High frequency corrosion fatigue crack propagation behavior of AISI 316L stainless steel was studied in 1M NaCl and 1M NaCl + 0.01M Na₂S₂O₃, under various anodically and cathodically polarized potentials, and dessicated air at 22°C and 1 atmosphere pressure. Constant load amplitude fracture mechanics techniques employing single edge notch specimens were used to assess the fatigue crack growth rate in the various environments. Unique specimen preparation procedures were developed which allowed near-threshold behaviors to be studied under gradually rising crack tip stress intensity conditions. Polarization studies showed that the presence of thiosulphate catalyzed the dissolution of stainless steel in low pH solutions(pH~1), due to reduction of thiosulphate species to H₂S, but had no effect in the near neutral solutions. Fatigue tests conducted in the neutral NaCl + Na₂S₂O₃ solution at cathodic potentials showed that the presence of thiosulphate had an insignificant effect. This indicated that high frequency fatigue produces efficient exchange of bulk solution with the crack tip environment, which prevented the lowering of pH in the crack by hydrolysis effects and prevented reduction of thiosulphate to H₂S. Fatigue crack retardation phenomena were very pronounced in the near-threshold regions in most of the fatigue tests. The cause of this retardation was attributed mainly to the surface-roughness- induced crack closure effect, which reduced the effective crack tip cyclic stress intensity ΔKth to a lower level. This closure effect only predominated in the near-threshold region where significant Mode II loading was present. The influence of various imposed anodic and cathodic potentials was found to be consistent with the surface-roughness-induced crack closure effects. The observed crack growth accelerating effect of high anodic potentials was attributed to the corresponding high removal rate of surface roughness in the wake of the crack, which kept the effective cyclic stress intensity level near the applied values. Crack fractography was studied by scanning electron microscopy. It showed that the fractography generally consisted of three regions; a crystallographic cleavage-like near-threshold region, a feathery and fibrous transition region, and a striated region. Using an etch pitting technique, it was determined that the crack plane and crack propagation directions in the near-threshold region were mainly those of {111} <110>, {110} <112>, {110} <001>, and other higher indexed planes. These crack orientations were effected by the activation of a single slip system or the alternate activation of two intersecting slip systems. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

The stress corrosion cracking (SCC) of an At ST C-1018 mild steel was investigated in three solutions, composed of 12.5 mol/kg NaOH, 3.35 mol/kg NaOH and 2.5 mol/kg NaOH + 0.42 mol/kg Na₂S, respectively. The potential of maximum susceptibility to SCC of steel in the latter two solutions was assessed by a slow strain rate technique. It was found to be slightly higher than the active-passive transition in each solution (-1.00 Vsce in 3.35 mol/kg NaOH and -0.88 Vsce in 2.5 mol/kg NaOH + 0.42 mol/kg Na₂S). A fracture mechanics technique, utilizing fatigue precracked double cantilever beam specimens, was then used to study the effects of stress intensity, temperature and electrochemical potential on crack velocity in all three solutions. Both stress intensity dependent (region I) and stress intensity independent (region II) crack velocity behavior was found. Apparent activation energies for region II of ~ 24 kJ/mol were determined at both Ecorr and -1.00 Vsce in 12.5 mol/kg NaOH. Crack velocities of the order of 10⁻⁹ m/s were measured at Ecorr in 12.5 mol/kg NaOH and at -1.00 Vsce and -0.88 Vsce 3.35 mol/kg NaOH and 2.5 mol/kg NaOH + 0.42 mol/kg Na₂S, respectively. The crack velocities measured at -1.00 Vsce in 12.5 mol/kg NaOH were of the order of 10⁻⁸ m/s. The fractography of the cracks was transgranular in 12.5 mol/kg NaOH at Ecorr. A mixed intergranular-transgranular fractography was observed at the active-passive transition in all three solutions. The results of the two techniques were compared and discussed, as was the role of stress intensity and passivation rate in fracture mechanics experiments. Anodic dissolution, hydrogen embrittlement and adsorption mechanisms were considered. It was decided that the results at Ecorr in 12.5 mol/kg NaOH could best be accounted for by a hydrogen embrittlement mechanism, perhaps assisted by anodic dissolution. Hydrogen embrittlement was eliminated as a possible mechanism at the active-passive transition in all the solutions. The most likely mechanism was thought to be one involving mixed activation-diffusion controlled dissolution. Applications of the results to the pulp and paper industry were considered. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

