Ishita Manjrekar, and Sourabh Manjrekar, Sunanda Speciality Coatings Pvt. Ltd. Mumbai

Introduction: Most of today's concrete construction relies on the composite interaction of concrete and steel, which is aided by the near equivalence of their thermal expansion characteristics. The alkaline environment within good quality concrete offers a high degree of protection to the embedded reinforcement against aggressive agents that promote the corrosion of steel. The most significant causes that lead to corrosion of reinforcement steel and corrosion induced damage such as cracking and spalling and consequent reduction in structural capacity are poor quality of concrete, inadequate cover to reinforcement, chlorides in the concrete or a combination of all these.

Reinforcement Corrosion
Figure 1: A ceiling showing delamination and spalling associated with reinforcement corrosion

Corrosion of steel in concrete has two effects1
  • Reduction of the cross sectional area of steel
  • Creation of local discontinuities in the steel surface.
These effects can reduce the tensile capacity of the steel in proportion to the loss of its cross sectional area and may reduce the steel's resistance to fatigue damage. When the steel or the concrete is degraded by corrosion or cracking to a point where the material can no longer support the stress imposed on them the structure fails. The risk of spalling of the concrete due to reinforcement corrosion should be carefully assessed since the falling lumps of concrete will have serious consequences.

Cement is highly alkaline with pH above 11-12. In this environment, steel is passive and a thin but dense protective oxide - gamma iron oxide film is formed on its exposed surface. Moreover the surrounding concrete restricts the ingress of carbon dioxide and chlorides which promote corrosion. The duration of this protection depends on a number of factors including high pH to maintain protective oxide film, the thickness and physical integrity of the cover concrete, and how well the concrete acts as a barrier to the ingress of aggressive species.

The influence of concrete pH, chlorides and the availability of oxygen on the corrosion reaction can be briefly explained as follows:
  • If the pH of the concrete adjacent to the reinforcement is above 10, a protective surface oxide layer forms on the metal surface. The rate of corrosion under these circumstances is insignificant. In Portland cement concrete the pH is maintained at levels of above at least 12.6 due to the presence of significant amounts of calcium hydroxide which is a product of hydration of the cement.
  • The presence of sodium and / or potassium salts can increase the pH further, for example: K2SO4 + Ca(OH)2 → CaSO4 + 2K+ + 2OH-
  • However, the protective surface layer can be broken down by Cl- ions, even at a high pH.
  • Removing hydroxide by the ingress of carbon dioxide can also depassivate the reinforcement:

    Ca(OH)2 → Ca2+ + 2OH-
    CO2 + 2OH- → CO32- + H2O
    Ca2+ + CO32- → CaCO3
  • The pH consequently falls to a level lower than 9 - 10 needed to maintain the protective surface oxide layer on the reinforcement.
  • In concrete where the supply of oxygen is restricted (eg. where the concrete is submerged or buried underground), the passive film may not be maintained. Corrosion can then theoretically occur through the reduction of water to hydrogen. Thankfully the kinetics of this process is extremely slow.

NBMCW February 2016