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This article used a high-temperature and high-pressure Hastelloy reactor to simulate the harsh corrosion environment. The effects of different Cl- concentrations(0, 5 000, 10 000, 15 000 mg/L)on the corrosion mechanism of N80 tubing steel in the extraction fluid was studied. The corrosion rate, corrosion morphology and corrosion film structure of N80 tubing steel was analyzed using a combination of macroscopic and microscopic means. The variability of corrosion in different Cl- content environments was clarified. The results showed that the corrosion rate rose significantly with the elevation of Cl- concentration, and the pits on the surface of the corrosion products gradually increased. The large amount of Cl- enrichment destroyed the originally dense corrosion film, exposing N80 tubing steel to the corrosion solution. The presence of trace O2 made the concentration difference battery occur, accelerating the corrosion process. [ABSTRACT FROM AUTHOR]

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]

Purpose: The salinity of the oilfield produced water has a significant effect on steel corrosion. The purpose of this paper is to study the influence of salinity on corrosion behavior of X60 steel and it also provides basic for material selection of gas wells with high salinity. Design/methodology/approach: The weight loss experiment was carried out on steel with high temperature and high pressure autoclave. The surface morphology and composition of corrosion scales were studied by means of scanning electron microscopy, energy dispersive spectroscopy and X-ray diffractometry. Findings: The results show that as salinity increases, the corrosion rate of X60 steel will gradually experience a rapid decline stage and then a slow decline stage. X60 steel is mainly exhibiting uniform corrosion in the first rapid decline stage and pitting corrosion in the second slow decline stage. The increase in salinity reduces gas solubility, which, in turn, changes the morphology and density of the corrosion scales of X60 steel. At low salinity, loose iron oxides generated on the surface of the steel, which poorly protects the substrate. At high salinity, surface of the steel gradually forms protective films. Chloride ions in the saline solution mainly affect the structure of the corrosion scales and initiate pitting corrosion. The increased chloride ions lead to more pitting pits on the surface of steel. The recrystallization of FeCO3 in pitting pits causes the corrosion scales to bulge. Originality/value: The investigation determined the critical concentration of pitting corrosion and uniform corrosion of X60 steel, and the new corrosion mechanism model was presented. [ABSTRACT FROM AUTHOR]

The authors have stated that at present, they believe that the conclusions of the published article are unreliable, and the corrosion behavior of 42CrMo steel after heat treatment needs to be comprehensively considered. According to the literature, these Cr-rich carbides can aggravate the local galvanic corrosion effect, and affect the corrosion rate and corrosion behavior of steel. [Extracted from the article]

• 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 influence of Cu addition on the microstructure evolution and substance transformation in corrosion product layer was explored through comprehensive microscopic characterization and immersion test. The Cu lowers the A r3 and M s temperatures of X80 steel, facilitating the Granular Bainite formation while impeding the development of Polygonal Ferrite. Furthermore, the Cu facilitates the development of stable corrosion products, leading to an increased proportion of these substances in corrosion product layer and a reduction in their dissolution and detachment during rust formation. And the formed oxide (CuO) not only enhances the stability of the corrosion product layer but also provides nucleation sites for CuFeO 2 production. CuFeO 2 carries a negative charge, which enhances the Cl− blocking ability of the corrosion product layer. The inclusion of these substances in corrosion product layer enhances its resistance to corrosive media. The corrosion product layer offers the best protection for the steel when the Cu addition is 1.5 wt%. • The Cu lowers the A r3 and M s temperature of X80 steel, facilitating the formation of GB while impeding the formation of PF. • The Cu addition increases the proportion of α-FeOOH and Fe 2 O 3 in the corrosion product layer to improve the thermodynamic stability of the layer. • The formed oxide (CuO) in inner corrosion product layer of X80 steel can provide nucleation sites for the synthesis of CuFeO 2. • CuFeO 2 is electronegativity, which enhances the Cl− blocking ability of the corrosion product layer. [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 Cr content on the corrosion behavior of Ni-Mo steel in the tropical marine atmospheric environment is systematically investigated. The increase of Cr content accelerates the aggregation and the enrichment of Ni element, and thus promotes the formation of an inner electronegative film which attracts positively charged ions including Mo6+, Mo4+, Ni2+ and Cr3+ ions, and repels negatively charged Cl- ions. Cr promotes the formation of more nano-FeOOH and the conversion of γ-FeOOH into α-FeOOH. A small amount of chromium promotes the formation of micro-pits at the initial stage, and with time, those micro-pits gradually accumulate into pieces. • Adding a small amount of Cr into Ni-Mo steel reduces the corrosion rate. • An electronegative inner film composed of Cr and Ni elements attracts Mo6+, Mo4+, Ni2+ and Cr3+ ions, and repels Cl- ions. • Cr 2 O 3 acts as the nucleation site of FeOOH, generating more nano FeOOH, and doping Cr promotes the transformation of γ-FeOOH to α-FeOOH. [ABSTRACT FROM AUTHOR]

