A comparative study of corrosion resistance of different coatings for mortar–embedded steel plates

In view of corrosion problem encountered in mortar–embedded steel plates used in joints of precast concrete cylinder pipes, an experimental study was undertaken at the initiative of the manufacture of the pipes to evaluate the relative performance of three chosen types of commercial coatings namely, red oxide, zinc primer, and epoxy coatings for protection of mortar–embedded steel plates against chloride-induced corrosion. Three different types of experiments were planned for this comparative performance evaluation: natural corrosion, accelerated corrosion and chloride exposure test. Test specimens were prepared using cement–sand mortar with cement–sand ratio of 0.5 (ASTM type I cement) and water–cement ratio of 0.5, using an embedded steel plate at the mid-depth. For natural corrosion, specimens admixed with 12% NaCl solution were placed in humidity and temperature controlled chamber to promote corrosion in hot–humid environment. In accelerated corrosion test, the weight loss and corrosion current densities were measured. Chloride penetration and corrosion initiation time were measured in specimens subjected to ponding with 10% NaCl solution. The results of all three tests convincingly showed that epoxy-coating, out of the three tested, performed as the best corrosion protection coating for mortar–embedded steel plates against chloride-induced corrosion.