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ANALYSIS
Corrosion in Concrete Constructions Luiz Paulo Gomes IEC-Instalações e Engenharia de Corrosão Ltda. – Rio de Janeiro, Brazil LPgomes@iecengenharia.com.br
C
oncrete, while new and in good conditions, has high pH and high electrical resistivity. These features provide excellent corrosion
protection for the reinforcing steel rebars. Over time, concrete can absorb water, CO2, chlorides and other
pollutants. This contamination reduces the pH and electrical resistivity of the concrete. When this happens the concrete turns into an excellent electrolyte, allowing the operation of the corrosion cells. The absorption of water, chlorides, CO2 and other pollutants is very
common to happen, being the main cause of deterioration of concrete works (ref. Opening photo).
Water, chlorides, and pollutants generally reduce the electrical resistivity of the concrete, allowing corrosion cells to run easily. The corrosion equations in these cases are well known:
Fe Fe++ + 2e½ O2 + H2O + 2e- 2OHFe++ + 2Cl- FeCl2 FeCl2 + 2OH- Fe(OH)2 + 2Cl2Fe(OH)2 + ½ O2 Fe2O3 + 2H2O
The resulting corrosion product, the rust, occupies a much larger volume than the mass of corroded steel and the concrete bursts, allowing more water, chlorides, CO2 and pollutants to enter, increasing the corrosion process.
On the other hand, the absorption of CO2 causes the carbonation of the
concrete, decreasing the pH and depolarizing the steel, according to the equation below, also well known: Opening photo: The absorption of water, chlorides, CO2 and other pollutants is very common, being the main cause of deterioration of concrete works.
Ca(OH)2 + CO2 (pH>12) CaCO3 + H2O (pH<9)
We are all accustomed to seeing this situation in bridges, viaducts, concrete roads, concrete piles and buildings in general (Figs. 1 and 2). To protect the steel rebars against corrosion the best solution is the use of a cathodic protection system, with galvanic anodes or impressed
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N.30 - 2019 JUNE - ipcm® Protective Coatings