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The localized corrosion behavior of different inclusions in a simulated solution of coastal industrial pollution is investigated in this study. It is indicated in the results that the Al2O3–MnS composite inclusions of ordinary carbon Q345B steel have microcracks and induce severe lattice distortions in the surrounding matrix. The MnS in the Al2O3–MnS composite inclusions dissolves preferentially to induce pitting corrosion. The microcracks and severe lattice distortion of the surrounding matrix accelerate the corrosion propagation. The preferential dissolution of CeAlO3–Ce2O2S inclusions induces pitting in No. 2 steel. The inherent physical and chemical properties of the inclusions and the chemical corrosion of the simulated fluid are vital in the initiation and expansion of pitting corrosion. The TiN in the test steel exhibits excellent resistance to pitting corrosion, and the double‐layer composite inclusions completely covered by TiN do not form corrosion pits in the simulated solution and do not cause the dissolution of the adjacent matrix. The coordinated addition of Ti–Ce can reduce the size of the inclusions and improve the probability of complete cladding of TiN. [ABSTRACT FROM AUTHOR]

The pitting corrosion of ZL101A aluminum alloy in the thin electrolyte layer containing Cl− was studied through the salt spray test. The pitting corrosion behavior was mainly characterized by scanning electron microscopy, electrochemical measurements and in-situ scanning Kelvin probe force microscopy. The results indicated that the second phases in ZL101A aluminum alloy had significant effects on the pitting corrosion behavior. The Al–Si phase acted as an anode phase induced the initiation of corrosion pits, which leaded to the decrease of polarization resistance (R) and the propagation of corrosion pits along the depth direction. Act as cathodic phase, Mg–Si–Fe phase and Si-rich/Al-poor phase have an 800 mV and 660 mV lower Volta potential than the matrix, respectively. The Volta potential difference between Mg–Si–Fe phase and matrix decreased more significantly than that between Si-rich/Al-poor phase and matrix which remained intact with the duration of corrosion. This verified the higher electrochemical stability of Si-rich/Al-poor phase than Mg–Si–Fe phase, which should be related to the presence of Mg and Fe in Mg–Si–Fe phase. [ABSTRACT FROM AUTHOR]

• Pit initiation in quenching and partitioning processed martensitic stainless steels was investigated. • The corrosion resistance of Q&P steels is controlled mainly by the presence of inclusions. • The lower surface potential at MnS results in selective dissolution of the MnS site. • The higher surface potential at TiN results in selective dissolution at the interface between TiN and the matrix. The present article investigates the influence of chemical composition and phase fractions on the corrosion behaviour of industrially produced quenching and partitioning (Q&P) martensitic stainless steels. Localised corrosion was analysed by scanning Kelvin probe force microscopy (SKPFM) and scanning electrochemical microscopy (SECM) in 3.5 wt.% NaCl solution. SKPFM revealed a Volta-potential difference of around 40 mV between inclusions and the matrix, which is larger than the Volta potential variations within the matrix. This difference in surface potential is a driving force for selective dissolution (corrosion initiation) at inclusions and inclusion/matrix interfaces. SECM detected early pitting initiation, particularly in alloys containing MnS and TiN inclusions. Results suggest that pitting initiation and propagation occur at those specific regions. This study emphasised that irrespective of chemical composition and phase fraction, localised corrosion initiation in Q&P-processed martensitic stainless steels is predominantly governed by the presence of inclusions. [Display omitted] [ABSTRACT FROM AUTHOR]

