Electrochemical Corrosion in Bars of AISI 304 Embedded in Concrete Immersed in Marine-Sulfated Environment

— The electrochemical behavior of the corrosion resistance of AISI 304 embedded in concrete manufactured as indicated by the ACI 211.1 method was evaluated. The specimens were exposed for more than 150 days to highly aggressive marine-sulfated environment, solution with a concentration of 5% NaCl and 5% Na 2 SO 4. The electrochemical technique of Resistance to Linear Polarization (Rp) was used for to determine the corrosion rate (I corr ) and monitoring of corrosion potential (E corr ). The E corr and I corr results indicate a high level of corrosion for AISI 1018 steel, on the contrary, the electrochemical behaviour of AISI 304 steel according to the values of E corr and I corr , indicate a corrosion resistance of up to 10 times higher when exposed to an environment with a high concentration of chlorides and sulfates.


I. INTRODUCTION
Worldwide, concrete is the most widely used material in the construction industry, due to its versatility for the manufacture of different structural elements of Civil Infrastructure such as bridges, roads, buildings, industrial warehouses, airports, tunnels, dams, canals, sewage treatment plants, houses, etc. [1]- [4].
However, if at first it was thought that reinforced concrete had unlimited durability, over time it was determined that one of the main causes of premature damage in civil works built with reinforced concrete was the corrosion of reinforcing steel, causing expenses for billions of dollars in the world [5]- [9], in 1986, it was estimated that more than 244,000 bridges in the USA had significant deterioration, the main cause being corrosion of the reinforcing bars of concrete structures [10]- [12].
Initially, reinforcing steel embedded in concrete is naturally protected from corrosion by the high alkalinity of the surrounding medium, pH=12.2 or higher. However, due to the behavior of concrete as a semi-permeable membrane, aggressive substances or ions enter, the most important being chlorides present in marine environments. [13]- [22], and sulfates that may be present in soils contaminated with agrochemicals, wastewater, etc. [23]- [34].
Unlike corrective or secondary actions, such as the use of corrosion inhibitors, galvanized steel, austenitic stainless steels, epoxy coating of the rods, electrochemical removal of chlorides, the production of more durable concrete based on industrial or agro-industrial residues [35]- [41], also pozzolanic materials when used in the construction industry have a beneficial environmental impact because the manufacturing process of Portland Cement is responsible for emitting 6 and 8% of CO2 in the world [42]- [45].
Therefore, the present research evaluated the corrosion resistance of bars of AISI 304 and AISI 1018 embedded in concrete immersed in a marine-sulfated environment.

A. Materials 1) Dosage and Proportioning of Concrete Mixtures
The ACI 211.1 method was used to determine the amount of materials for the preparation of the concrete mix. [46]. ASTM standards were used to determine the physical characteristics of the fine and coarse aggregates [47]- [50], see Table I.
A water/cement ratio = 0.65 was used for the design of the concrete mix.

B. Method 1) Quality control test of concrete mixture
The ASTM and ONNCCE standards were used to carry out the control tests of fresh and hardened concrete [51]- [54], the results obtained are within the specifications for conventional concrete, see Table III. 2) Characteristics of reinforcing steel In each specimen, two bars of AISI 304 and AISI 1018 were embedded, both to be used as working electrodes (WE) and auxiliary electrode (AE), and a third AISI 304 steel bar was embedded, but with a diameter of 1/8" as an auxiliary electrode. In the three bars was placed 5 cm strip of fluorocarbon tape on the upper part, to avoid differential aeration zones, the concentration of salts or crevice corrosion and an area susceptible to corrosion is delimited as indicated in the literature [55], see Fig. 1.

3) Nomenclature of the specimens
For the evaluation of corrosion, the nomenclature was assigned to the study specimens according to the type of steel and the exposure medium. Specimens exposed medium control (Water) and Marine-Sulfated environment (solution at 5% NaCl and 5% Na2SO4), see Table IV. • MS= Marine-Sulfated environment (solution at 5% NaCl and 5% Na2SO4).

4) Experimental arrangement
The ASTM G59 standard was used to determine the corrosion rate [56], using a potentiostat/galvanostat with a three-electrode arrangement, according to what was reported in the literature [57]- [58]. The characteristics of the concrete specimens that were used to evaluate the corrosion rate are detailed in Fig. 2, two WE (bars AISI 304 and AISI 1018 steel) and one auxiliary electrode (AE) of AISI 304. The three-electrode arrangement is also used in other electrochemical techniques but is used to a lesser extent in the steel-concrete system and more used in other industrial areas [59]- [60].

