceptibility in terms of constants M and C. / Master of Science

A study was conducted to evaluate the effect of specimen dimensions in slow strain-rate environmental effects testing. Tension tests of free machining brass were conducted in a mercuric nitrate solution at a constant crosshead displacement rate of 10⁻³(inch/sec). Thirty-six smooth round bar specimens with different dimensions were tested. It was shown that percent elongation to failure was inversely proportional to an effective ratio of length to diameter, ((D - 2a)L / D²), where D is the specimen diameter, L is the length of the reduced cross section of the specimen, and a is the environmentally induced crack depth. This effective length to diameter ratio correlates with the applied tearing modulus for a cracked round bar tension specimen as defined by P. C. Paris and co-workers in 1979. The results verify that the tearing modulus may be used as a parameter to evaluate tearing instability in terms of elastic-plastic fracture mechanics. More directly, these results show a possible source of error in evaluating the degree of susceptibility to environmentally induced cracking in a material-environment interaction. / Master of Science

<p dir="ltr">Irradiation-assisted stress corrosion cracking (IASCC) of austenitic stainless steels (SSs) remains one of the most critical material degradation issues in light water reactors (LWRs). This study presents new alloy design strategies and mechanisms to develop IASCC-resistant stainless steels. Additive manufacturing provides not only new mechanisms to suppress IASCC but also high-throughput means to support alloy exploration. New SS design concepts are demonstrated to significantly enhance IASCC resistance, and mechanistic insights are proposed.</p><p dir="ltr">In the first part of this study, we systematically explored the root cause of the superior IASCC resistance of additively manufactured 316L SS after the hot isostatic pressing (HIP) in high-temperature water, compared to 316L SS in other forms. It was found that the overall radiation hardening was not an accurate measure of IASCC susceptibility. A decreased strain localization along grain boundaries, caused by dislocation channel broadening, was identified as the main reason for the IASCC resistance. The phenomenon was further confirmed through <i>in situ</i> straining tests under the TEM. The second part developed a high-throughput approach utilizing directed energy deposition (DED) to accelerate alloy design and testing for improving IASCC resistance. We explored the effects of reactive elements (REs), such as Hf, Ti, and Y, on the IASCC of 316L SS. All of these REs suppressed the radiation hardening, radiation damage, and IASCC of 316L SS, although their contributions varied with concentrations. It is suggested that radiation-induced segregation is not necessary to cause IASCC, while hardness and strain localization exhibited a stronger correlation to the IASCC. Finally, based on the roles of these reactive elements, a new type of SS was developed, which exhibited superior resistance to stress corrosion cracking (SCC) and IASCC. The low level of radiation damage and high corrosion resistance were considered the primary factor.</p>