Corrosion Matters – how it really works

Corrosion is something that occurs all around us, all the time and in all environments and if there is no plan in place, then corrosion can cost you a lot more than money. 

But what exactly is corrosion?  According to NASA, corrosion can be defined as the degradation of a material due to a reaction with its environment.  Degradation implies deterioration of physical properties of the material. This can be a weakening of the material due to a loss of cross-sectional area, it can be the shattering of a metal due to hydrogen embrittlement, or it can be the cracking of a polymer due to sunlight exposure.

What this tells us is that because corrosion happens, reinforcements are necessary to ensure that the integrity of the component is maintained.  But sometimes, even the reinforcement can become corroded. 

Let’s take the example of steel – steel reinforcement may become rusted because it was initially exposed to the atmosphere.  When freshly, mixed concrete is placed around this steel, the mixing water penetrates through the rust pores, where it gradually forms hydrated calcium ferrite. Moreover, this water reacts with steel and forms a thin layer of iron and calcium hydroxides.

All these products in the vicinity of steel raise the pH of concrete pore solution, up to about 13. It should be noted that when on contact with initial rust, cement hydration is disturbed and a transition zone is locally formed. Concrete is more homogenous when far from this zone.

The concrete mixing water makes it possible to form some products, which protect the steel by passivation. More precisely, under atmospherically induced rust, reinforcement is covered with a thin protective layer of white products, containing ferrite and calcium hydroxides.

Such protection vanishes when large cracks reach reinforcement or the porosity of the concrete is large enough to allow aggressive elements to reach the steel surface.

Corrosion with rusting of reinforcement in concrete comprises two stages. In the first stage (or step), the aggressive elements such as chloride or carbon dioxide, present in the surrounding medium penetrates the concrete- this is the initiation stage. The second stage is propagation, which starts, when these aggressive bodies are in rather high concentrations at the reinforcement level. This corresponds to rust growth, which can break concrete cover.

These stages are described as follows:

• A first stage involves the transfers of aggressive agents (mainly carbon dioxide and chloride) of water and of oxygen, inducing the corrosion initiation (de-passivation of reinforcement).
• A stage of corrosion growth, leading to concrete damage, to spalling, cracks, etc. This stage starts when the contents of aggressive agents are high enough to reinforcing steel.
• To describe steel corrosion in concrete, it is advisable to define, on one hand, the penetration of the aggressive agents through concrete and on the other hand, the conditions of depassivation of reinforcement, then the dissolution rate of metal and rust growth.

Metallurgy is the science and technology of metals. It involves analyzing the physical and mechanical properties of metals and examining how they are affected by composition, mechanical working and heat treatment.

CARIRI’s Metallurgy Unit offers a wide range of tests and services that are comparable with established laboratories anywhere else in the world.

These include:

• Failure Analysis of Mechanical Components
• Corrosion Assessment & Prevention of Coatings and Components
• Mechanical Testing of Material

CARIRI is also home to the Industrial Materials Unit – Civil which is dedicated to testing and consultancy services, primarily to the Building and Construction industry and together Metallurgy and Civil have been able to ensure the best quality solutions for their clients.  As part of its commitment to ensuring that it continues to educate the population on various issues and topics, CARIRI will be hosting a session entitled ‘Quality in Construction’ on Tuesday 12^th April, 2016, which will be held under the auspices of the Minister of Works and Transport, the Honorable Fitzgerald Hinds.