Showing papers by "Andrej Atrens published in 2007"

Recent Insights into the Mechanism of Magnesium Corrosion and Research Suggestions

Abstract: The opportunity to give an invited presentation to the Gordon Research Conference - Aqueous Corrosion in 2006 and reflections on the discussion at the conference were used to review our understanding of Mg corrosion mechanism, to generate new insights, to identify gaps in our knowledge and to identify research opportunities.

Evaluation of the delayed hydride cracking mechanism for transgranular stress corrosion cracking of magnesium alloys

Abstract: This paper evaluates the important elements of delayed hydride cracking (DHC) for transgranular stress corrosion cracking (TGSCC) of Mg alloys. A DHC model was formulated with the following components: (i) transient H diffusion towards the crack tip driven by stress and H concentration gradients; (ii) hydride precipitation when the H solvus is exceeded; and (iii) crack propagation through the extent of the hydride when it reaches a critical size of similar to 0.8 mu m. The stress corrosion crack velocity, V-c, was calculated from the time for the hydride to reach the critical size. The model was implemented using a finite element script developed in MATLAB. The input parameters were chosen, based on the information available, to determine the highest possible value for Vc. Values for Vc of similar to 10(-7) m/s were predicted by this DHC model. These predictions are consistent with measured values for V, for Mg alloys in distilled water but cannot explain values for V, of similar to 10(-4) m/s measured in other aqueous environments. Insights for understanding Mg TGSCC are drawn. A key outcome is that the assumed initial condition for the DHC models is unlikely to be correct. During steady state stress corrosion crack propagation of Mg in aqueous solutions, a high dynamic hydrogen concentration would be expected to build up immediately behind the crack tip. Stress corrosion crack velocities similar to 10(-4) m/s, typical for Mg alloys in aqueous solutions, might be predicted using a DHC model for Mg based on the time to reach a critical hydride size in steady state, with a significant residual hydrogen concentration from the previous crack advance step. (c) 2007 Elsevier B.V. All rights reserved.

Experimental Measurement and Computer Simulation of Galvanic Corrosion of Magnesium Coupled to Steel

Abstract: The influence of geometrical factors on the galvanic current distribution for the magnesium alloy AZ91D coupled to steel was investigated using a Boundary Element Method (BEM) model and experimental measurements. The galvanic current distribution calculated from the BEM model was in good agreement with the experimental measurements. The study was extended to practical components by the study of the galvanic corrosion of AZ91D coupled to a steel fastener.

Stress corrosion cracking in magnesium alloys: Characterization and prevention

Abstract: The positive environmental influence of magnesium alloy usage in transportation applications could be compromised by catastrophic fast fracture caused by stress corrosion cracking (SCC). Transgranular stress corrosion cracking (TGSCC) of AZ91 has been evaluated using the linearly increasing stress test and the constant extension rate test. The TGSCC threshold stress was 55–75 MPa in distilled water and in 5 g/L NaCl. The TGSCC velocity was 7×10−10 m/s to 5×10−9 m/s. A delayed hydride-cracking model for TGSCC was implemented using a finite element script in MATLAB and the model predictions were compared with the experiment. A key outcome is that, during steady-state TGSCC propagation, a high dynamic hydrogen concentration is expected to build up behind the crack tip. In this paper, recommendations are given for preventing SCC of magnesium alloys in service. One of the most important recommendations might be that the total stress in service should be below a threshold level, which, in the absence of other data, could be estimated to be ∼50% of the tensile yield strength.

Influence of solution chemistry and surface condition on the critical inhibitor concentration for solutions typical of hot potassium carbonate CO2 removal plant

Abstract: This research studied the influence of steel surface condition and solution chemistry on the critical inhibitor concentration required for spontaneous passivation of carbon steel in solutions typical of hot potassium carbonate plant (HPC). The inhibitor was added to the solution as V2O5. The critical inhibitor concentration depended on solution composition and on the steel surface condition. An inhibitor concentration of 30 g/l may be required to ensure spontaneous passivation under all conditions. The spontaneous passivation of clean polished carbon steel surfaces required a critical inhibitor concentration of 0.5–1.8 g/l. A minimum level of V5+ is required for inhibition, so that monitoring the V5+ concentration may be crucial to successfully managing corrosion protection in plant.

Passivity breakdown of carbon steel in hot potassium carbonate solutions

Abstract: This research studied passivity breakdown of carbon steel in conditions typical of hot potassium carbonate (HPC) acid gas processing plants. The pitting potential, evaluated from polarisation curves, indicated resistance to pitting increased with increasing equivalent carbonate concentration. The pitting potential was much more positive than the free corrosion potential for uninhibited concentrated carbonate solutions with pH values of 8.5 to 9.5. Therefore pitting is not an issue in these solutions. However, the carbon steel was susceptible to pitting corrosion in the dilute 1.0% bicarbonate solution of pH 8.0 for chloride concentrations in excess of 0.5 wt%. The critical chloride concentration was considerably above the maximum concentration of 0.1 wt% recorded in a typical HPC plant. Consequently chlorides should not usually be of concern to plant integrity.