Influence of inorganic components in organic coating on the corrosion protection of galfan coated steel

摘要

Corrosion inhibitive pigments and barrier fillers play different roles in the corrosion protection mechanism within an organic coating. Usually inhibitive pigments slightly dissolve in water and release free ions which are able to migrate to metal surface and provide protection to metal from corrosion. On the other hand, fillers with high aspect shape ratio are claimed to prevent/reduce the permeation of species, Le. water, oxygen, aggressive ions, to the metal surface. In the present study, the behaviour of an organic coating with a strontium aluminium polyphosphate pigment and a barrier filler of talc were studied for the corrosion protection of zinc alloy (Galfan) coated steel using 3D electron imaging techniques, assisted with electrochemical impedance spectroscopy. It was found that employment of corrosion inhibitive pigment resulted in two conflicting effects. On one hand, the inhibitive species released from the solid pigment reduced the corrosion rate. On the other hand, the dissolution of the inhibitive pigment resulted in porosity in the coating that created easy pathways not only for the inhibitors to reach the substrate but also for the aggressive species to migrate inwards to metal surface. Further, the barrier filler was observed to restrict the movement of corrosion products from the substrate outwards to the coating surface.