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Alkali‐activated materials (AAMs) are binders that can complement and partially substitute the current use of conventional cement. However, the present knowledge about how AAMs protect steel reinforcement in concrete elements is incomplete, and uncertainties exist regarding the application of electrochemical methods to investigate this issue. The present review by EFC WP11‐Task Force 'Corrosion of steel in alkali‐activated materials' demonstrates that important differences exist between AAMs and Portland cement, and between different classes of AAMs, which are mainly caused by differing pore solution compositions, and which affect the outcomes of electrochemical measurements. The high sulfide concentrations in blast furnace slag‐based AAMs lead to distinct anodic polarisation curves, unusually low open circuit potentials, and low polarisation resistances, which might be incorrectly interpreted as indicating active corrosion of steel reinforcement. No systematic study of the influence of the steel–concrete interface on the susceptibility of steel to corrosion in AAMs is available. Less common electrochemical methods present an opportunity for future progress in the field. [ABSTRACT FROM AUTHOR]

This research evaluates the behavior corrosion of galvanized steel (GS) and AISI 1018 carbon steel (CS) embedded in conventional concrete (CC) made with 100% CPC 30R and two binary sustainable concretes (BSC1 and BSC2) made with sugar cane bagasse ash (SCBA) and silica fume (SF), respectively, after 300 days of exposure to 3.5 wt.% MgSO4 solution as aggressive medium. Electrochemical techniques were applied to monitor corrosion potential (Ecorr) according to ASTM C-876-15 and linear polarization resistance (LPR) according to ASTM G59 for determining corrosion current density (icorr). Ecorr and icorr results indicate after more than 300 days of exposure to the sulfate environment (3.5 wt.% MgSO4 solution), that the CS specimens embedded in BSC1 and BSC2 presented greater protection against corrosion in 3.5 wt.% MgSO4 than the specimens embedded in CC. It was also shown that this protection against sulfates is significantly increased when using GS reinforcements. The results indicate a higher resistance to corrosion by exposure to 3.5 wt.% magnesium sulfate two times greater for BSC1 and BSC2 specimens reinforced with GS than the specimens embedding CS. In summary, the combination of binary sustainable concrete with galvanized steel improves durability and lifetime in service, in addition to reducing the environmental impact of the civil engineering structures. [ABSTRACT FROM AUTHOR]

Electrochemical corrosion of thermo-mechanically processed (TMP) and recrystallized Fe−30Mn−5Al−0.5C twinning-induced plasticity (TWIP) steels containing 30 wt.% Mn was studied in a 1.0 wt.% NaCl electrolyte solution. The alkaline nature of the corrosion products containing manganese oxide (MnO) increases the dissolution kinetics of the TWIP steel in acid media, obtaining Mn2+ cations in solution, and producing the hydrogen evolution reaction (HER). X-ray photoelectron spectroscopy (XPS) surface analysis revealed an increased Al2O3 content of 91% in the passive layer of the recrystallized TWIP steel specimen, while in contrast only a 43% Al2O3 was found on the TMP specimen. Additionally, the chemical composition of the surface oxide layer as well as the TWIP alloy microstructure was analyzed by optical microscopy (OM) and scanning electron microscopy (SEM). The results indicate an enhanced corrosion attack for the TMP high-Mn TWIP steel. [ABSTRACT FROM AUTHOR]

A review by EFC-WP11 demonstrates that differences exist in this regard between AAMs and Portland cements, and between types of AAMs, mainly caused by differing pore solution compositions. These differences can lead to distinct anodic polarisation curves, low open circuit potentials, and low polarisation resistances for certain AAMs, which might be incorrectly interpreted as indicating active corrosion of the steel reinforcement. However, the present knowledge about how AAMs protect steel reinforcement in concrete elements, and about the significance of the outcomes of electrochemical analytical methods to study this subject, is incomplete. [Extracted from the article]

• Disentangle interphase inhibition mechanism of sodium borate for carbon steel. • Elucidation of the 3D structured stable double-layer Fe 2 O 3 /FeBO 3 passive oxide film. • Na 2 B 4 O 7 present a physicochemical adsorption process, which follows Freundlich isotherm. • Inhibition efficiency 86.3 % Na 2 B 4 O 7 ability to form a stable Fe–B–O interphase passive oxide film. • Mott-Schottky analysis revealed Na 2 B 4 O 7 produced less defective, and compact passive film. Sodium borate (Na 2 B 4 O 7) corrosion inhibition mechanism was studied on carbon steel rebars in 0.6 mol/L Cl– simulated concrete pore solution (SCPS) at different temperatures and concentrations. Electrochemical testing including cyclic potentiodynamic polarization (CPP), electrochemical impedance spectroscopy (EIS), and Mott–Schottky plots were utilized to understand the influence of inhibitor concentration and temperature on the thermodynamics of the adsorption and activation processes. It was found Na 2 B 4 O 7 imparts corrosion protection due to the formation of a 3D Fe–B–O stable interphase passive film, achieving an inhibition efficiency up to 86%. This can be attributed to the film having a lower donor current density and less vacancies, according to the Mott–Schottky tests. The formed interphase passive film was more stable and compact with less point defects compared to the blank due to Na 2 B 4 O 7 adsorption, this phenomenon was substantiated using the atomic packing factor, film thickness, and Mott–Schottky plots. It was revealed, using adsorption isotherms, that Na 2 B 4 O 7 interacted with the surface via physiochemical adsorption, creating a thinner/compact passive film composed of a rhombohedral calcite-type structure. SEM, XRD, and XPS were used to illustrate that Na 2 B 4 O 7 aides in the formation of a stable double-layered net structure Fe 2 O 3 /FeBO 3 passive oxide film. Finally, an interphase corrosion inhibition mechanism of Na 2 B 4 O 7 was proposed. [ABSTRACT FROM AUTHOR]

There is increasing interest in exploring the durability of structures manufactured with cements that are more environmentally friendly than traditional Portland. Accordingly, protection against chloride-induced corrosion of carbon steel reinforcement in mortars manufactured with environmentally friendly hybrid cements and alkali-activated materials (AAM) needs to be analyzed in-depth. Hybrid cements are innovative alternative binders with reduced amount of clinker in their formulation (20–30%). Their precursors are aluminosilicate wastes and they are very feasible in-construction implantation because they are not activated neither with liquid solutions of strong alkalinity nor external heating. In this research, AAM mortars were manufactured using slag from industrial by-products activated with sodium silicate. Hybrid mortars were manufactured either from fly ash or from slag, and activated with Na 2 SO 4. Portland cement (CEM IV) mortars were also included in this study as reference. Chlorides were made to penetrate into the four different mortars under study by diffusion. The porosity and chloride penetration were evaluated and related to the different materials. The electrochemical behavior of the steel reinforced mortar specimens was monitored by open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) measurements. The corrosion rate values obtained for steel embedded in AAM were similar to conventional CEM IV mortars, and the morphology of the attack is less localized. [ABSTRACT FROM AUTHOR]