Inclusions In Low-alloy Steels Research Articles

Local corrosion characteristics of CaS/CaO–MgO–Al2O3 inclusions in low-alloy steel under multi-factor mechanisms

The influence of inclusions corrosion behavior under multi-factor local corrosion mechanisms was researched by SKPFM, TOF-SIMS, EBSD, and in-situ corrosion tests. The model of the local corrosion behavior of CaO/CaS–MgO–Al2O3 inclusion under the multifactor mechanism was established. The structural defect area at the boundary of the inclusions acted as the anodic phase where corrosion occurs preferentially, and the CaS of the inclusion underwent a chemical dissolution process. TiN, TiS2 and TiO2 were gradually shed, and the MgO, Al2O3 and CaO inside the local corrosion area were not completely shed. The gap defect of inclusion was the preferred condition to induce local corrosion when the matrix contains galvanic corrosion effect, dislocation region and the gap defect, and the process of anodic dissolution occurred in the dislocation region. There was no gap defect, the environment could not completely provide the driving force for local corrosion, and the possibility of non-local corrosion existed. The existence of gap defect provided the condition for the existence of solution, and provided the double layer contribution of potential difference of inclusion/matrix and solution.

Local Corrosion Characteristics of Cas/Cao-Mgo-Al2o3 Inclusions in Low-Alloy Steel Under Multi-Factor Mechanisms

Local Corrosion Characteristics of Cas/Cao-Mgo-Al2o3 Inclusions in Low-Alloy Steel Under Multi-Factor Mechanisms

Local Corrosion Characteristics of Cas/Cao-Mgo-Al2o3 Inclusions in Low-Alloy Steel Under Multi-Factor Mechanisms

Local Corrosion Characteristics of Cas/Cao-Mgo-Al2o3 Inclusions in Low-Alloy Steel Under Multi-Factor Mechanisms

Synergistic effect of Cu and Cr on pitting behavior induced by MnS inclusions in low alloy steels

We designed three low alloy steels containing different Cu and Cr contents to investigate the synergistic effect of Cu and Cr on pitting initiation in low alloy steels. A metallographic analysis of inclusions in steels and electrolytic extracted inclusions was conducted using scanning electron microscopy (SEM)-energy dispersive X-ray spectroscopy (EDS) techniques. First-principles calculations were also performed to reveal the segregation behaviors of Cu and Cr at the α-Fe/MnS interface. Preferential dissolution of MnS was observed by in-situ ex-environment SEM. It was seen that severe pitting occurred in the steel containing both Cu and Cr. Statistical results indicated that Cu and Cr stimulate each other to accumulate on sulfides, while the stimulating effect of Cu on Cr is stronger than that of Cr on Cu. This synergistic effect intensifies the pitting corrosion of the steel. Experimental results were consistent with those of the first-principles calculations. From these results, a proposed pitting initiation mechanism by the synergistic effect of Cu and Cr on MnS inclusions is described.

Towards a better understanding of localised corrosion induced by typical non-metallic inclusions in low-alloy steels

Detailed understanding of localised corrosion phenomena induced by inclusions in low-alloy steel is extremely critical as any corrosion-induced premature failure can potentially lead to a catastrophe. In particular, there is still ambiguity as to whether the localised corrosion initiation induced by non-metallic inclusions is an electrochemical process or a chemical process. Herein, the localised heterogeneous electrochemical mechanism of typical non-metallic inclusions–Al2O3, ZrO2-Ti2O3-Al2O3, and (RE)AlO3-(RE)2O2S-(RE)xSy – was investigated using scanning Kelvin probe force microscopy and current-sensing atomic force microscopy. The chemical stability of typical non-metallic inclusions was characterized by Pourbaix diagrams. The results indicate that the localised corrosion behaviour shows no correlation to the Volta potential difference between the non-metallic inclusions and the matrix. Moreover, the localised corrosion initiation process is mainly a chemical corrosion process rather than an electrochemical corrosion process. Thus, the applicability of Volta potential difference in predicating the preferential corrosion area in different system (conductive/nonconductive system) should be given full attention.

