Corrosion Resistance of Anodic Layers Grown on 304l Stainless Steel at Different Anodizing Times and Stirring Speeds

: Different chemical and physical treatments have been used to improve the properties and functionalities of steels. The anodizing is one of the most promising treatments, due to its versatility and easy industrial implementation, since it allows obtaining different microstructures on the surface, in order to be employed in different industrial sectors. The present work has studied the influence of the anodizing time (15, 30, 45 and 60 min), as well as the stirring speed (0, 200, 400 and 600 rpm), on the morphology and the resistance corrosion behavior of anodic layers grown in 304L stainless steel. The anodic layers were characterized morphologically, compositionally and electrochemically, in order to determine the influence of these parameters on their corrosion behavior in a 0.6 M NaCl saline solution. The results show that the generation of anodic layers at different times propitiates the definition of nonporous morphologies, while the phenomena of dissolution of the layers that increase with the time of anodizing. However, the resistance to corrosion decreases respect to the non-anodized 304L SS. On the other hand, the effect of the stirring speed during the generation of these anodic layers does not seem to influence the corrosion behavior in the saline medium studied.

In photocatalysis applications, anodic layers have been considered a suitable way to achieve larger enhancement of surface area. First studies about semiconducting properties and growth of nanostructured layers on ferrous materials were reported by Prakasam et al. [31] who obtained thicknesses of 300-600 nm and pore diameters range from 50 to 250 nm on iron. Further, Zhang et al. has explored the photocatalytic decomposition of methylene blue [19] and the degradation of azo dye [20] by anodic iron oxides in different nanostructures.
Likewise, the nanostructures growth on iron and its alloys show great promise for water splitting reaction under solar or visible-light illumination [23,24]. Rangaraju et al. [25] studied different nanostructure configurations of photoanodes based on anodic iron oxide for photoelectrochemical water oxidation and they obtained better photocatalytic properties in 1 M KOH with AM 1.5 light illumination when the photoanode result in a two-layered oxide structure, a top layer of nano-dendrite morphology and a bottom layer of nanoporous morphology. Also, these same authors affirm that layer next to the metal substrate imparted corrosion resistance compared with a top layer [26].
On the other hand, in terms of biomedical applications, Asoh et al. [29] reported the growth of anodic layers on type 304 stainless steel using hydrogen peroxide and sulfuric acid that work as a highly suitable host for synthetic hydroxyapatite coating to improve bioactivity in biomedical implant devices; and although corrosion resistance is an important issue in medical devices and applications [32,33] and in other industrial applications, the literature is scarce in the electrochemical characterization of anodic layers on stainless steel.
The range of applications depends on the properties, which can be a tailor and improved by manipulating its nanostructure. Moreover, the morphology and structural characteristics of the anodic layers can be deeply correlated with the anodizing conditions. Controlling parameters such as potential applied, temperature, electrolyte composition and anodizing time is key to the formation of anodic layers [18,[34][35][36].
Various researchers have ventured into the growth of anodic layers on stainless steels, with 316L [25,26] and 304 [27][28][29][30] being the most commonly used. The studies on 316L SS indicate that in organic solutions the addition of water improves the diameter and thickness of the grown layers [37]. In addition, the use of fluoride-free solutions makes it possible to obtain anodic layers composed of a mixture of hydroxides-oxides-sulphates [38]. However, the growth on stainless steel 304 preferentially made in organic media of glycerol or ethylene glycol with NH4F and different additions of H2O, conclude three main points: (1) that the layers have similar Fe, Cr, Ni concentrations of substrate [39]; (2) that the H2O concentration in the anodized solution determines the morphological characteristics of the anodic layers [40]; and (3) that depending on the voltage applied, different growth mechanisms are favored, leading to the generation of anodic layers with different chemical compositions [29].
Nevertheless, most of the existing works so far are focused on the growth and chemical/ morphological characterization of the anodic layers on these ferrous materials [29,35,[39][40][41][42], but not in the electrochemical degradation to which these materials may be subjected . For that reason, the objective of this work is to explore the corrosion electrochemical response of anodic layers grown on 304L SS at different stirring speeds and anodizing time using an ethylene glycol-0.1 M NH4F-0.1 M H2O electrolyte, in order to be able to glimpse what would be its behavior in chlorides rich solutions.

