C.W. Du

Corrosion and Stress Corrosion Cracking Behavior of X70 Pipeline Steel in a CO2-Containing Solution

The electrochemical corrosion and stress corrosion cracking (SCC) behaviors of X70 pipeline steel in CO2-containing solution were studied by electrochemical measurements, slow strain rate tensile tests, and surface characterization. The results found that the electrochemical corrosion of X70 steel in aerated, alkaline solution is an activation-controlled process, and a stable passivity cannot develop on steel. Corrosion rate of the steel increases with the CO2 partial pressure. The enhanced anodic dissolution due to the additional cathodic reaction in the presence of CO2, rather than the film-formation reaction, dominates the corrosion process. The mass-transfer step through FeCO3 deposit is the rate-controlling step in corrosion of the steel. The susceptibility of steel to SCC and the fracture brittleness increase with the CO2 partial pressure. The enhanced fracture brittleness is attributed to the evolution and penetration of hydrogen atoms into the steel, contributing to crack propagation. The formed deposit layer is not effective in reducing hydrogen permeation due to the loose, porous structure.

Corrosion Behavior of E690 High-Strength Steel in Alternating Wet-Dry Marine Environment with Different pH Values

The corrosion behavior and mechanism of E690 high-strength steel in marine environment with different pH values were studied through electrochemical technology and long-term alternating wet-dry cycle experiments combined with SEM and XRD. Results showed that the corrosion current density of E690 high-strength steel gradually increased with decreased pH. After long-term tests in alternating wet-dry marine environment with various pH values, uniform corrosion mainly occurred on E690 steel, accompanied by vast corrosion pitting. Weight loss analysis demonstrated that corrosion rate decreased with increased pH. Moreover, corrosion mechanism varied with pH, and hydrogen-evolution reaction greatly increased the E690 steel corrosion rate at low pH. Meanwhile, the compositions of corrosion products slightly differed with pH; these products consisted of Fe3O4, Fe2O3, α-FeOOH, β-FeOOH, γ-FeOOH, and amorphous substances. However, the rust-layer density varied. Cr in the rust layer promoted the densification of rust layer and improved the decay resistance of E690 steel.

Crack growth behaviour and crack tip chemistry of X70 pipeline steel in near-neutral pH environment

In this paper, crack growth behaviour and the crack tip chemistry of X70 pipeline steel in the near-neutral pH environment were investigated using the in-situ measurements. The stress corrosion crack propagated forward under the cyclic load, with a mean crack growth rate (CGR) of 4.28 × 10–3 mm/cycle. The CGR da/dN obeys well with the stress corrosion fatigue mechanism. During the crack propagation, the crack tip environment differed significantly from the bulk solution. An aggressive electrolyte with low pH (about 4.0) and high concentration of Cl– (about 3.8 M) was produced near the crack tip. These results provided the direct evidence for simulating the crack tip solution and investigating the crack tip anodic dissolution.

CREVICE CORROSION BEHAVIOR OF X70 STEEL IN HCO3- SOLUTION UNDER CATHODIC POLARIZATION

The crevice corrosion behavior of X70 steel was investigated with a wedge-shaped crevice assembly under −1000 mV (SCE) cathodic polarization in the solutions with various HCO−3 concentrations. The potential, current, pH and the oxygen content within the crevice were measured with or without outside coupled specimen. The results indicated that the polarization potential of X70 steel in the crevice dropped with the increase of time under the cathodic polarization. There was a remarkable influence of HCO−3 concentration on the potential of X70 steel in the crevice. When HCO−3 concentration was up to 0.125%, the surface of the metal was covered with the corrosion products that resulted in the polarization extent of X70 steel decreased. The pH value in the crevice rose and it dropped gradually from the crevice mouth to the bottom under the cathodic polarization. With the increasing of HCO−3 concentration, the hydrolyzation reaction of metal in the crevice bottom aggravated. Most of the dissolved oxygen in the crevice was consumed by the cathodic current. The maximum cathodic current on the metal surface was at the crevice mouth and it was much more than that at the crevice bottom.

