W. Ke

Electrochemical measurements using combination microelectrode in crevice simulating disbonded of pipeline coatings under cathodic protection

Abstract A novel combination microelectrode was constructed based on an iridium pH microelectrode and an Ag/AgCl microelectrode, and was applied to simultaneously monitor the steel potential, pH and chloride ion concentration in a crevice simulating disbonding of pipeline coatings with cathodic protection (CP) applied or removed. The sensing characteristics and sensitivity of the microelectrode are described as well as the calibration procedures. A simulated crevice was formed between a pipeline steel surface and a polymethylmethacrylate plate. The results showed that in CP conditions, the evident potential gradient resulting from the IR potential drop was concentrated at the crevice opening. A protective local environment induced by CP under disbonded coatings was characterised by an increase in pH and removal of Cl−. After CP is removed, the protective effects can persist for a period of time, depending on the diffusion of soluble species.

Short-time Oxidation of Alloy 690 in High-temperature and High-pressure Steam and Water

The oxidation behavior of alloy 690 exposed to high-temperature and high-pressure steam and water at 280 degrees C for 1 h was investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). In high-temperature and high-pressure steam, the oxide film is composed of an outermost Ni-rich hydroxides layer, an intermediate layer of hydroxides and oxides enriched in Cr, an inner oxide layer. The film formed in high-temperature water is similar to that in steam, except for missing the Ni-rich hydroxides layer. Samples with different surface finishes (electropolished, mechanically polished, ground, and as-received) were prepared for comparison. A general increase of the oxide thickness with the degree of surface roughness is observed. The equivalent oxide thicknesses lie in the range of 100-200 nm for the as-received samples, 150-250 nm for the samples ground to 400(#) and 10-20 nm for the samples ground to 1500(#), mechanically polished, and electropolished.

Corrosion Evolution of Reinforcing Steel in Concrete under Dry/Wet Cyclic Conditions Contaminated with Chloride

The corrosion evolution of rebar in concrete was monitored by electrochemical impedance spectroscopy (EIS) under dry/wet alternated accelerated corrosion test. Four stages with different dynamic characteristics were observed during the corrosion evolution. They were passive stage, local corrosion controlled by the charge transfer step, accelerated corrosion controlled by the mass transfer step, and constant rate corrosion controlled by the mass transfer step through a barrier layer. X-ray diffraction (XRD) analysis showed that the corrosion product of rebar in mortar was composed of alpha-FeOOH, gamma-FeOOH and Fe3O4. The corrosion mechanisms of all four stages were discussed and the corrosion reactions were proposed according to the corrosion product and corrosion evolution characteristics.

Stress corrosion cracking of X-70 pipeline steel in near neutral pH solution subjected to constant load and cyclic load testing

Abstract Stress corrosion cracking (SCC) tests were conducted on American Petroleum Institute (API) grade X-70 line pipe steel using smooth specimens in a standard near neutral pH solution, NS4. Both constant loading and cyclic loading conditions were employed. There were some SCC cracks on the surfaces of specimens under constant loading conditions when the applied load and test duration were appropriate. These cracks were related to stress concentration at corrosion pits. Generally, the local strain rate around corrosion pits in constant load tests was not high enough for the transition from pits to cracks to take place, and so long test times were needed for crack initiation. In contrast, cyclic loading tests facilitated near neutral pH SCC by making the local strain rate at the crack tip change under the applied load spectrum and by enhancing the crack tip local strain rate, which appreciably shortened the crack initiation incubation.

Effect of Al alloying on corrosion performance of steel

Abstract To clarify some features of the atmospheric corrosion mechanism, the corrosion performance of Al bearing steels after wet/dry cyclic corrosion tests were studied by gravimetry, SEM, X-ray diffraction (XRD), potentiodynamic polarisation tests and electrochemical impedance spectra (EIS). The results showed that rusted Al bearing steel presented a better corrosion resistance than that of rusted plain carbon steel (Q235) in 0·3%NaCl (i.e. simulating a marine environment) solution; moreover, the corrosion resistance of Al bearing steel increased with increasing Al content. The formation of a spinel double oxide containing Al(FeAl2O4) in Al alloying steels was observed, which refined the structure of the inner rust layer, suppressed the anodic dissolution and accelerated the cathodic reduction. In addition, Al alloying increased the content of γ-FeOOH and decreased that of β-FeOOH. However, in NaHSO3 solution (i.e. simulating an industrial environment), the rust layer on Al bearing steel did not possess a protective ability, and the rust layer formed was not physically or chemically stable.

Corrosion behaviour of X52 pipeline steel in high H2S concentration solutions at temperatures ranging from 25°C to 140°C

Abstract High temperature and high pressure immersion tests in an autoclave were employed to study the corrosion behaviour of X52 pipeline steel in aqueous solutions containing high concentrations of H2S. The corrosion products generated were characterised using scanning electron microscope, energy dispersive spectroscopy and X-ray diffraction. It was seen that at a constant H2S concentration of 22 g/l, the corrosion rate increased with increasing temperature up to 90°C, thereafter decreased at 120°C and slightly increased again at 140°C while the corrosion rate increased with H2S concentration at a temperature of 90°C. When the temperature and H2S concentration increased, the corrosion product converted from iron rich to sulphur rich products in the following sequence: mackinawite→troilite→pyrrhotite, where the microstructure and stability of the corrosion products had an important effect on the corrosion rate. The corrosion film was formed through the combination of the outward diffusion of Fe2+ ions and the inward diffusion of H2S and HS− species.

