Electrical measurements of immersed paint coatings on metal. II. Effect of osmotic pressure and ionic concentration of solution on paint breakdown

Electrical capacitance and electrode potential measurements have been used to study the behaviour of painted steel panels immersed in solutions of sucrose, of sodium chloride and other electrolytes, and in sea water. The water uptake of the paint in the early stages of immersion depends only on the osmotic pressure of the solution. The time to reach breakdown of the paint in sodium chloride solutions depends primarily on water uptake and secondarily on ionic concentration. The time to reach breakdown in sea water is considerably greater than that in sodium chloride solutions of similar ionic concentration. Corrosion rate after breakdown is primarily a function of the anodic‐cathodic pore resistance, which is related to the electrolytic conductivity of the ambient solution; it is decreased in concentrated solutions by diminishing oxygen solubility. A mechanism of the course of paint breakdown is suggested.