A. Corrosion Potential (Ecorr)
The interpretation of the Ecorr results was carried out according to what is indicated in the ASTM C-876-15 standard [61] and in the literature [62], Table V shows the parameters for the analysis of the corrosion potentials of the present study.  Fig. 3 shows the behavior of corrosion potentials (Ecorr) of the specimens 304-C and 1018-C. The specimen 304-C reports values of corrosion potentials in the first 28 days from -170 mV to -192 mV, to maintain a stable behavior throughout the evaluation period with more positive values at -200 mV, which indicates a risk of corrosion of 10%. In the case of the specimen 1018-C has Ecorr values of -358 mV in in the same period, which would indicate a risk of corrosion of 90% to pass to more positive values, presented on day 28 a more noble corrosion potential of -292 mV, behavior associated with the formation of the passive layer, so there is a behavior with a tendency to passivation with the passage of time, remaining in an Ecorr range between -190 mV to -230 mV, behavior reported in literature because it is the nonaggressive control medium [63].
The results of the corrosion potentials of the specimens 304-MS and 1018-MS are presented in Fig. 4, when exposed to a sulfated marine environment (solution at 5% NaCl and 5% Na2SO4). The corrosion potential monitoring period was for more than 150 days, where the 304-MS specimen presented potentials of -174 mV to -162 mV from day 14 to 28, to maintain Ecorr more positive during the time of the electrochemical tests, with Ecorr values from -145 mV to -160 mV, indicates a 10% of corrosion risk. The specimen 1018-MS presents Ecorr from -430 mV to -390 mV in the first 28 days, with an Ecorr of -332 mV for day 112, reporting Ecorr greater than -350 mV, which is associated with a 90% risk of corrosion at the end of the exposure time, initiating the activation of the reinforcing steel when exposed to a marinesulfated environment, the results coincide with investigations in similar media in exposure media with a high concentration of chlorides and sulfates [64]- [65]. Fig. 3. Ecorr of AISI 304 vs AISI 1018 concrete exposed to control environment. Fig. 4. Ecorr of AISI 304 vs AISI 1018 concrete exposed to marine-sulfated environment.

B. Corrosion Current Density (Icorr)
To determine the level of corrosion present in the study specimens, DURAR NETWORK Manual criteria were used, see Table VI [66].  Fig. 3 presents the behavior of monitoring of intensity corrosion current Icorr of specimen 304-C and the specimen reinforced with 1018-C. It is found that specimen 304-C reports an Icorr of 0.038 µA/cm 2 for day 14, decreasing to 0.029 µA/cm 2 on day 28, normal behavior in the first four weeks of exposure to the control medium, to reach an Icorr of of 0.011 µA/cm2 in the day 56, and remain stable until the last monitoring in an Icorr range of 0.009 to 0.01 µA/cm 2 , values 10 times less than 0.1 µA/cm 2 , which represents the absence of corrosion in the evaluated system, according to what is indicated in Table VI.
For specimen 1018-C, a similar behavior is observed, with relatively high values of Icorr beginning to reach values that indicate a negligible corrosion level over time, however, the influence of the type of steel is observed in the Icorr ranges obtained, given that in the curing stage the values reported by specimen 1018-C are 10 times higher than specimen 304-C, with an Icorr from 0.30 µA/cm 2 to 0.20 in the first 28 days, ending with a value of 0.07 µA/cm 2 until the end of the exposure period. This behavior that coincides with what has been reported by some research [67]- [68].   6 presents the results of 304-MS and 1018-MS specimens, when exposed to a sulfated marine environment (solution at 5% NaCl and 5% Na2SO4) for more than 150 days. The specimen 304-MS reports in the curing stage values of Icorr from 0.034 to 0.026 µA/cm 2 , to decrease to 0.014 µA/cm 2 in the day 56, to present until the last day of the electrochemical test, Icorr values of 0.011 to 0.013 µA/cm 2 , with a variation of only 0.001 to 0.003 µA/cm 2 compared to specimen 304-C exposed to the non-aggressive medium (water), it is found that the 304-MS specimen exposed to the highly aggressive marine-sulfated environment offers great resistance to corrosion, behaving as if the exposure medium were not aggressive, this has been reported in various investigations worldwide, however, the number of studies on concrete durability in media with such a high concentration of NaCl and Na2SO4, are significantly less [69]- [70].  6. Icorr of AISI 304 vs AISI 1018 concrete exposed to marine-sulfated environment.
On the contrary to the great anticorrosive efficiency to the aggressive medium reported by the specimen with AISI 304 Stainless Steel, 304-MS, the specimen 1018-MS presented a good behavior in the curing stage, with Icorr of 0.28 to 0.16 µA/cm 2 , with a tendency to lower values of Icorr reporting on day 98 Icorr of 0.09 µA/cm 2 , to later report constant increases in Icorr until reaching at the end of the corrosion tests, day 150, an Icorr of 0.17 µA/cm 2 , but with an upward trend, behavior that indicates a much lower resistance when the this specimen is exposed to a highly aggressive environment (solution at 5% NaCl and 5% Na2SO4). Due to the results obtained in the present investigation, the intelligent use of AISI 304 stainless steel is justified for the construction of durable concrete structures, which would allow significant and sustainable savings to increase up to two times the useful life.

IV. CONCLUSIONS
The behavior in the control medium of both AISI 304 and AISI 1018 steels, is of a tendency to Ecorr values that indicate 10% risk of corrosion and Icorr values that confirm the level of negligible corrosion, with Icorr values below of 0.1 µA/cm 2 , with AISI 304 steel presenting the lowest values in all the monitoring.
The specimens with AISI 304 steel presented a great resistance to corrosion of more than 10 times when exposed to the marine-sulfated environment of the present study, reporting similar values of Icorr a of the specimens exposed to the control medium with only a difference of between 0.001 to 0.003 µA/cm 2 .
The use of AISI 304 stainless steel is recommended as reinforcement in concrete structures that will be exposed to environments with high concentrations of chlorides and sulfates, using it intelligently in critical areas of each structure.
ACKNOWLEDGMENT MA Baltazar-Zamora thanks to the National System of Researchers (CONACYT) for the support.