Corrosion Definition and Control

Corrosion is the breaking down of essential substances of a solid body due to chemical reactions with its surrounding. It can also be defined as loss of electrons of metal due to reaction with water and oxygen particles which also commonly known as rusting. Corrosion can be concentrated locally forming a pit or crack on concrete structure or it may extend across a wide area resulting in deterioration of the structure. Generally, corrosion in concrete structure can be divided into two stages.

The first stage also known as penetration concrete cover. In the present of aggressive elements such as chloride and carbon dioxide in the surrounding, concrete cover is penetrated. Beginning at the surface of concrete and moving gradually towards the inner zone. When these aggressive agents reach the steel bar, the protective layer may be locally destroyed if the concentration is at the surface of steel bar reaches critical level. Penetration of carbon dioxide in concrete is also known as diffusion process. The reaction rate is higher when the concrete is in dry condition, however the carbonation reaction will not occur without the present of pore solution in concrete.

The duration of this stage is affected by the cover depth and corrosion rate of the aggressive agent as well as the concentration required to depassivate the steel bar. Therefore, the quality of concrete cover in terms of porosity, permeability and microclimatic conditions at the concrete surface is important factors to be considered in controlling the rate of ingress of the aggressive agents.

Corrosion begins when the protective layer of the reinforcement is broken down. Once this layer is destroyed, corrosion will occur with the presence of oxygen and water on the surface of the reinforcement whereas corrosion rate determines the time taken to reach the minimal acceptable state of structure. The corrosion rate may vary depending on the temperature and humidity of surrounding.

During the second stage of corrosion, propagation process begins when these aggressive elements are in rather high concentrations at the reinforcement level. Then the steel bar may undergo depassivation process where the metallic surface is reduced by aggressive agents and initiates rust growth. When reinforcement corrodes, it undergoes a more or less localized dissolution, and it is covered with unstable corrosion products which ultimately lead to breaking of concrete cover. The reduction of steel cross sectional area and simultaneous rust swelling induce significant decrease of the bonding between steel and concrete.
Protective Measures Applied to control corrosion
1. Construction Practices
Steel frames or reinforcement are usually protected from premature corrosion by painting. Therefore good geometric design and good practice is the important factor in successfully protecting the concrete structures from corrosion. Environment at building site are not conducive to refinements such as chemical, precleaning, conversion coatings, and automatic paint application. A rugged approach is inevitable and the quality of the results depends on the skill, conscientiousness, and supervision of those who carry out the work.
2. Design
Good design of concrete structures may protect the structure from corrosion. The lesser time water spends in contact with structure, the less is the chance of corrosion regardless of what type of painting used. Sources of water in this context are usually from rain, snow and internal sources by condensation. Therefore, to avoid trapping of water; angled sections must be oriented to drain freely. For example, box sections are end-capped or fitted with drainage hole. Additionally, crevices must also be eliminated to avoid oxygen-depleted water traps so that crevice corrosion will not occur. Traps in which dust and debris can accumulate and absorb condensate must also be eliminated.
3. Protection
In practice, bare rolled steel sections may carry patches as strongly adherent mill scale from hot rolling as purchased. These patches of mill scale can absorb water through the paint, forming an electrolyte that stimulates corrosion of the steal underneath the paint. Therefore, these patches should be removed by leaving the steel in a stockyard open to weather before assembly.
Painting or coating is expensive because it is labor intensive. Thus, in practice no more is applied than is needed. The treatment varies with the position within the structure. As an illustration, for building in contact with external side where conditions are most aggressive; it is necessary to use galvanized steel sections overlaid with a thick paint coating. However, concealed steel sections in the interior of a warmed air-conditioned building can sometimes be left uncoated. In short, thickness of paint should be adjusted to suit intermediate situations.