| Metastable pitting of stainless steel,as a typical early stage of localized corrosion phenomena,such as stable pits,intergranular corrosion and stress corrosion cracking,has been an active subject in the research area of corrosion science.Due to stochastic characteristics and small size of metastable pits,it is usually characterized as tiny transient signals under an applied anodic potential.The in-situ characterization of metastable pitting events and on-line monitoring of electrochemical transient signals are rather difficult,which limit in-depth understanding the mechanism of metastable pitting.At present,numerous studies reported research of metastable pitting based on the statistical analysis of electrochemical transient signals.Further more,the formation of metastable pitting affected by many factors,such as inclusions,precipitates,matrix structure and deformation states,etc.Therefore,how to systematically analyse the initiation and growth of metastable pitting behavior under different conditions,and clarify the correlation between transient signals and pit morphologies is the key to understand the metastable pitting mechanism of stainless steel.To systematically investigate the metastable pitting,304 stainless steel(304 SS)and 2205 duplex stainless steel(2205 DSS)were employed which subject to different conditions,such as solution-treatment,sensitisation and U-bending deformation.The effects of sensitisation,tensile deformation and compressive deformation on the metastable pitting initiation and growth behavior were revealed.The discriminating parameter was proposed to distinguish different types of transient signals.The correlation between the types of transient signals and the morphologies of metastable pitting was clarified.The formation mechanism of metastable pitting was further elucidated.The detailed contents are as following:(1)By statistical analysis on the characteristic parameters of metastable pitting in potentiostatic polarization curves,it was found that the initiation frequency of metastable pitting in 304 SS and 2205 DSS was as follows:solution-treated<sensitised,and the sensitised 304 SS had the highest initiation frequency,which can reach 1.35/(s·cm2).For samples under different deformation states,the initiation frequency of 304 SS increased in the order:tensile deformation<as-received<compressive deformation;while for 2205 DSS,as-received<tensile deformation<compressive deformation.The characteristic parameters,such as the peak current(Ip),pit lifetime(tlifetime)and integral charge(Qpit),followed log-normal distribution.Sensitisation,tensile deformation and compressive deformation had only a minor influence on the growth rate of most metastable pits.The results showed that the maximum volume of metastable pits(Vmax)calculated based on the current transients followed two-slope of extreme distribution.It was found that sensitisation,tensile deformation and compressive deformation promoted the growth of few metastable pits,thus increased the transition probability from metastable pitting to stable pitting,and degraded the pitting resistance.(2)Based on the analysis of transient signals characteristics in potentiostatic polarization curve,it was proposed that the shape of transient signals can be comprehensively and objectively characterized based on the correlation relationship between the two correlated parameters of(tg/trep)and[(Ip/Qg):(Ip/Qrep)].The characteristic parameter overcomes the confusion of types recognition between the growth time(tg)and the repassivation time(trep),or the(Ip/Qg)and(Ip/Qrep),and effectively identifies three types of current transients.It was found that type I transient signals relate to a slow rise and fast decay,characterized by(tg/trep)>1 and[(Ip/Qg):(Ip/Qrep)]<1;type ? transients correspond to the current transient characterized as a fast rise and rapid drop,both(tg/trep)and[(Ip/Qg):(Ip/Qrep)]were close to 1,showed a symmetrical shape;when(tg/trep)<1 and[(Ip/Qg):(Ip/Qrep)]>1,it represents type ?transient characterized as sudden rise and slow decay in current.(3)Four patterns of pitting morphologies were found by analyzing numerous of metastable pits after potentiostatic polarization tests.The results showed that pattern A morphologies relate to round or oval shape of metastable pits,which has a wide range of size distribution and presents relatively discrete state,and mainly initiates at the sites of inclusions or ? precipitates;morphologies of pattern B characterized as long strip of metastable pits;pattern C morphologies of metastable pits are smaller than 1.16?m2,and correspond to metastable pits distributed in chain shape,often initiated at small sulfide inclusions;pattern D morphologies characterized with micro-cracks around the metastable pits,which were initiated at sites of the composite oxide inclusions.According to the clustering analysis on types of transient signals and morphologies of metastable pitting,the correlation between the transient signals and metastable pitting morphologies was clarified.Type I current transients correspond to pattern A and pattern B metastable pitting morphologies.Type II transient signals relate to pattern C morphologies of metastable pitting in sensitised 304 SS.Type III transitent signals correspond to pattern D metastable pitting morphologies in 2205 DSS under tensile and compressive deformation.(4)The correlation between the formation of metastable piting and the passive film compactness was studied by electrochemical impedance tests.It showed that the film stability was degraded by the sensitisation,tensile deformation and compressive deformation,the reactions at the matrix/film interface increased,consequently promoting the nucleation and growth of metastable pitting.Metastable pits mainly initiated at the sites of inclusions.For sensitised 304 SS,small size of MnS inclusions can also induce the initiation of metastable pitting due to the degradation of passive film.The Cr23C6 precipitates were not considered to initiate metastable pits in 304 SS,while ? phases can induce the initiation of metastable pitting in 2205 DSS.The debonding and crevice defects formed at the matrix/MnS inclusion interface accelerated the dissolution of the inclusion during the open circuit potential before polarization,decreased available sites for initiation of metastable pitting,therefore,the frequency of metastable pitting decreased in 304 SS;the crevices and micro-cracks formed at the matrix/oxide inclusions interface by tensile deformation,promoted the initiation frequency of metastable pits in 2205 DSS.The results showed that the MnS inclusions were still tightly bonded with the matrix,while the oxide inclusions were fractured and de-bonded from the matrix under compressive deformation.At the same time,due to the degraded stability of the passive film,the metastable pitting initiation rate in 304 SS and 2205 DSS increased by compressive deformation.The preferential pit initiation sites transferred from y phase to a phase by tensile deformation and compressive deformation.(5)The effects of the compactness of passive film,inclusions,precipitates and matrix structure on the formation mechanism of metastable pitting were analyzed by radar maps.It was found that the susceptibility of metastable pitting greatly depends on the compactness of passive film.Metastable pitting initiation generally occurs at inclusions and precipitates because they were the susceptible sites of passive film on the surface.The growth behavior was affected by the matrix structure.Based on this,a duplex passivation treatment was proposed to improve the metastable pitting behavior:Firstly,removed the inclusions through acid pickling,followed by the formation of a highly-protective of Cr-enriched passive film maintained by electrochemical passivation in sodium nitrate solution.After the duplex passivation treatment,the pitting potential(Epit)was close to 0.99 VSCE in 0.6 M Cl-solution at 70? for 304 SS,more excellent pitting resistance than that of pickling passivation which was 0.31 VSCE,the susceptibility to metastable pitting also decreased significantly for 304 SS. |
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