Electrochemical and Microstructural Characterization of CRUD

摘要

Amongst the postulated mechanisms for CRUD deposition, electrokinetic effects are believed to cause sufficiently large polarisations across the solution/metal interface to accelerate the electrochemical reactions that may be involved in the deposition process. To investigate this postulated mechanism, a miniature cell design was built to map the solution potential in regions of accelerated flow in an orifice. The setup contained a reference electrode that was isolated in a region of non-flowing solution to prevent it from being affected by electrokinetic effects, but capable of moving vertically through a narrow annulus in a sample washer where the flow accelerated. The potentials measured between this reference electrode and the cell body when the probe was moved through regions of accelerated flow showed rapid transient spikes when the probe was repositioned, possibly originating from a capacitative effect. The potentials rapidly settled to reproducible values, with the largest negative deviations up to 500 mV from the equilibrium potential found away from the accelerated flow region. The potential profiles were consistent with the degrees of polarisation expected for anodic and cathodic areas in light of the polarisation behaviour measured using conventional electrochemical characterisation. To assess the validity of miniature scale apparatus for measuring CRUD deposition around an orifice, micro scale flow cells were also designed to evaluate the CRUD build up rate as a function of the solution chemistry. Hydraulic modelling of the micro flow cells was performed and showed that the pressure drop is inversely proportional to the fourth power of the diameter of the orifice and this was in good agreement with direct measurements. Therefore, measuring the pressure drop in these systems has shown excellent sensitivity to the annular diameter and it is useful a technique with which to follow the progress of CRUD build up inside annular flow restrictions. Under the conditions employed, rates for CRUD deposition of up to 6 μm/hour were observed at the entrances to the orifices. The effect of water chemistry and fluid velocity were investigated. The evidence found from this work is consistent with the previous observations that CRUD deposition is accelerated by electrokinetic effects.