Case study: Analysis of coating failure on hinged steel boxes

Abstract

A systematic failure analysis was performed on three large steel boxes in which the coating system had blistered, extensively delaminated, and subsequently corroded. These boxes were a sample from a larger batch of boxes that had been recalled due to severe corrosion. Despite the fact that limited information on the process history was provided, we were able to determine that two out of three boxes were neither galvanized nor protected by a zinc-rich primer to provide cathodic protection. The coating system comprised an epoxy primer, zinc-rich intermediate coat followed by an acrylate urethane top coat. A zinc-rich coating should never be applied over a nonmetallic coating because cathodic protection of the underlying steel can only take place if the zinc-rich coating is in direct electrical contact with clean steel. The third box was galvanized and primed with a urethane-alkyd primer followed by an acrylate urethane top coat. Alkyd-type coatings should not be applied over zinc (galvanizing) because the corrosion products of zinc are alkaline. Alkyd-modified coatings are very sensitive to alkalinity and a saponification reaction occurs at the zinc-alkyd interface. This degrades the alkyd and causes it to peel or delaminate from the zinc alloy used for the galvanization. The epoxy primer showed unusually severe air bubbles (or pockets) that could have been due to saponification and/or solvent entrapment. In either case, these large pockets weakened the bond between the primer and galvanized layer and allowed delamination to occur. Our finding that chlorides were present on the surface of the epoxy primer indicated that the boxes might have been exposed to a marine or coastal atmospheric environment. Marine or coastal atmospheric corrosion is generally considered to be one of the most severe atmospheric corrosion environments and the presence of chlorides explains the severity of the corrosion. The boxes were presumed to have been powder coated, yet microscopic and chemical analysis showed the coatings were probably applied as liquids. This was further supported by the thinning of the coatings at the sharp edges and corners of the boxes.