Electrochemical Evaluation of AISI 304 SS and Galvanized Steel in Ternary Ecological Concrete based on Sugar Cane Bagasse Ash and Silica Fume ( SCBA-SF ) exposed to Na 2 SO 4

In the present research, was studied the electrochemical behavior of AISI 304 stainless steel and Galvanized Steel embedded in Ternary Ecological Concrete made with partial substitution of Portland Cement (PC) by combination of Sugar Cane Bagasse Ash and Silica Fume (SCBA-SF) in 10, 20 and 30% and exposed to a 3.5% solution of Na2SO4 as an aggressive medium. For the design of the concrete mixtures was used ACI 211.1 method. Quality control tests of fresh and hardened concrete were carried out in accordance with the ONNCCE and ASTM standards. The electrochemical evaluation was carried out for a period of 6 months, using the techniques of corrosion potential Ecorr (ASTM C-876-15) and Linear Polarization Resistance-LPR(ASTM G59) to determine the corrosion rate Icorr. The results indicate that AISI 304 SS has a high corrosion resistance from the curing stage to the end of monitoring, with values of Ecorr lower than -200 mV and negligible corrosion levels with values of Icorr below 0.1 μA/cm, greater protection is identified in the Ternary Ecological Concrete with replacement of 30% of PC by SCBA-SF.


I. INTRODUCTION
Corrosion is a phenomenon that destructively attacks reinforced concrete structures, being one of the main factors causing the reduction or shortening of their useful life, durability and operation [1][2]. This problem is due to the exposure of structures in environments where aggressive or deactivating ions such as sulfates and chlorides are found. [3][4]. It is known that the problem of corrosion in reinforced structures is for the community responsible for such civil works a problem of billions of dollars, however, in Mexico there are no data to give us an idea of the problem [5][6]. Among the causes causing the onset of corrosion are sulphates ions. The presence of sulphates in water that is in contact with a hardened cement paste, can considerably increase solubility of the components of such paste and cause, on the one hand, the development of the degradation of concrete by leaching and on the other hand the steel is left unprotected and that is where the corrosion process is triggered [7][8].
The objective of present research is to evaluate the corrosion behavior of bars of Galvanized Steel and AISI 304 stainless steel embedded in Conventional Concrete (CC) and Ecological Concrete (EC) made with partial substitution of Portland Cement (PC) by combinations of Sugar Cane Bagasse Ash (SCBA) and Silica Fume (SF) in different percentages (10%, 20% and 30%), agroindustrial and industrial waste of pozzolanic characteristics as indicated in various investigations [9][10].
All specimens of four concrete mixtures CC and TEC were exposed in sodium sulphates solution at 3.5%, as an aggressive medium. The dosage of the concrete mixtures, Comun Concrete (CC) and Ternary Ecological Concrete (TEC) was carried out according to the ACI 211.1 [11], which is based on the physical characteristics of the aggregates fine and coarse. Table I shows the results of the physical characterization of the aggregates used in the present study, the tests were performed according to the ONNCCE standards.

2) Dosage of Sustainable Concrete mixtures.
For the dosing of the four concrete mixtures (MC, M10, M20, M30), a ratio w/c = 0.65 was used.

B. Method 1) Characterization of Fresh and Hardened of Ternary Ecological Concretes
According to the ONNCCE and ASTM standars [12][13][14][15], the characteristics of the fresh state concrete and its mechanical strength (Hardened Concrete) were determined. See Table III. 2) Characteristics of the reinforcement bars AISI 304 SS and Galvanized steel bars were used as reinforcement in the study specimens made with Comun Concrete and Ternary Ecological Concretes. Each of the bars were placed 5 cm paint at the top and 5 cm at the bottom, in order to delimit the area of exposure to corrosion of steel in concrete with a length of 5 cm, see Fig. 1, as they have reported other researchers [16][17][18]. All specimens were made according to standard ASTM C 192 [19].

3) Nomenclature of study specimens
The nomenclature used to keep track of the electrochemical monitoring of Ecorr and Icorr of and AISI 304 and Galvanized Steel as reinforcement in Ternary Ecological Concrete exposed to sodium sulfate as aggressive medium, is shown in Table IV.

) Electrochemical cell
For the evaluation of corrosion, galvanized steel bars and steel AISI 304 were embedded in each study specimen, these steel bars were used as working electrode (WE), a third bar of AISI 304 steel and 1/8" diameter, was also embedded as an auxiliary electrode (AE), and using a standar copper-copper sulfate (Cu/CuSO₄) as reference electrode (RE), this type of arrangement or electrochemical cell, allows to evaluate the corrosion current density (Icorr) by the technique of linear polarization resistance (LPR) as indicated by the ASTM-G59 standard [20], see Fig. 2. The tests were performed with equipment Gill AC Galvanostat/Potentiostat/ZRA from ACM Instruments, as they have done in different investigations [21][22][23][24][25].

A. Corrosion potential (Ecorr)
The standard ASTM C876-15 [26], considering a more interval according to the literature [27], was used to perform the monitoring of the corrosion potential (Ecorr) and interpretation of the probability of corrosion (see Table V).

