Search

The initial and long-term corrosion behaviors of 3Ni steels with different microstructures including ferrite (F), ferrite + bainite (F + B), and bainite (B) were studied. The results showed that the corrosion process of steels can be divided into an initial state and a stable state, and the corrosion rates increased continually in the initial stage and gradually decreased in the stable stage. Due to the larger relative potential difference between the F and B in the F + B steel, the corrosion rate was the highest among three steels. The B steel first formed the protective rust layer and presented the best protectiveness. The initial corrosion of B steel mainly developed longitudinally and led to small and deep corrosion pits. The initial corrosion of the F and F + B steels mainly developed laterally, and was contributing to the large and shallow corrosion pits. [ABSTRACT FROM AUTHOR]

The effects of silty sand and pH on the corrosion characteristics of 1Cr steel were investigated. The results revealed that silty sand changed the nucleation behaviour of FeCO3 crystal from homogeneous nucleation to heterogeneous nucleation through increasing the critical supersaturation of FeCO3. The compactness and protection of corrosion film were improved by silty sand. The amorphous FeCO3 content decreased as the pH increased in the CO2 environment with silty sand, while the Cr(OH)3 content increased. The critical pH for the good corrosion resistance of 1Cr steel was 4.5. The amorphous FeCO3 dominated the protective properties of corrosion film when the pH was lower than the critical pH, whereas the Cr(OH)3 was predominant when the pH was higher than the critical pH. Therefore, the corrosion resistance of 1Cr steel was reduced and then enhanced with increasing pH under silty sand condition. [ABSTRACT FROM AUTHOR]

The corrosion behaviors of carbon steel and Ni-advanced weathering steel exposed to tropical marine atmosphere were investigated by using electrochemical impedance spectroscopy, x-ray diffraction, transmission electron microscope and x-ray photoelectron spectroscopy measurements. The results showed that Ni had almost no effect on corrosion kinetics and electrochemical performance of Ni-advanced weathering steel in mild atmosphere. Conversely, Ni-advanced weathering steel in marine atmosphere showed a significant superiority in improving corrosion resistance compared with carbon steel. Moreover, the mass formation of NiFe2O4 in the inner rust layer promoted the stability of the corrosion resistance improvement rate of Ni-advanced weathering steel with the increase in exposure time. Therefore, Ni-advanced weathering steel seemed to be more suitable for the harsh marine atmospheric environment. [ABSTRACT FROM AUTHOR]

The effect of silty sand on the pre-passivation behaviour of 1Cr steel in a CO2 aqueous environment was investigated by electrochemical measurements and immersion tests. The results indicated that the cathodic reaction was accelerated by silty sand at the initial corrosion stage, but it was inhibited during the late corrosion stage. The corrosion film presenting as one single layer structure under silty sand condition was mainly composed of Cr(OH)3, amorphous FeCO3, and silty sand. The content of Cr(OH)3 in the film with silty sand was lower and then higher than that without silty sand with increasing corrosion time. The film was separated by silty sand at the early stage and it was filled with silty sand during the late stage. In addition, the pre-passivation behaviour of 1Cr steel under silty sand condition was weakened and then strengthened compared to sand-free condition. [ABSTRACT FROM AUTHOR]

The effect of Cu and Ni content on the corrosion behaviors of weathering steels in a corrosive atmosphere was investigated using electrochemical measurement techniques, such as electrochemical impedance spectroscopy and linear polarization resistance methods. Scanning electron microscopy with energy-dispersive spectrometry (EDS) and x-ray diffraction (XRD) were used to analyze the samples. The corrosion rates, which are above 2.0 mm a−1 at 48 h and approximately 1.12 mm a−1 at the range of 144-240 h, show that Cu and Ni slowed down the electrochemical reaction at the initial corrosion stage. With an increasing immersion time, the protective properties of the rust layer exhibited an increase in its impedance, and it was enhanced compared to that in the initial stage. The enrichment phenomenon of Cu and Ni elements in the inner rust layer was more evident in the 0.4Cu-0.4Ni and 0.7Cu-0.7Ni steels than that of 0.3Cu-0.3Ni steel after 240 h. The XRD and EDS results show that the inner rust layer contained compositions of CuO and Ni compounds, which filled up the pores located on the corrosion products contributing to the densification of the rust layer. [ABSTRACT FROM AUTHOR]

