Effect of potassium-chromate and sodium-nitrite on concrete steel-rebar degradation in sulphate and saline media
Introduction
Corrosion induced degradation of steel reinforcement (steel-rebar) in concrete is the major cause of deterioration of concrete structures including building facilities, bridges, parking garages and other civil infrastructures [1], [2], [3]. Industrial, coastal and marine environments of the world are well dispersed with rust stained, cracked and spalled concrete structures caused by this form of degradation of the reinforcing steel and the composite constituents of the concrete structures [2], [4], [5]. The environments in which these structures exist provide more than adequate agents necessary for initiating and propagating the corrosion failure process. These corrosion inducing agents include oxygen, carbon dioxide and moisture from the atmosphere [5], sulphate ions from microbial or sewage environment [2], [6] and chloride ions from saline or marine environment [1], [4], [7], [8]. This is such that the corrosion destruction of concrete steel-rebar has contributed more than 80% of all the damages of reinforced concrete structures globally [9]. Consequent repair, maintenance and rehabilitation, to ensure in-service continuance and durability, of concrete infrastructures gulp substantial resources of intensive labour and escalating costs annually [3], [5], [10]. This, coupled with the inevitability of steel-reinforced concrete as the material of choice for various construction works in modern society [10], [11], [12], necessitates needs of research activities aimed towards corrosion prevention for the durability of concrete facilities [13].
Several methods could be used for the protection of steel-reinforced concrete structures from corrosion degradation. Some of the methods include: paintings, coatings or waterproofing membranes of the outer surface of the concrete structure [9], [14], [15], [16]; coating of the steel rebar [17]; cathodic protection of steel rebar [18]; and corrosion inhibitors admixed in concrete [19], [20]. While each of these methods had been employed in studies with varying degrees of success, the use of corrosion inhibitor admixtures had been described to be an effective method, which combines the advantages of lower cost with easy application compared to other methods, for controlling concrete steel-rebar corrosion [19], [21].
Corrosion inhibitor is a chemical substance which, in the presence of corrosive agent, decreases the corrosion rate in a corroding system when used at suitable concentration [21], [22], [23]. Several studies have investigated the inhibiting effectiveness and the compressive strength effect of different corrosion inhibitor admixtures in reinforced concrete [3], [9], [22], [24], [25], [26], [27]. Many of these studies linked the repressive capabilities to corrosion degradation and the strength properties of the reinforced concrete due to the applied inhibitors studied to the inhibitor types and their admixed concentration. Not many studies, however, had been carried out on potassium-chromate and sodium-nitrite and their synergies as well as the effect of their admixture in concrete on the compressive strength of the concrete structures. Also, there is dearth of work using the statistical tools, either of the normal probability density function (PDF) or the Weibull PDF to analyse electrochemical potential readings, which could be attended with fluctuations that could make interpretation from such readings difficult.
The focus of this study is to comparatively investigate the effect of potassium-chromate and sodium-nitrite as inhibitors on the corrosion degradation of steel-rebar in concrete. The corrosion monitoring technique of the half-cell (or open circuit) potential [22], [24], [25], [26] was employed in the work, in accordance with ASTM C876-91 R99 [28], to study the performance of the different concentrations of the two inhibitors and their synergies in sulphuric-acid and sodium-chloride media. The statistical modelling tools of the Normal PDF and the Weibull PDF were used to analyse the quality, uniformity and reliability of the comparative effectiveness of each inhibitor concentrations and goodness-of-fit criteria, by the PDF’s, were determined using Kolmogorov–Smirnov statistics [29], [30]. Also, the effect of the different concentrations of inhibitor admixtures on the compressive strengths of the concrete specimens were investigated and reported.
Section snippets
Concrete blocks preparation
Schematic drawings and pictorial representations of the materials and experimental set-up employed for this study are shown in Fig. 1. Each concrete block, Fig. 1(a), used for the experiment was made using Portland cement, sand and gravel mixed with water at a mix ratio of 1:2:4 (cement, sand and gravel). The formulation used for each reinforced concrete specimen was (in kg/m3): cement 320, water 140, sand 700 and gravel 1150. The water/cement (w/c) ratio was 0.44.
Thirty concrete blocks were
Experimental results
Graphical plots of averaged readings of open circuit potential (OCP) against time, obtained during the experiment for the two-set specimens of reinforced concrete samples are presented in Fig. 2 for the two media – Fig. 2(a) for H2SO4 and Fig. 2(b) for NaCl.
The open circuit potential readings from the graphical plots shown in Fig. 2(a) and (b) could be observed as being predominated with fluctuations in the form of spikes of varying amplitudes for each concentration of inhibitors presented.
Conclusion
The effect of potassium-chromate and sodium-nitrite on concrete steel-rebar degradation in sulphuric-acid and sodium-chloride media had been investigated in this study. From the results in the study, the following conclusions can be drawn:
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In a modelling agreement by the Normal and the Weibull PDF, steel-reinforced concrete sample with 4.5 g K2CrO4 (0.145 M) inhibitor admixture was estimated as exhibiting the lowest probability of corrosion risk in sulphuric-acid medium. The corrosion condition
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