Abstract
Thermally sprayed zinc (TSZ) with an organic coating system on top has mainly given very long lasting corrosion protection on Norwegian road bridges. Even in coastal areas up to 40 years lifetime has been achieved. However, there are also examples of bridges where the coatings have failed after about 20 years. Seven bridges in various environments, 12 - 23 years old, were inspected in this study. Two types of coating failure was found: (1) corrosion of the TSZ in an area where the thickness of the organic coating was too thin, and (2) single spots where the TSZ corroded, probably caused by pinholes in the organic coating. Pinholes are easily formed when organic coatings are applied on TSZ. By diluting the sealer coat to about 30% solids and applying less than 25 µm dry film thickness, the formation of pinholes was avoided.
Introduction
In Norway there are more than 3000 steel bridges and about 350 of them are more than 100 m long. They typically have a design life of 100 years, and most of them are located in coastal areas, crossing fjords and straits. The corrosivity varies depending on elevation over the sea and climatic conditions at the site, but corrosion class C4 and C5 according to ISO(1) 12944 are estimated for many bridges1.Some floating bridges are also located in the splash zone. Hence, corrosion is a considerable threat. Due to the very long lifetime of the bridges, a long life coating system is also desired in order to keep the life cycle costs as low as possible.
Since 1965 the coating specification for Norwegian road bridges has been to apply 100 µm thermally sprayed zinc (TSZ) with a paint system on top, a so called duplex coating system. The paint system has changed over the years, depending on environmental legislation and paint technology development in general. From 1965 to 2005 the paint system applied consisted of four layers of alkyd or alkyd/chlorinated rubber mix with total specified film thickness of 160-200 µm. Since 2005 a coating system consisting of epoxy sealer, epoxy barrier coat and polyurethane topcoat with minimum film thickness 185 µm has been specified on top of the TSZ.
