Effect of Vertical Length on Corrosion of Steel in the Tidal Zone

The corrosion loss of mild steel in the marine tidal zone varies considerably with elevation relative to mean tide. Classical studies on steel piles and electrically isolated coupons show quite different vertical corrosion profiles. This usually is attributed to the effect of differential aeration. In many practical applications, steel structures may be confined to just part of the tidal zone or may extend only partly into either the atmospheric zone or the immersion zone. This paper presents mass-loss profiles for steel strips of different lengths (0.20, 0.50, 1.0, and 3 m) exposed at various elevations in the atmospheric, tidal, and immersion zones at Jervis Bay, Australia, where the tidal range is around 1.2 m. Relatively little difference in the corrosion profile was observed in the corrosion profiles for 100-mm-long coupons and for the shorter strips, mainly because each strip or coupon was confined to exposure only in a single zone. However, very high, localized corrosion was observed for a 1-m strip with the lower end extending just into the immersion zone. The 3-m-long strip and the individual coupons, all extending from the immersion zone to the atmospheric zone, displayed corrosion loss profiles generally consistent with those reported in classical studies.