J. Q. Wang

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.

Stress Corrosion Cracking of X80 Pipeline Steel in Near-Neutral pH Environment under Constant Load Tests with and without Preload

Constant load tests in NS4 solution purged with N(2)-5%CO(2) gas mixture were conducted on American Petroleum Institute (API) X80 pipeline steel applied in the 2nd West-East Gas Pipeline project with and without preload. The results show that cracks could initiate and propagate in X80 pipeline steel in near-neutral pH environment under a constant load condition. The life of crack initiation and propagation increased with decreasing applied stress. Preload did not change its corrosion behavior obviously. However, preload reduced the time for crack initiation.

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 temperature on initial corrosion of AZ91 magnesium alloy under cyclic wet-dry conditions

Abstract The effect of temperature on the initial corrosion of an as cast AZ91 magnesium alloy with surface deposition of salt particles in a water–vapour atmosphere under cyclic wet–dry conditions was studied using an environmental scanning electron microscope (ESEM). The results showed that the corrosion on the surface accelerated with increase in temperature and wetting time. The corrosion rates at various temperatures were a function of time, which expressed as with A &SetFont Typeface=44;representing the initial corrosion rate and n, usually in a range of 0 to 0·5, the exponent for the protective ability of corrosion products.

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.

Synthesis of nanocrystallized zirconium carbide based on an aqueous solution-derived precursor

Nanocrystallized zirconium carbide (ZrC) powder was synthesized by an aqueous solution-based process using zirconium acetate and sucrose as starting reagents. Polyvinyl pyrrolidone (PVP) was used to combine the reactants to form a suitable precursor for ZrC. Through such a novel aqueous solution-based process, fine-scale mixing of the reactants is achieved in an environmentally friendly manner. The formed precursor can be converted to ZrC by carbothermal reduction reaction at 1600–1650 °C. Thanks to a limited amount of residual free carbon, the as-synthesized ZrC powder had an ultra-fine particle size (50–100 nm) and a low oxygen content less than 1.0 wt%. The conversion mechanisms from as-synthesized pre-ceramic precursor to ZrC powder were investigated systematically and revealed by means of FTIR, TG-DSC, XRD, Raman, SEM and TEM.

Correlation between particle size and porosity of Fe-based amorphous coating

ABSTRACT The relationship between the particle size and porosity of an Fe-based amorphous coating that is fabricated using a high-velocity oxygen fuel (HVOF) process was revealed by both numerical and experimental methods. A computational fluid dynamics model was implemented in order to understand the gas-flow field and the behaviour of in-flight particles with different particle sizes during the HVOF spray process. By analysing both particle velocity and temperature, the optimal particle size range (20–30u2005μm) to achieve low-porosity coatings was predicted. These calculations were experimentally verified by the production of a low porosity (0.95%), Fe-based amorphous coating that was fabricated by HVOF using the predicted optimal particle size.