3.1. Corrosion Potential
Figure 2 shows
Ecorr versus exposure time of AISI 1018 CS embedded in concrete for specimen 4AN (100% CPC) (control specimen), specimen 4BN (80%CPC + 20% SF), and specimen 4CN (80% CPC + 20% SF). The evolution of
Ecorr with time for 4AN (control specimen) was analyzed according to the ASTM C 876-15 standard (see
Table 5) [
36,
37]. Thus, the
Ecorr values throughout the exposure time were in the severe corrosion region, with values from −580 mV to −480 mV vs. CSE from day 60 to 180. At day 210, an
Ecorr value of −415 mV vs. CSE was reported, and, during the last weeks (365 days),
Ecorr tended toward a more active value, −550 mV vs. CSE, in agreement with the literature [
42], reporting that reinforced concrete exposed to complete or partial immersion in 3.5% NaCl solution presents
Ecorr values between −550 mV and −600 mV vs. CSE from day 50 to 275. It is noted that, in the referenced study, a concrete specimen with 15% FA was evaluated [
42], presenting an
Ecorr value of −400 mV vs. CSE for 125 days of experimentation.
It was considered that the corrosion criteria (
Ecorr) indicated in
Table 5 could be used to analyze the different systems in the present study and an AISI 304 SS reinforcement.
Figure 2 shows that
Ecorr values were in the range of −250 mV to −600 mV vs. CSE from the first week up to 365 days of immersion in 3.5 wt.% NaCl solution. The samples made with AISI 1018 CS and SF (specimen 4BN) or FA (specimen 4CN) showed severe corrosion and similar behavior to those made with 100% CPC (specimen 4AN), with
Ecorr values ranging from −600 mV to −450 mV vs. CSE from day 60 to 210. For the final weeks (360 days),
Ecorr values remained between −380 mV and −480 mV vs. CSE for specimens 4AN and 4BN (not presenting severe corrosion) and at a value of −600 mV vs. CSE for specimen 4CN (indicating severe corrosion). At the end of the experiment (385 days), the three specimens showed the probability of severe corrosion.
Concrete specimens reinforced with AISI 304 SS (
Figure 3) made with 100% CPC (specimen 4AI), 80% CPC + 20% SF (specimen 4BI), and 80% CPC + 20% FA (specimen 4CI) showed that, during the first weeks (curing step) until day 180, the three specimens presented a 10% probability of corrosion, with
Ecorr > −200 mV vs. CSE. Specimen 4BI presented an uncertain corrosion probability from day 210 to 245 with
Ecorr values between −300 mV and −286 mV vs. CSE, associated with a period of uncertain corrosion probability or small rupture (initiation of pitting corrosion) of the passive layer [
43,
44,
45,
46,
47]. From day 300 to 360, a passivation process was generated, with
Ecorr vales of −200 mV vs. CSE. The
Ecorr values for specimen 4CI with 80% CPC + 20% FA presented a more stable
Ecorr value than specimens 4AI and 4BI, from day 30 to 365, where their
Ecorr values remained in a range from −120 mV to −190 mV vs. CSE, indicating a 10% probability of corrosion.
Crouch et al. [
48] stated that one of the most attractive properties of FA is its influence on the improvement of durability, which is the result of the reduction in calcium hydroxide, the most soluble of the hydration products, and changes in the pore solution. In the case of specimen 4BN with 20% SF, the protection provided to the reinforcing steel against corrosion may be attributed to the SF, which reacts with calcium hydroxide released during the hydration of the cement and forms additional hydrated calcium silicate (C–S–H), which improves the durability and mechanical properties of the concrete [
49]. Comparison of the
Ecorr values in
Figure 2 and
Figure 3 indicates that a difference in
Ecorr behavior can be observed, with the AISI 1018 CS reinforced specimens (4AN, 4BN, and 4CN) (
Figure 2) exhibiting non-efficient corrosion resistance regardless of the addition of SF or FA. These specimens had more active potentials than −500 mV vs. CSE, indicating 90% probability of corrosion and severe corrosion. In contrast, the concrete specimens reinforced with AISI 304 SS (4AI, 4BI, and 4CI) (
Figure 3) had
Ecorr values corresponding to a 10% probability of corrosion and uncertainty of corrosion.
3.2. Corrosion Kinetics
Figure 4 and
Figure 5 show
icorr versus exposure time.