A New Understanding on AC Corrosion of Pipeline Steel in Alkaline Environment

In this work, the corrosion behavior of X80 pipeline steel at various frequencies AC was investigated in carbonate/bicarbonate solution using the polarization curve, EIS test, Mott-Schottky curve and immersion tests. A new understanding on AC corrosion of the steel in alkaline environment is proposed. Decreasing AC frequency negatively shifts the corrosion potential and increases the corrosion rate of steel, as well as corrosion pits occur more readily. The superimposed AC shifts the critical pitting potential negatively and degrades the passivity of the steel. AC reduces the compactness and uniformity of the passive film formed on the steel and increases the possibility of the breakdown of the film, as well decreases the film thickness. The application of AC could prevent the passive film forming on the surface of X80 steel and result in a destructive effect on the film formed on the steel surface, especially at the low-frequency AC.

Effect of pH Value on the Electrochemical and Stress Corrosion Cracking Behavior of X70 Pipeline Steel in the Dilute Bicarbonate Solutions

In this work, effects of pH value on the electrochemical and stress corrosion cracking (SCC) behavior of X70 pipeline steel in the dilute bicarbonate solutions were investigated using electrochemical measurements, slow strain rate tensile tests and surface analysis techniques. Decrease of the solution pH from 6.8 to 6.0 promotes the anodic dissolution and cathodic reduction simultaneously. Further decrease of the pH value mainly accelerates the cathodic reduction of X70 pipeline steel. As a result, when the solution pH decreases form 6.8 to 5.5, SCC susceptibility decreases because of the enhancement of the anodic dissolution. When the solution pH decreases from 5.5 to 4.0, SCC susceptibility increases gradually because of the acceleration of cathodic reactions.

Influence of alternating voltages on passivation and corrosion properties of X80 pipeline steel in high pH 0.5 mol L− 1 NaHCO3+0.25 mol L− 1 Na2CO3 solution

Abstract X80 steel exhibits two different steady states in 0.5 mol L− 1 NaHCO3+0.25 mol L− 1 Na2CO3 solution with and without precathodic polarisation, which are activation and passivation respectively. Different amplitudes of alternating voltages were superimposed to the pre- and non-prepolarised steels. The results demonstrated that passivation of the non-prepolarised steel was enhanced by low amplitude alternating voltages but was destroyed by high amplitude alternating voltages. Moreover, the corrosion of the prepolarised steel was worsened with the application of alternating voltages.

Study on the Crack Propagation Behavior of ×80 Pipeline Steel Under AC Application in High pH Solution

The crack propagation behavior of pipeline steels with or without AC application was studied in high pH solution using the crack propagation experiment (cyclic load). The results show that there is a significant difference in the crack propagation behavior of steels with or without AC interference. The crack growth rate (CGR) of steel under superimposed AC is considerably greater than that without AC. AC could cause an obvious effect on the crack propagation behavior, and enhance the CGR. The crack propagation behavior of steel under AC application in high pH solution is analogous to that in near-neutral pH solution.

Influence of AC waveforms on stress corrosion cracking behaviour of pipeline steel in high pH solution

The effect of various waveforms AC on stress corrosion cracking (SCC) behaviour and mechanism of X80 pipeline steel was investigated in high pH carbonate/bicarbonate solution by polarisation curves and slow strain rate tensile tests. Under various waveforms AC, the passivity of the steel degrades severely, and pitting corrosion easily forms on the electrode surface. With or without AC application, the SCC behaviour and mechanism of the steels in the solution are distinctly different. Without the presence of AC, the cracks propagation are intergranular, and the mechanism is anodic dissolution. AC significantly increases the SCC susceptibility of pipeline steel. The order of the SCC susceptibility of the steel at various waveforms AC is: sine wave < square wave < triangular wave. Under a long term AC interference, the fracture mode is transgranular, and the mechanism of the steel in high pH solution is controlled by anodic dissolution and hydrogen embrittlement.