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The effect of nanosized NbC precipitates on the stress corrosion cracking (SCC) of high-strength low-alloy (HSLA) steel in a simulated deep-sea environment was studied through electrochemical tests, hydrogen permeation tests and slow strain rate tension (SSRT) tests. The results show that in a simulated deep-sea environment, the presence of nanosized NbC precipitates significantly decreases electrochemical corrosion and SCC susceptibility. Nanosized NbC precipitates increased the amount of irreversible and reversible hydrogen traps, which pinned the hydrogen induced by the hydrogen evolution reaction, and weakened the effect of hydrogen on the AD and HE during SCC of HSLA steel. In addition, whether in deep- or shallow-sea environments, nanosized NbC precipitates improved the SCC resistance of HSLA steel by means of the hydrogen-trapping and microstructural optimization effect, but the microstructural optimization played a minor role. Once the steels were with higher hydrogen concentration, the hydrogen-trapping function of NbC precipitates increased, which resulted in larger SCC resistance improvement.

We investigated the effect of nanosized NbC precipitates on hydrogen-induced cracking (HIC) of high-strength low-alloy steel by conducting slow-strain-rate tensile tests (SSRT) and performing continuous hydrogen charging and fracture analysis. The results reveal that the HIC resistance of Nb-bearing steel is obviously superior to that of Nb-free steel, with the fractured Nb-bearing steel in the SSRT exhibiting a smaller ratio of elongation reduction (Iδ). However, as the hydrogen traps induced by NbC precipitates approach hydrogen saturation, the effect of the precipitates on the HIC resistance attenuate. We speculate that the highly dispersed nanosized NbC precipitates act as irreversible hydrogen traps that hinder the accumulation of hydrogen at potential crack nucleation sites. In addition, much like Nb-free steel, the Nb-bearing steel exhibits both H-solution strengthening and the resistance to HIC.

In this work, the stress corrosion cracking (SCC) behavior of E690 steel base metal (BM) and different heat-affected zone (HAZ) microstructures, i.e., coarse grain HAZ (CGHAZ), fine grain HAZ (FGHAZ), and intercritical HAZ (ICHAZ), was investigated at different cathodic potentials in artificial seawater by slow strain rate tensile tests, scanning electron microscopy and electron back-scattered diffraction measurements. The results show that the HAZ microstructures and BM exhibit different SCC susceptibilities: FGHAZ

The environment at crack tip and its effect on the crack growth behaviour of low alloy steel-E690 steel were studied at cathodic potentials in artificial seawater. The results showed that the micro environment at crack tip and crack growth behaviour were related to the electrochemical reactions at crack tip, which were affected by the stress state and applied potentials. The crack tip environment was acidified under cyclic loading, resulting from the crack tip anodic dissolution reaction and corresponding hydrolysis reaction. Because of the hydrogen evolution and the inhibited anodic dissolution inside the crack, the crack tip p H increases as the cathodic potential decreases. The effect of cathodic potentials on the electrochemical reactions caused the variation of the hydrogen content, which influenced the crack growth rate because the crack growth behaviour was controlled by hydrogen embrittlement mechanism. This resulted in a fact that with the negative decrease of potential, the crack growth rate first decreased and then increased,with the minimum rate at-0.75 V. And the crack growth path exhibited transgranular fracture.

In this study, simulated heat-affected zone (HAZ) of Nb-free and Nb-bearing steel were obtained, and SEM, TEM, and slow strain rate tensile (SSRT) tests were performed to investigate the effect of Nb on the stress corrosion cracking (SCC) behavior of high-strength low-alloy (HLSA) steel in simulated seawater with or without hydrogen charging. The addition of Nb significantly refined the grains and uniformed the microstructure of HLSA. Nb hardly affected the SCC susceptibility of BM and HAZ without hydrogen-charging. However, after charging with 10 mA cm−2, the SCC resistance of Nb-bearing steel, especially the coarse grain HAZ (CGHAZ) improved drastically, and the process of crack initiation and propagation was inhibited owing to the hydrogen trap function of NbC precipitates.

