Inhibition Effect of Natural Pozzolan and Zinc Phosphate Baths on Reinforcing Steel Corrosion

Zinc phosphate (ZnP) baths are widely used for increasing corrosion resistance and surface preparation for painting. Studies on exploiting these baths in the reinforced concrete (RC) are still in the early stages. This is probably due to the shortcomings, such as the alkaline instability and high porosity of the obtained coatings. Use of natural pozzolan (NP) as cement replacement is growing rapidly due to its economic, ecological, and technical benefits. The combined effect of using ZnP baths and NP-based cement on the resistance of concrete against damage caused by corrosion has been investigated. Four phosphating baths were prepared: ZnP, ZnP-Ni, ZnP-Cu, and ZnP-Mn. Steel specimens were phosphated at 55-60°C for 15 min. Concrete specimens were produced with four different levels of NP: 0% (control), 10 %, 20%, and 30%. The investigation was carried out using RC specimens where a constant anodic potential was impressed after 28 and 90 days of concrete curing. The electrochemical behavior of the coated steel has further been evaluated in chloride contaminated Ca(OH)2 saturated solution (CH-Cl) using the open circuit potential (OCP), the potentiodynamic polarization, and the polarization resistance with time. The bond strength between the coated steel and concrete has been evaluated by the pull-out test. Test results showed that concrete containing NP at higher replacement levels and steel specimens treated in bication baths exhibited corrosion initiation times several times longer than the control concrete with uncoated steel. In addition, the best corrosion performance was noted in the steel specimen treated in the ZnP-Cu bath. Its corrosion density was about twentyfold lower with respect to the bare steel, and its inhibition efficiency exceeded 95% in (CH-Cl) solution. In addition, its polarization resistance was about fifteenfold lower with respect to the bare steel. SEM, EDX, and XRD techniques have been employed, as well.