Study of the scale inhibition by HEDP in a channel flow cell using a quartz crystal microbalance

An electrochemical quartz crystal microbalance apparatus was inserted in a channel flow cell in order to allow in-situ electrochemical and gravimetric measurements in well-defined electrolyte tube flowing conditions. An expre ssion of the mass transfer towards the electrode was established where the diffusion-limited current is a function of Re-1/3 in the laminar regime and was verified experimentally using the redox couples Fe[CN](6)(4-)/Fe[CN](6 )(3-) and O-2/OH-. This hydrodynamically controlled experimental device was developed to investigate scale deposit processes and to evaluate scale inh ibitor efficiency. Experiments were performed on a mineral water at differe nt flow rates; in the laminar regime, an increase in the flow rate accelera tes the scaling process. The inhibiting properties of a well-known phosphon ate (HEDP) were studied and the efficiency of this anti-scale additive was tested at different concentrations and an optimum concentration could be es tablished (0.7 mg dm(-3)). The effect of inhibitor injection during the sca le formation could also be evaluated as well as the influence of flow rate on the inhibiting efficiency. HEDP has shown a strong effect on inhibiting crystal growth and that affected the morphology of CaCO3 crystals as well a s the coverage of the metallic surface. (C) 2001 Elsevier Science Ltd. All rights reserved.