This work investigated the use of ZnAl-layered double hydroxide (LDH) intercalated with nitrate or nitrite ions for controlling the corrosion of steel in reinforced concrete. The work started by analyzing the stability of the powder in the 1-14 pH range and the capacity for capturing chloride ions in aqueous solutions of different pH. The effect of the ZnAl-LDH on the corrosion of steel was studied in aqueous 0.05 M NaCl solution and in mortars immersed in 3.5% NaCl. It was found that the LDH powders dissolved partially at pH > 12. The LDH was able to capture chloride ions from the external solution, but the process was pH-dependent and stopped at high pH due to the partial dissolution of LDH and the preferential exchange of OHions. These results seemed to imply that ZnAl-LDH would not work in the alkaline environment inside the concrete. Nonetheless, preliminary results with mortars containing ZnAl-LDH showed lower penetration of chloride ions and higher corrosion resistance of the steel rebars. dioxide leads to the carbonation of concrete, which is accompanied by a decrease in pH. When the front of low pH reaches the steel surface, the passivity is lost and uniform corrosion starts. Chloride ions, on the other hand, can disrupt the passive layer even at high pH, if a threshold concentration is surpassed [5]. In these conditions, the corrosion is localized in the form of pitting, crevice, or stress corrosion cracking. The iron corrosion products are more voluminous than steel, which creates expansive stresses, leading to cracking at first and spallation of the concrete cover at the end. After that, steel becomes directly exposed to the atmospheric environment, and corrosion proceeds at a much faster rate.Since the corrosion of steel rebars dramatically limits the service life of reinforced concrete, many forms of corrosion control are being explored [2][3][4]6,7], including the use of stainless steel or galvanized steel rebars, application of epoxy coating on steel rebars, cathodic protection, and addition of corrosion inhibitors to concrete (e.g., calcium nitrite, sodium benzoate, chromates, phosphates, polyphosphates, silicates, polycarboxylic acids, fatty acids emulsions, and alkanolamines). Corrosion can also be delayed by painting the surface of the concrete structure or using a thicker layer of concrete, separating the rebars and the environment. Less porous concrete (lower water/cement ratio), high quality cement, and the use of water and aggregates without soluble salts also contribute to extending the durability of the structure.The direct addition of corrosion inhibitors to concrete may affect the curing process or the mechanical properties of the hardened material. Because of this possibility, the encapsulation of inhibitors in nano-or micro-reservoirs to be released only when needed (either with the onset of corrosion or in the presence of aggressive species) is a line of investigation worth pursuing.Layered double hydroxides (LDHs) are one example of such nanostructured reservoirs. The LDH structure...

A numerical model for enhanced service life prediction of concrete infrastructure is presented which includes transient analysis of processes during corrosion initiation as well as propagation stage. The temporal and spatial transition of Steel-Concrete Interface during depassivation events is described by a randomly varying chloride threshold function. As such random activation events can be accounted for, rather than having to pre-describe the anode size and location as in many existing models. The aim of the study is to investigate random spatial activation events in concrete structures in submerged zones based on dynamically changing boundary conditions on the rebar surface to control transition from passive to active state. Investigations are carried out to realize the sustainability of corrosion processes in limiting oxygen concentrations in dissolved seawater. The model showcases the numerical architecture, the associated concept of randomly varying chloride threshold and predicts that among other factors, the rate of oxygen strongly influences corrosion rate in submerged locations.

In this work, the ZnAl-NO2 LDH (layered double hydroxide) is investigated as a possible additive for mitigating the chloride-induced corrosion of steel in reinforced concrete. The investigation focused on the stability and chloride binding capacity of this LDH in the pH range typical of cementitious materials. Until pH = 12.5 the material was stable and effective in capturing chloride ions from the surrounding aqueous environment. For higher pH, precisely that of hydrated cement, the LDH was partially dissolved and OH− preferentially entrapped instead of Cl−. These results suggested that ZnAl-NO2 has excellent chloride entrapping capability at neutral pH, but this is reduced with increasing pH. However, when the LDH was incorporated into mortars, the chloride ingress was delayed, signifying that the dissolution of LDH leads to a secondary mechanism responsible for chloride capture.

The sulfuric acid attack is a common form of degradation of reinforced concrete in contact with industrial wastewater, mine water, acid rain, or in sewage treatment stations. In this work, new pH-sensitive IrOx electrodes were developed for monitoring the pH inside mortar or concrete. To test their ability, the pH sensors were embedded in mortar samples at different depths and the samples were exposed to sulfuric acid solution. In another set of experiments, iron wires were placed at the same depths inside similar mortar samples and their corrosion was monitored as the acid attacked the mortar. Severe acid attack led to cement dissolution and formation of gypsum. The new pH sensors succeeded in measuring the pH changes inside the mortars. The pH gradient, from the high acid environment to the high alkaline mortar interior, occurred in a narrow region. Corrosion of the iron electrodes started only when the acidic solution was in their close vicinity.