Sodium silicate incorporated into mortars as corrosion inhibitor for construction steel.

L. MORAN AYALA; M. A. FRONTINI; VAZQUEZ M.; M. B. VALCARCE

Congreso; 34th Topical Meeting of the International Society of Electrochemistry.; 2023

International Society of Electrochemistry

Inorganics corrosion inhibitors are commonly used to prevent carbon steel corrosion in concrete. Amongthe inorganic inhibitors that can be admixed into concrete, nitrite ions are efficient and widelyinvestigated. However, recent regulations limit their use, given that they can lixiviate and contaminatesurrounding soil or water [1, 2]. Other inorganic ions have also shown promising results. Silicate ions areparticularly interesting given their low cost and low toxicity. To evaluate the efficiency of silicate ions asinhibiting agents for reinforcements in concrete, preliminary results have been obtained in poresimulating solutions [3, 4]In contrast, there is scarce information in the bibliography where silicate ions are evaluated in mortars orcement paste as corrosion inhibitors. The aim of this work is to evaluate the efficiency of silicate ions asinhibiting agents for carbon steel in mortar.The results obtained in mortar are compared with data obtained in an alkaline solution (AS) that simulatesgood quality concrete contaminated with chlorides ion, with and without silicate ions also present [3].Carbon steel behavior was evaluated by electrochemical impedance (EIS) in AS solution pH=13containing KOH 0.08 mol/L, NaOH 0.02 mol/L and Ca(OH)2 0.001 mol/L. Chloride ions 0.3 mol L-1were incorporated corresponding to [Cl-]/[OH-]=3 and 0.3 mol/L of Na2SiO3.5H2O, corresponding to[inhibitor]/[Cl-]=1. Also, a weight loss test involving long exposure times was carried out.In this work, polished carbon steel bars were included in mortar prepared with water/Portland cement=0.6and sand/cement ratio =3. Mix R (for reference) had no admixed inhibitor and mix S incorporatedNa2SiO3.5H2O using a 0.3 mol/L solution to replace the water in the mix. Once cured for 7 days, sampleswere immersed in aerated 0.3 mol/L NaCl solution for 3 months, and periodically evaluated by EIS. Thealkaline reservoir in the mortar (content of insoluble Ca(OH)2) was investigated by thermogravimetricanalysis. Also, a lixiviation test was designed using Phenolphthalein and Yellow Alizarin to verify thefinal pH after lixiviation process (15h of boiling in tap water).Results shows that in AS, carbon steel corrosion can be prevented, with efficiencies higher than 99% andno localized attack observed after 30 days of immersion. Similar results are observed in mortars at 15days. However, after 90 days of exposure the charge transference resistance for mortar S is similar thanthat for mortar R. This is indicating that the inhibitor effect is lost at longer times. Thermogravimetric testindicates that silicates consume Ca+2 ions in mortar, increasing the NaOH content. As this is moresoluble, it is lixiviated faster than Ca(OH)2 producing a drop in the pH. Lixiviation tests agree with thisexplanation.1. Gaidis, J.M., Chemistry of corrosion inhibitors. Cement & Concrete Composites, 2004. 26(3): p.181.2. B. Elsener, Corrosion inhibitors for steel in concrete. State of the art report. London, ed. M.Publishing. 2001.3. Montes, F., et al., Silicate Ions as Corrosion Inhibitors for Carbon Steel in Chloride-Contaminated Concrete Pore Simulating Solutions. Corrosion, 2020. 76(12): p. 1147-1154.4. Ming, J., et al., Effects of stray current and silicate ions on electrochemical behavior of a highstrengthprestressing steel in simulated concrete pore solutions. Corrosion Science, 2022. 197