Figure 4 shows AISI 1018 CS embedded in concrete for specimen 4AN (100% CPC) (control specimen), specimen 4BN (80% CPC + 20% SF), and specimen 4CN (80% CPC + 20%FA). The
icorr values obtained using Equation (1) could be used to obtain the corrosion rate (CR). As indicated above, the criteria used to analyze the
icorr results for specimen 4AN (control specimen) (
Figure 4) were based on the state of corrosion of carbon steel in Portland cement-based concrete reported in Reference [
41], as shown in
Table 6. It was considered that the corrosion criteria indicated in
Table 6 could be used to interpret the
icorr of the different systems in the present study, for partial replacement of CPC by SF or FA, and for AISI 304 SS reinforcement.
Figure 4 shows
icorr results for the three types of specimens reinforced with AISI 1018 CS. During the curing period,
icorr values from 1 to 3 μA/cm
2 could be observed. In general, it was seen that the medium was highly aggressive for specimens 4AN and 4BN, with
icorr values from 3 to 8 μA/cm
2 up to day 105, decreasing to 0.8 μA/cm
2 for specimen 4BN at day 210 because of the passivation of the AISI 1018 CS reinforcement. Specimen 4AN presented a decrease of
icorr from 12 to 3 μA/cm
2 from day 140 to 245.
The specimen containing 20% FA (specimen 4CN) showed a tendency toward more active
icorr values than specimens 4AN and 4BN until day 280, where the corrosion behavior was similar to that of specimen 4AN, with values above 10 μA/cm
2 indicating high corrosion. Concrete specimens reinforced with the AISI 1018 CS presented critical
icorr values above 10 μA/cm
2; this corrosive aggressiveness to reinforced concrete was also demonstrated in soils with a concentration of 3 wt.% NaCl, presenting, for the reinforcing steel AISI 1018 CS, values of
icorr between 3.3 μA/cm
2 and 3.6 μA/cm
2 after 260 days of exposure to the soil environment [
50].
Figure 5 shows
icorr results for AISI 304 SS embedded in concrete for specimen 4AI (100% CPC), specimen 4BI (80% CPC + 20% SF), and specimen 4CI (80% CPC + 20% FA). The
icorr results for AISI 304 SS showed lower values than for the AISI 1018 CS reinforced specimens (see
Figure 4). The specimen made with concrete containing 100% CPC (specimen 4AI) had an
icorr of less than 0.1 μA/cm
2 until day 105; then, at day 140, it was depassivated with an
icorr of 2 μA/cm
2. From day 175 to 350, it was in the passive state, presenting
icorr values lower than 1 μA/cm
2, thus indicating a low corrosion level. It was reported that the passive film formed on AISI 304 SS consists of a duplex layer structure, with an inner layer of chromium oxide covered by an iron-oxide outer layer [
43,
44,
47]. In the alkaline environment of the present study, the high Ni content of the AISI 304 SS reinforcement (8.13 wt.%) may have contributed to the corrosion resistance [
46].
In the last monitoring period (365–385 days), specimen 4AI manufactured with AISI 304 SS presented an
icorr value of 3 μA/cm
2 (see
Figure 5), showing a benefit in accordance with Bautista et al. [
51], indicating that the use of stainless steel in environments with high chloride content is one of the only viable options for achieving more durable reinforced concrete structures. Knudsen et al. [
52] also noted that the intelligent use of stainless steel is advisable to repair damaged structures in coastal zones and is a cost-effective option. For a conventional building of 40 flats (~80 m
2 each) the use of AISI 304 SS instead of the conventional AISI 1018 CS had an additional structure cost of the order of 5–10% [
53].
Specimen 4BI with 20% SF showed better corrosion behavior than specimen 4AI manufactured with 100% CPC, with
icorr values of 0.03 μA/cm
2 until day 105 and less than 1 μA/cm
2 from day 280 to 350. Specimen 4CI containing 20% FA exhibited similar corrosion behavior to specimen 4BI, with lower
icorr values during the monitoring period compared to the specimen made without addition of admixtures (specimen 4AI). A comparison between the specimens reinforced with AISI 1018 CS and AISI 304 SS (see
Figure 4 and
Figure 5), embedded in concrete without addition and with a mixture of 20% SF or FA, indicates that, after 385 days of immersion in 3.5 wt.% NaCl solution, there was a benefit in terms of corrosion behavior in using the specimens reinforced with AISI 304 SS rather than with AISI 1018 CS. This benefit was further increased by partially replacing CPC with SF or FA, whereby the specimens made with SF showed better corrosion behavior.