Answer
Edge corrosion is a localized, accelerated form of structural steel degradation that typically occurs along the sharp, un-rounded edges of structural members inside harsh environments like water ballast tanks, cargo holds, and pipe tunnels.
Cross-section of structural steel elements showing vulnerability to edge corrosion.
Why It Happens
When liquid protective coatings (paints) are applied to a steel plate with sharp 90-degree edges, the natural physical effect of surface tension pulls the wet paint away from the sharp corner before it can cure.
As a result, the Dry Film Thickness (DFT) directly at the apex of the sharp edge is extremely thin—often only a fraction of the thickness found on flat steel surfaces. This thin coating breaks down prematurely under mechanical stress, vibration, or exposure to corrosive mediums, leaving the bare steel edge exposed to rapid oxidation and heavy steel loss.
Prevention and Mitigation (IMO PSPC Standards)
To prevent edge corrosion from compromising the structural integrity of the hull, the IMO Performance Standard for Protective Coatings (PSPC) mandates specific surface preparation protocols before painting:
Edge Rounding: All sharp structural steel edges must be mechanically ground or radiused to a rounded profile of at least R = 2 mm, or subjected to a three-pass grinding process. Rounding the edge allows the liquid paint to flow evenly and maintain uniform thickness across the entire profile.
Application of Strip Coats: Prior to spraying the full, structural coats of paint, a manual strip coat must be applied using a brush or roller along all edges, welds, and complex geometries. This builds up the protective barrier thickness specifically where surface tension would otherwise thin it out.