Effect of process variables on the corrosion resistance of super duplex stainless steel

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 3.5m. 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 850C, 1000ºC and 1300ºC on its microstructure and its localised corrosion resistance. Heat treatment at 850ºC and 1000C caused formation of intermetallics such as sigma () and chi () while at 1300C 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.EThOS - Electronic Theses Online ServiceGBUnited Kingdo