## 3 Results
### Analysis of Chemical Corrosion by Linear and Cyclic Voltammetry  
Tafel diagrams and cyclic voltammograms were presented to exemplify how to evaluate the electro-chemical parameters of the corrosion process generated by the unaltered 2.5% CMC quenching medium and the thermally fatigued one for the selected steel (Figure 7). In the case of linear voltammetry, the parameters according to which we could appreciate the degree of corrosion of an environment with respect to a certain metal were the corrosion potential (Ecor (mV)); the oxidation-reduction potential on the Tafel diagram; the anodic branch ba (mV), which provided indications about the oxidizing characteristics of the environment; the cathodic branch bc (mV), which provided indications about the reducing characteristics; the corrosion rate (Vcor (mm/year)); and the current density (Jcor (mA/cm\({}^{2}\))). From the Tafel slopes, the transfer coefficient could be determined, which was different from 0.5 in the case of slow reactions. From the Evans diagram, it could be seen that the anodic branch of the polarization curve had a steeper slope than the cathodic branch in both cases. The high value of the ba constant, especially at CMC 2.5% (ba = 1023 mV, thermally fatigued) indicated that the anodic oxidation process was high, so the spent quenching medium had a strong oxidizing effect. It was observed that the altered quenching medium had more significant oxidation characteristics (ba = 1023 mV) than the fresh quenching medium (ba = 773 mV). The reduction potential was higher (cathodic branch; bc = \(-\)399 mV) for the 2.5% CMC quenching medium unaltered by thermal wear compared with the reduction potential of the worn 2.5% CMC (bc = \(-\)380 mV). The corrosion potential provided  
Figure 6: FTIR spectrum of CMC and CMC 50.  
indications about the thermodynamic stability of the metal-corrosive medium interface and specified the physical-kinetic conditions for breaking through the superficial layer for the appearance of the corrosion effect. The instantaneous corrosion current indicated that the current flow that was present at the equilibrium potential was E = Eo (when the superficial film on the metal sample was pierced). In the used medium, it was Ecor = 1102 mV, whilst in the fresh medium, it was Ecor = 1028 mV. The higher the Jcor, the higher the effort required to damage (corrosion) the surface layer. From Table 4, it can be seen that the Jcor for unaltered 2.5% CMC was 0.1 mA/cn\({}^{2}\), whilst for the treated medium with 50 thermal cycles, it was 0.28 mA/cm\({}^{2}\), which was significantly high. The higher the current density (Jcor), the lower the polarization resistance, so the more aggressive the liquid medium was on the metal surface. Figure 7: Linear voltammetry (Tafel curves) and cyclic voltammetry for the different types of quenching media. The polarization resistance is used to determine the instantaneous corrosion current that occurs at the metal-corrosive medium interface and cannot directly be measured by electro-chemical methods. The polarization resistance provides indications about the thermodynamic stability characteristics of the metal. It can be seen from Table 4 that the metal-CMC clean interaction was \(\text{Rp}=594\)\(\text{ohm}/\text{cm}^{2}\), whilst the CMC-used metal action was \(\text{Rp}=454\)\(\text{ohm}/\text{cm}^{2}\), which highlighted the corrosive nature of the thermally fatigued biological tempering environment. The corrosion rate was determined as the thickness of the metal layer removed by corrosion per unit time. From the table, it can be seen that the corrosion rate was 2.5 times higher in the case of the altered quenching medium compared with the clean quenching medium. This may have been due to the decrease in molecular mass and the formation of acids in the CMC 50 solution following the stronger thermal degradation of the CMC compound [22]. Cyclic voltammetry showed the nature of the corrosion.