CATION ENHANCEMENT OF INTERNALLY COATED METAL CONTAINER CORROSION FAILURE

The effect of different rations on corrosion rates of internally coated metal containers at rest potential was studied. Time before the failure of 30% of the internally coated metal containers (T30) decreased in the order: deionized water > lithium cation > sodium cation > potassium cation > calcium cation for salt concentrations from 0.0 M to 0.1 M. This trend was similar to one reported previously for coated-steel cathodic delamination rates. The relationship between mean activity coefficients and T30 may help resolve the seeming inconsistency between the 2 M and lower concentration data. Pore resistance values obtained using electrochemical impedance spectroscopy also followed the trend for T30. Potassium salt caused earlier failure than the calcium salt when the salt concentration was 2 M. The correlation between trends observed for pore resistance and T30 suggested that a polymer coating degradation mechanism proposed previously was operative when the coated metal was at rest potential However, the current data suggested the coating morphology changed at salt concentrations below the 0.5 M level originally hypothesized. Cation water sheath structure may determine how effectively a cation alters coating morphology. However, lack of general agreement on the number of water molecules in a cation water sheath and lack of information on group II cation water sheath structure prevented formulation of a rigorous hypothesis. It was speculated that the thermodynamic state of a cation in a coating may be similar to its thermodynamic state in the bulk electrolyte.