**3. Results and Discussion**

All the obtained polarization curves have a course characteristic of steel in the passive state (Figure 3). In all cases, the gamma irradiation of the samples results in a shift of the corrosion potential values towards more negative potentials, indicating an increase in the probability of corrosion. The steel in all mortars is characterized by low jcor values. However, only the non-irradiated mortars in cans in an environment with an RH = 50% jcor value met the requirements for steel in the passive state (jcor < 0.1 μA/cm2; negligible corrosion level). In other cases, the jcor value indicates a low corrosion level. The corrosion current density increased after irradiation (Table 1), which means that the steel in irradiated mortars has a higher corrosion rate. In the case of samples stored in environment with 100% RH, gamma radiation caused a small increase in current density (1.1–1.3 times), while in the case of RH 50%, significantly higher jcor values (4–12 times) for steel in irradiated samples were observed. The effect of RH on the course of the polarization curves of steel in irradiated mortars was small—the courses of polarization curves of steel in samples in environments with 50 and 100% RH were comparable. In contrast, in the case of non-irradiated samples, significantly lower values of jcor and anodic current density were observed for 50% RH than for 100% RH. The highest values of jcor and jp were obtained for steel in mortar stored under laboratory conditions, which demonstrates the worst protective properties of the passive layer and the highest corrosion rate of steel. It may be related to the unrestricted access of CO2 to the samples stored in air, while other samples were sealed in cans.

**Table 1.** Characteristic parameters of polarization curves in plain mortars (50/100—RH value; ir—irradiated; can—in a can in a climatic chamber; air—laboratory conditions).


**Figure 3.** Polarization curves for steel rebar embedded in Portland cement mortar (50/100—RH value; ir—irradiated; can—in a can in a climatic chamber; air—laboratory conditions).

The characteristics of the steel in mortars on the Bode plots of the EIS spectra for samples conditioned under five different conditions are shown in Figure 4. The shape of the EIS spectra (wide phase angle peak) and high impedance at a low frequency indicates the presence of a passivation layer on the steel surface. The gamma radiation is responsible for shifting the impedance and phase angle to smaller values in the low frequency range. The above-mentioned effects are observed for all investigated specimens and indicate a deterioration in the quality of the passivation layer.

**Figure 4.** EIS spectra: (**a**) impedance, (**b**) phase shift for steel in mortars (points—measurement data; lines—fitting for equivalent circuit from Figure 2; 50/100—RH value; ir—irradiated; can—in a can in a climatic chamber; air—laboratory conditions).

The parameters determined on the basis of the equivalent circuit and the EIS measurement showed similar conclusions compared to polarization curve parameters (especially jcor). The polarization resistance Rp (Rct + Rpas) is the highest for the non-irradiated specimen at 50% RH (Table 2), which should be interpreted as the best protection of the passivation layer among the analysed specimens. In contrast, the smallest Rp values were present in mortar stored in air (air). The gamma irradiation of steel in mortar caused a decrease in Rp values—it was larger for RH = 50% than for RH = 100%—which indicates an acceleration of steel corrosion. It should be emphasized that the interpretation of changes takes place at a low level of steel corrosion. Increasing RH from 50% to 100% decreased Rp and therefore accelerated corrosion. It can be seen that the change in Rp due to an increase in RH was small for irradiated samples and significant for non-irradiated samples.


**Table 2.** Electrical circuit parameters for steel in mortar (50/100—RH value; ir—irradiated; can—in a can in a climatic chamber; air—laboratory conditions).

The gamma irradiation of mortars with steel bars caused a three-fold increase in capacity of the double layer (Ydl) compared to non-irradiated specimens stored under 50% RH conditions. The physical interpretation of such a determination is an increasing thickness of the electrical double layer and consequently loses the stability of the passivation layer and iron dissolution [25]. In the case of fully saturated mortar (RH-100), the opposite relationship was observed—Ydl after gamma irradiation decreased by about 15–30%. A 15% decrease in the *n* parameter is also observed for steel in mortar at 50% and exposure to gamma radiation. This effect is due to the reduced uniformity of the passive layer on steel, and thus a degradation of its protective properties. In the case of RH-100 mortar, no effect of gamma radiation on the *n* parameter was observed.

#### **4. Conclusions**

The electrochemical parameters of steel rods in mortar conditioning in specific environmental conditions (the dose of gamma radiation reached 1.8 MGy and environmental conditions favoured the progress of carbonation phenomena) result in the following conclusions:


**Author Contributions:** Conceptualization, M.D.; methodology, M.D. and M.A.G.; validation, M.D.; formal analysis, M.A.G.; investigation, M.D., J.K., K.D.; data curation, M.D. and J.K.; writing original draft preparation, M.D.; writing—review and editing, M.D., J.K.; visualization, M.D. and J.K.; supervision, M.A.G.; project administration, M.A.G.; funding acquisition, M.A.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Centre for Research and Development, Poland, grant number Project V4-Korea/2/2018.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** Publication cost of this paper was covered with founds of the Polish National Agency for Academic Exchange (NAWA): "MATBUD'2023—Developing international scientific cooperation in the field of building materials engineering" BPI/WTP/2021/1/00002, MATBUD'2023

**Conflicts of Interest:** The authors declare no conflict of interest.