• Corrosion resistance of 3Cr steel was first reduced, followed by enhancing and then reduced as the rotation speed increased. • Silty sand changed from participating in the corrosion film to destroying the corrosion film. • Increasing rotation speed converted a factor that played a dominant role in corrosion resistance from mass transfer process to shear stress of silty sand. • Critical rotation speed for corrosion mechanism transformation of 3Cr steel was 600 r/min. The 3Cr steel used as the tubing has serious corrosion damage in a flowing CO 2 environment with silty sand. Hence, the investigation for the influence of flow velocity on the corrosion behavior of 3Cr steel has an important and practical significance. It could not only unveil the corrosion damage mechanism of the steel under dynamic conditions, but also provide a theoretical basis for the corrosion protection of the tubing. In this paper, the effect of flow velocity on the corrosion film of 3Cr steel in a CO 2 environment with silty sand was investigated by adjusting rotation speed. The results revealed that the corrosion resistance of 3Cr steel was first reduced, followed by enhancing and then reduced as the rotation speed increased. There was a critical rotation speed for the transformation of corrosion mechanism of 3Cr steel, i.e., 600 r/min. When the rotation speed was lower than the critical speed, the mass transfer process of corrosive medium dominated the corrosion resistance of 3Cr steel, and silty sand participated in the formation of corrosion film. The increasing rotation speed accelerated the mass transfer process, which first reduced and then improved the corrosion resistance. When the rotation speed was higher than the critical speed, the shear stress of silty sand played a predominant role. At the same time, silty sand impinged on the corrosion film, and then destroyed the stability of the film, leading to a significant decrease of the corrosion resistance. Silty sand changed from participating in the corrosion film to destroying the corrosion film with increasing rotation speed. [ABSTRACT FROM AUTHOR]

The characteristics of corrosion scales on 3Cr and X60 steel were investigated by using weight-loss measurement and electrochemical test. The results manifested that corrosion scales of X60 steel was composed of a S-rich layer, while that of 3Cr steel was composed of an outer Cr-rich layer and an inner S-rich layer. Cr(OH) 3 and FeS deposited competitively on the surface of 3Cr steel. The deposition of FeS, which inhibited the precipitation of Cr(OH) 3 , preferentially accumulated closely to the surface to form the inner layer, while Cr(OH) 3 deposited away from the surface to form the outer layer. The corrosion degree of X60 and 3Cr steel was perceived as serious corrosion under the test conditions. In the selection of tubing and casing for sulphur-bearing gas wells, 3Cr steel should be further evaluated according to specific working conditions. • Characteristics of corrosion scales of 3Cr and X60 steel exposed to CO 2 -H 2 S environment are studied. • Cr(OH) 3 competes with FeS for deposition at the near surface of 3Cr steel. • Formation mechanism of corrosion scales of 3Cr steel in CO 2 -H 2 S environment is proposed. [ABSTRACT FROM AUTHOR]

The relationship between microstructure parameters and mechanical properties was studied in this paper. The steel was heat-treated at different austenitizing temperatures to acquire varying microstructure. The results showed that austenite grain size increases with austenitizing temperature, while the pearlite colony size was relatively constant. The strength followed a Hall–Petch relationship with the austenite grain size, but the austenite grain size has nothing to do with the impact toughness. The control unit for determining the impact toughness of pearlitic steel is the pearlite colony size using a comparison method. Further studies have found that, in the hypoeutectoid steel and hypereutectoid steel, the pearlite colony size changes with the austenitizing temperature. However, when the eutectoid steel with a carbon content of 0.81% undergoes the isothermal transformation, the number of grain boundary precipitates is very few. There are many nucleation sites at the grain boundary. The pearlite colonies randomly nucleate at the grain boundaries and grow into the interior of the grains. Simultaneously, new pearlite colonies nucleate by the side of the existing pearlite colony. The intragranular pearlite colonies are also randomly nucleated. These nucleation sites increase the chance of the growing pearlite colonies colliding with each other, eventually resulting in a constant pearlite colony size. [ABSTRACT FROM AUTHOR]

In this study, the effects of Ti addition on the mechanical properties and corrosion resistance of X80 pipeline steel were investigated using tensile impact, electrochemical and neutral salt spray tests, scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The results show that the addition of Ti can promote the formation of acicular ferrite (AF), resulting in significant grain refinement. It can improve the tensile strength, yield strength and impact toughness of X80 pipeline steel, with the highest value of tensile strength at 0.04 wt % Ti content, which is 8% higher than the original; yield strength is 18% higher; and impact work is 9% higher. The best corrosion resistance of the pipeline steel is achieved at a Ti addition of 0.04 wt %, with a reduced corrosion weight loss rate, 28% less than the original. The Tafel curve is shifted forward, the self-corrosion potential E corr increases and the corrosion current density I corr decreases, improving by 10% from the original. The capacitive arc radius and R ct increase, the resistance to corrosion reaction increases. The corrosion products are mainly loose and porous γ-FeOOH and β-FeOOH in the early stage of corrosion, and more α-FeOOH and Fe 3 O 4 are generated in the later stage. • The addition of Ti can inhibit the transformation of polygonal ferrite (PF) and quasi-polygonal ferrite (QF), promote the formation of needle-like ferrite (AF), and make the grains significantly finer. • The addition of Ti can improve the tensile strength, yield strength and impact toughness of X80 pipeline steel, with the highest value of tensile strength R m at a Ti content of 0.04 wt %, which is 8% higher than the original; yield strength R t is 18% higher; and impact work is 9% higher. • When the Ti content is 0.04wt %, the Tafel curve moves forward. Ti can further transform γ-FeOOH in the rust layer into denser α-FeOOH, and the corrosion resistance of pipeline steel is the best. [ABSTRACT FROM AUTHOR]

