How can steel deteriorate

Once corrosion damage does strike, performing durable repairs on corrosion-damaged reinforced concrete can be challenging. When corrosion initiates on the steel, the area that is actually corroding is anodic, while the non-corroding areas surrounding the corroding site are cathodic. Essentially the corroding area is cathodically protecting the non-corroding areas around it, Broomfield explains. When the corroding area is repaired with a patch, typically the damaged concrete is removed and new, good-quality concrete is used to replace it.

This, in turn, converts the corroding anodic area into a non-corroding cathodic area. If chloride-contaminated concrete remains on the reinforcing steel adjacent to the patch repair, however, the steel in areas around the patch will become anodic and start to corrode.

Unless all chloride contamination around the patch has been removed which can be a difficult process, particularly in a marine environment even a high-quality patch repair can promote the formation of corroding anodes around it and spread the corrosion process rather than control it. Broomfield notes that several industry standards, such as NACE standard SP, 3 provide maintenance and rehabilitation considerations for concrete repairs. Models can be used for determining the durability and corrosion resistance of reinforced concrete structures, and industry standards and guidance documents can help ensure these structures achieve their desired service life.

Contact John P. Become a Member Sign In. Become a Member Sing In. White Papers. More Articles. No other contaminant is documented as extensively in the literature as a cause of corrosion of metals in concrete than chloride ions.

The mechanism by which chlorides promote corrosion is not entirely understood, but the most popular theory is that chloride ions penetrate the protective oxide film easier than do other ions, leaving the steel vulnerable to corrosion. The risk of corrosion increases as the chloride content of concrete increases.

When the chloride content at the surface of the steel exceeds a certain limit, called the threshold value, corrosion will occur if water and oxygen are also available. However, only water-soluble chlorides promote corrosion; some acid-soluble chlorides may be bound within aggregates and, therefore, unavailable to promote corrosion.

Work at the FHWA Clear found that the conversion factor from acid-soluble to water-soluble chlorides could range from 0. Arbitrarily, 0. Although chlorides are directly responsible for the initiation of corrosion, they appear to play only an indirect role in the rate of corrosion after initiation.

The primary rate-controlling factors are the availability of oxygen, the electrical resistivity and relative humidity of the concrete, and the pH and temperature. This reaction reduces the pH of the pore solution to as low as 8.

Carbonation is generally a slow process. In high-quality concrete, it has been estimated that carbonation will proceed at a rate up to 0. The amount of carbonation is significantly increased in concrete with a high water-to-cement ratio, low cement content, short curing period, low strength, and highly permeable or porous paste.

Carbonation is highly dependent on the relative humidity of the concrete. The highest rates of carbonation occur when the relative humidity is maintained between 50 and 75 percent. Below 25 percent relative humidity, the degree of carbonation that takes place is considered insignificant.

Above 75 percent relative humidity, moisture in the pores restricts CO2 penetration. Carbonation-induced corrosion often occurs on areas of building facades that are exposed to rainfall, shaded from sunlight, and have low concrete cover over the reinforcing steel. Carbonation of concrete also lowers the amount of chloride ions needed to promote corrosion.

In new concrete with a pH of 12 to 13, about 7, to 8, ppm of chlorides are required to start corrosion of embedded steel. If, however, the pH is lowered to a range of 10 to 11, the chloride threshold for corrosion is significantly lower—at or below parts per million. Like chloride ions, however, carbonation destroys the passive film of the reinforcement, but does not influence the rate of corrosion.

Zinc 5. Nickel 9. Aluminum 6. Tin Steel 7. Lead Occasionally, the lack of oxygen surrounding the steel bar will cause the metal to dissolve, leading to a low pH liquid property. Civil engineers around the world have created strict quality assurance standards to make sure the materials being used are of good quality. The mixture of materials is another way to keep them strong and retain their strength for a maximum period of time. It is necessary to protect the concrete from damage by freezing and thawing cycles.

Air also reduces exudation and is increased permeability due to exudation water channels. Peeling the concrete surface can accelerate the corrosion damage of embedded reinforcement bars. A great solution to completely remove the chance of corrosion in building structures is the use of Glass Fiber Reinforced Polymer GFRP , otherwise known as fiberglass rebar.

This is an alternate material used for steel in the construction industry. The qualities of GFRP range from being corrosion free, being lightweight, having a long product life and being stronger than steel itself. Toggle navigation. What is steel corrosion? Why is the corrosion of steel a concern?