Investigation Of Marine Microbiologically Influenced Corrosion On Duplex Stainless Steel

Duplex stainless steel(DSS),with a well-balanced composition of the ferrite(a)phase and the austenite(y)phase,possesses excellent mechanical properties and corrosion resistance.The strong corrosion resistance extends the use of DSS in more aggressive chloride environments,such as marine environment.The development of DSS is mainly characterized towards two directions in the future.One is the development of economical duplex stainless steel with lower concentrations of nickel,molybdenum and other precious metal elements(like SAF 2304 and 2101 DSS).This kind of DSS can replace 304,316 austenite stainless steel due to its low cost and outstanding comprehensive performance.The other trend is based on the thermal stability of microstructure,further improving the strength(1000 MPa)and corrosion resistance of the conventional DSS.The super duplex stainless steel(SDSS)and hyper duplex stainless steel(HDSS)were developed and applied in some special and harsh environments.With the implementation of national maritime strategy,the rapid development stage of steel materials used in marine engineering is coming.Until now,there is a wide application of DSS in marine environment,and the discussion about the marine microbiologically influenced corrosion(MIC)of DSS is a hot topic in the research of stainless steel.Research of the marine MIC of DSSs can provide a theoretical basis for the development of alloys used in the marine environment,and possess significant values both in scientific and practical applications.In this work,the MIC characteristics of 2707 HDSS and 2304 DSS caused by the marine aerobe Pseudomonas aeruginosa were investigated by using electrochemical methods and surface analysis techniques.The main aim of this study is to observe the change of the P.aeruginosa biofilm morphology,analyze the corrosion behavior of DSSs caused by P.aeruginosa,and reveal the mechanism of MIC caused by P.aeruginosa.The main conclusions in this paper can be obtained as follows:A dense and porous P.aeruginosa biofilm could be found on the surfaces of 2707 HDSS and 2304 DSS coupons.The formation of the biofilm formation was affected by hydrophobic and morphology of the coupon interface.XPS analysis showed that the presence of Cr6+ underneath the biofilm may involve in the removal of Cr from the steel surface by the P.aeruginosa biofilm,leading to the chromium depleted regions and destroying the passive film.The loss of Cr made it more difficult to repair the passivation as well,thus induced the localized pitting corrosion.By combining with EDS results,the rapid dissolution of the anode metal caused the formation of metallic oxide in the corrosion products and led to the severe pitting corrosion.The P.aeruginosa biofilm formation should be responsible for the localized pitting corrosion on the coupon surface.CLSM results showed that the significant pits formatted on the coupon surface after 14-day exposure in the P,aeruginosa broth.The largest pitting depths on 2707 HDSS and 2304 DSS coupon surfaces reached to 0.69 ?m and 11.0?m respectively,suggesting that both of the materials cannot completely immune to MIC.And it was obviously that 2707 HDSS possesses better MIC resistance property by comparison.In the sterile medium,no significant pits were found on the surface of 2707 HDSS coupons,it can immune to the chloride ion corrosion in seawater.While many pits can be observed on 2304 DSS coupon surface and the largest pitting depth was 4.8 ?m,confirming that it suffered chloride ion corrosion and the presence of P.aeruginosa may considerably accelerate the corrosion process.So the P.aeruginosa biofilm played a major role in the pitting corrosion of DSS.The electrochemical test showed that the presence of the P.aeruginosa biofilm changed the corrosion potential,increased the corrosion current density.2707 HDSS possessed a relatively stable passivation film structure.The biocatalysis of the P.aeruginosa was regarded as the key factor that led to the positive move of the corrosion potential and the acceleration of the cathodic oxygen reduction.The MIC phenomenon on 2304 DSS was more severe and the anodic metal dissolution was accelerated.The P.aeruginosa biofilm was capable of accelerating electrons from the metal matrix,resulting in serious pitting corrosion.Comparing 2707 HDSS,2304 DSS and 2205 DSS in the P.aeruginosa containing medium,the order of MIC degree after 14 days of incubation was:2304 DSS>2205 DSS>2707 HDSS,suggesting that optimization of the material properties can improve the MIC resistance.