The inhibition of corrosion-driven coating delamination on iron by novel cation-exchange pigments studied using the scanning Kelvin probe technique

Low-cost, environment friendly, cation exchange pigments derived from naturally occurring (Wyoming) bentonite clay are shown to significantly enhance resistance to corrosion-driven cathodic delamination in organic coatings adherent to iron surfaces. A scanning Kelvin probe (SKP) is used to study the delamination kinetics of pigmented and unpigmented polyvinyl-butyral (PVB)-based coatings adherent to polished iron substrates. The bentonite clay is used both in its native form and exhaustively exchanged with a range of divalent alkali earth (Mg2+, Ca2+, Sr2+ and Ba2+) and trivalent rare earth (Y3+ and Ce3+) metal cations. The dependence of coating delamination rate on bentonite pigment volume fraction is determined and compared with that of a conventional strontium chromate (SrCrO4) inhibitor. An inhibition mechanism is proposed for the bentonite pigments whereby underfilm cation release and the precipitation of sparingly soluble hydroxides reduces the conductivity of underfilm electrolyte.