Basics of Steel Corrosion in Concrete

In my professional engineering practice in Fix Concrete Technologies, I’ve seen massive number of steel corrosion in the concrete. I’ve also discussed with a number of very good engineers, who know only a little about such rusting process. This inspired me to start writing this blog and share my experience. Fingers crossed, so let’s start…

What is steel corrosion?

In very general words, corrosion of steel is a process that involves progressive removal of iron atoms from the steel. By electrochemical reaction the iron is dissolved in the surrounding water solution. As a result of this dissolution process, steel cross-section becomes smaller and rust deposits around the steel.

There are risks associated with steel corrosion in concrete:

1. Due to smaller cross-section of reinforcement bars, stresses carried by the remaining area will increase under load. In extreme cases it can lead to failure of the structure. Practically it starts with serious safety risk prior to failure.

2. The rust, which is a solid product of the corrosion reaction, is formed in very restricted space around the steel. Being confined by the concrete, it sets up expensive stresses which may crack the concrete cover. This may result in progressive deterioration of the concrete.

Corrosion process

Before discussing beauty of safety hazards, let’s come back to the basis of electrochemical reactions and let’s try to answer the question how steel corrodes.

As I mentioned before the corrosion of steel in concrete is a process. An electrochemical process in which chemical reactions and flow of electrical current are involved. Both chemical and electrical processes are coupled. I will try to describe both processes in simple terms, to avoid making it too complex on the beginning.

The corrosion process involves two separate chemical simultaneous reactions on steel surface. Those two electrochemical reactions are known as anodic and cathodic reactions, which occur respectively on anodes and cathodes. Anodic reaction is related to formation of ferrous ions (Fe2+). Cathodic reaction is related to formation of hydroxide ions (OH-). As indicated on the first drawing, an electric current must flow in a closed loop between the two sites for the reactions to proceed.

The flow of electrons from the anodic areas (where they are produced by anodic reaction) to the cathodic areas (where they are used in the cathodic reaction) and its counter-current ionic flow in the external concrete pore solution constitute the corrosion current. Any interruption in corrosion current flow will cause the corrosion process to slow down or stop.

On the second drawing electrochemical character of corrosion process is introduced by the flow of current in the closed loop as electron flow from the anode to the cathode in the steel bar. External current flows through the solution through the pores of the concrete. This external current consists of hydroxide ions moving from the cathode to the anode and ferrous ions moving from the anode to the cathode. Water in concrete pores serves as a vehicle for ionic flow.

Important note

It is very important to realize, if pores are dried out or the concrete is very dense (lack of pores), the flow of ions becomes difficult. To initiate corrosion reaction, conditions that give rise to the formation of anodic and cathodic sites must occur. If these conditions will never be present, corrosion will be no practical concern.

I hope I explained basis of steel corrosion mechanism in not too geeky way. If my summary of corrosion initiation of reinforcement gave you any value, I definitely invite you to the other interesting posts on our blog, which are coming soon. Do not miss it!