CORROSION OF CONSTRUCTION STEEL

MARCELA VAZQUEZ; ANGEL VICO; WALTER MORRIS

New Topics in Electrochemistry Research

Nova Science Publishers, Inc.

Año: 2005; p. 1 - 27

The article focuses on the corrosion of reinforcing steel bars in concrete and mortars exposed to marine environments. Four different concrete mix designs characteristic of those typically used at coastal cities in Argentina are studied. Two exposure conditions were investigated: immersed and marine environment. Two water/cement ratios and various chloride contents were investigated. Several electrochemical parameters characteristic of the corrosion process were evaluated, such as corrosion potential, corrosion current, polarization resistance, among others. Also, mechanical, chemical and physical properties of the concrete mixes were evaluated. From the results, it can be concluded that steel bars in contact with a good quality concrete exposed for 1000 days to the seashore environment remained in the passive state, even when its surface chloride concentration reached 0.75 % relative to the cement content. The corrosion behaviour in this condition was even better than the one observed on a standard-quality uncontaminated concrete. In contrast, all the reinforcements immersed in saline solution present an active behaviour, although the corrosion rate depended primarily on the concrete quality and the initial chloride concentration. When analysing the results, it was found that the electrical resistivity of concrete can be used as an effective parameter to evaluate the risk of reinforcing steel corrosion independently of mix composition and exposure conditions. Rebars are likely to achieve an active state of corrosion when resistivity is lower than 10 kohm, and prone to present a passive behaviour when concrete resistivity is higher than 30 kohm. The electrical resistivity can also be used as an indirect indicator of the chloride threshold value for rebar corrosion initiation. According the proposed correlation, the value of Cl- may vary from 0.44 to 2.32 % relative to the weight of cement when the electrical resistivity of concrete increases from 2 to 100 kohm. As regards protective methods, the performance of three coatings suitable for protecting reinforcing steel bars is analyzed by means of electrochemical methods. The coatings are representative of those commonly used to repair concrete structures affected by corrosion in the coastal regions of Argentina: an epoxy rust conversion coating, a zinc-rich epoxy, and a sprayed zinc coating. Two exposure conditions were investigated: immersed in aerated saline solution and exposed to an indoors atmosphere. The performance of the coatings was compared to bare steel by following the rebar corrosion potential, the corrosion rate and the electrical resistance between bars during approximately three years. In the dry condition the three coatings presented a satisfactory performance characterized by passive Ecorr values and CR values lower than 1 mm/year. On the other hand, the performance of the coatings in the wet condition depended on their formulations. The rust conversion coating showed active Ecorr values and CR values higher than those measured on the uncoated bars. The zinc-rich epoxy and the sprayed zinc coatings presented Ecorr values typical of active zinc. This indicates a certain degree of cathodic protection provided to the reinforcing steel. Besides, i values showed that in this condition, coatings do not provide a barrier type of protection. Finally, the performance of a surface-applied migrating corrosion inhibitor (MCI) based on an alkylaminoalcohol was evaluated on concrete specimens containing reinforcing steel bar (rebar) segments. Two water/cement ratios, various chloride and inhibitor contents and two exposure conditions were investigated. The inhibiting efficiency was followed over a period of 1000 days. When concrete is exposed to the marine environment the inhibitor is able to reduce the corrosion rate only if the initial chloride content is below 0.16% in weight relative to the cement content. Higher levels of protection are reached as the water/cement ratio (w/c) increases. There is no beneficial effect when the initial chloride content is greater than 0.43%. When concrete is immersed in a saline solution, no beneficial effect associated to the use of the inhibitor could be appreciated, regardless of w/c or initial chloride content in concrete. The efficiency of the product increased when the amount of inhibitor being applied increased by a factor of two.