Adopting twin-roll casting (TRC) technique, lean Mg-1.2Zn-0.3Ca-0.3Al-xMn (x = 0.25, 0.38, 0.5 wt%) alloys with different Al/Mn ratio were prepared. The stoichiometric ratio of Al 8 Mn 5 is taken as dividing line. Under Al/Mn > 1.6, rhombohedral Al 8 Mn 5 is formed, while under Al/Mn ≤ 1.6, cubic β-Mn is generated. Because of lower Volta potential of Al 8 Mn 5 than β-Mn, the alloy with Al/Mn > 1.6 shows the lowest average corrosion rate (4.55 mm y−1). The formation of fine rather than coarse Al 8 Mn 5 weakens micro-galvanic corrosion and relieves stress concentration, thus simultaneously endowing the alloy (Al/Mn > 1.6) with admirable corrosion resistance and mechanical properties (UTS:∼266 MPa, YS:∼193 MPa, EF:∼20 %). [Display omitted] • Effect of Al/Mn atomic ratio on corrosion behavior of Mg-Zn-Ca-Al-Mn alloy is studied. • Rhombohedral Al 8 Mn 5 is formed (Al/Mn>1.6), while cubic β-Mn is formed (Al/Mn ≤ 1.6). • Fine Al-Mn phase are obtained by twin-roll casting and multi-pass rolling. • β-Mn is nobler than Al 8 Mn 5 and larger Al/Mn ratio leads to less film breakdown. • Alloy with Al/Mn> 1.6 shows good corrosion resistance and mechanical property synergy. [ABSTRACT FROM AUTHOR]

The effect of tellurium (Te) on the localised corrosion mechanism in 15–5PH stainless steel is studied in a simulated marine environment. The addition of Te changed the composition and size of the inclusions, which significantly affected the localised corrosion mechanism of the steel. Scanning Kelvin probe force microscopy and current-sensing atomic-force microscopy results show that the inclusions have good electrical conductivity and are always electrochemically dissolved. During the corrosion process, MnS acted as the anodic phase and was preferentially dissolved. The portion of Te in Te-containing complex inclusions increased the Volta potential of the inclusions and changed the localised corrosion mechanism. The appropriate addition of Te increased the pitting potential of 15–5PH stainless steel, thereby improving its localised corrosion resistance. However, superfluous addition of Te resulted in corrosion-resistance attenuation. This can be attributed to the continuous cathodic effect of the larger Te portion in the complex inclusion. The dissolution of Te oxides destabilised the passivation film. • The addition of Te to stainless steel changes the localised corrosion mechanism. • Galvanic coupling effects between the MnS-Te inclusion and steel matrix were revealed. • Appropriate Te addition enhanced the pitting potential and corrosion resistance of 15–5PH stainless steel. • Superfluous Te addition attenuated the corrosion resistance of 15–5PH stainless steel. [ABSTRACT FROM AUTHOR]

The degradation of passive films of a stainless steel by S. algae was investigated at the single-cell level. The focused ion beam-scanning/transmission electron microscopy showed that the passive film underneath the adherent S. algae cell was significantly thinner than that without bacterial coverage after 3 days of inoculation. Atomic force microscopy (AFM) in the nano manipulation mode was used to move the bacterial cell and AFM in the dynamic mode revealed the nanoscale corrosion pit underneath. Peak force tapping-frequency modulation mode of AFM showed high Volta potentials at the pit center and low Volta potentials around the bacterial cell. • FIB-SEM/TEM and multimode AFM are used to study MIC at the single-cell level. • FIB-SEM/TEM shows degradation of stainless steel passive film by a S. algae cell. • Multimode AFM is used to investigate pitting corrosion underneath a S. algae cell. [ABSTRACT FROM AUTHOR]

Weld joints can experience numerous defects, which may become enriched in hydrogen and form pits after an attack. However, few in situ studies have been conducted on the relationship between hydrogen enrichment and pitting initiation sites. Moreover, the mechanism by which hydrogen enrichment promotes the initiation and propagation of pitting is unknown. In this study, the pitting susceptibility of austenitic stainless steel weld joints that had been used for 6 years in a hydrocracking unit was investigated. Scanning Kelvin probe force microscopy (SKPFM) was performed to characterize the Volta potential before and after hydrogen removal. The α/γ phase boundaries of the as-received samples were mostly enriched in hydrogen. Furthermore, the α/γ phase boundaries were the preferential locations for the initiation and propagation of pits, which were located at sites of hydrogen enrichment. The results of focused ion beam analysis revealed that the pits deepened along the α/γ phase boundaries and achieved stability. Hydrogen enrichment promotes the transformation of metastable pits to stable pits, which may be the mechanism underlying the promotion of pitting propagation by hydrogen enrichment. [Display omitted] (a) The Volta potential of α, γ and α/γ phase boundary, (b) SIMS map of hydrogen, SEM high magnification images of pits on (c) the as-received samples and (d) the dehydrogenation samples (baked at 300 °C for 10 h) after immersion test, (e) longitudinal propagation of pits and (f) the pit morphologies in the horizontal direction. [ABSTRACT FROM AUTHOR]

