# Direct Solar Thermal Water-Splitting Using Iron and Iron Oxides at High Temperatures: A Review
## 3 Mechanisms of High-Temperature Oxidation  
Despite extensive scientific knowledge about the oxidation of iron at both environmental and high temperatures due to the presence of oxygen ions and despite efforts to reduce or avoid iron corrosion, the electrochemical reaction mechanism that explains the \begin{table}
\begin{tabular}{c c c} \hline \hline
**Equation** & **Observation** & **Refs.** \\ \hline Logarithmic reaction rate: & & \\ \(X=K_{reaction}\text{ln}\ (t+t_{o})+A\) & It represents the initial oxidation states at low temperatures. & [41] \\ \(X=K_{reaction}\text{ln}\ (Bt+1)\) & - & [41] \\ \hline Linear reaction rate: & & \\ \(\frac{dy}{dt}=K_{Lineal}\) & It represents a constant rate of oxide growth applicable at very & [41] \\ \(X=K_{Lineal}t+C_{Lineal}\) & - & [41] \\ \(\Delta W\big{/}_{A}=\ K_{L}\ t\) & - & [52] \\ \hline Parabolic reaction rate: & & \\ \hline \(\frac{dy}{dt}=\frac{K_{break}}{x}\) & It adjusts to the processes controlled by the diffusion of species. & [41] \\ \hline \(X^{2}=K_{parab}t+C_{parab}\) & - & [41,53] \\ \hline \(\left(\
icefrac{{\Delta W}}{{A}}\right)^{2}=K_{parab}*t\) & - & [52] \\ \hline \(\frac{dy}{dt}=k\ f(X)\) & The function f(X) depends on the reaction mechanism for & [54] \\  & diffusion in one, two, or three dimensions. & \\ \hline \(\frac{dy}{dt}=k(T)*f(X)\) & The reaction rate of a sample of iron slag in water vapour. & [55] \\ \(k(T)=k_{o}*e^{(\frac{x}{RT})}\) & The reaction rate of a sample of iron slag in water vapour. & [55] \\ \hline \hline \end{tabular} Where X is the thickness of oxide consumed per surface unit or the weight gain per area unit; \(\Delta\)W/A is the mass gain per unit area (g/cm\({}^{2}\)); t is the time; k(T) is the reaction rate constant; f(X) is the function that represents the reaction mechanism; ko, Kreaction, K1 Lineal, and KPARab are reaction constants; E\({}_{\text{a}}\) is the activation energy; R is the gas constant; T is the absolute temperature; A and B are constants; C\({}_{\text{Lineal}}\) and C\({}_{\text{PARab}}\) are constants of integration.  
\end{table} 
Table 3: Reaction-rate equations applicable to high-temperature oxidation. presence of oxides such as FeO, Fe\({}_{3}\)O\({}_{4}\), and Fe\({}_{2}\)O\({}_{3}\)[58], especially when iron interacts with the hydroxyl ion (OH\({}^{-}\)), remains insufficiently understood. [PERSON] et al. [59] developed a multi-step optical system that enables water decomposition for OH\({}^{-}\) ion detection, demonstrating that rate constants can be accurately measured at lower temperatures, such as 500 K. This approach contrasts with alternative methods that require temperatures exceeding 2570 K [60]. [PERSON] et al. [16] proposed a kinetic mechanism of thermal dissociation of H\({}_{2}\)O between 2000-3000 K, as represented by the following expressions:  
\[H_{2}O+G\leftrightarrow H+OH+G \tag{11}\]  
\[H_{2}O+H\leftrightarrow H_{2}+OH \tag{12}\]  
\[OH+H\leftrightarrow H_{2}+O \tag{13}\]  
\[OH+O\leftrightarrow O_{2}+H \tag{14}\]  
where G is the extinguishing gas (argon at room temperature or steam at 400-450 K and between 2.5-9.4 bar), H is the H\({}^{+}\) ions, and OH is the OH\({}^{-}\) ions. Fe corrosion is a process of electrochemical dissolution involving electron transfer to an intermediate species formed by the interaction between ferrous ions and water. In the reduction of Fe, this intermediate species is found on the iron surface and must undergo a subsequent reaction to form Fe [61]. The initial rate of oxidation of an Fe or steel surface newly exposed to H\({}_{2}\)O\({}_{(\mathrm{g})}\) is always lower compared to the rate of corrosion induced by O\({}_{2}\). Researchers [PERSON] et al. [12; 52] suggested that while H\({}_{2}\) tends to dissolve in solid metals in atomic rather than molecular form, it is unlikely to dissolve in an oxide network due to its large size, which impedes diffusion. Therefore, they propose that the cathodic reaction could occur within the scales.