Reinforcement Corrosion to Aggressive Environment

Abstract

Corrosion of reinforcing steel in concrete leads to the premature failure of many structures exposed to harsh environments. Rust products form on the bar, expanding its volume and creating stress in the surrounding concrete. In this study we will present how metal acts into an aggressive environment and how we can adopt the best solutions to reduce the attack of corrosion. First we should understand how corrosion occurs. Corrosion occurs when two different metals, or metals in different environments, are electrically connected in a moist or damp concrete. This will occur when: steel reinforcement is in contact with an aluminium conduit; concrete pore water composition varies between adjacent or along reinforcing bars; where there is a variation in alloy composition between or along reinforcing bars; where there is a variation in residual/applied stress along or between reinforcing bars. Loss of alkalinity due to carbonation or chlorides, crack due to mechanical loading, stray currents, agents from atmospheric pollution, moisture pathways, low concrete tensile strength, electrical contact with dissimilar metals are some of the most important reasons of corrosion. Electro-chemical corrosion, which plays a subordinate role in air, is of greater significance in liquids. The extent of electro-chemical corrosion depends on the electrical conductivity of the liquid, which affects the protective influence of the zinc layer over greater or smaller areas. The pH value of the liquid is of most significance. The corrosion rate of zinc is normally low and stable in the pH range of 5,5—12,5, at temperatures between 0 and 20 °C. Corrosion outside this range is usually more rapid. Hard water, which contains lime and magnesium, is less aggressive than soft water. Together with carbon dioxide these substances form sparingly soluble carbonates on the zinc surface, protecting the zinc against further corrosion. Soft water often attacks zinc, since the absence of salts means that the protective layer cannot be formed. In some waters, polarity reversal can occur at about 70 °C so that the zinc coating becomes more electro-positive than the steel and pitting occurs. Oxygen, sulphates and chlorides counteract polarity reversal, which means that the problem may exist only in very clean water. Water temperature is of great significance to the rate of corrosion. Above approximately 55 °C, the layer-forming corrosion products acquire a coarse-grained structure and lose adhesion to the zinc surface. They are easily dislodged and expose new, fresh zinc for continued and rapid corrosion attack. The rate of corrosion reaches a maximum at about 70 °C, after which it declines so that at 100 °C it is about the same as at 50 °C. Keywords : composite materials, corrosion, reinforcement, water corrosion, reinforcement bars.