The inhibition effects of several inhibitors on rebar in acidified concrete pore solution

https://doi.org/10.1016/j.conbuildmat.2011.08.048Get rights and content

Abstract

The inhibitive effects of several inhibitors for carbon steel in acidified pore solution (pH 8.0) were studied. Sodium molybdate shows good effect to promote passivation at low concentrations (0.013 wt.%), but for sodium tungstate and sodium nitrite higher concentration (0.045 wt.%) is needed to obtain passivation. Sodium phytate only shows minor inhibitive effects at the concentration level of 0.045 wt.%. At higher concentration level (0.045%  wt.%  2%), the inhibitors sodium molybdate, sodium tungstate and sodium nitrite behave well in the acidified pore solution. For sodium phytate, to obtain obvious inhibition the inhibitor concentration must be above 0.5 wt.%. Molybdate and tungstate are present on the steel surface according to Raman spectrum analysis, which supports the adsorption mechanism of the inhibitors.

Highlights

► Inhibitive effects of Na2WO4 and Na2MoO4 for steel in pH 8 pore solution are studied. ► Na2MoO4 shows good inhibitive effect at low concentration (0.013%). ► Higher concentration (0.045%) is needed for Na2WO4 to inhibit corrosion. ► Sodium phytate also shows inhibitive effect when the concentration is above 0.5%. ► Raman spectrum analysis supports an adsorption mechanism for the two inhibitors.

Introduction

Reinforced concrete as one of the most important construction materials is widely used in various fields. However, many concrete structures exposed to aggressive environments show early deterioration. There are several factors affecting the durability of concretes, such as steel corrosion, freeze–thaw damage and physicochemical reactions with aggressive environments [1]. Steel corrosion is one of the main factors which cause the damage of the reinforced concrete structures [2]. The common protection measures against corrosion of the reinforcing steel include the use of corrosion resistant steel, cathodic protection, surface treatment of the reinforcing steels (e.g., epoxy coating) and the use of inhibitors in the concretes [3]. Inhibitors are chemical substances which can prevent or slow corrosion of the steel in concretes. They work by being mixed in the concretes or coated on the concrete surface [4]. The use of inhibitors is particularly attractive due to its simplicity and relatively low cost. The urban and industrial areas are characterized by the presence of SO2 and NO2 in the forms of atmospheric contaminants, which can react with water from the atmosphere and form sulfuric and nitric acids. Sulfuric acid attacks the calcium hydroxide and hydrated calcium silicates which constitute hydrated concrete, and the formation of the products with greater volumes can introduce stresses and produce cracks in concretes. The penetration of nitric acid into concrete causes the pH of the pore solution to decrease and results in hydrated calcium nitrate, which is highly soluble and leaches from the concrete. As the result the porous matrix would be susceptible to other types of attack. With the pH decrease of the pore solution the surface of the reinforcing steel depassivates and the corrosion begins [5], [6]. Statistical data have shown that more than 30% areas in China are affected by acidic atmosphere, so it is necessary to investigate the effects of inhibitors for reinforcing steel in concretes polluted by acid rain environments [7].

So far, numerous investigations have been carried out to determine the effects of inhibitors on the corrosion behavior of reinforcing steel in simulated concrete pore solutions, but there are only a few studies regarding the situations in acid environments. Tommaselli et al. [6] studied the inhibitive effects of sodium molybdate and sodium nitrite on carbon steel at low concentrations (less than 0.040 wt.%) in acid atmosphere and pointed out that a small amount of sodium molybdate shows high efficiency in preventing rebar corrosion. Sodium tungstate and sodium molybdate are two kinds of environmental friendly inorganic inhibitors [8], [9], [10], [11], [12], [13], which have been widely used for many applications, including engine coolants, paints and coatings, metalworking and hydraulic fluids, and cooling waters. However, these inhibitors are rarely used in acid polluted concrete environments. It is also reported that sodium phytate, an organic inhibitor, shows inhibitive effect for carbon steel in simulated pore solutions, and its strong chelate ability is available in a large pH range [14], [15], [16].

The purpose of this study was to evaluate the effects of sodium tungstate, sodium molybdate and sodium phytate as inhibitors at various concentrations in acidified saturated calcium hydroxide solution. Sodium nitrite and a commercial inhibitor named RSA were chosen as the reference inhibitors.

Section snippets

Materials and solutions

Carbon steel was used for the experiments. The composition of the steel is as follows (wt.%): C 0.37, S 0.053, P 0.026, Si 0.16, Mn 0.32, Cr 10.9, Fe bal. The steel rod with 10 mm diameter was cut into 10 mm length to make the steel electrodes for electrochemical tests. The electrical contact was achieved through a copper wire soldered to the end of the steel sample. The electrode surface was abraded with emery papers up to 800 grit, rinsed in deionized water and degreased with acetone, finally

The inhibitive effects of the inhibitors in the pore solution (pH 12.5)

Fig. 1 shows the potentiodynamic polarization curves for steel samples in pore solution (pH 12.5) containing different concentrations of sodium nitrite (0.013%, 0.027% and 0.040%). In the solution steel shows active–passive behavior. In the presence of sodium nitrite the corrosion potential shifts to more positive value with an extensive passivation region of −0.2 to 0.6 VSCE, and the anodic current density distinctly decreases. Therefore, in the low concentration range sodium nitrite can

Conclusions

  • (1)

    In ordinary pore solution (pH 12.5) the polarization curve of carbon steel shows an active–passive transition. Sodium molybdate, sodium tungstate and sodium nitrite at low concentrations (wt.%  0.045%) reveal effective inhibition effects by promoting passivation of the steel.

  • (2)

    In the acidified pore solution (pH 8.0), the steel shows active polarization behavior. Sodium molybdate still shows good effect to promote passivation at low concentrations (0.013 wt.%), but for sodium tungstate and sodium

Acknowledgment

The authors would like to thank The National Natural Science Foundation of China (Contract No. 50731004) for financial support to this work.

References (30)

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