Investigation On The Crevice Corrosion Behavior Of 316L Stainless Steel In Seawater By Marine Organisms

Crevice corrosion in seawater is the most complex corrosion process in ocean engineering,with biological factors exacerbating the complexity and severity of crevice corrosion.However,the impact of marine organisms on the crevice corrosion behavior of stainless steel is still unclear,which limits its widespread application in marine environments.This paper took 316L stainless steel commonly used in ocean engineering as the research object.The crevice corrosion behavior of 316L stainless steel in natural seawater was analyzed,and the characteristics of bacterial community and solution composition in the crevice microenvironment were investigated.Meanwhile,the effects of acidification of crevice solution and marine Sulfate-reducing bacteria（SRB）on crevice corrosion behavior in laboratory simulation experiments were studied.Finally,the crevice corrosion behavior of 316L stainless steel in seawater under the action of marine organisms was summarized.The main research content and results were as follows:The crevice corrosion behavior of 316L stainless steel in natural seawater was studied through field experiments conducted in the Xiamen sea area.The results showed that after 36 months of immersion,the maximum corrosion pit depths of artificial crevice corrosion and biological crevice corrosion were 1892μm and 2132μm,respectively.The initiation process of artificial crevice corrosion was mainly controlled by the IR drop theory,while the development process was mainly controlled by the critical crevice solution theory.The corrosion pit with the maximum depth was located at the edge of the crevice,and the internal morphology showed austenite grain with grain boundary corroded.Barnacle was the typical marine organism that induced crevice corrosion.Corrosion rings appeared on the surface of stainless steel along the periphery of the shell substrate,and the microscopic corrosion morphology underwent a process of changing from metastable pitting morphology to grooved morphology to austenite grain with grain boundary corroded morphology.The erosion of grain boundaries by barnacle glue accelerated the initiation of crevice corrosion caused by barnacles.From the perspective of long-term corrosion behavior,the acidification of the solution inside the crevice was the main factor for the sustained development of the above two types of crevice corrosion.High-throughput sequencing technology was used to explore the bacterial community structure and diversity in the crevice environment of natural seawater,and the solution composition in the crevice microenvironment was measured in situ by microelectrode.The results showed that after soaking for 12 months,there were differences in the abundance and dominant bacterial species of bacterial communities inside and outside the crevice.With the increase of the crevice depth,the bacterial abundance in the crevice gradually decreased,which could be 2～3 orders of magnitude lower than that at the crevice mouth.The dominant bacteria in the crevice were Vibrio,Halomonas and Alteromonas in turn,of which the average relative abundance of Halomonas was about 20%,significantly higher than that outside the crevice.At the same time,bacteria involved in sulfur cycle corrosion such as Thiomonas and Desulfurization Vibrio were also found inside the crevice.The main characteristics of the solution environment inside the crevice were low oxygen concentration and acidity.The dissolved oxygen concentration inside the crevice was always below 0.5 mg/L.The farther away from the crevice mouth,the lower the pH value,and the lowest measured value could be reduced to around 4.4.Additionally,there was a phenomenon of enrichment of anions and cations such as Cl^-,Na⁺ and K⁺.The effect of solution acidification inside the crevice on the crevice corrosion behavior of 316L stainless steel was studied by simulating the environmental difference inside and outside the crevice in a double electrolytic cell device.The results indicated that under the condition of aerobic neutrality outside the crevice and anaerobic acidity inside the crevice,there was a galvanic effect inside and outside the crevice.When the pH value of the solution in the anaerobic environment inside the crevice was 7,the open circuit potential difference between the inside and outside of the crevice could reach 0.49 V,and the driving force of the galvanic effect was strong,with a greater tendency to cause crevice corrosion.However,at the same time,the galvanic current density was low,only 6.2 × 10^-9 A·cm^-2.In addition,the galvanic effect would increase the potential of the metal inside the crevice to the galvanic potential,thereby affecting the repassivation process of the metal inside the crevice.When the solution was acidified to a pH value of 1,the open circuit potential difference between the inside and outside of the crevice was only 0.15 V,and the driving force of the galvanic effect was weakened.However,at this time,the galvanic current density was two orders of magnitude higher than that at a pH value of 7,reaching 2.01 × 10^-7 A·cm^-2,implying that the corrosion rate of metal inside the crevice accelerated.Under this condition,the crevice corrosion behavior of 316L stainless steel would be more dependent on the degree of acidification of the solution inside the crevice,rather than the galvanic effect.The growth characteristics of a typical marine SRB Desulfovibrio ferrophilus inside the crevice were studied using prefabricated biofilms.The results showed that the growth of the SRB could be divided into an exponential growth period,a stable period and a declining period.The presence of the crevice affected the transport process of nutrients and in turn affected the growth process of the SRB inside the crevice.Under conditions without the crevice,biofilms were distributed in large clusters,while under conditions with the crevice,biofilm clusters showed smaller sizes but higher densities.At the initial stage of the experiment,the main nutrient inside the crevice,sodium lactate,was abundant,and the SRB obtained its required energy by oxidizing sodium lactate.Subsequently,the nutrients inside the crevice were depleted,and a decrease in the metallic Fe⁰ content on the surface of stainless steel and a decrease in the concentration of SO₄^2-in the crevice solution was observed,indicating the existence of a "biological cathode" phenomenon,where the SRB directly obtains electrons from the metal matrix while reducing SO₄^2-to obtain the energy required for its metabolism and accelerate metal corrosion.The crevice corrosion behavior of 316L stainless steel in the seawater containing Sulfate-reducing bacteria（SRB）was further studied by the separation of cathode and anode.The results showed that the presence of marine Sulfate-reducing bacteria accelerated the crevice corrosion process of 316L stainless steel in seawater,but weakened the galvanic effect inside and outside the crevice.The entire crevice corrosion process of 316L stainless steel was jointly controlled by the enhancement of crevice solution corrosiveness and the microbial concentration corrosion process.The SRB biofilm outside the crevice was distributed in clusters,and the area covered by the biofilm generated FeS and FeS₂ due to the metabolic activity of SRB,resulting in the formation of numerous local concentration batteries under the biofilm.The numerous small anodes formed outside the crevice weaken the galvanic effect inside and outside the crevice,and accelerated corrosion outside the crevice.The internal corrosion of the crevice was due to the enhanced corrosiveness of the solution inside the crevice and the presence of the "biological cathode"phenomenon of SRB,resulting in continuous corrosion.