The Science Behind Concrete Corrosion

This post is based on an article from the September/October 2011 issue of Inspectioneering Journal by Pieter VanderWerf at Building Works. You can find the original article here.

Deterioration of concrete structures is a constant plague on petrochemical production facilities. Concrete corrosion occurs when concrete is exposed to a sulfur compound, causing it to weaken continuously over time and lose functionality. Eventually the strcuture will require either a repair or replacement of the affected sections. Concrete corrosion can cause all sorts of problems, productivity can be lost and product quality can suffer. Likewise, repairing damaged concrete can be an expensive process and can lead to downtime.

Thankfully, a greater understanding of the chemistry of concrete and how it corrodes has begun to alleviate this problem. Alternative materials that possess greater resistance to severely corrosive environments are being produced. Likewise improved inspection practices mean that inspectors are able to spot problems in their early stages and prevent minor problems from getting out of hand.

Problem

Concrete is a commonly used material in the process industries due to its durability and longevity. Unfortunately though the conditions present in most petrochemical facilities can speed its decay significantly.

This is not helped by the fact that concrete is a porous material. This means that it has many tiny capillaries (long, thin, interconnected tube-shaped channels) running through it. These capillaries provide pathways for corrosive materials to penetrate below the surface and into the body of the concrete, enabling corrosion from within.

What makes this condition particularly insidious is that the corrosion won't always stop with the concrete. Most concrete structures are reinforced by an embedded grid of steel bars which provide a great deal of the structure's strength. Steel is highly vulnerable to corrosion and can deteriorate and weaken rapidly should it come in contact with a corrosive substance. If the corrosion of steel and concrete continue for too long, the structure may lose the ability to support its own weight, causing it to suffer structural cracks and shift under the pressure of its contents or the ground.

New Materials

The main reason concrete is particularly susceptable to corrosion is because it contains calcium. Thus one method of making concrete more resistant to acid corrosion is to use a special portland cement with reduced calcium. With less material for acid to attack, the concrete corrodes less quickly.

Another approach is to mix silica fume or fly ash into the concrete. These are both waste products from other processes made up of ultra-fine mineral particles. The tiny particles are able to fill up the capillaries in the concrete so that corrosive materials cannot penetrate as rapidly. Silica fume and fly ash also have virtually no calcium so they themselves are quite durable. 

A more recent development is a different form of cement with no significant calcium at all. These new chemical-resistant cements are made up almost entirely of recycled fly ash, with a small amount of special additives. Without calcium the concrete is inherently resistant to corrosion throughout.

Inspection

Depending on the structure involved and the potential for deterioration, petrochemical facilities tend to schedule regular inspections of their concrete structures. It is difficult to see the extent of damage when the structures contain product, so proper inspection requires that operations be temporarily shut down and the structures emptied in these cases.

One early indicator of acid attack is discoloration of the concrete. If there is no further damage, no repairs are likely necessary. However, the color change may alert one to areas to examine carefully at the next inspection.

When deterioration becomes more advanced, the concrete may actually begin to fall apart, dropping stones and sand. If untreated this can even progress to the point where the steel reinforcing bars become exposed. Once it gets to that point continuing to use of the structure is very risky, and in fact it probably was so for some time already.

Conclusion

Careful inspection of concrete structures in a petrochemical facility can spot problems in the making. Understanding how and why concrete corrodes helps the inspector know what to look for and how to interpret the findings to assess the nature and extent of the damage. With the damage assessed a proper repair can be planned.