•SCC properties of X70 steel in simulated deep-sea environment were investigated.•Hydrostatic pressure promotes SCC, particularly in an acidified solution.•Hydrostatic pressure promotes pitting propagation and linkage.•Much more hydrogen was involved in the SCC process under hydrostatic pressure.•The SCC mechanism is controlled by both anodic dissolution and HIC.The stress corrosion cracking (SCC) properties of X70 pipeline steel in simulated deep-sea environments was investigated in a high-pressure vessel with a constant loading device. The results reveal that the threshold stress, σSCC, for SCC is 0.89σb, 0.78σb and 0.68σb in a 3.5% NaCl solution (pH = 7) of 0.1 MPa, 5 MPa and 10 MPa, respectively. When the 3.5% NaCl solution pH value decreased to 3, σSCC decreased further to 0.83σb, 0.68σb and 0.53σb, corresponding to 0.1 MPa, 5 MPa and 10 MPa, respectively. These results indicate that the hydrostatic pressure (HP) promotes SCC, particularly in an acidified sodium chloride solution. Scanning electron microscopy (SEM) and confocal scanning laser microscopy showed that on the sample surface, the HP promoted pitting propagation and linkage, which then initiated microcracks in SCC. The hydrogen concentration was 0.10 ppm, 0.17 ppm and 0.23 ppm after a 200-h immersion in a 3.5% NaCl solution (pH = 7) of 0.1 MPa, 5 MPa and 10 MPa, respectively, revealing that the hydrogen concentration increased approximately linearly with the HP. This result indicated that much more hydrogen was involved in the SCC process in a solution under a higher hydrostatic pressure. The SCC mechanism is controlled by both anodic dissolution and hydrogen-induced cracking in deep-sea environment.

The formation condition of hydrogen blister in 18Ni maraging steel without any inner or external stress was investigated. The results show that the critical diffusible hydrogen concentration of a blister forming is about 1.4 × 10−5, which is corresponding to the current density of 30 mA/cm2 during cathodic charging in a sodium hydroxide solution. For a 0.1 cm thick sample, no matter the current density is equal to or much larger than the critical value, it spends at least about 132 h to form a hydrogen blister when hydrogen charging in single direction. It is approximately equal to the time for hydrogen atom to diffuse throughout the sample, which exactly depends on the hydrogen diffusion coefficient and the penetration depth. The very first clear suggestion was reported that the incubation period for hydrogen blister nucleation was necessary. According to the Fick’s laws, calculations show that the normalized hydrogen concentration in the escaping surface almost reaches 0.96 times of the charging surface, which means that the diffusion almost reaches a dynamic balance. A model was illustrated to describe the competitive relationship between hydrogen diffusion and blister formation.

•Hydrogen embrittlement can be assessed by combining CH and σys on KIH.•KIH was found to increase and da/dt decrease with D if σys < 1360 MPa.•KIH and da/dt were found to be independent of D when σys ⩾ 1360 MPa.•Hydrogen diffusion coefficient is affected by temper temperature.The threshold stress intensity of hydrogen-induced cracking KIH, the crack propagation rate da/dt and the hydrogen diffusion coefficient D have been studied for 30CrMnSiNi2 steel. The results show a combined effect of the hydrogen concentration CH and the yield strength σys on KIH. KIH and da/dt are independent of D when σys ⩾ 1360 MPa, but KIH increases and da/dt decreases with increasing D when σys < 1360 MPa. This phenomenon is explained by the effect of the temper temperature on the microstructure. Based on the relation of KIH with CH and σys, the reliability of the material could be assessed.

•SKPFM was employed to characterize the surface potential induced by hydrogen.•The work function decreased due to the presence of hydrogen.•The real-time distribution of hydrogen at the micro-level was detected.•Hydrogen enrichment and the crack propagation were observed by SKPFM.It has been a challenge to detect the real-time distribution of hydrogen at the micro-level. In this study, a scanning Kelvin probe force microscopy (SKPFM) was employed to characterize the surface potential changes induced by hydrogen. Variations of the surface potential were used to assess (a) the real-time distribution of hydrogen, (b) hydrogen enrichment in reverted austenite and (c) hydrogen enrichment around the crack tip induced by the stress concentration.

In this paper, the effect of pv factor on the tribological behavior of a copper-impregnated metallized carbon brush sliding against a CuCrZr ring was investigated. Friction and wear tests were conducted on a brush-on-slip ring type wear tester. The tests were performed at two electrical currents (0 and 20 A) and seven different combinations of contact pressure (p) and sliding velocity (v), herein referred to as the pv factor to study the relationship between electrical and mechanical wear. The results show that mechanical wear aggravates electrical wear. Higher interfacial temperatures appear to be responsible for the higher wear rate. Worn surfaces of brush specimens were observed and analyzed by scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDS). It is apparent that the degree of tribo-oxidation at the brush increases with an increase in pv, which has a significant influence on the tribological behavior of the brush specimen.Highlights► The mechanical wear aggravates the electrical wear. ► The tribo-layer containing oxides plays a key role in friction and wear. ► A higher interfacial temperature is responsible for a higher wear rate.

The effect of humidity and hydrogen on the growth of an unloaded indentation crack in lead-free potassium sodium niobate (KNN) ferroelectric ceramics has been investigated. The results indicate that crack growth of an unloaded indentation occurs in humid air of 70 and 90% RH without the presence of an electric field or mechanical stresses but not in air with RH ≤ 30%. Growth of an indentation crack also occurs in dry air when the field is larger than the threshold field Eth(y) = 0.01Ec (normal to the poling direction) or Eth(z) = 0.05Ec (parallel to the poling direction), and the incubation time decreases for larger fields. The change in crack growth length in humid air under a sustained field, Δc, is composed of three parts; i.e., Δc = Δc1 + Δc2 + Δc12, where Δc1 is the incremental growth in humid air without a field, Δc2 is that in dry air under a sustained field and Δc12 is that induced by the coupling effect of the electric field and humidity since humidity promotes domain switching. Crack growth of an unloaded indentation can occur during hydrogen charging, and the threshold stress intensity factor of hydrogen-induced delayed cracking, KIH, as well as the fracture toughness of hydrogenated ceramics, KIC(H), decreases with increasing hydrogen concentration.