Electrical Resistivity and Half-Cell Potential Studies to assess organic and inorganic corrosion inhibitors’ effectiveness in concrete

Very limited guidelines are available on the use of corrosion resistant inhibitors as a constituent material in concrete due to availability of statistics on its corrosion inhibiting efficiency. So the study is conducted on the M25 grade concrete mixed with few selected corrosion inhibitors of organic and non-organic nature to comprehend the impact of these corrosion inhibitors on the conductivity of electricity in these concretes. Based on the other researchers’ work, four well know corrosion inhibitors such as Calcium nitrate, Di-ethanolamine, Sodium nitrite and Hexamine and are chosen for study. Dosages of 1%, 2%, 3%, 4% and 5% the weight of cement are chosen. Measured electrical resistivity and half-cell potential values of all corrosion inhibitors admixed M25 grade concrete mixes indicates the superior corrosion inhibition ability of calcium nitrate and Di-ethanolamine’s with shows high electrical resistance.


Introduction
The best generally implemented method to mitigate the corrosion of the rebars in concrete structures is applying epoxy or zinc coating on the rebars or by coating with water repellent agents, or regulating the water/cement ratio or by adopting sufficient cover thickness or by coating with chemicals or by using cathodic protection etc. Although experts suggest practice of supplementary cementitious materials (SCMs) as concrete admixtures which offers a worthy solution for deterring corrosion in rebars. The cost-effective and recently accepted practice is to prevent the rebar corrosion in concrete is admixed concrete with corrosion inhibiting agents. The sustainable organic inhibitors are non-toxic green-plant extracts which are environmentally decomposable, nonpresence of heavy metals, environment-friendly and are natural available. It is postulated that the concrete admixed with corrosion inhibitors will deter the start of corrosion and also effects the properties of concrete in a big way. The process of corrosion in typically governed by features such as presence of Clions and H + ions. The corrosion admixtures' inhibitive properties can be assessed in the laboratory using electrical resistivity and half-cell potential measurements.

Mechanism of Inhibitors
Corrosion inhibitors' ions gets absorbed on the surface of rebars due to high alkaline environment of concrete forming a hydrophobic layer which prevents the rebars from corrosion by opposing the vital electrochemical reactions such as rebar oxidation and oxygen reduction of corrosion mechanism. The effect of inhibitors is governed principally by the molarity or the concentration of inhibitors'. If the molarity is high then the effect of inhibiting corrosion is significant.

Corrosion Inhibition efficiency Studies
In this phase corrosion inhibition efficacy of various corrosion inhibitors considered for study are evaluated in terms of resistivity measurements and potentials measurements of M25 grade reinforced concrete specimens.

Electrical resistivity studies
This experiment is performed in the research laboratory at GRIET Hyderabad, India. Electrical resistivity method is a nondestructive technique which indicates the ability of the medium to conduct electric current depending on the presence of electrolyte, water and salts present in the pore solution of the concrete. So once the passive layer is broken the electrical resistivity of the concrete acts as indicator for probable corrosion rate in concrete (CEB 192). Resistivity of concrete depends on its electrolytic resistance. So low electrolytic resistance indicates the high probable corrosion rate in concrete. A saturated concrete cube specimen of 100mm size is used as test specimen using direct two probe technique where the wet piece of sponge or multi-folded cloth and electrodes (brass plate) are firmly secured on either side of the cube to guarantee a uniform current flow as shown in the Fig. 1. The electrical resistance is measured on the saturated surface of the cube is Resistivity ( Ωcm), = ( * )/ Where R= Resistance measured in kilo ohms A = Surface area of the cube = 100 cm 2 L= Distance between two electrodes= 10 cm  . High half-cell potentials (less negative value) indicates high resistance to corrosion. The correlation between the rate of corrosion and the half-cell potential is that the level of corrosion increased with the decrease of the half-cell potentials (high negative value). Concrete cylinders of size 100x200 mm was casted with 8 mm diameter rebar twisted high yield strength deformed bar (Tor steel) of 100 mm length embedded centrally (with at least 30 mm length of rebar exposed). This exposed end is insulated with coated copper wires of low resistance to facilitate the measurement of half-cell potentials. The potentials were measured with reference electrode and digital voltmeter (1mV accuracy) for all the specimens. These potentials indicate the probability of corrosion to occur. The lead of the specimen is to be connected to the positive terminal whereas the electrode is connected to the negative terminal. The reference electrode is kept in contact with the wet cloth surface throughout the testing.

Test Results and Discussions
Based on the experimental investigations, the test results are presented as follows-

Corrosion Inhibition Studies
In this phase corrosion inhibition efficacy of various corrosion inhibitors considered for study are evaluated in terms of resistivity measurement, potentials measurements and accelerated corrosion test on M25 grade reinforced concrete structures.