Corrosion Protection of Construction Steel

Introduction

The most readily available and economic material for construction is reinforced concrete. One of the major causes of loss of durability of concrete structures is due to corrosion of the embedded steel rebars. Corrosion is a chemical attack of aggressive species that deteriorates the mechanical strength of the construction steel and rebars. Hence, it is harmful and at times may lead to catastrophic failure also. Corrosion at the steel-concrete interface has been a field of extensive research since decades. This is one of the main reasons for limited life span of any concrete member and a consequent increase in maintenance cost of reinforced structures. It causes spalling, cracking of the concrete and thus reduces its strength. Early failure of reinforced and pre-stressed concrete structures leads to very high cost of maintenance, restoration and replacement worldwide (Bertolini, 2008).

The corrosion mechanism at the interface of steel and concrete is somewhat different from the normal corrosion of steel. There are many factors which are important for governing steel corrosion in concrete. The steel inside the concrete is normally corrosion resistant due to the formation of a passivation layer of oxide in the highly alkaline atmosphere i.e. often >13 (Sagüés et al., 2003). This happens mainly because of Ca(OH)₂ generated due to hydration of cement (Koleva et al., 2006). For steel to remain corrosion resistant this passive state needs to be maintained. Further, the concrete should be free from porosity which is mostly unavoidable since it is a matter of fact that no concrete is fully free of porosity (Shi & Ming, 2017). So, when any corrosive solution (Cl^-, CO₂^2-, SO₄^2-) penetrates the concrete and their concentration at the interface increases above a certain critical level, it starts reducing the alkalinity and then de-passivates the steel. In fact, the defects in concrete play a dual negative role i.e. loss of stability of passive films due to reduction of Ca(OH)₂ and accumulation of higher chloride species through the defects (Shi & Ming, 2017). Once the steel embedded in concrete de-passivates, it is corroded at a severe rate due to harsh atmosphere (Söylev & Richardson, 2008; Ožbolt et al., 2011). Once corrosion of the rebar occurs, cracking and damage of the concrete soon follows. Since the corrosion products of steel are quite voluminous, it results in swelling and generates stresses of sufficient magnitude to cause tensile failure of the concrete (Yeomans, 2004). Further expansion leads to delamination and spalling of pieces of concrete from the surface. The corrosion behavior of structural steel and rebars is also dependent on the external load as well as temperature (Deus et al., 2012; Shi et al., 2017). Therefore, corrosion of reinforcement steel necessities remedial action to increase the life of the structure or in certain cases complete replacement of the element. Globally, the cost associated with such replacements is expected to increase at an alarming rate (Yeomans, 2004).

Numerous researches have been done to reduce steel corrosion in concrete. Either the composition of concrete and surroundings is tailored or a suitable material is selected. The rebars may be also protected from pitting corrosion by application of coatings. The present chapter summarizes the corrosion phenomenon at the concrete-rebar interface and steps taken for prevention of corrosion mainly by suitable material selection or surface engineering. Future trends and scopes for improvement of corrosion resistance of reinforcement steel are also discussed. The present work would be also beneficial for adopting a suitable course of action for improving corrosion behavior of steel and leading to economic benefits in the long run.