Dissolution mechanism of Fe3O4 scale by 1-hydroxyethane-1,1-diphosphonic acid: an ab initio molecular metadynamics study.

Using the ab initio molecular metadynamics method, the adsorption of the structure of 1-hydroxyethane-1,1-diphosphonic acid (HEDP) on the Fe3O4 surface and subsequent detachment of Fe atoms from the surface were simulated, and the dissolution mechanism by which HEDP dissolves Fe3O4 scale at room temperature while other organic acids cannot was elucidated. The adsorbed hydroxyl groups, water and HEDP on the Fe3O4 surface play a synergistic role in detaching the Fe ions, which increases the coordination number of the Fe atoms and weakens the original Fe-O bond strength. In addition, the strong coordination ability and flexible molecular structure of HEDP also facilitate dissolution of Fe3O4 scale by breaking down the chemical bonds and forming Fe-HEDP complexes. The free energy surface for the dissolution reaction shows a low barrier, and the descaling reaction is easily accomplished.