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 expression 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 inhibitor efficiency. Experiments were performed on a mineral water at different flow rates; in the laminar regime, an increase in the flow rate accelerates the scaling process. The inhibiting properties of a well-known phosphonate (HEDP) were studied and the efficiency of this anti-scale additive was tested at different concentrations and an optimum concentration could be established (0.7 mg dm −3). The effect of inhibitor injection during the scale 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 CaCO 3 crystals as well as the coverage of the metallic surface.