Corrosion inhibitors for reinforced concrete: 15 years of experiments @PoliLaPP

Corrosion of carbon steel reinforcement is the most important cause of premature failure on reinforced concrete structures. Prevention of corrosion is primarily achieved in the design phase by using high quality concrete and adequate cover. Additional prevention methods are adopted when severe environmental conditions occur or on structures requiring very long service life. Among these methods, corrosion inhibitors seem to offer a simple and cost effective prevention technique. They may be used both as a preventative techniques, if added to fresh concrete, and as a repair system, if applied on hardened concrete. The performance of corrosion inhibitors for reinforced concrete structures has been studied at PoliLaPP, Laboratory of Corrosion of materials "P. Pedeferri" of the Department of Chemistry, Materials and Chemical Engineering "G. Natta", Politecnico di Milano, in the last 15 years. Organic commercial admixed corrosion inhibitors delayed the occurrence of chloride induced corrosion. This result is related to two effects: reduction of the rate of chloride transport into concrete (most relevant in our experiments) and increase of the critical chloride threshold. Commercial migrating corrosion inhibitors (MCI) are able to delay time-to-corrosion of passive rebars in concrete subjected to chlorides ponding; this effect is mainly related to the reduction of chlorides diffusion coefficient. MCI can reduce the corrosion rate after corrosion initiation only in carbonated concrete, nevertheless efficiency is low and residual corrosion rate is not negligible. The results obtained with nitrite-based inhibitor confirm literature data. Among the tested organic substances, compounds containing carboxylic group showed the best results solution tests: pitting potential, time-to-corrosion and critical chloride content are similar to those obtained with nitrite. In concrete tests, only one amine and one amino acid showed good performance increasing the critical chlorides threshold with respect to the reference condition. The paper is dedicated to the memory of prof. Giordano Trabanelli, with which we cooperate at the beginning of this research, and prof. Pietro Pedeferri, our "maestro" and friend.