To optimize Ni, Cu, Mo element content of low Cr steel rebars in tropical marine atmosphere environment, five newly steel rebars were exposed in Dalat to carry out the outdoor exposure tests including weight loss method, surface characterization method and electrochemical test. The results show that when 0.15 wt% Mo is added in the steel rebar, the corrosion resistance is 31.8% higher than that of the steel rebar with 0.3 wt% Cu. The corrosion resistance of 0.3 wt% Cr added steel rebar is 63.4% higher than that of 0.2 wt% Ni added steel rebar. With the addition of 0.3 wt% Cu and 0.3 wt% Cr, the corrosion resistance is 46.3% higher than that of the steel rebar with 0.2 wt% Ni and 0.15 wt% Mo. Mo and Cr are the main elements to improve the protection of steel rebar rust layer. The corrosion products of the five steel rebars are composed of α-FeOOH, γ-FeOOH and Fe 2 O 3 or Fe 3 O 4. The higher the content of Cr is, the higher the ratio of α - FeOOH to γ-FeOOH (α/γ) in the rust layer becomes. The protective layer composed of CaMoO 4 and Fe 2 (MoO 4) 3, formed by molybdate in alkaline environment hinders the further corrosion of steel rebars. Cr(OH) 3 produced by hydrolysis of Cr in low alloy steel inhibits the growth of FeOOH in rust, resulting in the decrease of FeOOH particle size and the rust layer more compact. On the whole, the newly designed low alloy steel rebar with a little of 1.6Cr-0.15Mo elements can be well used in tropical marine atmospheric environment. [ABSTRACT FROM AUTHOR]

• Newly designed low-cost steel bar is well used in tropical marine atmospheric environment. • Corrosion product films of newly designed low alloy steel bar and carbon steel bar is studied by outdoor exposure and immersion test. • Fixture for simulating concrete steel rebar is designed. • Improvement mechanism of Cr and Mo on the compactness of corrosion product films of steel rebar is proposed. Carbon steel rebar seriously corrodes in severe tropical marine atmospheric environment, threatening the safety of building structures. Hence, it is of great significance to design and manufacture low-alloy steel rebar which can be used in tropical marine atmospheric environment. In the paper, both of the self-designed low alloy (LA-1) steel rebar and carbon steel (HRB400E) rebar has been exposed in Sriracha for two years. The corrosion rate of LA-1 steel rebar is only half of that of carbon steel rebar after two years. However, it is difficult to study the root cause of low alloy steel rebars' enhanced corrosion resistance by alloy elements due to the presence of a large amount of CaCO 3 in the rust layer. Therefore, the mechanism of improving the protective effect of alloy elements on rust layer is further studied by dry/wet cycling corrosion tests to simulate a tropical marine atmosphere environment. The formation of dense product film on LA-1 steel surface is mainly because of the addition of Cr and Mo elements, leading to the corrosion rate of LA-1 steel rebar far less than that of HRB400E steel rebar. The protective film formed by molybdate in alkaline environment hinders the further corrosion of steel rebars. Cr(OH) 3 formed by hydrolysis of Cr in low alloy steel rebars inhibits the growth of FeOOH in rust, resulting in the generation of smaller size of FeOOH particles. The experiment proves that the newly designed LA-1 steel rebar can be well applied in tropical marine atmospheric environment. [ABSTRACT FROM AUTHOR]