Initiation and propagation of localized corrosion induced by (Zr, Ti, Al)-Ox inclusions in low-alloy steels in marine environment

The effects of inclusions on localized corrosion of Zr–Ti deoxidized low-alloy steels in marine environment were investigated by various analytical techniques including scanning electron microscopy with X-ray microanalysis (SEM/EDS), confocal Raman microscopy (CRM), and in situ scanning vibrating electrode technique (SVET). It was found that complex (Zr, Ti, Al)-Ox inclusions were responsible for the initiation of localized corrosion. Localized corrosion preferentially occurred at Fe matrix adjacent to these inclusions and formed micro-gaps. In the early stage of corrosion, catalytic-occluded cells and the diffusion of chloride ions played a major role in the propagation of corrosion, further accelerating the dissolution of Fe matrix and (Zr, Ti, Al)-Ox inclusions. Combining SVET and CRM results, it revealed that the maximum anodic current density in local area gradually decreased with prolonged exposure time, indicating that corrosion products covered the steel surface and lowered the propagation rate of corrosion. In the later stage of corrosion, the barrier effect of corrosion products played an important role in inhibiting localized corrosion.

Effect of surface-modified MgO nanoparticles on intragranular ferrite nucleated on inclusions in low-alloy steel

Despite extensive research into steel refining with nanoparticle addition, some aspects remain unclear, such as the low utilization ratio of the nanoparticles. In this study, application of surface-modified nanoparticles in the steel refining process was investigated. The effects of their addition amount on inclusion modifications and microstructure refinements were investigated and the mechanism of intragranular ferrite formation was clarified. Initial experiments were performed to determine the appropriate steelmaking holding time, and the utilization ratio of MgO@C nanoparticle was 65% from the calculated total oxygen content. After nanoparticle addition, the inclusions were mainly composed of MgAl2O4 spinel or MgAl2O4–TiN-based composite inclusions. The inclusion size sharply decreased with nanoparticle addition. The number of sub-micrometer inclusions accounted for 77.2% of all inclusions in S2 steel sample, which contributed to inhibit the migration of prior austenite grain boundaries, and effectively induced acicular ferrite nucleation. Based on the theory that inclusions act as heterogeneous nuclei to induce ferrite nucleation, the results from thermodynamic calculations showed that the critical nucleation diameter for the TiN inclusions is 0.346 μm, which is consistent with experimental data of the minimum inclusion size for ferrite nucleation in actual steel samples.

Interaction between Cu and Cr coadsorption on MnS inclusions in low alloy steels

Interaction between alloy elements Cu and Cr on MnS inclusions was studied by Scanning Transmission Electron Microscope (STEM)/X-ray energy-dispersive spectrometer (EDS) and the first principles calculation method. Assuming that the enrichments of Cu and Cr play a key role on pitting corrosion of low alloy steel, STEM/EDS were used to observe the enrichments of Cu and Cr on MnS inclusions in this study. The first-principles calculation, which is based on density functional theory was used to investigate the segregation behavior of Cu and Cr in the Fe matrix and the adsorption behavior of Cu and Cr on MnS surface. The atomic structures, electronic and energetic properties of clean and Cu or Cr adsorbed MnS surface were calculated. The results showed that both Cu and Cr tend to segregate on the Fe (1 1 0) surface rather than solid dissolved in the Fe bulk. MnS (1 0 0) is the most stable surface amongst the low-index surfaces, and H (hollow) is the most stable adsorption site for both Cu and Cr. Coadsorption of Cu and Cr on MnS (1 0 0) surface was also studied. The results showed that Cu and Cr promote the adsorption of each other on MnS (1 0 0) surface. However, this interaction is effective only within a certain distance between them. Overall, these findings will be helpful to understand the mechanism of pitting corrosion induced by MnS.