In this study, a series of χNb-X80 pipeline steels were obtained by adding different contents of Nb to X80 pipeline steel and after different cooling rates. The effects of Nb on the microstructure and corrosion resistance of X80 pipeline steel were studied by metallographic microscope, scanning electron microscope, differential scanning calorimeter, electrochemical corrosion, and salt spray corrosion. The results show that the high cooling rate will hinder the phase transformation, reduce the diffusion ability of Nb element at the grain boundary, inhibit the growth rate of ferrite grains, and refine the grains of the sample steel structure. The grain refinement effect is the best when the cooling rate is 40 °C/min. In addition, the reason why Nb improves the corrosion resistance of X80 pipeline steel is discussed from two aspects: the change of corrosion products and the effect of grain refinement. The analysis shows that Nb promotes the formation of a dense corrosion protective film, and this positive effect of the protective film overcomes the negative corrosion effect caused by the grain refinement caused by the precipitated Nb (CN), thereby enhancing the corrosion resistance of the substrate. The corrosion resistance is the best when the Nb content is 0.16 wt %. • The higher the cooling rate, the smaller the grain size of the sample steel and the more uniform distribution of the metallographic structure. The optimum cooling rate is 40 °C/min. • Appropriate addition of Nb is beneficial to promote the formation of dense corrosion product layer and improve the corrosion resistance of X80 pipeline steel. • The solid solution strengthening and precipitation of Nb(CN) refine the grains of the X80 pipeline steel structure. [ABSTRACT FROM AUTHOR]

The effect of Cl- concentration on the corrosion behavior of Cr-containing steel in tropical marine atmospheric environment is systematically studied. Cl- ions affect the adsorption amount of water film and then affected O 2 diffusion. They reduce the generation of NiFeO 4 and Cr(OH) 3 and destroy the electronegativity of inner film, further weakening the homologous enrichment of Ni, Mo and Cr and intensifying the penetration of Cl- ions. The amount of NiO and Cr 2 O 3 decreases due to oxygen reduction, which reducrs FeOOH nucleation sites and promotes the large-size FeOOH generation, leading to increase the pores in the corrosion product films. • Adding a small amount of Cr to Ni-steel reduces its corrosion rate. • Cr element promote more NiFe 2 O 4 forming and attract more Ni2+ and Cr3+ ions, repelling and shielding negatively charged Cl-. • High Cl- concentration reduces the content of O 2 , inhibiting the formation of NiFe 2 O 4 and Cr(OH) 3. [ABSTRACT FROM AUTHOR]

The corrosion behavior of weathering steel in the environments with different NaCl concentrations was systematically investigated. The results indicated a decrease of corrosion resistance of steel as NaCl concentration increased. While, the weight gain rate in high concentration was lower than that in lower conditions, which was attributed to the inhibition on the diffusion of O 2 caused by the residual NaCl solution. Cu, Ni and Mo can mutually promote the agglomeration process and improve the performance of the rust layer in low NaCl concentrations, which was severely inhibited in high NaCl concentrations, resulting in a loose rust layer. • Corrosion behavior of the weathering steel in different NaCl concentration conditions was studied. • The phase compositions and valence states of Cu, Ni and Mo in the rust layer were characterized. • Based on the principle of the Raoult's law, the formation mechanism of rust layer was proposed. [ABSTRACT FROM AUTHOR]

• The effect mechanism of Cl− on corrosion resistance was investigated. • The effect of Cl− on corrosion resistance was related to the Cl− concentration. • The corrosion resistance reduced with the inhibition of Cl− on NiFe 2 O 4 enhanced. • The transformation from γ-FeOOH to α-FeOOH was limited in high-Cl− environment. The effect of chloride ion on corrosion resistance of Ni-advanced weathering steel in simulated tropical marine atmosphere was investigated by surface analysis and electrochemical methods. The results showed that the effect of chloride ion on corrosion resistance of steel was primarily related to chloride ion concentration. As the chloride ion concentration increased, the inhibition of chloride ion on the formation of NiFe 2 O 4 was gradually enhanced and the corrosion resistance of steel was reduced. Meanwhile, the main composition phase of inner product film changed from α-FeOOH to γ-FeOOH, whereas that of outer film changed from γ-FeOOH to β-FeOOH. Moreover, Ni was mainly concentrated in the inner film instead of being distributed in the whole product film. [ABSTRACT FROM AUTHOR]