Microbiologically Influenced Corrosion Of Friction Stir Welded 654SMO Super Austenitic Stainless Steel

Recent years microbiologically influenced corrosion(MIC)has attracted a lot of attention.MIC is widespread around our life and engineering practice and can lead to huge losses.New corrosion resistant alloys were invented to prevent the corrosion failure of engineering materials.Super austenitic stainless steel(SASS)is a kind of stainless steel which contains high contents of Cr,Ni,Mo and possesses excellent corrosion resistance and mechanical properties.SASS has great potential to be used in ocean,oil and soil field to take place of conventional stainless steel.Welding is a must process when metal material is applied.Nevertheless,welding process may deteriorate the properties of welding joint.So it isessential to use a better welding method to get good welding joint.Till now the MIC of SASShas not been reported and we know nothing about the MIC behavior,characteristics and regularity of SASS.It is necessary to investigate the MIC behavior of SASS,which is important for the understanding of MIC of high alloy stainless steel and broadening applications of SASS.The research and conclusions are as below.Microbiologically Influenced Corrosion of 654SMO Super Austenitic Stainless Steel in the presence of Pseudomonas aeruginosa.The main conclusions are as follows.(1)EOCP shifted to negative direction in the presence of P.aeruginosa.P.aeruginosa also causes the decent of polarization resistance Rp,charge transfer resistance Rct and increasing of corrosion current density icorr.The results show that the corrosion rate of 654SMO SASS was greatly accelerated by P.aeruginosa.(2)The result of polarization curves shows that P.aeruginosa led to the decreasing of pitting potential,which demonstrates that SASS was easier to form pitting corrosion in the presence of bacteria.The CLSM result also proves this conclusion.The deepest pitting corrosion for sample in biotic broth was 2.83 ?m,which is much higher than that of sample in sterile medium.(3)The X-ray photoelectron spectroscopy analysis result shows that P.aeruginosa can catalyze the formation of CrO3 from stainless steel surface.The unstable CrO3 will dissolve in the solution and cause the depletion of Cr from passive film,which will lead to the breaking down of passive film.Microbiologically Influenced Corrosion of friction stir welded 654SMO Super Austenitic Stainless Steel in the presence of Acidithiobacillus caldus SM-1.The main conclusions are as follows.(1)No obvious welding flaws were founded in the welding joint by observing the cross section.The welding process formed four regions in the joint:base metal(BM),stir zone(SZ),heat affected zone(HAZ)and thermo-mechanically affected zone(TMAZ).The friction stir welding process led to extremely fine grain of stir zone,which may be attributable to the low heat input and the breaking down of coarse grain.The grain in HAZ did not grow up compared to those of BM.(2)The biofilm and corrosion morphology were observed by SEM,live/dead staining and CLSM.And the pits depth was measured.Very dense A.caldus SM-1 biofilm formed on the sample surface.The biofilm promoted the pitting corrosion of stainless steel significantly.The deepest pits on weldment and BM were 3.86 ?m and 3.74 ?m,respectively.HAZ was easier to be attacked by pitting corrosion by observing the pits distribution on different regions.(3)The results of electrochemical measurements including LPR,EIS and polarization curves show that A.caldus SM-1 accelerated the corrosion rate of SASS significantly and deteriorated the passive film property,which is related to the high concentration of H+in local place.BM and FSW weldment show similar corrosion resistance,while welding joint was easier to be attacked by pitting corrosion in the presence of A.caldus SM-1.