John Postlethwaite

An Electrochemical Model for Prediction of Corrosion of Mild Steel in Aqueous Carbon Dioxide Solutions

Abstract A predictive model was developed for uniform carbon dioxide (CO2) corrosion, based on modeling of individual electrochemical reactions in a water-CO2 system. The model takes into account the electrochemical reactions of hydrogen ion (H+) reduction, carbonic acid (H2CO3) reduction, direct water reduction, oxygen reduction, and anodic dissolution of iron. The required electrochemical parameters (e.g., exchange current densities and Tafel slopes) for different reactions were determined from experiments conducted in glass cells. The corrosion process was monitored using polarization resistance, potentiodynamic sweep, electrochemical impedance, and weight-loss measurements. The model was calibrated for two mild steels over a range of parameters: temperature (t) = 20°C to 80°C, pH = 3 to 6, partial pressure of CO2 (PCO2) = 0 bar to 1 bar (0 kPa to 100 kPa), and ω = 0 rpm to 5,000 rpm (vp = 0 m/s to 2.5 m/s). The model was applicable for uniform corrosion with no protective films present. Performance of...

Erosion-Corrosion in Slurry Pipelines

Abstract Electrochemical techniques have been used to determine the corrosion component of the erosion-corrosion of steel pipe carrying sand, iron ore, potash, limestone, and coal slurries, at commercial concentrations and velocities. The tests have shown that the presence of the solids can bring about a sharp increase in the corrosion rate, and that the effect is dependent on the solids concentration and slurry velocity. An explanation for this effect is given in terms of the erosive disruption of surface films which normally hinder the diffusion of oxygen to the corroding surface.

Erosion in Disturbed Liquid/Particle Pipe Flow: Effects of Flow Geometry and Particle Surface Roughness

Abstract Erosion rates were measured along the length of a tubular flow cell of type 304 (UNS S30400) stainless steel (SS) carrying dilute slurries of silica sand (0.43 mm diam) and smooth glass be...

Calculation of wall-mass transfer rates in separated aqueous flow using a low Reynolds number κ-ε model

Abstract Mass transfer in aqueous, turbulent flow through a sudden pipe expansion is simulated with a low Reynolds number (LRN) κ-e eddy viscosity model. The predicted wall-mass transfer rates are tested against experimental data, obtained with electrochemical measurements ( Re = 2.1−13 × 10 4 and Sc = 1460). LRN modifications to the turbulence model in the near-wall regions, coupled with the turbulent Schmidt number concept, enable successful predictions of wall-mass transfer rates to be obtained. For the specific case of a high Schmidt number fluid, the mass transfer boundary layer is much thinner than the hydrodynamic boundary layer. Furthermore, even low levels of turbulence in the near-wall region are shown to have significant influence on the overall wall-mass transport.

Predictive models for erosion-corrosion under disturbed flow conditions

Abstract Recent advances in the development of predictive models for erosion-corrosion in disturbed turbulent flows are reviewed. The application of turbulence models permits the structure (velocity, pressure, turbulence fields) of the complex flow to be determined along with the calculation of local mass transfer rates of reactants and products. Particle/wall interaction statistics that are required for the application of erosion models are also determined.

CO2 CORROSION OF CARBON STEEL - FROM MECHANISTIC TO EMPIRICAL MODELLING

Many different models for C02 corrosion are used by engineers in the oil and gas industry. Some are described in the open literature and only those are covered by this review. In order to classify the CO2 corrosion models, we have arbitrarily decided to group them into three categories based on how firmly they are grounded in theory: mechanistic, semi-empirical and empirical models. A few most representative examples are discussed from each group. The performance of four different models covering all three groups is tested by comparing the predictions with a large experimental C0 2 corrosion database.

Effect of the Crevice Gap on the Initiation of Crevice Corrosion in Passive Metals

Abstract Past research into the mechanism governing the time to active crevice corrosion—the incubation period—of a passive metal crevice has produced theoretical models coupled with the B-dot model, the Debye-Huckel limiting law, and other activity models to correct for nonideal behavior at moderately high concentrations. In this research, the transport model of Watson and Postlethwaite is coupled with the ionic interaction model of Pitzer to predict the effect of the crevice gap on the iR drop and chemical activity of the crevice solution. Two cathodic reactions, crevice external oxygen reduction and crevice internal hydrogen ion reduction, are assumed to balance metal dissolution. To validate the model, the experimental Type 304 (UNS S30400) stainless steel crevice of Alavi and Cottis is simulated. Model predictions improve upon predictions of past models and match observations of this experimental work within experimental uncertainty. The effect of crevice gap on a titanium crevice immersed in 0.5 M a...

The application of low Reynolds number k-ε turbulence model to corrosion modelling in the mass transfer entrance region

Abstract Effects of mass transfer on the corrosion of metals are simulated throughout the mass transfer entrance region in turbulent pipe flow. A low Reynolds number k-e turbulence model is used to determine local mass transfer coefficients and kinetic E/log icorrosion diagrams which take into account effects of mass transfer resistance on the anodic and cathodic reactions are constructed. Iron is chosen to represent metals with a low exchange current density and copper those whose anodic dissolution is more reversible. Profiles of corrosion rate, surface metal ion concentration and surface pH for corrosion under charge transfer control; oxygen-mass transfer control; and anodic partial mass transfer control are presented. The concentration profiles along the entrance region are discussed in terms of their effect on film formation.

Effects of Corrosion on the Wear Patterns in Horizontal Slurry Pipelines

Abstract Electrochemical techniques have been applied to the study of the effect of corrosion on the wear patterns in a 10 cm diameter pilot plant slurry pipeline carrying a freely settling Newtonion slurry of 20 Vol% 30 to 50 mesh silica sand in water. Comparison of specimens with and without cathodic protection made after periods of exposure from 3 to 6 weeks showed that erosion-corrosion can give very rough surfaces with pitting in the earlier stages later developing into irregular wavy roughening, in contrast to the erosion behavior where hydrodynamically smooth polished surfaces were produced. Electrochemical corrosion rate measurements show that the effect of time on the corrosion component with an abrasive slurry is very much different to that with water where rust films are formed.

Wear and CO2 corrosion of steel couplings and tubing in heavy oil screw-pump wells

Abstract A prototype apparatus was used to investigate the effect of the conjoint action of CO2 corrosion and rotary sliding wear on steel couplings and tubing used in heavy oil screw-pump wells. The apparatus is versatile in design to simulate the oil-well conditions such as temperature, rotary speed, and the side forces exerted by the couplings on the tubing. Oil-well gases can also be passed through the system, and the apparatus can be converted to a three-electrode corrosion cell for conducting electrochemical studies. All tests were carried out in a produced heavy oil and formation water mixture. A comparison of the performance of various types of commercially available steel couplings and tubing grades through material loss measurements is presented. The couplings tested were made of plain carbon steel, and ‘hard metal’ coated steel. The steel tubing grades L-80, N-80, and P-110 as well as plain carbon steel were among the tubing materials tested. It was found that the martensitic tubin suffered the least thickness loss but caused the most damage to the couplings. Also, ‘hard metal’ coated couplings yielded the highest tubing thickness loss. The results are attributed to the microstructures of the various steels as well as their hardness values. The morphologies of the different materials surfaces were characterized using scanning electron microscopy (SEM) following testing.