No description available.

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)

Thesis (Ph. D.)--Lehigh University, 2002. / Includes vita. Includes bibliographical references (leaves 143-159).

This dissertation is a part of a collaborative work between the University of Texas, Austin-Texas, the University of Oulu, Oulu-Finland, and Outokumpu Stainless Oy, Tornio-Finland, , to develop commercial austenitic stainless steels with high strength and ductility. The idea behind this work involves cold-rolling a commercial metastable austenitic stainless steel -- AISI 301LN stainless steel to produce strain-induced martensite, followed by an annealing treatment to generate nano/sub-micron grained austenite. AISI 301LN stainless steel sheets are cold-rolled to 63% reduction and subsequently annealed at 600°C, 700°C, 800°C, 900°C and 1000°C for 1, 10 and 100 seconds. The samples are analyzed by X-Ray diffraction, SQUID, transmission electron microscopy, and tensile testing to fundamentally understand the microstructural evolution, the mechanism for the martensite [implies] austenite reversion, the formation of nano/sub-micron austenite grains, and the relationship between the microstructure and the strength obtained in this stainless steel. The results show that cold-rolled AISI 301LN stainless steel consist of dislocation-cell martensite, heavily deformed lath-martensite and austenite shear bands. Subsequent annealing at 600°C for short durations of 1 and 10 seconds leads to negligible martensite to austenite reversion. These 600°C samples exhibit a similar microstructure to the coldrolled sample. However, for samples annealed at 600°C for 100 seconds and those annealed at higher temperatures (700°C, 800°C, 900°C and 1000°C) exhibit equiaxed austenitic grains of sizes 0.2[mu]m-10[mu]m and secondary phase precipitates. The microstructural analysis also reveals that the martensite [implies] austenite reversion occurs via a diffusion-type reversion mechanism. In this regard, a generalized form of Avrami's equation is used to model the kinetics of martensite [implies] austenite phase reversion. The results from the model agree reasonably well with the experiments. Furthermore, the activation energy for grain growth in nano/sub-micron grained AISI 301LN stainless steel is found to be ~ 205kJ/mol, which is comparable to values observed in coarse grained commercial stainless steels (AISI 304, 316). However, the driving force for grain growth in nano/sub-micron grained AISI 301LN stainless steel is considerably higher when compared to other stainless steels. Finally, the average grain sizes in AISI 301LN stainless steels are related to the mechanical properties obtained, through the Hall-Petch relationship. / text

Abstract Purpose – The purpose of this paper is to supplement the scant previous investigations on the corrosion behaviour of 2205 and 2507 duplex stainless steels in selected organic acids containing chloride additions. Design/methodology/approach – Microstructural examination of the alloys was first carried out, after which the corrosion behaviour of the alloys in citric, oxalic, formic and acetic acids containing chloride additions at varying temperatures was studied using electrochemical techniques. Findings – The alloy 2507 material had a larger grain size than did the alloy 2205 sample. The corrosion resistances of the alloys generally are highest in acetic acids and lowest in citric acid. The addition of chloride had a pronounced effect on their corrosion resistance. Alloy 2507 generally exhibited higher corrosion resistance in all of the acids than alloy 2205, with the exception of acetic acid at room temperature. The 50:50 ratio of ferrite to austenite composition, as revealed by phase compositional analysis, indicated no significant possibility for galvanic corrosion between the phases. This suggests that the corrosion behaviour of the alloys is controlled by their grain sizes and chemical compositions. Originality/value – Although the corrosion behaviour of duplex stainless steels in some organic acid media has been reported, this investigation covers the major organic acids not previously reported. Since in real industrial systems a mixture of both organic and minerals acids/salts may typically exist, investigations of the combined effect of chloride ions with the organic acids reported in this paper typify real industrial operations. The paper thus provides a basis for material selection for the application of 2205 and 2507 in industrial systems where organic acids are mostly used.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

No description available.

