| In retrospect,corrosion has been a subject of intense research,and is reported as one of the causative agents of nuclear power plant(NPP)and its components deterioration worldwide.The impact of corrosion in the nuclear sector is detrimental both structurally and economically.One major area of concern is the recent clamour to extend the operational license of NPPs beyond their anticipated lifespan(customarily 40 years),to about 50%longer than the initial period.In order for this to happen,there would be additional demand of the reactor’s components to maintain their structural integrity for a time longer than expected.The prolonged extension in the service life of NPP reactors may introduce a variety of potential material aging issues such as galvanic corrosion occurring in dissimilar metal weld(DMW)components that should be looked into in the NPP’s license renewal process,in order to mitigate potential hazards and risk to human life and the environment.However,to date,the menace of the impact of galvanic corrosion of these DMWs components like the reactor pressure vessel and piping still leaves researchers with the craving to seek better understanding of the galvanic corrosion mechanism involved.In order to understand in micro-scale and macro-scale the galvanic corrosion damage of the reactor pressure vessel dissimilar metal weld in pressurized water reactor primary water environment,this thesis studies the microstructural constituents and characteristics of the DMW,the galvanic corrosion behaviour of the coupled materials,mechanism of corrosion between the interacting coupled dissimilar metals in ambient and high temperature environment,the effects of various factors that play important roles in the galvanic corrosion of the galvanic couple,and the evolution of the galvanic corrosion morphology of the corroding material.The main work includes the following aspects:The microstructure and micro-hardness of low alloy steel(LAS)A508 welded with 309L/308 L stainless steel(SS)are investigated.The microstructure evaluations show that the heat affected zone of the A508 base metal changes,from tempered bainite to an uneven mixture of bainite and martensite,depending on the intensity of the heat flow from the welding process.Different recrystallization processes at the heat affected zone of the base metal are responsible for the changes of the microstructure,which lead to differences in the grain sizes,strain level,and yield strength in this region.The greatest deformation level is observed in the grain refined region with the highest heat influx,which leads to a high level of grain refinement and stress in this region,followed by the partial grain refined region and base metal matrix region,respectively.The interface region between the base metal and weld metal is observed to be very complex.At some regions of the 309L/308L stainless steel,adjacent to the fusion boundary type Ⅱ boundaries,resulting from the allotropic ferrite(δ)-austenite(γ)transformation and migration of steel phases/boundary at high temperature of welding were observed,alongside the type Ⅰ boundaries.The 309L/308L stainless steel consists of vermicular ferrite distributed along the austenite matrix,while ternary sigma secondary phase was observed to precipitate at the interface of the ferrite/austenite boundary.Finally,the heat affected zone of the base metal showed the highest hardness followed by the base metal matrix and weld metal,respectively.The microscopic scale correlation between microstructure and galvanic corrosion of low alloy steel A508 and its welded 309L/308L stainless steel overlayer is investigated.This change in the microstructure leads to a heterogeneous corrosion attack on the base metal(anode),while the weld metal(cathode)is protected in a galvanic interaction.The galvanic interaction across the DMW surface analysed with scanning vibrating electrode techniques shows that the most transformed coarse grain refined region adjacent to the fusion boundary was the most susceptible to corrosion attack,owing to its complex surface constituents.Furthermore,the extent and rate of corrosion attacks on the surface constituents are quantified by AFM,of which the MnS induced pitting corrosion which occurred simultaneously with the galvanic corrosion between A508 base metal and 309L/308L stainless steel welded couple,was the most precarious mechanism of corrosion attack.The synergistic effects of area ratio and microstructure on the galvanic corrosion of LAS A508/309L/308L SS dissimilar metals weld are studied by a multi-analytical approach.It was demonstrated that decreasing the anode/cathode surface area ratio obviously enhances the corrosion rate of A508,both locally and globally.Deeper analyses of the AFM results enabled quantitative comparison of the corrosion behaviour of the different surface constituents.It was revealed that in the galvanic interaction of the DMW,the grain refined region corrodes most,followed by the partial grain refined region and base metal matrix of the A508,respectively.The electrochemical localization index(LI)estimation method and AFM analysis both confirmed the presence of a mixed(localized and uniform)corrosion phenomenon occurring on the surface of the A508 anode metal in the galvanic interaction of the dissimilar metals.In addition,the degree of synergism equation was utilized to describe the synergistic effects of anode/cathode area ratio and the microstructure of the samples on the galvanic corrosion of LAS A508/309L/308L SS DMW.The effects of oxide film and exposure time on the galvanic corrosion between low alloy steel A508 and 309L/308L stainless steel dissimilar metals are studied by several electrochemical techniques and surface composition analytical tools.The zero resistance ammeter(ZRA)result shows that the trend of galvanic corrosion current between the coupled LAS A508 and 309L/308L SS changes with respect to exposure time.Furthermore,from the ZRA measurement,the quantitative relationship between galvanic current and dissolution rate of the coupled LAS A508 anode metal was established by statistical approach.Electrochemical impedance spectroscopy(EIS)results and analyses demonstrated that the increasing galvanic corrosion rate observed between the coupled dissimilar metals as exposure time prolonged was suppressed by the oxide film effect.The suppression occurred as a result of the transformation of unstable oxides to stable oxide forms,and the oxide film thickening phenomenon experienced on the LAS A508 surface as exposure time prolonged.Finally,the galvanic activity of the LAS A508/309L/308L SS DMW in high temperature primary water environment are studied with electrochemical techniques and long-term immersion test.The results show that the LAS A508 acts as the anode,while the SS acts as the cathode in a galvanic interaction of both materials.However,the influence of the driving force between the coupled metals(potential difference)is greatly reduced in high temperature environment and at some point reverse trend was recorded.Also,the ZRA and long-term immersion tests shows that increasing the area ratio of cathode to anode can promote galvanic corrosion,especially the corrosion rate of the fusion line base metal(LAS A508).The high temperature immersion test shows that the maximum corrosion rate in the depth direction at the DMW interface is 0.350 mm/y,which is 15.22 times that of uniform corrosion of LAS A508 under uncoupled conditions.Galvanic corrosion at the interface mainly extends along the welding fusion line,rather than along the depth of A508 low alloy steel,while the LAS A508 local corrosion rate far away from the fusion line is mainly affected by its own microstructure. |
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