Characterisation of rust formed on carbon steel after exposure to open atmosphere in Qinghai salt lake region

Abstract The rust layer formed on carbon steel after 1 year exposure to Qinghai salt lake atmosphere was characterised by the following complementary techniques: X-ray diffraction, infrared transmission spectroscopy, scanning electron microscopy and energy dispersive X-ray. The crystalline components of the rust layers consisted primarily of β-FeOOH, γ-FeOOH and iowaite [Mg4Fe(OH)8OCl.4H2O]. δ-FeOOH, ferrihydrite and amorphous rust were also found. Most corrosion products were contained in the inner layer, while foreign particles were mainly distributed in the outer layer. Energy dispersive X-ray and linear scanning results indicated that Cl and Mg elements were rich in the whole rust layer, in which case they will have had an important influence on the corrosion process of carbon steel in the salt lake atmospheric conditions.

Influence of strain rate on the stress corrosion cracking of X70 pipeline steel in dilute near-neutral pH solutions

Publisher Summary This chapter intends to evaluate the stress corrosion cracking (SCC) behavior of line pipe steel in the NS4 solution, traditional for near-neutral pH SCC tests, and also in a Chinese actual soil environment. Potentiodynamic polarization experiments are conducted at the scan rate of 0.5 mV/s using Solartron 1287 Electrochemical Interface. The specimens for these experiments are mounted in epoxy and then wet-ground using 1200 grit aluminum oxide (A1203) waterproof abrasive paper. SCC tests are performed using the slow strain rate tensile (SSRT) technique at room temperature. At higher strain rates, there are few cracks on the specimen surface, and SCC occurs at more negative potentials. At lower strain rates, SCC susceptibility increases with decreasing cathodic potentials. SCC susceptibility of X70 steel at cathodic potentials increases with decreasing strain rates. The time-to-failure of X70 pipeline steel with a coarse-grain heat-affected zone (HAZ) suggests that susceptibility to SCC, with all other conditions being equal, is higher in the than in the base metal. This may be related to a number of factors, including coarse-grain sizes, heterogeneous microstructure, and high residual stresses in HAZ, and increased localized hardness caused by welding.

EIS study on corrosion evolution of chemical quenched rebar in concrete contaminated with chloride

Abstract The corrosion evolution of scaled rebar cooled by a chemical reagent FM in concrete immersed in 3·5%NaCl aqueous solution was studied by electrochemical impedance spectra and compared with bare rebar. The results show that the corrosion evolution process of FM cooled rebar consists of three stages during the immersion: the passive stage, the accelerated corrosion stage dominated through the charge transfer step and the steady corrosion stage dominated through the mass transfer step. The corrosion rate of FM cooled rebar reaches a steady rate (<0·7 μA cm−2) after 80 days. However, the corrosion rate of bare rebar always increases at all immersion times. After immersion for ∼1 year, the corrosion rate of bare rebar is nine times larger of that of FM cooled rebar. Therefore, the corrosion resistance of the rebar has been improved significantly by applying the FM cooling process.

Stress Corrosion Cracking of X70 Pipeline Steel in Near-Neutral pH Soil Solution

Stress corrosion cracking behavior of X70 pipeline steel was studied using slow strain rate tests (SSRT) and cyclic loading at high R and low frequency in a nearneutral pH soil solution saturated with 5% CO2 +95% N2 . The soil was from Xinjiang Uygur Autonomous Region where the Chinese West-East natural gas transmission pipeline started. Electrochemical tests including a potentiodynamic polarization technique and electrochemical impedance spectrum (EIS) were also conducted in order to analyze the effect of the concentration of bicarbonate, bubbled gas and the addition of chloride ion on the polarization behaviors. The results of SSRT showed that transgranular stress corrosion cracking (TGSCC) occurred in Xinjiang soil solution. Crack initiation was associated with pitting, inclusion and streamline of rolling. The susceptibility to SCC increased with the decrease of the applied electrochemical potential and strain rate. Cyclic loading tests with smooth specimens showed that some cracks initiated after certain cycles and cracking mode was transgranular. Under the cyclic loading of high R and low frequency, the crack propagation rate (CPR) of precracked specimens did not increase at some region of stress intensity factor range (ΔK), which showed that the crack propagation process was dominated by SCC. The results of the electrochemical tests showed that the polarization behaviors were influenced greatly by the concentration of bicarbonate, bubbled gas and the addition of chloride ion. Low concentration of chloride ion in bicarbonate could cause the elimination of passivity and SCC behavior to that in near-neutral pH soil solution.Copyright