No description available.

The title of this research suggests the importance of manufacturing variables and their influence on the behavior of super duplex stainless steel (SDSS) alloy when exposed to corrosive aqueous environments. Studies show that SDSS alloys have many applications due to their good physical, mechanical and corrosion resistance properties. However, there are a number of cases of in-service failures of SDSS alloys, such as pitting corrosion, stress corrosion cracking and environmentally assisted cracking failures. The root cause of these failures was due to the impact of one or more manufacturing variables such as surface finish, microstructure defects or deficiencies after deformation. From this concept, a detailed study into the effect of manufacturing variables on the corrosion resistance of a UNS S39274 super duplex stainless steel has been carried out. The manufacturing variables studied are as follows: Effect of surface condition; The corrosion resistance of stainless steel components is very sensitive to the final surface condition. The effect of surface condition on the corrosion resistance was investigated by measuring the critical pitting temperature (CPT) and the pitting potential (Epit) of in-service UNS S39274 super duplex stainless steel tubular sections with three different surface roughness values of Ra =2.8, 3.2 and 3.5m. Experimental results confirmed that the surface roughness parameter Ra is a useful parameter as a general and basic guideline of surface texture but it’s not sufficient on its own to predict the likely effect of surface roughness on the in-service corrosion behaviour of SDSS materials. The manufacturer of super duplex stainless steel products should include the electrochemical corrosion evaluation such as the CPT or Epit values beside the surface roughness average value Ra and PRENs in SDSS alloy assessment. Effect of microstructure; The thermo-mechanical handling of super duplex stainless steels is an important factor in their performance. This is due to the complex nature of the material which is highly alloyed with Cr, Ni, Mo, and N. Exposure to certain temperature ranges leads to undesired secondary phases formation. The presence of intermetallic phases has been found to be harmful to super duplex stainless steel properties. A study was undertaken to evaluate the effect of thermal exposure of a solution annealed UNS S39274 super duplex stainless steel to temperatures of 850C, 1000ºC and 1300ºC on its microstructure and its localised corrosion resistance. Heat treatment at 850ºC and 1000C caused formation of intermetallics such as sigma () and chi () while at 1300C caused precipitation of secondary austenite and enlarged grains and percentage of ferrite was observed. The formation of intermetallics of sigma () and chi () had a drastic effect on the corrosion resistance of the super duplex stainless steel. Electrochemical measurements confirmed that the intermetallics affected the corrosion resistance by reducing the pitting potential by around 80% in the active direction. Evaluation of the corrosion morphology using SEM revealed depletion at the ferrite/austenite interfaces created intergranular corrosion path with preferential attack of the ferrite phase. The present results confirmed that PRENs is not suitable for corrosion assessment of SDSS after exposure to intermetallic temperature ranges. Effect of cold work; Cold work introduces metallurgical changes like dislocations and deformation bands which affect the corrosion resistance of stainless steel components. Cold work is still a complicated phenomenon for ferritic/austenitic alloys such SDSS. The effect of cold work (0%, 4%, 8%, 12% and 16% plastic strain) on the pitting corrosion behaviour of UNS S39274 SDSS was evaluated. The Ferrite phase of the material exhibits greater degrees of plastic deformation under straining providing a greater number of initiation sites than the austenite phase. The hydrogen embrittelement of the cold worked specimens was investigated after 48 hours cathodic charging in 0.1M H2SO4. Experimental results indicated that the increase in cold work caused reduction in mechanical properties such as elongation and strain to failure. The depth of embrittlement increased as the degree of cold work increased and revealed brittle surfaces of cleavage fracture mode. Manufacturer and users should consider the degree of plastic strain when cold worked SDSS components going to work in corrosive environments or hydrogen containing environments. The main conclusion of the work is that the interactions between corrosive environments and SDSS components containing one or more of the manufacturing variables evaluated must be considered if reductions in in-service life are to be avoided.

No description available.