## 4 Results and Discussion  
As shown in Figure 1, agitation contributed substantially to the corrosion mechanism. With static exposure in the standard test solution (curve 1), the reaction occurred under the diffusion control and, with a surface layer saturation near the metal, the process retardation took place. As we could expect, the decrease in the corrosion rate during the first 20 days was apparently due to the gradual shielding of the metal surface with hydrated oxides and other corrosion products. The agitation probably removed the diffusion limitations on the reaction rate [30; 31]. We assume that the increase in the corrosion rate after 40 days was due to kinetic features. With every subsequent agitation, the saline solution saturation by oxygen and CO\({}_{2}\) up to the constant values of concentration occurred. A similar nature of relationship between the corrosion rate and the exposure time for the sample with a weld joint (Figure 2) showed that the metal microrelief and the likely standard electric potential difference of two different alloys were also not the reason of the observed peculiarity. The above-mentioned factors impacted the overall dissolution rate, but this impact was invariable in time. One could cautiously surmise that, in this case, there were kinetic difficulties caused by the occurrence of the following reaction:  
\[\text{CaHCO}_{3}+2\text{OH}^{-}=\text{CaCO}_{3}+\text{HCO}_{3}^{-}+\text{H}_{2 }\text{O} \tag{4}\]  
With that, metal surface passivation by the deposition of formed calcium carbonate would be likely to occur. It would gradually cease as the standard test solution would absorb CO\({}_{2}\) from the air in the quantities, sufficient for the reaction described by Equation (2). Another possible reason for the abrupt change in the reaction rate during intermittent agitation after 20 days was the development of porosity on the sample surface as the metal was oxidized. Within the framework of these concepts, it was easy to explain the  
Figure 1: Variation of St3 (sample No. 1) corrosion rate in the solution containing NaCl. 1—in static conditions; 2—with agitation. Figure 2: Variation of the corrosion rate of the plate with a weld joint (sample No. 4) in the solution containing NaCl. 1—in static conditions; 2—with agitation. significant difference in the dynamics of the corrosion rate changes over time for cases with intermittent stirring and without it. It was agitation that helped saturate the pores with \"fresh\" portions of the solution. Existing porometry methods require surface dewatering. Unfortunately, drying the sample caused very significant changes in the chemical structure of hydrated oxides on the metal surface. The complete dehydration of surface compounds inevitably leads not only to a change in the size of the pores, but also to a complete change in the relief of the surface. These circumstances caused it to be impracticable to correctly take into account changes in the porous structure of the corrosion products in the solution environment. Due to these reasons, it was also not possible to provide an adequate mathematical interpretation of the diffusion in the case under consideration. It cannot be ruled out that the noted peculiarity of the process with periodic agitation was associated with the transition of the iron ion to the solution and its further completion of oxidation. However, the data on the iron content in the solution (Figures 3 and 4) prevented us from stating this with confidence. Figure 4: The history of steel grade St3 (sample No. 1) corrosion rate in the NaCl-containing solution, determined by iron availability in the saline solution with agitation. Figure 3: The history of steel grade St3 (sample No. 1) corrosion rate in the NaCl-containing solution, determined by iron availability in the saline solution without agitation. Incidentally, it is known that iron oxides have a nonstoichiometric structure, and the degree of corrosion in time depends on the residual porosity (capillarity) of FeO [34].