•Establishing a model for describing the corrosion behavior of multiphase steel.•Quantitatively assessing the corrosion rate of each phase in multiphase steel.•Establishing a function to predict the surface roughness of corrosion morphology.•Defining an index to assess galvanic corrosion at phase-scale in multiphase steel.•The phase distribution affects the service safety of multiphase steel.In situ observation of the initial corrosion behavior of a low carbon microalloyed ferrite–bainite dual-phase steel showed that the corrosion originated from the inside of ferrite and ferrite boundary. In addition, a model for describing the corrosion behavior of each phase in multiphase steel was established. Based on this model, a method to quantitatively assess the corrosion rate of each phase was presented by white light interference, and the relationship between the surface roughness and corrosion morphology was also established. Meanwhile, the galvanic corrosion at phase-scale and the influence of phase distribution on service safety of multiphase steel were discussed.

This paper focuses on the interaction of macroelastic stress and electrochemical corrosion of low carbon steel in 3.5 wt% NaCl solution. The electrochemical impedance spectra (EIS), corrosion current density and open circuit potential (OCP) tests were used to represent the electrochemical behavior of low carbon steel. The microplastic deformation was studied through acoustic emission (AE) techniques and surface morphology observation. The results indicated that there was non-linear relationship between the applied stress and electrochemical corrosion of low carbon steel in 3.5 wt% NaCl solution. At the beginning of macroelastic deformation, the applied mechanical energy would activate the metal surface, so the corrosive solution would more easily to cover the metal surface and become thin liquid film, then accelerate the electrochemical corrosion process. When the gradually increased stress came to a critical value, the extra mechanical energy would not only affect the surface corrosion behavior of samples, but also enhance the microplastic deformation at some local regions of metal surface even the applied stress was far below the yield strength. Usually these two processes competed with each other. The dominated process at the beginning of elastic deformation is stress accelerating corrosion, and when exceeding the critical stress, the applied energy would induce microplastic deformation.

To investigate the quantitative relation between the temperature and the microstructure of low carbon steel, the low carbon microalloyed ferrite–bainite dual-phase (FBDP) steel was prepared by heat treatment. On a series of positions chosen in the sample which was cut from the treated FBDP steel, the volume fractions of bainite in the microstructures were measured and the corresponding temperatures were also obtained through the finite element method. The results show that not only the volume fraction of bainite increases linearly, but also the size of bainite increases with the increase of the outset water-cooling temperature. In addition, by comprehensively evaluating the microstructure and the temperature, it can be estimated that the lowest bainite formation temperature for this steel is about 550 °C, which had been proved by the experiment.Highlights► Temperature distribution within steel during heat treatment. ► The microstructure changing process during heat treatment. ► The lowest bainite transformation temperature of the low carbon microalloyed steel. ► The quantitative relation between the temperature and the microstructure of the low carbon microalloyed steel.

The effect of hydrogen on the fracture behaviors of Incoloy alloy 825 was investigated by means of slow strain rate testing (SSRT). Hydrogen was introduced into the sample by electrochemical charging. The results show that surface microcracks form gradually during aging at room temperature when desorption of hydrogen takes place after hydrogen charging at a current density of 5 mA/cm2 for 24 h. SSRT shows that the increase of ductility loss is significantly obvious as the hydrogen charging current density increases. Scanning electron microscopy (SEM) images reveal ductile fracture in the pre-charged sample with low current densities, while the fracture includes small quasi-cleavage regions and tends to be brittle fracture as the hydrogen charging current density increases to 5 mA/cm2.

The fracture toughness of CO2 corrosion scale in X65 pipeline steel has been measured using Vicker's indentation on a polished cross-section of the scale and the variation of the fracture toughness with scale forming temperature has been investigated. The results show that CO2 corrosion scale formed at 65 °C to 90 °C is (Fe,Ca)CO3 and that at 115 °C is (Fe,Ca,Mg)CO3, respectively. The scale thickness decreases and the amount of Ca in the scale increases with increasing scale forming temperature. The fracture toughnesses of the outer and inner layers of the scale formed at 65 °C are 0.68 MPam1/2 and 1.46 MPam1/2, respectively. The fracture toughness of the CO2 corrosion scale decreases with increasing scale forming temperature.

The flow stress of a specimen of α-Ti before unloading is different with the yield stress of the same specimen after unloading and forming a passive film through immersing in a methanol solution at various constant potentials. The difference is the passive film-induced stress. The film-induced stress and susceptibility to stress corrosion cracking (SCC) in the methanol solution at various potentials were measured. At the stable open-circuit potential and under anodic polarization, both film-induced tensile stress σp and susceptibility to SCC had a maximum value. The film-induced stress and SCC susceptibility, however, decreased steeply with a decrease in potential under cathodic polarization. When the potential V≤−280 mVSCE, the film-induced stress became compressive; correspondingly, susceptibility to SCC was zero. Therefore, the variation of film-induced stress with potential was consistent with that of susceptibility to SCC. A large film-induced tensile stress is the necessary condition for SCC of α-Ti in the methanol solution. The symbol and amount of the film-induced stress were related to the compositions of the passive film, which have been analyzed using the X-ray photoelectron spectrum (XPS).

The spiral welded pipe for oil transport failed catastrophically fracturing along the spiral welding line. The failure was in the base metal close to the heat-affected zone. Passive inclusions in the rolled metal are responsible for the pipe failure. Microstructure examination and Charpy-V notch tests reveal that the crack follows the oxide inclusions and is associated with their distribution, since the base metal toughness is insufficient to retard cracking. Improvement of the steel purity is suggested to prevent future pipe failures. Improving the welding technique to reduce the residual stress acting on the base metal also decreases the risk of pipe failure.Highlights► Spiral welded pipe for oil transport failed catastrophically fracturing along the spiral welding line. ► Crack propagation is following inclusions and is associated with their distribution in base metal. ► Improvement of steel purity is suggested to prevent future pipe failures.