Anodic polarization of mild steel in saturated Ca(OH)2 contaminated with NaCl in presence of NaNO2
The corrosion behavior of mild steel in saturated aerated and de-aerated Ca(OH)2 solution was investigated using electrochemical measurements . The work was carried out with small coupons immersed in solutions containing different quantities of NaCl in presence of various NaNO2 concentrations as corrosion inhibitor. It has been found that:
1- In absence of NaCl , the time required to reach O2 evolution potential in de-aerated Ca(OH)2 polarized anodiclly at 10μA/cm2 is function of inhibitor concentration and it becomes less as NaNO2 increases compared with zero presence indicating the effectiveness of NaNO2 as anodic corrosion inhibitor .
2- In absence of NaCl, the time required to reach O2 evolution potential in de-aerated solutions is less than in aerated solutions when inhibitor increases from 0 to 0.3%wt .
3- In presence of sufficient chloride in de-aerated Ca(OH)2 solution, the passive film may breakdown locally.
Key words :
Anodic polarization, alkaline solution, galvanostatic technique, O2 evolution potential.
The electrochemical behavior of steel in stagnant alkaline solutions, especially saturated Ca(OH)2, has been previously investigated (1,2). It was found that the oxidation processes that take place previously on steel are determined by the degree of surface oxidation of the sample and by dissolved oxygen, but not by the type of cation present. In aerated solution, ferrosoferric oxide (Fe3O4) is the intermediate oxidation product on the steel surface while ferrous hydroxide Fe(OH)2 is the intermediate product in de-aerated solution (1) , therefor immersed steel in aerated or de-aerated solution of Ca(OH)2 always becomes passivated. Such passivity is destroyed, however, when chlorides Cl- and Other corroding materials are present as admixtures in the alkaline solution (3) . Corrosion inhibitor by NaNO2 is a problem of both practical and theoretical significance. Corrosion of steel can be prevented in nutral and alkaline solutions by NaNO2. It is of advantage over chromates in that it has no known effect on the skin. NaNO2 is of theoretical interest in that it is an inhibiting material which does not form an insoluble with iron (4) .
It is believed, that the present study of the corrosion behavior of steel in aerated & de-aerated saturated Ca(OH)2 is similar in many respects to the aqueous phase of cement in reinforcement, so it will contribute to better understanding of the behaviour of embedded reinforcement . Keeping in mind that the induced applied current which causes polarization of the immersed mild steel in the anodic direction compared to its free corrosion potential in this study was about 10μA/cm2 using galvanostatic pulse technique (5).
Experimental Work :
The evaluation of an anodic corrosion inhibitor for mild steel immersed in stagnant alkaline solution was investigated using Half-Cell and GALVANOSTATIC polarization techniques.
Steel coupons of (3*1*0..1)cm3 dimensions and chemical composition ( C:0.04, Mn:0.309, Si:0.004, P:0.005, S:0.007, Cr:0.021, Ni:0.01, Mo:0.009, Cu:0.012, Al:0.004, Fe: The reminder ) %wt were used .
The coupons were first polished using emery paper of grades no : 220,320,400 and 600 , degreased by benzene and acetone , rinsed with distilled water & then pickled in 50% HCl acid .The clean steel was coated with epoxy in such a way that only about 3cm2 area was always exposed to corrosive media . De-aerated and aerated Ca(OH)2 at 30 C0 were used as a corrosive media (i.e., saturated Ca(OH)2 was prepared by dissolving 1.53g Ca(OH)2 crystals in 1 liter of pure distilled water . All potential values mentioned are with respect to saturated calomel electrode. For de-aerated test solution, high purity (99.9%) N2 was bubbled into the test solution for ≈ 1.5 h.
Polarization Measurements :
GALVANOSTATIC technique was used to make these measurements. The circuit diagram and procedure is shown in details (6). The potential of the steel electrode was recorded relative to that of the saturated calomel electrode, at current density of 10μA/cm2 as function of time. The experiments were carried out at 30 C0.
Result and Discussion:
Typical corrosion potential measurements as referenced to saturated calomel electrode (S.C.E), for low carbon steel totally immersed in aqueous solutions of saturated Ca(OH)2 , in absence and presence of both sodium nitrite (NaNo2) and sodium chloride (NaCl) at different concentrations are tabulated in table (1) , with pH value for each experiment between brackets . The variation of electrode potential with time is shown in figures (1, 2, 3, 4, and5). These figures associated with corrosion potentials in table (1) showed that:
(1) In absence of NaCl. Increasing the inhibitor concentration (NaNO2) from 0% to 0.3% wt., shifts the Ecorr to more positive direction indicating the effectiveness of NaNO2 to polarize the metal anodically and shifting the Ecorr to open circuit potential of the cathode . While increasing the concentration of NaNo2 to 3%wt through 1% leads to shift the Ecorr to negative direction.
(2) Generally, the variation of steel potential with time in alkaline solution containing 0.1,0.3,1&3%wt NaCl, shifts to more positive direction at all inhibitor concentration levels , compared with 0% NaNO2 .
It can be concluded that: Under the particular conditions of interest here, the low carbon steel at high pH, a film of oxides probably of few nino meters (nm) thickness covers the surface and is responsible for the passive nature of the metal at the low levels of %NaCl concentrations ******* with increasing NaNO2 from 0.1 to 0.3%wt. This situation becomes more pronounced at high levels of NaCl %wt.
Rosenberg and Gaidis(7) in their study on the mechanism of nitrite inhibition of Cl- attack on reinforcing steel in alkaline aqueous environments concluded that: Nitrite ion rapidly oxidizes Fe+2 ion to Fe+3 ion, blocking further passage of Fe+2 ion from the metal into the electrolyte.
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