Chloride-induced corrosion on reinforcing steel: from the fundamentals to the monitoring techniques

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Abstract

One of the most important causes for reinforcing steel corrosion is the presence of chloride ions. They cause localised breakdown of the passive film that initially forms on steel as a result of the alkaline nature of the pore solution in concrete. The harmful chloride ions can be originated from the use of contaminated mix constituents or from the surrounding environment. The determination of a critical level, above which serious problems can occur, has been one of the main goals of investigation. Unfortunately, it is difficult to establish such a value since the chloride level is influenced by several factors. Thus, after concrete contamination, it is of fundamental importance to follow the activity of chlorides and the state of the reinforcing rebars. In this respect, the use of electrochemical techniques such as polarisation resistance, electrochemical impedance, galvanostatic pulse and potential measurements have shown to be powerful tools. Nevertheless, the interpretation of the results becomes sometimes a difficult task. A large number of authors have dedicated several studies to the interpretation of such measurements and a highly dispersed number of interpretations can be found in literature. The aim of this paper is to present an overview on the state-of-the-art of the most important aspects of the corrosion process initiated by chlorides, its development and monitoring techniques.

Introduction

The use of cement, the most important ingredient of concrete, is known since the construction of pyramids in old Egypt, where it was used as a binding agent. Nowadays concrete is one of the most widely produced materials on the earth, with consumption above dozens of billions of tons. The concrete industry involves millions of euros being the basis of the modern society development. Contrary, to the common belief, concrete is not free of severe degradation problems. Apart from structural design failures, the most important cause of concrete degradation is the corrosion of the reinforcing steel. This problem has reached alarming proportions in the past three decades, leading to very high repair costs, sometimes above the initial construction cost, or in extreme situations, to the final collapse of the structure.

The most important causes of reinforcement corrosion are (i) localised depassivation of the reinforcing steel due to the ingress of chloride ions and (ii) complete depassivation of the reinforcement due to acidification of the interstitial solution in consequence of reactions of the cement matrix with carbon dioxide present in the atmosphere.

The harmful chloride ions may be present in concrete, as result of the use of contaminated ingredients on the manufacture of the mix, or as result of an external contamination prior to construction. This situation arises from exposure of the structures to water and marine atmospheres or to the use of de-icing salts (NaCl, CaCl2 and MgCl2) a necessary practice in cold climates.

After initiation of the corrosion process, the accumulation of corrosion products (iron oxides and hydroxides), occupying a volume several times larger than that of the original iron [1] leads to internal stresses that result in cracking and spalling of the concrete cover. At this stage the intrusion of aggressive agents, oxygen and humidity is facilitated and the next step can be the total loss of the structural integrity.

Section snippets

The corrosion process

Corrosion is an electrochemical process with cathodic and anodic half-cell reactions. In the absence of chlorides and in good quality concrete, in which the pH is usually in the range 12.5–13.5, the anodic reaction (1) leads to the formation of iron cations, according to:FeFe2++2e

This reaction is balanced by the cathodic reduction of oxygen, which produces hydroxyl anions according to reaction (2).1/2O2+H2O+2e→2OH

The products of both reactions combine together and in a last stage they

Potential measurements

The detection of corrosion by using potential measurements is one of the most typical procedures for the routine inspection of reinforced concrete structures. The technique is very well known, being described in the American National Standards ANSI/ASTM C876. An oversimplified interpretation of the potential readings is described by:

Ecorr (vs. Cu/CuSO4)Probability of corrosion
<−0.35 V>95%
>−0.20 V<5%
−0.20 to −0.35 V≈50%

Potential readings, however, can be more negative than −350 mV (SCE), without

Conclusions

The composition of the passive film formed on reinforcing steel and the mechanism of its breakdown by chlorides can be explained by more than one model. However it can be assumed that chloride ion form soluble complexes with iron leading to localised acidification and consequent pit growth.

Chlorides can bind with concrete, being partially immobilised in the matrix. In spite of this, bound chloride may participate in the corrosion process if the pH falls, leading to dissolution of the chloride

Acknowledgements

The authors acknowledge the support from POCTI.

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