Differences between Using Mortars and Solutions When Testing Sodium Silicate as Corrosion Inhibitor for Construction Steel
Authors: Morán Ayala Lucía , Frontini Maria Alejandra , Vazquez Marcela , Valcarce Maria Beatriz 
Published: February 2024

Corrosion inhibitors are regularly used to minimize corrosion progress in reinforced concrete. Nitrite ions are very effective inorganic inhibitors but not environmentally friendly, since they can leach and contaminate the surrounding soil and water [1, 2]. Among other promising inorganic ions, silicate ions are of particular interest due to their low cost and low toxicity, although most results have been reported in pore simulating solutions. There is little information in the literature where silicate ions are evaluated as corrosion inhibitors in mortar or cement pastes. The purpose of this study is to show some contrasts between the effectiveness of silicate ions as carbon steel inhibitors when they are tested in solution or in mortar.
The results in solution (ASCl) were obtained in an alkaline electrolyte (pH=13) that simulates good quality concrete contaminated with chloride ions [3,4]. ASCl contains KOH 0.08 mol/L, NaOH 0.02 mol/L, Ca(OH)2 0.001 mol/L and NaCl 0.3 mol/L, so that [Cl-]/[OH-]=3. The solution containing silicate ions (SSCl) incorporates Na2SiO3.5H2O 0.3 mol/L, corresponding to [inhibitor]/[Cl-]=1. The electrochemical evaluation included standard tests such as cyclic voltammograms, polarization curves, electrochemical impedance spectroscopy (EIS) and weight loss tests involving long exposure times. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were used to investigate the composition of the surface films. The findings for bare rebars demonstrate that the presence of silicate ions in solution may effectively prevent carbon steel corrosion, with an inhibition efficiency greater than 99% and no localized attack seen after 30 days of immersion. It has also been observed that a 24-hour pretreatment at open circuit potential (OCP) in silicate solutions (SS) improves the corrosion resistance of construction steel when tested in SSCl. The polarization curves show higher pitting potential and lower corrosion current intensities when compared to untreated electrodes. XPS spectra show that silicate ions are incorporated into the surface film. Pretreatments in SS produce passive films that are rich in Fe(II), present mainly as oxohydroxides. EIS shows that the film is more resistive and compact. In contrast, the reference pretreatment in the absence of silicates led to surface films composed mainly of Fe(III) oxides and oxohydroxoxides.
Tests in mortars were carried out including reinforcement bars (rebars) in mortar prepared with water/Portland cement=0.6 and sand/cement ratio =3. Mix R (for reference) had no admixed inhibitor and mix S incorporated Na2SiO3.5H2O using a 0.3 mol/L solution to replace the water in the mix. Also, samples P contained rebars pre-treated by previous immersion in silicate ions solutions, in mortars with the same composition as mix R. Once cured for 7 days, the mortar samples were immersed in aerated 0.3 mol/L NaCl solution for 3 months. The electrochemical follow-up included periodical evaluation by EIS and Rp. The alkaline reservoir in the mortar (which is the content of insoluble Ca(OH)2) was investigated by thermogravimetric analysis. Also, a lixiviation test was designed using pH indicators to verify changes in basicity after a lixiviation process. In mortars R and S, charge transfer and polarization resistance (Rt and Rp) remained lower than 50 kΩ cm2 after the first 15 days. However, Rt and Rp higher 100 kΩ cm2 were measured for rebars P after 15 and 60 days of exposure. The differences between P and R tend to flatten after 90 days of exposure. This shows that the inhibitor effect of pretreating the rebars (P) is lost over a longer period of time. According to the results from thermogravimetric tests, silicate ions in mortar consume Ca+2 ions, producing NaOH. Since this is more soluble than Ca(OH)2 it lixiviates quicker, causing the pH to decrease. Lixiviation tests support this theory.
In summary, silicate ions are very effective corrosion inhibitors when tested in alkaline solutions. When silicates are used to pretreat the rebars the beneficial effect is lost after 90 days. When solutions containing silicate ions replace water in the mortars´ preparation, the inhibitory effect is lost faster due to the consumption of portlandite and the consequent acidification of the mortars.
References

Gaidis, J.M., Chemistry of corrosion inhibitors. Cement & Concrete Composites, 2004. 26(3): p. 181.

Elsener B., Corrosion inhibitors for steel in concrete. State of the art report. London, ed. M. Publishing. 2001.

Montes, F., et al., Silicate Ions as Corrosion Inhibitors for Carbon Steel in Chloride-Contaminated Concrete Pore Simulating Solutions. Corrosion, 2020. 76(12): p. 1147-1154.

Rolandi A.C. et al. Corrosion Engineering, Science and Technology 57(5) (2022) 455–463.

Figure 1