Corrosion Inhibition of Steel Rebar in Concrete


Prepared by Behzad Bavarian, PhD and Lisa Reiner

March 2000

Department of Civil and Manufacturing Engineering

College of Engineering and Computer Science

California State University, Northridge

Corrosion of steel rebar is the major cause of concrete deterioration in reinforced and prestressed concrete structures. This type of corrosion results when salts, water and air are able to penetrate through the pores of concrete to the surface of the steel. The key to inhibiting rebar corrosion is to restrict the permeability of the concrete. Migrating corrosion inhibitors (MCI) can reduce the corrosive ion mobility and neutralize these corrosive species.

The corrosion inhibition of several commercially available migrating corrosion inhibitors was investigated over a 400-day period. These investigations were made on steel rebar in concrete totally immersed in 3.5% NaCl in ambient temperatures using electrochemical monitoring techniques. Due to low conductivity of concrete, the corrosion behavior of steel rebar had to be monitored using AC electrochemical impedance spectroscopy (EIS). During this investigation, changes in the resistance polarization and the corrosion potential of the rebar were monitored to determine the degree of effectiveness for Cortec MCI-2022.

This final report contains the corrosion test results, specimen preparation, and the theory behind electrochemical impedance spectroscopy (EIS) for the 10 specimens that were investigated. The reinforced rebars (class 60 #4 steel wire) were cast in concrete at one and two inches from the surface and allowed to cure for 28 days. The samples were then submerged in 3.5% NaCl solution for the duration of the experiment. According to the ASTM (C876) standard, if the open circuit potential (corrosion potential) is between 0 and -200 mV, this indicates a 90% probability that no reinforcing steel has corroded. Corrosion potentials more negative than -350 mV are assumed to have a greater than 90% likelihood of corrosion. However, this standard alone is not sufficient to demonstrate the corrosion inhibition effectiveness of these MCI products in concrete and should be used in concert with the EIS results.

During this investigation, samples were tested once a week to measure the open circuit potential and resistance polarization (Rp). Corrosion potentials were recorded at the beginning of each AC impedance test. The Rp values obtained by curve fitting data from the Nyquist plots in the EG&G M398 corrosion software were used to estimate corrosion. The RP values are inversely proportional to the rate of corrosion. A high Rp value could be an indication of a low corrosion rate. The bode plots were also obtained from this software to verify the RP value for each sample.

The MCI-2022 products have successfully demonstrated corrosion inhibition of steel rebar in a 3.5% NaCl solution by maintaining a high resistance polarization (low corrosion rate) for the rebar in concrete. The observed corrosion rate was equivalent to the performance of 304 & 3 16 stainless steel rebar in concrete. The non-treated concrete samples showed a gradual decline in their resistance polarization, indicative of the increasing corrosion.

The MCI-2022 products have successfully inhibited rebar corrosion. The steel rebar corrosion potentials were maintained at approximately -120 mV, and rebar resistance polarization reached as high as 250,000 ohms. Both results indicate excellent corrosion resistance performance. The observed corrosion rate was equivalent to the performance of 304 & 316 stainless steel rebar in concrete. The reference samples (non-treated) have shown a decline in their resistance polarization and corrosion potentials, indicative of corrosion.

In summary, the experimental results demonstrate that the MCI-2022 products offer an excellent inhibiting system for protecting reinforced concrete in an aggressive 3.5% NaCl solution. The results are extremely promising for this product in its ability to protect steel rebar in concrete in aggressive environments. The construction industry should be duly informed on the advantages of using this product.