Alkali-activated fly ash (AAFA) can reduce the carbon emissions from the production of ordinary Portland cement (OPC) by >80%. The utilization of AAFA and the research on the electrochemical behavior of steel in AAFA have thus attracted much attention. Compared with OPC, a higher resistance to chloride-induced corrosion can be achieved in AAFA due to its high alkalinity and zeolite-like adsorption layer. However, the corrosion of steel in AAFA cannot be completely inhibited in harsh environments contaminated with chlorides.

Carboxylate groups-based corrosion inhibitors (such as sodium citrate) improve the pore structure and compressive strength of concrete, and have a high inhibition efficiency for steel. Especially at a high chloride concentration, the pitting potential (E_pit) of steel treated with carboxylate groups-based corrosion inhibitors is greater than that treated with amine salt and amino acid. Therefore, sodium citrate can be considered as a promising organic corrosion inhibitor to enhance the corrosion resistance of steel in AAFA. However, the inhibition mechanism of sodium citrate for steel in AAFA is still unclear. In this paper, the effect of sodium citrate on the corrosion behavior of reinforcing steel in extracted AAFA solution under chloride attack was investigated by electrochemical measurements and surface characterization techniques, and the inhibition mechanism of sodium citrate was revealed.

In this study, the preparation of AAFA solution was carried out at room temperature ((25±2) ℃). Firstly, a low calcium fly ash was mixed with an alkali-activated solution (80 g/L NaOH solution) at a mass ratio of 1∶1, then the mixture was stirred in a closed vessel for 24 h, and finally it was filtered to remove the residues. The concentration of sodium citrate added into the AAFA solution was 100 mmol/L.

Conventional ribbed carbon steel (HRB400E) with a diameter of 16 mm was used. The electrochemical measurements, namely electrochemical impedance spectroscopy (EIS) and the cyclic potentiodynamic polarization (CPP), were performed in a classic three-electrode electrochemical system, i.e., the working electrode (HRB400E steel), the reference electrode (saturated calomel electrode, SCE) and the counter electrode (platinum electrode). The surface morphology and chemical composition of steel passive films were characterized by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), respectively. After chloride attack, the corrosion morphology and corrosion products of steel were analyzed by scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) in a model FEI Quanta 3D FEG environmental scanning electron microscope.

In the absence of protective passive film, the capacitive loop of the non-passivated steel (the Blank specimen) decreases in AAFA solution containing chlorides, thereby gradually reducing the corrosion resistance of steel. However, the capacitive loop and impedance modulus (|Z|) of steel treated with citrate (the Cit specimen) increase. Citrate can effectively improve the corrosion resistance of the non-passivated steel in AAFA solution containing chlorides.

Moreover, the EIS results of pre-passivated steel in chlorides-free AAFA solution show that the capacitive loop of steel treated with citrate (the P-Cit specimen) is somehow smaller than that of the blank group (the P-Blank specimen), indicating that citrate is unfavorable to the formation of steel passive films in AAFA solution. However, in the presence of chlorides, an enhanced corrosion resistance can be obtained for the P-Cit specimen, which is the same as that of the non-passivated steel.

According to the results of AFM and XPS measurements, citrate can modify the surface morphology and chemical composition of steel passive film in AAFA solution. There are a large number of fine particles on the steel surface in AAFA solution, and the surface roughness of steel is less and more uniform due to the adsorption effect of citrate, compared with that without citrate. For steel with citrate, the intensities of C—C/C—H and C==O peaks increase and the oxidation of Fe may be suppressed to a great extent. Furthermore, O^2-/OH^- ratio of steel passive film treated with citrate slightly decreases, and the corresponding content of bound water in steel passive film decreases.

Clearly, citrate ions can combine with iron ions to form an adsorption film on the steel surface, therefore suppressing the formation of passive film to some extent. However, when steel is attacked by chlorides, the adsorption film of citrate can prevent chloride ions from reaching steel surface. In addition, the negatively charged carboxylic acid group is also dominant in the competition with chloride ions, effectively inhibiting the depolarization of chloride ions, and finally improving the corrosion resistance of steel in AAFA solution.

Although sodium citrate had a negative effect on the formation of a stable steel passive film, it was suitable for improving the corrosion resistance of steel in AAFA solution. Citrate ions could be adsorbed on the steel surface to form an adsorption film, thereby acting an effective physical barrier. Also, citrate ions played a dominant role in the competitive adsorption with chloride ions on the steel surface. The corrosion resistance of steel in AAFA solution contaminated with chlorides was improved by the addition of sodium citrate.