Pitting behavior and mechanism caused by Ca-Al-O-S composite inclusions in low-alloy steels

PurposeThe purpose of this study is to study the pitting caused by Ca-Al-O-S composite inclusions of low-alloy steel in 3 Wt.% NaCl solution and 0.01M NaHSO3 solution.Design/methodology/approachThe corrosion in 0.01M NaHSO3 was much weaker than in 3 Wt.% NaCl 3D display of the pitting formation and development process that has been calculated using scanning electron microscope (SEM) and laser scanning confocal microscopy (LSCM). In addition, a corrosion mechanism of pitting formation by galvanic interaction of composite inclusion and base metal has been proposed.FindingsResults show that in immersion test, metal base around inclusions was dissolved due to corrosion. Corrosion on the metal base closer to inclusions was more severe.Originality/valueA corrosion mechanism of pitting formation by galvanic interaction of composite inclusion and base metal has been proposed.

Influence of Inclusions in Low Alloy-Steels on Pitting Process

Two low alloy steels were selected. Inclusions in them were analyzed by electronic probe micro-analyzer. The Influence of inclusions in low alloy-steels were discussed by polarization tests and simulated occluded corrosion cell combined with micro-analyse. The results showed that the inclusions in low alloy-steels were the main pitting initiation and the sulfide inclusions were easier to cause pitting corrosion than other inclusions. The inclusions also accelerated pitting corrosion propagating.

Direct Observations of Cracks and Voids in Structural Materials by X-ray Imaging Using Ultra-bright Synchrotron Radiation

Refraction contrast X-ray imaging experiments were conducted on acrylic resin with an artificial cylindrical hole, A7075 aluminum alloy, A6063 aluminum castings, mild steel with cracks or voids, and low alloy steel with inclusions, using a ultra-bright synchrotron radiation X-ray beam in BL24XU hutch C of SPring-8. Conventional absorption contrast X-ray imaging experiments were also done for the comparison. The X-ray beam was controlled to be monochromatic by Si double-crystals and collimated by a slit. The distance between the sample and the detector was changed from 0 to 3 m, and the X-ray energy was 15 to 25 keV. Photographs were taken by X-ray film and/or X-ray CCD camera. As a result, the refraction imaging method gave a much more distinct image of the artificial cylindrical hole in acrylic resin as compared with the absorption method. The fatigue cracks in aluminum alloy and mild steel were also distinctly observed. The X-ray imaging revealed the presence of MnS nonmetallic inclusions in low alloy steel. Void defects in aluminum castings were clearly detected by the imaging. In addition, in-situ observation of tensile fracture of aluminum alloys using a high resolution X-ray CCD camera system was successfully conducted. The observations by use of asymmetric reflection technique for X-ray imaging experiment were also well performed. From above, the X-ray imaging method using ultra-bright synchrotron radiation is concluded to be very useful for fracture research of materials.

Effect of desulphurization and calcium treatments on the inclusion morphology of 0.4C-Cr-Mo-Ni steel

Several 0.4C-Cr-Mo-Ni steels with different sulphur and calcium content levels have been studied to determine the effect of desulphurization and calcium treatments on the morphology of non-metallic inclusions in low-alloy steels. The volume fraction and size parameters of the inclusions were determined using optical microscopy. The inclusions were identified by electron probe microanalysis. The volume fraction and mean aspect ratio of a stringy type of inclusion, which consisted predominantly of MnS, was reduced significantly with a decrease in sulphur content through desulphurization. A cluster type of composite inclusion was also decreased with decreasing sulphur content. Calcium treatment of steel with a commercial sulphur level was not very effective for modification of the inclusions, producing two types of cluster composite inclusion. However, the calcium treatment of desulphurized steel modified dramatically the stringy type to a particle type (mean diameter: 1.3 μm) which consisted predominantly of CaS-CaO, while small amounts of Ti and Al were also detected.

Dissolutions of Inclusions in Low-Alloy Steel Exposed to Chloride-Containing Environments

Abstract The role played by non-metallic inclusions in the nucleation of pitting corrosion in low-alloy steels is determined by the detailed analysis of the inclusions in an AISI 1536 steel and the...

Homogenization of sulfide inclusions in low alloy steel

Homogenization of sulfide inclusions in low alloy steel