Sulphate-reducing bacteria are known to play an important role in anaerobic corrosion processes. They are often found associated with metal surfaces and their activities can be of particular economic significance in many industrial areas. The aim of this thesis was to investigate anaerobic corrosion of metals by the sulphate-reducing bacteria in the presence of mixed microbial consortia in various heterogeneous marine environments. A laboratory model system was designed, based on part of an offshore oil storage system. An extensive bacteriological analysis and comprehensive study of the consequent physicochemical parameters involved in the microbial corrosion process was carried out. Particular attention was paid to the activity of the sulphate-reducing bacteria on metal surfaces. A method was developed to measure both acid-volatile and non-acid-volatile sulphur formation, produced by the activity of the sulphate-reducing bacteria on mild steel coupons. The importance of this method is firmly stressed. Previous results involving rates of sulphate reduction estimated without considering non-acid-volatile sulphur product formation, must be interpreted with caution. A study of non-biological methods of analysing corrosion and their various limitations was carried out to assess their usefulness in determining the effect of microbial corrosion in various environments. It must be stated that no single technique can be used to study anaerobic microbial corrosion. Therefore, it is recommended that a series of tests should be utilised. These should include microbiological, chemical and metallurgical methods.

Abstract SIMULATION, MEASUREMENT AND IMAGE ANALYSIS OF CORROSION INITIATION AND GROWTH RATE OF ALUMIUM 2024 AND STEEL 304 By Yan Fang, MS A thesis submitted in partial fulfillment of the requirements for the degree of M.S. of Mechanical and Nuclear Engineering at Virginia Commonwealth University. Virginia Commonwealth University, 2011 Major Director: Dr. Brian Hinderliter, Ph. D Associate professor, Mechanical and Nuclear Engineering Time: 11 a.m. Place: Room E3210 Engineering East Hall Date: Friday August 12, 2011 Corrosion initiation and growth rate are important properties in maintaining structural integrity, especially for surface and pit corrosion of common infrastructure and transportation metals. In this study, the surface corrosion pit initiation and growth rate on Aluminum 2024(common aerospace alloy) and Steel 304(common alloy for infrastructure) in different pH solutions was measured and values were analyzed by image analysis over a scheduled time. A MATLAB algorithm was developed for detecting the initiation and growth rate of pits as a function of time. The developed algorithm was validated with simulated specimen as well as experiments conducted corrosion specimen. Based on the result of obtained, the MATLAB algorithm predicts the right trends and power law radial corrosion pit growth rates and should be useful for corrosion initiation and growth predictions in various metals.

The sweet corrosion profiles of X-52 carbon steel in 0.l% NaCl at 30, 40, 50 and 60°C were analysed. The corrosion tests lasted for a period of one month. They were performed using a flow loop under a total gas pressure of 1 atm. containing carbon dioxide (99.95% & 100 vpm O2). The corrosion profiles were obtained by scanning the corroded surfaces with a stylus. The results were analysed and compared with linear polarisation resistance (LPR) data. lt has been found that the (LPR) data at 40°C, i the absence of significant pitting, progressively underestimated the actual metal loss values. It has been proposed that the above discrepancy is due to the occurrence of small scale localised corrosion. The above effect was found to be exaggerated at 60°C, where extensive pitting occurred. Attempts to find a correlation between the (LPR) measurements and the actual metal loss distributions were unsuccessful. Furthermore, although sudden increase of the instantaneous corrosion rates for passive samples appeared to signal the onset of localised attack, i the absence of prior knowledge of the relative spread of the localised corrosion, it was not possible to quantify the observed errors. The range of the metal loss distributions was found to increase with time. This was most notíceable at 60°C. The progress of localised corrosion therefore, appears to have extended the distributions towards the extreme values. It has been proposed that, in the absence of severe pitting, the sweet corrosion profiles can be approximated by the Weibull statistical function. For pitted samples, where the localised attack resulted i metal loss on different scales, the upper 10% of the values were also found to be approximated by the Weibull function. The Weibull function was used for predicting the progress of the metal loss values with time. The analysis of the top 10% values, for pitted surfaces at 60°C, however, suggested the occurrence of an accelerating rate of attack. The original model was therefore improved to provide a time dependent sweet corrosion model. It has been estimated that the corrosion rate at 60°C increases with time at intervals of approximately 8 days. The effect of methanol on the sweet corrosion of X-52 carbon steel in 1% NaCl was also studied under stagnant conditions at 4, 40 and 60°C. The partial pressure of carbon dioxide was maintained at 0.3 bar. lt has been found that the corrosion rate decreases with increasing methanol concentration. The data was used to develop a predictive model within the range of 10 to 60% vol. methanol.

