Abstract
Abstract Low-cost, environmentally friendly, cation exchange pigments derived from naturally occurring 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 poly-vinyl-butyral (PVB)-based coatings applied to polished iron substrates. The bentonite clay is used both in its native form and exhaustively exchanged with a range of divalent alkali earth and trivalent rare earth metal cations. For the best performing divalent cation-exchanged pigment, the dependence of coating delamination rate on pigment volume fraction is determined and compared with that of a conventional strontium chromate (SrCrO 4 ) inhibitor. An inhibition mechanism is proposed for the bentonite pigments whereby underfilm cation release and subsequent precipitation of sparingly soluble hydroxides reduces the conductivity of the underfilm electrolyte.
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