Development and evaluation of thin film electrical resistance sensors for monitoring CO2 top of the line corrosion

Abstract

CO2 top of the line corrosion of carbon steel was studied using an in-house Electrical Resistance (ER) monitoring system. The system comprised a thin film ER sensor made of 50 μm thick carbon steel foil, and a high precision constant current circuit. Compensation for the variation in metal resistivity with temperature was achieved by the standard method of using two different circuit paths (corroding and reference). The offset-compensated ohms method using a switched relay circuit was found to effectively overcome thermoelectric junction potentials between the copper connecting wires and the steel sensor element, thus achieving reliable and reproducible top of the line corrosion rates at short time scales. The possible current short circuits by droplet or continuous thin film pathways through the condensate layer were modelled using finite element techniques and confirmed to be insignificant even for a fluid with an electrical conductivity equivalent to that of seawater, more than two orders of magnitude above that of the condensed fluid. The monitoring results successfully demonstrated the role of increasing gas temperature in accelerating top of the line corrosion. The accuracy and the reproducibility of the ER technique for the measurement of corrosion in low conductivity fluids are shown to be at least as good as other published results, and the ability to give a cumulative measurement of corrosion rather than an instantaneous rate is shown to allow the true long term total corrosion depth to be measured, unaffected by short term variations due to cyclic droplet formation.