A variation of the double cantilever beam specimen has been calibrated and used to study the propagation of stress corrosion cracks as a function of stress intensity in 316 and 310 stainless steels, and a TRIP steel exposed to hot aqueous magnesium chloride solutions. The effects of cold work, temperature and applied potential on both the fractography and cracking rates have been examined. The effects of cold work and crack path on crack branching were also investigated. Bpth stress Intensity dependent (Region I) and stress intensity independent (Region II) cracking were observed. Region II having apparent activation energies from 15.1 kcal/g.mole to 18.1 kcal/g.mole. The crack velocities of 25% cold rolled 316 were found to be independent of applied potential over a range of more than 50mV in Region I and 75mV in Region II. In the same material the crack path changed from solely transgranular at low stress intensities and noble potentials to more than 80% intergranular at high stress intensities and active potentials. The topography of the transgranular fracture was similar to that observed by others except in the case of the TRIP steel where nodular features were observed. These observations have been discussed with respect to mechanisms involving the following (i) electrochemical dissolution, (ii) absorption of hydrogen and (iii) adsorption of a damaging species. Of these, an adsorption assisted process is most compatible with the observations. Qualitatively the adsorbed species are envisioned as modifying the behaviour of the surface atoms at the crack tip. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Steel is widely used as building material for large-scale structures, such as oil and gas pipelines, due to its high strength-to-weight ratio. However, corrosion attack has been long recognized as one of the major reasons of steel pipeline degradation and brings great threat to safety in normal operation of structure. To mitigate the corrosion attacks, coatings are generally applied to protect steel pipelines against corrosion and improve durability of the associated structures for longer service life. Although have higher corrosion resistance, coated pipelines will still get corroded in a long run, as coatings may subject to damages such as cracks. Cracks on coatings could lower the effectiveness of protection for associated structures. Timely updates of up-to-date corrosion rate, corrosion location, and coating conditions to the pipeline risk management model and prompt repairs on these damaged coatings would significantly improve the reliability of protected structures against deterioration and failure. In this study, a corrosion risk analysis system is developed to detect and locate the corrosion induced coating cracks on coated steel using embedded fiber Bragg grating (FBG) sensors. The coatings investigated include high velocity oxygen fuel (HVOF) thermal sprayed Al-Bronze coating, wire arc sprayed Al-Zn coating, and soft coating. Theoretical models of corrosion risk assessment system were carried out followed by systematic laboratory experiments, which shows that the developed system can quantitatively detect corrosion rate, corrosion propagations, and accurately locate the cracks initialized in the coating in real time. This real-time corrosion information can be integrated into pipeline risk management model to optimize the corrosion related risk analysis for resource allocation. To place the sensing units of the system in the most needed locations along the huge pipeline systems for an effective corrosion risk assessment, an example case study is conducted in this study to show how to locate the most critical sensor placement locations along the pipeline using worst case oil and gas discharge analysis. Further applications of the developed system can be integrated with pipeline management system for better maintenance resource allocations. / USDOT-PHMSA

Stainless steels were developed in the early 20th century and are used where both the mechanical properties of steels and corrosion resistance are required. There is continuous research to allow stainless steel components to be produced in a more economical way and be used in more harsh environments. A necessary component in this effort is to correlate the service performance with the production processes. The central theme of this thesis is the mechanical grinding process.  This is commonly used for producing stainless steel components, and results in varied surface properties that will strongly affect their service life. The influence of grinding parameters including abrasive grit size, machine power and grinding lubricant were studied for 304L austenitic stainless steel (Paper II) and 2304 duplex stainless steel (Paper I). Surface integrity was proved to vary significantly with different grinding parameters. Abrasive grit size was found to have the largest influence. Surface defects (deep grooves, smearing, adhesive/cold welding chips and indentations), a highly deformed surface layer up to a few microns in thickness and the generation of high level tensile residual stresses in the surface layer along the grinding direction were observed as the main types of damage when grinding stainless steels. A large degree of residual stress anisotropy is interpreted as being due to mechanical effects dominating over thermal effects. The effect of grinding on stress corrosion cracking behaviour of 304L austenitic stainless steel in a chloride environment was also investigated (Paper III). Depending on the surface conditions, the actual loading by four-point bend was found to deviate from the calculated value using the formula according to ASTM G39 by different amounts. Grinding-induced surface tensile residual stress was suggested as the main factor to cause micro-cracks initiation on the ground surfaces. Grinding along the loading direction was proved to increase the susceptibility to chloride-induced SCC, while grinding perpendicular to the loading direction improved SCC resistance. The knowledge obtained from this work can provide a reference for choosing appropriate grinding parameters when fabricating stainless steel components; and can also be used to help understanding the failure mechanism of ground stainless steel components during service.