Materials and Methods
Samples of 304L SS with a nominal composition (wt.%) of: 18.3% Cr, 8.11% Ni, 1.52% Mn, 0.27% Si), were ground using successive grades of SiC paper up to 2000 grade, degreased with detergent and rinsed with tap water followed by deionized water.
Anodic layers were formed in a two-electrode cell by anodizing the specimens at constant voltage at 50 V in the electrolyte containing 0.1 M H2O, 0.1 M NH4F and ethylene glycol for 15,30,45 and 60 min at a constant temperature of 5 °C with uniform stirring and platinum mesh was used as cathode. Likewise, anodic layers were grown for 15 min, under the afore-mentioned conditions, modifying the agitation of the solution at 0, 200, 400 and 600 rpm.
Plan view morphology of oxide films was examined by field emission gun scanning electron microscopy (FEG-SEM) utilizing JSM6500F JEOL instrument (JEOL, Tokyo, Japan) equipped with EDX facilities, operated at 15 keV for EDX analysis and 7 keV for secondary electron imaging. Each of the area and local EDX analysis results are quoted as an average of 3 measurements.
The electrochemical measurements were done in triplicate in a conventional three-electrode cell. The working electrodes were the non-anodized and anodized samples, an Ag/AgCl electrode (3 M), was used as reference electrode and the counter electrode was a platinum wire. The electrolyte was a 0.6 M NaCl at room temperature. Corrosion behavior was evaluated by potentiodynamic polarization using a Gamry Reference 600 potentiostat (Gamry Instruments, Warminster, PA, USA). The potentiodynamic curves were conducted at a scan rate of 0.16 mV/s. Before starting the scan, the sample remained in the solution for 15 min to stabilize the open circuit potential (OCP). The potential scan was started in the anodic direction from a potential value of 0.3 V with respect to the OCP to 1 V with respect to the Ag/AgCl electrode or when the sample reached a current density of 0.5 A/cm 2 .

Influence of Stirring Speed on the Growth of Anodic Layers
To investigate the effect of the stirring speed on the growth of the layers, anodized at 4 different stirring speeds (0, 200, 400, 600 rpm) were performed. Figure 1 shows the curves of current density as a function of time recorded during the anodizing treatments. The shape of the potentiostatic curve is typical in the formation of porous morphology anodic layers reported for different valve metals [19,[43][44][45] and it comprises three stages: (I) an accelerated decrease in the current density during the first seconds of the process, related with the growth of a barrier layer on the surface of the material; (II) a gradual increase of the current density associated with the generation of preferential dissolution zones randomly on the surface of the initial layer; and (III) a stability of the current density related to the growth/dissolution of the anodic layer [46].
Moreover, the charge density that passes through the system during the different treatments at 0, 200, 400 and 600 rpm for 15 min is ~0.750, ~0.864, ~1.027 and ~1.061 C/cm 2 , respectively. As from 400 rpm, the anodized curves present an increase in current density and, consequently, in the charge density that may be associated with a greater transport of ionic species from the solution to the metal/electrolyte interface and the acceleration of the chemical dissolution mechanism on the layer as has been reported for the growth of anodic layers on other metallic materials [47,48]. The electrochemical response was studied by polarization curves in 0.6 M NaCl solution, Figure 2. The non-anodized 304L SS substrate present a corrosion potential (Ecorr) of ~110.2 mV vs. Ag/AgCl (3 M), a pitting potential (Epit) of ~834.2 mV vs. Ag/AgCl (3 M) and a passive current density (ipass) of 7.71x10-8 A/cm2. On the other hand, the anodic layers showed an unfavorable electrochemical response against corrosion with respect to the substrate, since the Ecorr was moved to values close to -312.1 mV vs. Ag/AgCl (3 M) while the Epit decreased to ~329.5 mV vs. Ag/AgCl (3 M). Nevertheless, the layer grown at 600 rpm had a much lower pitting potential, with a value of ~140.5 mV vs. Ag/AgCl (3 M), indicating that it is more prone to localized corrosion. In addition, the polarization curves of the anodizing treatments showed narrower passive regions and ipass values around 4.10 × 10 −7 A/cm2, which indicates that the layers grown have less corrosion resistance due to the density of passivation increased approximately one order of magnitude compared to the nonanodized sample.  Figure 3 shows the current density-time curves recorded during the anodizing treatments in the electrolyte at 0 rpm and constant agitation of 600 rpm for different anodizing times (15,30,45, 60 min). The same stages described for anodizing curve at the same time and different stirring speeds are identified, Figure. 1, but the duration of each one varies significantly with the stirring speed.