1) Ecorr behavior AISI 304 SS and Galvanized Steel in control medium
The Fig. 3 present Ecorr values of galvanized steel and the AISI 304 Stainless steel embedded in the four concrete mixtures. In the curing stage all specimens with bars of AISI 304 present corrosion potential of -144 mV to -86 mV at 90 days, this values of Ecorr indicate 10% probability of corrosion, maintains a trend of values up to 180 days. In the case of specimens with galvanized steel, during the days of curing stage the MC-GAL, M20-GAL and M30-GAL presented Ecorr values from a range of -610 mV to -463 mV, however the MC-10 specimen presented Ecorr values in the curing stage of -765 mV a -755 mV. All the specimens after the curing stage presented Ecorr values with a tendency to more positive reaching values of -340 mV and 435 mV for day 90, to continue with the trend and end the monitoring period with Ecorr values between -290 mV and -340 mV, indicating according to ASTM C-876-15, uncertainty of the presence of corrosion.

2) Ecorr behavior AISI 304 SS and Galvanized Steel in Na2SO4
In Fig. 4 show the performance of galvanized steel and AISI 304 stainless steel 304 in the presence sodium sulphates, All the specimens with bars of AISI 304, MC-304, M10-304, M20-304 and M30-304, show the tendency from more negative to more positive values in the curing stage, going from -180 mV a -100 mV, to present during the entire period of exposure to sodium sulphates,, Ecorr values more positive than -100 mV, indicating a 10% probability of corrosion or passivity of the steel, in agreement to ASTM C-876-15 standard. In the case of specimens with galvanized steel as reinforcement, MC-GAL, M10-GAL, M20-GAL and M30-GAL, they also have a tendency towards more positive corrosion potential values from the curing stage, going from values from -630 mv to -482 mV for day 28, continuing with a very homogeneous trend in all specimens, presenting throughout the period of exposure to sulfate of sodium, stable Ecorr values in a range of -360 to -380 mV, indicating according to ASTM C-876-15, 90% probability of corrosion, evidencing the sulfate attack when comparing the values of Ecorr reported in the control environment, see Fig. 3.

B. Corrosion Current Density (Icorr)
The criteria of the the DURAR Network Manual [28], were used to interpret the results of the Corrosion Current Density (Icorr), see Table VI.  Table VII shows the corrosion rate of the study specimens with Galvanized Steel and AISI 304 Stainlesss Steel, embedded in Comun Concrete and Ecological Concrete exposed for 180 days to the control medium.  20 and 30% substitution of PC by SCBA-SF (M20-304 and M30-304), with values of 0.004 and 0.003 µA/cm 2 , in the case of galvanized steel specimens, the MC-GAL and M10-Gal specimens have Icorr values less than 0.1 µA/cm 2 , indicating a negligible corrosion level or passivation, but the specimens with M20-GAL and M30-GAL have Icorr values greater than 0.1 µA/cm 2 , indicating a moderate level of corrosion. As the literature indicates, this behaviour of the Icorr values of negligible levels of corrosion are expected in non-aggressive environments [29][30].

2) Icorr behavior AISI 304 SS and Galvanized Steel in Na2SO4
Table VII shows the corrosion rate or Icorr presented by the study specimens after 180 days of exposition to a 3.5% solution of Na2SO4, as an aggressive medium, all the specimens with AISI 304 SS, MC-304, M10-304, M20-304 and M30-304, have excellent corrosion performance with values between 0.005 to 0.015 µA/cm 2 , Icorr values indicating a negligible level of corrosion, presenting the specimens made with Ecological Concrete based on 20 and 30% of replacement of the PC by SCBA-SF the highest resistance. This indicates a contribution to the sulphate corrosion resistance of SCBA and SF, the benefit of the use of this residual agroindustrial, which is consistent with what was reported in other investigations [31].
In the case of specimens with galvanized steel and exposed to sodium sulphate, corrosion resistance or behavior in this aggressive environment is good in the MC-GAL and M10-GAL specimens, with Icorr values less than 0.1 µA/cm 2 , indicating a negligible corrosion level or passivation, but the specimens with M20-GAL and M30-GAL sulphate attack is observed, as Icorr values increase have to 0.209 and .207 µA/cm 2 , twice what was reported in the control medium and indicating a moderate level of corrosion. The results of increased corrosion of galvanized steel are consistent with those reported in other investigations [32][33][34], which indicate a good protection of galvanized steel, but which depends a lot on the exposure conditions or the aggressiveness of the contact environment.

IV. CONCLUSION
Corrosion resistance in sulfate environments is good in the MC-GAL and M10-GAL specimens, with Icorr values less than 0.1 µA/cm 2 , indicating a negligible corrosion level or passivation.
The specimens with M20-GAL and M30-GAL present Icorr values increase have to 0.209 and .207 µA/cm2, twice what was reported in the control medium and indicating a moderate level of corrosion and signs of sulfate attack after 180 days of exposure.
The results of increased corrosion of galvanized steel are consistent with those reported in the literature, which indicate a good protection of galvanized steel, but which depends a lot on the exposure conditions or the aggressiveness of the contact environment.
The specimens made with Ecological Concrete based on 20 and 30% of replacement of the PC by SCBA-SF presented the highest resistance, this indicates a contribution to the sulfate corrosion resistance of SCBA and SF, agroindustrial and industrial wastes, in addition to contributing to a reduction of CO2 emissions, derived from the manufacture of Portland cement and which are responsible for 7% of total emissions worldwide.
It can be concluded by last, that it is feasible to use Sugarcane Bagasse Ash in combination with Silica Fume for the manufacture of Ternary Ecological Concrete resistant to sulphates corrosion