## 1 Introduction  
The discovery of reinforced concrete at the end of the 19 th century allowed the civil engineering field to make remarkable advances. Indeed, it made it possible to achieve more complex structures and increase their lifetime, thanks to the fact that the steel embedded in concrete is surfaced with a passive film, which will protect it against corrosive agents. The passive film is the result of the interactions between the steel and its direct environment. For example, the exposure of iron and steel to an alkaline environment promotes the formation of a protective passive film [1; 2; 3; 4; 5; 6; 7]. However, several factors such as humidity, temperature [3; 8], pH and halogens like chloride ions lead to the destruction of the passive films, reducing the lifetime of reinforced concrete structures. Since the physical and electrochemical properties of a passive film depend mainly on both the environment (temperature, pressure, and pH) and the chemical composition of the steel [9; 10], the replacement of standard steel by more resistant alloys became crucial during the 20 th century. Among these alloys, types of stainless steel invented by [PERSON] [11] have the ability to form passive films that will occur by very complex training processes not fully understood and are subject to many controversies. Nevertheless, stainless steel remains a good solution for engineering purposes instead of traditional carbon steel [10] and continues to be a subject of research. Recently, it has been shown that the industrial water pollutant named Red Mud (RM) had positive effects on the corrosion resistance of steel in concrete, which was more pronounced in the presence of chlorides, offering a promising new way for the cement industry to reduce their environmental impact [12]. Moreover, other alloys, such as nickel-based alloys [13], have been developed for various application fields. Nickel-based alloys are used for equipping chemical and nuclear power plants, especially steam generators in pressurized water reactors (PWR). However, the literature is less abundant on nickel-based alloys exposed to very alkaline media at room temperature, which is why we studied it specifically in a previous paper [14]. In this paper, we compare the behavior of Inconel 600 with that of stainless steel. In an alkaline medium, films formed on pure iron show the presence of magnetite Fe\({}_{3}\)O\({}_{4}\), which oxidizes into hematite Fe\({}_{2}\)O\({}_{3}\)[15, 16]. Other investigations have also been conducted on Fe-Cr alloys and revealed the presence of Cr (III) oxy-hydroxide in the inner layer of the passive film [17, 18]. In the case of stainless steel, it has been demonstrated that the protective oxide film has a bilayer structure: an outer layer very enriched in iron oxide and hydroxide in contact with the electrolyte and an internal layer enriched with chromium oxide in contact with the alloy [19, 20]. Both layers forming the electrical equivalent of an n-p junction behaved like a diode, stopping the future diffusion of corrosive chemical species and thus forming an effective protection layer against corrosion [21]. Given these properties, stainless steel is more reliable for the construction of complex structures in hostile environments. In recent decades, electrochemical impedance spectroscopy (EIS) has been one of the most widely used techniques to identify the different processes occurring during the growth of passive films [10, 22, 23, 24, 25, 26]. This non-destructive technique makes it possible to simulate the behavior of a given system (metal, film, and solution) by an equivalent electrical circuit (EEC) without altering the composition and structure of the studied films. The components of the EEC (resistances, capacitors, etc.) correspond to the different processes (double electric layer, adsorption, oxidation-reduction reactions, etc.) taking place at the electrode-electrolyte interface.