Hydrogen-induced cracking (HIC) susceptibility tests, hydrogen permeation tests and the electron backscatter diffraction (EBSD) measurements were used to study the effect of Nb on the HIC resistance of the high-strength low-alloy (HSLA) steel. The Nb microalloying significantly improved the HIC resistance and decreased the proportion of intergranular cracks through the hydrogen trapping effect and microstructure optimization. Dispersed nanometer-sized NbC precipitates effectively increased the number of irreversible traps, hindered hydrogen aggregation, and enhanced the resistance to crack initiation and propagation. Moreover, Nb optimized the microstructure by increasing the proportion of the low-angle grain boundaries (LAGBs), refining the prior austenite grain size, decreasing the dislocation density, and changing the distribution of Σ3 boundaries.

The investigation of this paper aimed at finding the failure cause of a heavily cracked geothermal water convection tube, utilizing electron backscattered diffraction, double loop potentiodynamic polarization test, and energy-dispersive X-ray spectroscopy. Results show the failure was attributed to stress corrosion cracking (SCC). Chloride in the service environment has induced the initiation of SCC cracks. The sensitization of matrix increased the susceptibility to intergranular cracking, which may result from improper heat treatment before service. The residual stress arising from the manufacturing process promoted the failure process. The SCC cracks originated from the internal tube. In the early stage, the cracks propagated along austenite grain boundaries. Thereafter, they transformed into a coexisting mode of intergranular cracking and transgranular cracking. The failure was attributed to the synthetical effect of sensitization, chloride, and residual stress.

Immersion tests and magnetic property measurements were used to study the long-term effects of electrochemical corrosion on the magnetic properties of sintered NdFeB magnets. The results revealed that, during electrochemical corrosion, uneven distribution of the Nd-rich phase caused local lattice expansion in the magnets, in which the priority nucleation of reversal domains occurred. By analyzing the measurement results, about 90% of the remanence (Br) loss was ascribed to the matrix phase loss, while the remaining 10% was attributed to the deterioration of the surface structure of the magnets during electrochemical corrosion.

The effect of nitrate reducing bacteria (NRB) on corrosion of X80 steel in near-neutral pH solution was studied using various electrochemical and surface techniques. NRB promoted the uniform and pitting corrosion of X80 steel, especially the depth and density of pits. The biofilm formed by NRB hinders the anodic reaction, but still accelerates the corrosion rate. The corrosion of NRB on X80 steel is not only related to the extracellular electron transfer, but also the concentration difference of the intermediate metabolite nitrite. When the nitrite concentration difference is lower than 6 mmol/L, the extracellular electron transfer of bacteria plays a dominant role in differential concentration corrosion, while when the nitrite concentration difference is higher than 6 mmol/L, it is the corrosion inhibition effect of γ-Fe2O3 oxidized by nitrite.

The effect of Cr content in steel on its passivation behavior in a CO2 aqueous environment containing silty sand was investigated. The results indicated that the stability of passive film was improved with increasing Cr content. High Fe3C content accelerated the rapid formation of a passive film as well as precipitates and dislocations. Increasing Cr content enhanced the contents of Cr2O3 and Cr(OH)3 in the passive film. Silty sand increased carrier densities of passive films, thereby reducing their protective effects. The role of the contents of Fe3C and Cr on corrosion mechanism of Cr steel with silty sand was emphasized.

Iron-based soft magnetic composites with phosphate-polyimide (PI) insulating layer were successfully fabricated by phosphating and imidization. The effect of phosphoric acid content on microstructure and compressibility of iron powder, and magnetic properties of the cores were investigated. The results demonstrate that the inner inorganic coated layer mainly consists of iron phosphate and ferrous phosphate. With increasing phosphoric acid content, the thickness of phosphate increases gradually, resulting in the reduction of density, maximum permeability and magnetic induction intensity. When the content of phosphoric acid reaches above 0.3 wt%, the bonding strength between the PI coating and iron particles is reduced, leading to the breakage of insulating layer and the augment of core loss. Owing to the excellent thermal stability of PI, magnetic powder cores can be cured at the temperature up to 400 °C. The sample coated with 0.1 wt% phosphoric acid and 1.5 wt% PI exhibits excellent magnetic properties. The maximum permeability reaches 201.4, and the core loss (186.6 mW/g) is less than 15% of monolayer phosphate coated sample (measured at 50 mT and 100 kHz). Additionally, the corrosion resistance of the sample is significantly improved.