Les systèmes Cu /Ni-P/Au sont utilisés comme contacts électriques car ils présentent une conductivité électrique élevée, alliée à un bon comportement mécanique et une résistance à la corrosion. Le Cu possède une conductivité électrique unique qui en a fait le métal le plus utilisé en électronique. Cependant, sa faible résistance à la corrosion nécessite l’application de revêtements protecteurs. Les sous couches de Ni (généralement Ni-P) permettent essentiellement d’éviter la diffusion entre Cu et Au. Enfin, la couche de finition en Au est utilisée pour garantir la durée de vie des contacts électriques. Pour des raisons économiques, ce film de faible épaisseur est poreux, entrainant ainsi un couplage galvanique entre l’Au et le Ni au détriment du nickel. Ainsi ce travail est dédié à l’identification et la mise en œuvre des stratégies visant à améliorer la durée de vie des contacts électriques et plus globalement des modules électroniques.Lors de cette thèse, nous avons développé 3 stratégies : (1) améliorer les propriétés de la couche barrière de Ni, (2) remplacer l’or par un métal moins onéreux, (3) sceller les pores de la couche d’Au les propriétés du film barrière Ni-P améliorées notamment par des additifs tels que la glycine. Sn a également imposé un effet similaire. D’autres couches de finition nobles NiAg et NiPd ont été étudiées. Bien que des films hautement adhésifs aient été formés par potentiel pulsé, ces films poreux n'offraient pas un comportement correct à la corrosion. Enfin, il a été découvert que les pores de la couche de finition en Au peuvent être efficacement scellés par électrodéposition de poly méthacrylate de méthyle / Cu/Ni(Ni-P)/Au systems are used as electrical contacts due to their combination of electrical conductivity, corrosion resistance, and mechanical behavior. Cu has a unique electrical conductivity that made it the most used metal in electronics. However, protective coatings must be applied on Cu due to its poor corrosion resistance. Au films are used to secure a proper lifetime of electrical contacts. Ni films are essential to avoid the diffusion of Cu into Au. Electrodeposition is the method of choice to form these multi-layer systems. The Au top-coat is notably thin and hence porous. The corrosive media penetrate through these pores, hence electrical contacts are suffering from a galvanic coupling. This work is dedicated to identify and test the strategies to enhance the lifetime of electrical contacts and electronic modules. Three strategies were detected, (1) improve the properties of the Ni barrier layer, (2) replacing the Au film with a thicker but cheaper alternative metal, and (3) seal the pores of Au top-coat using a post-treatment process. It was found out that the properties of the Ni-P barrier film can be notably improved by additives such as glycine. Sn also found to be highly advantageous for forming NiSn barrier coatings. NiAg and NiPd noble top-coats were investigated as alternatives to Au thin films. Although highly adhesive films were formed using the pulse deposition, the films were porous and thus did not offer a proper corrosion behavior. And finally, a cathodic electropolymerization was employed as a post-treatment method. It was found out that the pores of Au top-coat can be effectively sealed by the electrodeposition of polymethyl methacrylate

In this Master of Science thesis screws with different coatings were exposed to an accelerated corrosion test in order to investigate the corrosion development. The test matrix constituted of hexagonal and flange screws (fastened on frames) coated with either zinc/iron (ZnFe) or a flake system of aluminum and zinc (GEOMET). The corrosion results were analyzed by means of X-ray diffraction (XRD). In this study the screws were also crosscut and analyzed by means of FEG-SEM in order to determine the thickness and the elemental content of the surface coating. The result of the corrosion test showed that the screws coated with GEOMET showed a very good corrosion resistance whilst the screws coated with ZnFe failed to fulfill the demands on corrosion resistance of Scania.