A thesis submitted to the Faculty of Engineering and the Built Environment, University of Witwatersrand, Johannesburg in fulfilment of the requirements for the degree of Doctor of Philosophy (Engineering), 2017 / Chromium oxide provides an inexpensive and practical means of increasing the corrosion resistance of austenitic stainless steel in most environments. However, the oxide is prone to dissolve in reducing acids and in chloride containing solutions, which compromises the durability and effective operation of structures made of austenitic stainless steel. This research project explored the use of thin ruthenium surface alloys produced by ion implantation, RF sputtering and pulsed electrodeposition (PED) to improve the corrosion resistance of AISI 304L austenitic stainless steel in reducing acids and chloride solutions via a technique known as cathodic modification. The properties of the alloyed 304L stainless steel were evaluated using a number of tools including X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), potentiodynamic polarisation, and electrochemical impedance spectroscopy (EIS). Preliminary tests in 1 M sulphuric acid showed that the ruthenium surface alloys sufficiently raised the corrosion potential of 304L stainless steel to ranges where the stability of chromium oxide is guaranteed. Surface alloys produced by RF sputtering and PED were associated with the best corrosion resistance, and protection efficiencies of at least 85%, but they spalled during corrosion exposure rendering them unsuitable for corrosion application. The corrosion of the ruthenium implanted surface alloys exhibited a strong dependence on the surface roughness of the stainless steel, with the least corrosion rates achieved on rough 304L stainless steel samples implanted with 1016 Ru/cm2 at 50 keV. Corrosion characterisation of these ruthenium implanted surface alloys was studied in various corrosive media including sulphuric acid, sodium chloride, magnesium chloride and simulated fuel cell solutions. Their corrosion rates in sulphuric acid decreased with increase in acid concentration, and exhibited non-Arrhenius behaviour in the acid solutions; corrosion rates were unaffected by increasing exposure temperature from 25 to 50°C. In 3.5 wt% sodium chloride, addition of ruthenium via ion implantation changed pit morphology from elongated to circular, indicating a diminished tendency for pits to initiate at manganese sulphide stringers. Corrosion rates of the ruthenium implanted stainless steels in the simulated fuel cell solutions were at least 69% lower than the target corrosion rate for use in polymer electrode membrane fuel cells (PEMFCs), thus presenting a possible practical application of ruthenium surface alloyed austenitic stainless steel. / CK2018

University of Macau / Faculty of Science and Technology. / Department of Electromechanical Engineering

The penetration of chloride ions plays a crucial role in reinforcement corrosion and affects the durability and service life of marine structures. The problem is even more acute once cracking occurs in a concrete member. The presence of cracks is shown to have a significant impact on chloride penetration into concrete. It has been recognized, for concrete cracked in flexure, the chloride diffusion coefficient in the tension zone is higher than that in the compression zone. Even a single crack in the tensile zone could contribute to a higher diffusion coefficient compared to that of uncracked members. In this study, the influence of flexural cracks on chloride ion penetration into reinforced concrete beams has been investigated experimentally and a two-dimensional theoretical model is developed. The experimental study investigated the influence of multiple flexural cracks on the chloride diffusivity of reinforced concrete beams and on the corrosion rate of reinforcing bars. The size of the reinforced concrete beams tested is 200 x 250 x 2200 mm with 20mm concrete cover. The beams were pre-cracked using a compression testing machine and the crack widths induced were maintained between 0.1 mm to 0.2 mm. The chloride profile was determined after 1 month, 1 year and 2 years of immersion in 3.0% NaCl solution. The measured chloride profiles of the beams with multiple cracks are compared to those obtained from the uncracked (control) beam. An observation was also made on the effect of cracks on the corrosion development of steel bar after two years of immersion in salt solution. Chloride concentration varied linearly with the depth of crack (at crack planes). A twodimensional model using finite element analysis is developed to evaluate the chloride penetration into cracked concrete. This model was used to verify the chloride penetration data which is obtained from the experimental results. The prediction of chloride ingress, using the two dimensional model agrees well with the short-term and long-term chloride profiles.

Passivation, local anodic depassivation and pitting corrosion of stainless steels in bulk and under thin phase layers of acid sulphate and neutral chloride solutions have been studied. A number of electrochemical, chemical, metallographic and physical techniques were employed in this study. The choice of stainless steels, test solutions and methods of investigations was based on scientific expediency and practical availability. For the first time in electrochemical and corrosion characteristics of passivation, local anodic depassivation and pitting corrosion of stainless steels were obtained in thin layers and compared with those in bulk electrolytes. It was demonstrated that the thickness of the electrolyte layer can influence the passivation process and, under certain conditions, the kinetics of pitting erosion. Several tests were conducted and results given. It was shown that the prognosis of corrosion behaviour of some stainless steels in sulphur dioxide contaminated atmosphere based on electrochemical measurements in bulk electrolytes can be erroneous. The same measurements carried out in thin phase layers enable more accurate prediction. However, the prognosis of the relative resistance of the stainless steels to pitting corrosion in the clean chloride-containing atmosphere may be based on conventional electrochemical measurements in the bulk of corresponding electrolytes / Doctor of Philosophy (PhD)