• Strain mainly concentrated within Mg 2 Si particles and at Al/α(Al-Fe-Cu) interface. • Volta potentials of Mg 2 Si and α(Al-Fe-Cu) decreased slightly in the elastic stage. • Volta potentials of Mg 2 Si and α(Al-Fe-Cu) increased in the plastic stage. • The pitting corrosion resistance was improved under elastic deformation. • The pitting corrosion resistance was degraded under plastic deformation. Micro-galvanic corrosion behaviors between two primary intermetallic particles (Mg 2 Si, α(Al-Fe-Cu)) and the Al matrix were investigated under elastic and plastic deformations in AA7075-T6. Strain localization was mainly concentrated within Mg 2 Si and at the interface of α(Al-Fe-Cu)/Al matrix under deformations. The experimental Volta potential differences between Mg 2 Si, α(Al-Fe-Cu) and Al decreased by 0–16 mV and 2–27 mV in the elastic stage but increased by 0–39 mV and 0–81 mV in the plastic stage, which were consistent with the calculation results. Accordingly, the pitting corrosion resistance was improved under elastic deformation but deteriorated under plastic deformation. [ABSTRACT FROM AUTHOR]

For the first time, we present that the heterogeneous microstructure of Ce/Y/La-based multi-main-phase (MMP) magnets, the origin of intergrain and intragrain Volta potential discrepancy (a), initiates peculiar corrosion mechanisms: (b) preferential corrosion of RE-rich intergranular phase yields an abnormal increase in the corrosion resistance, which is ascribed to high surface integration of stable REFe 2 and RE 2 Fe 14 B phases; (c) local Ce-rich region with lower Volta potential, including Ce-rich shell (type I, upper red rectangle) and Ce-rich core (type II, bottom blue rectangle), evolves as crucial proceeding source of pit corrosion within 2:14:1 grain. • Whole-process corrosion behavior of multi-main-phase magnet was firstly investigated. • Heterogeneous microstructure is the origin of intergrain/intragrain potential discrepancy. • Abnormally increased anti-corrosion is closely correlated to intergranular heterogeneity. • Local Ce-rich shell/core with lower potential evolves as preferential sites for pit corrosion. Heterogeneous microstructure of multi-main-phase (MMP) Nd-Ce-Y-Fe-B sintered magnet is unraveled to generate local electrode potential discrepancy of intergranular multi-phases and intragrain core-shells, being the origin of peculiar corrosion behaviors relative to conventional Nd-Fe-B. i) For intergranular heterogeneity, the preferential corrosion of active RE-rich phase yields an abnormal enhancement of anti-corrosion performance. ii) For intragrain heterogeneity, local Ce-rich shell and Ce-rich core with lower Volta potentials evolve as preferential sites for pit corrosion, resulting in large cavities and apparent Ce-oxide accumulation. Above findings unlock the close correlation between heterogeneous microstructure and whole-process corrosion behavior of MMP magnets for the first time. [ABSTRACT FROM AUTHOR]

• There are the combined inclusions and precipitates within the steel sample, and the morphologies of the combinations are different. • The precipitated (Ti, Nb)N phases have the highest Volta potential, followed by that of the matrix. The inclusions have the lowest Volta potential. • When precipitated phases completely encapsulated inclusions, the rates of pit initiation and propagation are low. • When inclusions are in contact with the matrix, pitting initiation occurs very quickly and is accompanied by the generation of corrosion products. In this work, the effects of combined inclusions and precipitates on the initiation and propagation of pits in weathering steel were studied. The SKPFM results showed that the precipitated (Ti, Nb)N phases had the highest Volta potential, followed by that of the matrix. The inclusions had the lowest potential. The corrosion initiation and propagation processes were observed, in situ, at the site of the combined inclusion and precipitate; the matrix around the precipitated phase dissolved slowly. When inclusions were in contact with the matrix, they rapidly dissolved, forming pits. Subsequently, the surrounding matrix dissolved rapidly, causing the pits to expand. [ABSTRACT FROM AUTHOR]