Stainless steels were developed in the early 20th century and are used where both the mechanical properties of steels and corrosion resistance are required. There is continuous research to allow stainless steel components to be produced in a more economical way and be used in more harsh environments. A necessary component in this effort is to correlate the service performance with the production processes. The central theme of this thesis is the mechanical grinding process.  This is commonly used for producing stainless steel components, and results in varied surface properties that will strongly affect their service life. The influence of grinding parameters including abrasive grit size, machine power and grinding lubricant were studied for 304L austenitic stainless steel (Paper II) and 2304 duplex stainless steel (Paper I). Surface integrity was proved to vary significantly with different grinding parameters. Abrasive grit size was found to have the largest influence. Surface defects (deep grooves, smearing, adhesive/cold welding chips and indentations), a highly deformed surface layer up to a few microns in thickness and the generation of high level tensile residual stresses in the surface layer along the grinding direction were observed as the main types of damage when grinding stainless steels. A large degree of residual stress anisotropy is interpreted as being due to mechanical effects dominating over thermal effects. The effect of grinding on stress corrosion cracking behaviour of 304L austenitic stainless steel in a chloride environment was also investigated (Paper III). Depending on the surface conditions, the actual loading by four-point bend was found to deviate from the calculated value using the formula according to ASTM G39 by different amounts. Grinding-induced surface tensile residual stress was suggested as the main factor to cause micro-cracks initiation on the ground surfaces. Grinding along the loading direction was proved to increase the susceptibility to chloride-induced SCC, while grinding perpendicular to the loading direction improved SCC resistance. The knowledge obtained from this work can provide a reference for choosing appropriate grinding parameters when fabricating stainless steel components; and can also be used to help understanding the failure mechanism of ground stainless steel components during service. / <p>QC 20160203</p>

A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Master of Science in Engineering Johannesburg, 2018 / The use of austenitic stainless steels in harsh environments at elevated temperatures has increasingly become a global problem, these alloys can fail unpredictably when subjected to tensile stresses and chlorides. Hence the study was focused on understanding the environmentally assisted cracking of Ru enriched laser alloyed layers on 304L stainless steel in a corrosive environment at elevated temperatures. The Ru composition of laser alloyed samples was 0, 0.96, 1.96, 4.74 and 9.2 wt%. Microstructural analysis and microhardness measurements were performed in order to understand the grain orientation and resistance to indentation respectively. The bend beam SCC test was conducted by stressing the samples to 350 MPa and exposing them to 50 ppm sodium chloride with 10 ppm dissolved oxygen at 160°C for 172 hours. The results revealed a significant improvement in the SCC resistance. The samples with lower Ru content (0, 0.98 and 1.96 wt%) were less susceptible to SCC when compared to as-received 304L stainless steel. Cracks initiated from pits and propagated transgranularly on the alloyed layer. The crack growth rate decreased as the Ru content was increased. The samples with 4.74 and 9.2 wt% Ru were immune to SCC. Electrochemical test results showed improved corrosion resistance when the Ru level was increased to 1.96 wt%. Thereafter, there was a gradual increase in corrosion rates for samples with 4.74 and 9.2 wt% Ru. However, these corrosion rates were lower when compared to as-received 304L stainless steel. Another SCC test was conducted to investigate fractography of vacuum remelted samples alloyed with Ru. The results showed ductile failure for most of the samples and the maximum stress threshold of 580 MPa was archived on samples with 1.07 wt% Ru. There was a sudden increase in failure time, % elongation and % reduction in area when the Ru content was increased to 1.07 wt%. In essence, laser surface alloying 304L stainless steel with higher Ru content (more than 2wt%) improves SCC resistance, but does not improve the general corrosion resistance, therefore a careful selection for any application is necessary. However, the cost analysis revealed the laser surface alloying of 304L stainless steel with Ru to be more efficient over other corrosion resistant materials. / MT 2018

Steel corrosion is one of the major distress mechanisms that causes the deterioration of reinforced concrete structures around the world. It is an electrochemical reaction between the reinforcing steel and the surrounding concrete that produces a mass loss of the metal. Through the process of corrosion in reinforced concrete, iron ions get oxidized to form corrosion products (CP). Although multiple experiments and studies have been developed to understand the rheological behavior of corrosion products, this topic stays inconclusive. This work aims to characterize corrosion products at micro-scale in order to trace the progress of the formation of rust, to determine its nature and to analyse its rheological behavior in reinforced concrete. An experimental procedure to produce CP in the laboratory is also presented in this research. In addition, material characterization methods have been used to identify the iron oxide phases present in CP, determine their viscosity and rheological behavior and to study how CP flows in a porous media. In order to identify the different stages in the corrosion process, the CP was analysed at 2, 4, 6 and 8 weeks. The experiments identified four phases of iron oxide for each period. Furthermore, it was found that CP behaves as a shear-thinning slurry and as a result, its viscosity decreases with the applied shear rate. In addition, the damage caused by CP on concrete depends on the w/c ratio of the concrete mix and the exposure time to a corroding environment. The rebar mass loss results show that CP is formed in layers around the rebar, and the flow of each CP layer can differ.