T. Hodgkiess

A study of the erosion-corrosion behaviour of engineering steels for marine pumping applications

This paper presents results of a series of electrochemically based tests under liquid and liquid-solid erosion conditions on three generic materials, a C-Mn steel, an austenitic and a duplex stainless steel. Separation of the components of pure erosion, pure corrosion and synergy has been achieved under conditions imposed by an impinging saline flow containing a high burden of suspended solids. Although the pure corrosion component of deterioration on the stainless steels has been shown to be very small, the synergistic processes ensure that corrosion has a significant effect on these martials as well as on the less corrosion resistant C-Mn steel. The results have shown that complex hydrodynamic conditions impose mixed modes of attack since local variations in impingement angle exist.

An assessment of the corrosion behaviour of high-grade alloys in seawater at elevated temperature and under a high velocity impinging flow

Abstract Laboratory experiments were carried out to investigate the effect of elevated temperature (up to 60 °C) and a high velocity impinging flow on the corrosion behaviour of stainless steels and related Ni-base and Co-base alloys. DC-electrochemical techniques were employed in conjunction with microscopy to determine the extent and mechanisms of attack under the severe imposed conditions. Experiments demonstrated the clear effect of increased temperature in facilitating premature breakdown of passivity on all materials and the effect of the high velocity impinging flow was to further shift the passivity breakdown potential to more active values but not necessarily to result in greater depth of attack. In addition, the study has indicated that the relative resistances of the different materials to attack under static, ambient temperature conditions is altered significantly when they are subjected to elevated temperatures and/or high velocity seawater impingement.

Mechanisms of wear on a Co-base alloy in liquid–solid slurries

Abstract The efficient performance of drilling tools in aqueous environments requires materials to be used, which can effectively resist excessive material loss due to the combined effects of erosion and corrosion. In this paper, the interactions between electrochemical corrosion and mechanical erosion have been assessed in liquid–solid slurries on a Co-base material (Stellite X40), which is currently used in the manufacture of oil tools. The material degradation is assessed as a function of aqueous environment and sand loading. Corrosion has been shown to represent an important factor in the degradation of this material even though it essentially exhibits passivity in static saline conditions. The relative importance of corrosion and its synergistic effect when combined with erosion are quantified and the implications in terms of material performance and of potential methods of reducing oil tool degradation are discussed.

Assessment of the corrosion rates and mechanisms of a WC-Co-Cr HVOF coating in static and liquid-solid impingement saline environments

Abstract The corrosion behaviour of a WC–Co–Cr high-velocity oxy-fuel (HVOF)-sprayed coating has been examined in static saline conditions and under the influence of an impinging jet of liquid free from solids and containing solids. The corrosion behaviour in static conditions is neither passive nor active, and its complexity results from the composite ceramic/metal nature of the coating surface. The influence of exposure to the impinging jet free from solids is to slightly increase the corrosion rate, and a further increase in corrosion rate is observed when solid particles are added to the liquid stream. This paper assesses the corrosion behaviour of this cermet system and discusses the factors controlling the electrochemical material loss.

Electrochemical and mechanical interactions during erosion–corrosion of a high-velocity oxy-fuel coating and a stainless steel

Abstract This paper describes the results of a study of the erosion–corrosion behaviour of a thermally sprayed Ni–Cr–Si–B–C cermet coating in saline solution in the presence and absence of solid particles. The water, at ambient temperature, impinged perpendicularly onto the specimen surface at a velocity of 17 m/s. Emphasis was placed on the assessment of the contribution of corrosion processes to the overall erosion–corrosion deterioration. For comparison purposes in terms of performance and degradation mechanisms, parallel tests were carried out on a standard stainless steel alloy (UNS S31603). The study provided clear evidence of the crucial role of corrosion processes in governing the extent (totalling about 30% in solid/liquid impingement) and mechanisms of erosion–corrosion of the cermet material. The cermet coating was found to undergo less weight loss than the stainless steel in solid/liquid conditions but vice-versa during solid-free impingement and this was attributed to the relative influences of erosion and corrosion.

Characterisation of high-grade alloy behaviour in severe erosion–corrosion conditions

Abstract A study has been made of the erosion–corrosion behaviour of two nominally corrosion resistant alloys (a superduplex stainless steel, UNS S32760 and Inconel 625 (UNS N00625) and a wear-resistant material Stellite 6 (UNS AMS5387). The experiments comprised exposure to an impinging jet of 3.5% sodium chloride solution at 100 m/s, 18°C and 50°C in the absence of entrained solids and also with 1000 parts per million sand particles at 25 m/s and 50°C. It was found that the passive film on the corrosion resistant materials remains essentially intact under solids-free impingement but that these alloys exhibit active corrosion under liquid–solid impingement and that the direct and indirect effects of corrosion on the erosion–corrosion behaviour of such alloys can be extensive. The study also demonstrated that there is a potential for significant galvanic interactions between materials when under different erosion regimes.

Electrochemical assessment of calcium carbonate deposition using a rotating disc electrode (RDE)

An electrochemically-based technique, which uses assessment of the oxygen reduction reaction at a rotating disc electrode, has been devised which shows promise as a method for studying nucleation and growth of mineral scale at a solid surface. In this paper the background and development of the technique are described for the study of deposition of CaCO3 from a supersaturated solution. Results are presented which illustrate the good correlation between the surface coverage predicted by electrochemical analysis and the actual coverage quantified by image analysis. The potential of this technique for mechanistic studies of surface scaling and for assessment of inhibitors is discussed.

A comparison of the corrosion behavior of WC-Co-Cr and WC-Co HVOF thermally sprayed coatings by in situ Atomic Force Microscopy (AFM)

The corrosion behavior of WC-Co-Cr and WC-Co high velocity oxygen fuel (HVOF) sprayed coatings were examined in static saline conditions. Direct current (DC) polarization tests were conducted and the electrochemical corrosion behavior was shown, by in situ atomic force microscopy (AFM) and scanning electron microscopy (SEM), to be complex because of the composite ceramic-metal nature of the coating. The addition of chromium to the matrix greatly enhanced the corrosion resistance of the coating.

Studies of the deposition of CaCO3 on a stainless steel surface by a novel electrochemical technique

This paper describes the application of a novel electrochemical-based technique for the investigation of scale deposition directly on a metallic surface. The work, also involving conventional bulk chemistry, has comprised a comparison of scale control inhibition by polyacrylic acid on bulk precipitation and on deposition at an electrode surface. It has been shown that the new technique can monitor the influence of an inhibitor on scale deposition. Moreover the new experimental approach has provided evidence that the efficiency of inhibitors may be substantially different in relation to surface deposition and bulk precipitation.

An electrochemical and microstructural assessment of erosion–corrosion of cast iron

Abstract An extensive study has been carried out on grey cast iron (BS 1452) in liquid erosion and liquid–solid erosion conditions in which weight loss tests have been used in conjunction with electrochemical measurements and microscopy. The study has revealed interesting effects of the impingement velocity in solid-free conditions and of solids loading at constant velocity. This experimental approach has enabled important interactive effects between mechanical and electrochemical processes to be quantified. It has been demonstrated that corrosion and erosion, when acting together, accentuate the weight loss above what can be accounted for by pure erosion or pure electrochemical corrosion. Sequential periods of erosion–corrosion affect the subsequent static corrosion behaviour and pre-corrosion of grey cast iron significantly affects the erosion–corrosion behaviour.