Effect of Phosphate Ion Eluted from Ni-P Coated Steel on CaCO₃ Scale

抄録

The ion eluted from the metal surface is expected to utilize as the scale inhibitor. In previous study, we found that the eluted phosphate ion from Ni-P coated steel prevents the adhesion amount of CaCO₃ scale. However, its scale inhibiting mechanism is unclear. The scale formation process should be explained as the nucleation (the generation of crystal on the surface), its growth and transformation. In present study, we investigated the effect of nucleation and crystal growth on the adhesion amount of CaCO₃ to understand the mechanism of inhibiting the adhesion by the eluted phosphate ion. Test materials are Ni, Ni-5%P, Ni-8%P, Ni-12%P, and Ni-14%P coated steels. CaCO₃ scale adhesion testing was conducted to the Ni-P coated steels. The adhesion morphology of calcium carbonate was observed by scanning electron microscopy. There were four kinds of polymorphic crystal forms on the test materials: calcite, aragonite, vaterite, and irregular shape crystal. The adhesion amounts, the crystal numbers of their polymorphs, and their sizes were measured. There was almost no correlation between the adhesion amount and the crystal number of polymorphs. Inhibiting nucleation does not affect on the inhibiting adhesion amount. On the other hand, there was a strong correlationship between total adhesion amount and the size of irregular shape crystal. The reason why the scale adhesion amount became small may be that the crystal growth by the eluted phosphate ion has been suppressed.