Influence of Anodizing Time on the Growth of Anodic Layers
As shows in Figure 3a, the first stage associated with the formation of the barrier layer occurs up to 500 s for the anodized at 0 rpm, while at 600 rpm ( Figure 3b) the mass transfer is favored leading to out this stage in 200 s. As was expected, higher current densities were observed during the anodization performed in the agitated electrolyte. This increase is also reflected in the charge density, which rose to 4 times its value for the same anodizing time, going from ~0.810 to ~3.626 C/cm 2 for 15 min and from ~1.031 to ~4.438 for 60 min.
The charge density values obtained allow to estimate a theoretical thickness of the anodic layer assuming that the entire charge is used for the formation of FeOOH, which the literature mentions as the main component of the anodic layers grown in ferrous materials [35,41,49], obtaining thicknesses of compact amorphous FeOOH of 581 nm (15 min), 1.07 μm (30 min), 1.83 μm (45 min) and 2.60 μm (60 min) at 0 rpm; and 739 nm (15 min), 1.43 μm (30 min), 2.27 μm (45 min) and 3.18 μm (60 min) at 600 rpm. The above could indicate that to greater anodizing time and greater stirring speed would be obtaining thicker anodic layers; however, this approach does not contemplate the processes of chemical dissolution or dissolution assisted by the electric field during the generation of the anodic layers, which results in oxide layers with less thickness [42,50].  The increase of anodizing time to 60 min shows the collapse of outer porous structures giving rise to an ordered pattern of nanostructures in the lower anodic layer, which remains adhered to the substrate (Figure 4g,h). This outer layer (Zone 1) has a thickness of approximately 500 nm. These results reveal that the anodizing time controls both the morphology and the pore diameter, obtaining values of 42.56 ± 5.03 at 15 min and 63.0 ± 11.0 nm at 60 min for anodizing.
On the other hand, Figure 5a shows the results of EDS analysis for anodic layer obtained at 0 rpm for different time of treatment. The distribution of the elements is analyzed in two zones: zone 1 corresponds to the outer layer located at the oxide/electrolyte interface; and zone 2 corresponds to the nanoporous layer adhered to the substrate that is observed after 45 min of treatment, Figure 5b. Both zones show a similar trend since the content of F and O elemental increases while the percentage of other elements such as Cr, Fe and Ni tends to decrease with the time of the anodizing process .The above is caused by a fluorine and oxygen enrichment from the electrolyte, which leads to the formation of fluoride compounds, mainly FeF2, FeF3 and FeOOH [40,49]. Nevertheless, at 45 min of treatment, F values decrease abruptly from 52.69% to 17.34% and O values from 12.53% to 4.53% between zone 1 and zone 2, respectively. This suggests the possible presence of an immobile C-O layer that limits the migration of ionic species within the anodic layer [50] as reported in previous works on anodic layers grown on Mg using a glycerol-based electrolyte containing 0.35 M NH4F and 5 vol.% H2O. The presence of the immobile layer would explain the decrease of anions in zone 2 and the cations in zone 1.
The corrosion behavior of the anodic layers is shown in Figure.  Performing of a thermal treatment could be an alternative to improve the electrochemical response of the oxide layers since it allows the obtaining of more stable oxides and the elimination of fluoride inside the anodic layers as they have reported it for layers of Titanium and its alloys [52].

Conclusions
Anodic layers nano-structured are obtained on 304L SS using ethylene glycol-0.1 M H2O-0.1 M NH4F at different stirring speeds and anodizing times. Increasing the stirring speed results in a higher current density response and, as a consequence, an increase in the charge that passes through the system, which is used for the growth of the anodic layers, as well as, in the processes of chemical dissolution and dissolution assisted by the electric field. The increase in anodizing time favors the obtaining of oxide layers with more defined morphologies, superior pore diameters (~63.0 nm) and thicknesses around 500 nm. The chemical composition reveals that exist a dependency with the anodizing time associated with the phenomena of dissolution and migration through the anodic layer, where after 45 min of treatment, a higher content of F and O is identified in the outer zone of the anodic layer. However, the electrochemical responses of the layers grown for all conditions exhibited a less passive and analogous behavior among them. This result is a direct consequence of the high presence of fluoride compounds in the anodic layer that induces a lower corrosion resistance by approximately one order of magnitude with respect to the nonanodized 304L SS substrate.

Conflicts of interest:
The authors declare that they do not have any conflicts of interests.