Adhesion Tests and Failure Modes Study on Structural Steel Coatings

Abstract Pull-off adhesion behaviors of 15 coating systems in three groups were studied utilizing test method ASTM D4541. Three groups of coating systems included coating systems with organic or inorganic zinc-rich primers, polymeric polyurea coatings, and overcoating systems applied on an existing coating system. The adhesion strength and failure mode of each coating system under different conditions were investigated and compared. The advantages and drawbacks for each coating group regarding adhesional performance were summarized in this paper. For most coating systems in this study, the impact on adhesion loss due to 3000-hour salt-fog exposure was more severe than that due to cyclic freeze-thaw testing. For the group having zinc-rich primer, adhesion strengths after freeze-thaw or salt fog exposure remained at the same level compared to adhesion strengths before the exposure. However, they had lower strength values to begin with. Most failure modes occurred due to low cohesive or adhesive strength within the zinc-rich primers. Coating systems having polyurea had adhesion strengths up to 2800 psi at the beginning, but were prone to lose adhesion to steel substrate easily during 3000-hour salt-fog testing. When steel substrate underneath the epoxy or urethane was under chloride attack, the bonding strength dropped dramatically and bond failure happened at the steel substrate. The corrosion was occasionally observed underneath the primer. In the third group of overcoating systems, debonding of the overcoat from existing coating system was an issue, especially in the case of calcium sulfonate alkyd. The cohesion strength of the calcium sulfonate alkyd was also problematic. Polyaspartic polyurea showed some advantages on overcoat in terms of the adhesion strength initially. However, blistering showed at the scribe after 1500-hour salt-fog exposure. Waterborne acrylic had an adhesion problem with its own designated primer. Its topcoat was too permeable to protect the existing coating system during the freeze thaw or salt-fog testing. In this group, the condition of the existing coating system was also an important factor to achieve desirable adhesion performance of the overcoat. Two different types of existing coating systems showed different results on adhesion, especially after the salt-fog test.