Solid oxide fuel cells operate at elevated temperature, oxidising fuel gases to generate electricity. The fuel gas streams in the fuel cell systems are rich in carbon and have very low oxygen potential. Under these conditions, alloys can undergo metal dusting, which causes pitting or general thinning of the alloys. This process is not yet fully understood. It is, hence, not possible to accurately predict the susceptibility of a particular alloy in the atmospheres relevant to SOFC. Model Fe-Cr and Fe-Ni-Cr alloys were exposed to test the hypothesis that cementite formation and its decomposition is necessary for metal dusting to occur. A series of ferritic and austenitic engineering alloys were also exposed to compare their dusting rates. Two specimens of each alloy were studied, one was etched in a H3PO4-15%H2SO4-21%H2O solution and the other was ground to a 600-grit finish. The alloys were exposed to a CO-26%H2-6%H2O gas mixture at 680oC under thermal cyclic conditions. The hot gas composition corresponded to ac = 2.9 and an oxygen potential high enough to oxidise chromium, but not iron or nickel. All the alloys were shown to undergo internal carburisation, metal dusting and coking once the protective chromium oxide scale was damaged. Fe-25Cr was less resistant than Fe-60Cr because of its lower chromium content. However, ferritic Fe-25Cr-based steels are more resistant to dusting than austenitic Fe-25Cr-25Ni. The present findings are consistent with the earlier conclusions that cementite formation is essential for dusting on ferritic steels and that dusting of austenitic alloys does not involve the prior formation of cementite and its decomposition. The onset of metal dusting was more accelerated for most austenitic engineering alloys (Alloy 800, Inconel 601, 690, 693 and Alloy 602CA) than for engineering ferritic steels (Fe-27Cr-0.001Y). However, the alloy with the best performance was austenitic Inconel 625, which was still protected by its Cr2O3 scale after 500 one-hour cycles. In both ferritic and austenitic chromia-formers alloys, the surface ground specimens were more resistant to metal dusting than the electropolished specimens. In contrast, ferritic alumina-formers with electropolished surfaces did not dust during the entire experimental periods of 1200 one-hour cycle, but the alloys with ground surfaces slowly underwent dusting attack. The coke deposits formed consisted largely of graphite nanotubes, containing small particles at the tube tips. These particles were identified as single crystal cementite, in the case of ferritic steels, and austenite, for the austenitic alloys. This is not in agreement with the currently accepted dusting model for ferritic steels that cementite decomposition yields iron particles, which catalyse coke deposition. EDX analysis of the cementite particles, showed that the only metal detected was iron, thus differing in chemistry from the (Fe,Cr)3C surface layer. Similarly, the austenite particles contained only nickel and iron, differing in chemistry from the disintegrated alloy surfaces. These results suggested that the particles were formed in the coke in the carbon-supersaturated gas, rather than disintegration of the alloy surface layer. Strong orientation relationships were determined between the graphite and cementite particles; however, no clear crystallographic relationship was deduced between the graphite and austenite. Relative alloy performance appears consistent using the present multiple one-hour cycle and the results of others using a smaller number of lengthy cycles. Hourly thermal cycling was shown to accelerate the dusting onset for both electropolished chromia-formers and surface ground alumina-formers. Protective oxide scales spall at a critical thickness and carbon attack results when the alloy surfaces are depleted of scale-forming elements and healing becomes impossible. On this basis, analytical models were developed and used to predict the incubation periods for oxide failure and the subsequent carbon attack. Upon testing, these models were, however, found to be not qualitatively meaningful in predicting the onset of dusting observed in the present study. Gross oversimplifications involved in the model and the absence of reliable data for many parameters required for the computations prevented even an approximate quantitative prediction.

This thesis describes the stress corrosion cracking of a series of aluminium bronze alloys and attempts to characterize the cracking in relation to the various mechanisms proposed. The theories of cracking applicable to the aluminium bronze/ammonia system include; the anodic dissolution, the film rupture and the embrittlement theories, which are described in the literature review. The copper aluminium alloy system was selected for investigation as it exhibits a martensite transformation, making the cracking of these alloys in ammonia solutions a usefulful comparison with the cracking of stainless steels in chloride solutions. The study used 'U-bend' specimens stressed in ammonia solution preconcentrated with copper, while the oxidation of the alloys under the same conditions was examined using unstressed specimens. Oxidation in this thesis is taken to mean an anodic dissolution process with or without the formation of a metallic oxide layer. The tests showed three binary alloys to be susceptible to intergranular and transgranular cracking, while two commercial alloys only cracked transgranularly and only in pH = 11 ammonia solution. Stress corrosion conditions gave rise to sharp, narrow cracks traversing all the phases in the alloy with matching features on opposing fracture surfaces, whereas oxidation gave rise to less distinct, more rounded features with a definite phase dependence. Further experiments showed the limited penetration and reactivity of a corrosive solution along narrow cracks. It is concluded that stress corrosion cracking and oxidation occur by different mechanisms and that the mechanism of stress corrosion cracking is unlikely to involve oxidation as a necessary step to cracking. The results of the present study support an embrittlement mechanism of stress corrosion cracking in aluminium bronze alloys.

Thesis (Ph. D.)--University of Florida, 2000. / Title from first page of PDF file. Document formatted into pages; contains xxii, 263 p.; also contains graphics. Abbreviated abstract copied from student-submitted information. Vita. Includes bibliographical references (p. 252-262).