Effect of surface damage induced by cavitation erosion on pitting and passive behaviors of 304L stainless steel

The corrosion behavior of 304L stainless steel(SS)in 3.5wt%NaCl solution after different cavitation erosion(CE)times was mainly evaluated using electrochemical noise and potentiostatic polarization techniques.It was found that the antagonism effect resulting in the passivation and depassivation of 304L SS had significant distinctions at different CE periods.The passive behavior was predominant during the incubation period of CE where the metastable pitting initiated at the surface of 304L SS.Over the rising period of CE,the 304L SS experienced a transition from passivation to depassivation,leading to the massive growth of metastable pitting and stable pitting.The depassivation of304L SS was found to be dominant at the stable period of CE where serious localized corrosion occurred.

海洋环境中碳钢和不锈钢螺栓紧固件的腐蚀机制差异研究

通过对碳钢和不锈钢紧固件在海洋环境中的腐蚀特征、腐蚀产物和电偶极化等内容的比较性研究,提出了两种不同的腐蚀机制。对于碳钢紧固件,锈层增加了额外的IR降,削弱了阴极区对阳极缝隙区的极化作用,供氧的差异导致螺纹曝露部位的腐蚀更为严重,腐蚀形式以均匀腐蚀为主。对于不锈钢紧固件,缺氧的环境导致螺纹缝隙部位的钝化膜性能劣化,螺杆曝露区对螺纹缝隙区的电偶极化作用促使缝隙区腐蚀更为严重,腐蚀形式以点蚀为主。针对碳钢和不锈钢紧固件在海洋环境中的不同腐蚀机制,提出了差异化的腐蚀防护技术思路。

Insights into the characteristics of corrosion products formed on the contact and exposed regions of C1045 steel bolt and nut fasteners exposed to aqueous chloride environments

This study investigated the characteristics of corrosion products formed on the contact and exposed re-gions of C1045 steel bolt and nut fasteners exposed to aqueous chloride environments.The corroded sur-face morphology,rust compositions,and corrosion kinetics of the bolt specimen were studied by visual observation,optical microscopy(OM),scanning electron microscopy(SEM),X-Ray diffractometry(XRD),micro-Raman,electron probe micro-analyser(EPMA),and potentiodynamic polarization techniques.Re-sults obtained showed a variation in corrosion kinetics,morphology,and composition of the rust layer which were driven by differential aeration and concentration effects.Due to the availability of sufficient dissolved oxygen,the oxyhydroxide compound,lepidocrocite(γ-FeOOH)was detected in the outer rust layer in the exposed region,whereas the inner rust layer was composed of magnetite(Fe_(3)O_(4)).How-ever,the oxygen-deficient contact surface revealed the presence of akaganeite(β-FeOOH)and magnetite(Fe_(3)O_(4))as dominant oxide phases.The most stable phase,goethite(α-FeOOH)was also detected in the rust formed in both regions,though in significantly low amounts.Furthermore,owing to variation in environmental conditions,the amount and density of the rust layer varied in the different regions.The estimated corrosion stability values for the different regions revealed that the corrosion products formed on the steel surfaces were non-protective,suggesting the need for specific surface treatment as a protec-tive measure.

Effects of rare earth modifying inclusions on the pitting corrosion of 13Cr4Ni martensitic stainless steel

In this study,the pitting corrosion behavior of 13Cr4Ni martensitic stainless steel(BASE)and that modified with rare earth(REM)in 0.1 mol/L Na Cl solution were characterized.Techniques such as automatic secondary electron microscope(ASPEX PSEM detector),scanning electron microscope(SEM),transmission electron microscope(TEM),scanning Kelvin probe force microscope(SKP),potentiodynamic and potentiostatic polarizations were employed.The results obtained indicate that BASE steel contains Al_(2)O_(3)/Mn S,Al_(2)O_(3) and Mn S inclusions,while REM steels contain(La,Ce,Cr,Fe)-O and(La,Ce,Cr,Fe)-O-S inclusions.Compared with BASE steel,REM steel is more susceptible to induce the metastable pitting nucleation and repassivation,whereas it restrains the transition from metastable pitting to stable pitting.Adding 0.021%rare earth element to BASE steel can reduce the number and area of inclusions,while that of 0.058%can increase the number and enlarged the size of inclusions,which is also the reason that pitting corrosion resistance of 58 REM steel is slightly lower than that of 21 REM steel.In the process of pitting corrosion induced by Al_(2)O_(3)/Mn S inclusions,Mn S is preferentially anodic dissolved,and also the matrix contacted with Al_(2)O_(3) is subsequently anodic dissolved.For REM steels,anodic dissolution preferentially occurs at the boundary between inclusions and matrix,while(La,Ce,Cr,Fe)-O inclusions chemically dissolve in local acidic environment or are separated from steel matrix.The chemically dissolved substance(La^(3+) and Ce^(3+))of(La,Ce,Cr,Fe)-O inclusions are concentrated in pitting pits,which inhibits its continuous growth.

Corrosion Behaviour of Carbon Steel Fasteners in Neutral Chloride Solution

An experimental model for simulating the corrosion of carbon steel fasteners(bolt and nut) composed of a contact carbon steel electrode(CCSE) and an exposed bare carbon steel plate electrode(BCSE) was designed. The effect of coupling on the corrosion process of the galvanically coupled carbon steel electrode was evaluated and compared with the self-corrosion process observed independently at the exposed and contact regions. Results obtained indicated that at an equal area ratio and uncoupled conditions, the corrosion rate is accelerated in the surface directly exposed to bulk solution compared to the bolt surface in contact with the nut. A coupling current was recorded when the exposed surface(BCSE) was electrically connected with the contact surface(CCSE);with the CCSE acting as the anode thereby suppressing the corrosion process in the exposed surface. By implication, the galvanic coupling between CCSE and BCSE increased the corrosion rate of CCSE. The diff erence in oxygen supply was responsible for the coupling effect observed in the system as there was no decrease in the solution pH. Moreover, varying the cathode-to-anode area( S c/S a) ratio significantly influenced the corrosion current density as increased S c/S a ratio resulted in an accelerated galvanic corrosion process. The corroded surfaces and interfaces were analysed using stereomicroscopy and scanning electron microscopy. X-ray diff ractometry was adopted for corrosion product characterization. The results obtained showed supportive evidence of the corrosion behaviour in carbon steel fasteners.

Active anticorrosion and self-healing coatings:A review with focus on multi-action smart coating strategies

The development of smart coatings with potential for active anticorrosion and self-healing protection of metals is essential for long-term performance of metallic structures in aggressive chemical environments.Presently,emphasis has been placed on the development of advanced smart coatings for corrosion protection in different applications.Innovative multifunctional coatings with fascinating stimuliresponsive functionalities are considered“smart”.The stimuli-responsive functionalities of these smart coatings when properly harnessed result in a class of coatings with inherent autonomous control of corrosion.Fundamentally,when metals are exposed to aggressive environments,occurrences at the metalsolution interface cause environmental changes.These changes can be controlled when triggers from external environment set off active components of smart coating,thereby enhancing coating’s life and functionality.Common triggers include the availability of moisture,concentration of chloride ion,p H gradient,mechanical damage,impact,fatigue,light,redox activity and temperature.In this review,recent technological trends in active anticorrosion and self-healing coatings as functional routes for metal protection are summarized,stimuli responsiveness and mechanisms of inhibition are discussed,and recent multi-action protective systems are particularly focused on.