Hydrophobicity and Improved Localized Corrosion Resistance of Grain Boundary Etched Stainless Steel in Chloride-Containing Environment

Localized corrosion of stainless steels by chloride ions in seawater leads to metal degradation while fouling of marine organisms increases the occurrence of localized corrosion. We describe a simple method to increase hydrophobicity of austenitic stainless steel using grain boundary etching that can also inhibit adhesion of bio-organisms present in seawater as well as increase the localized corrosion resistance of stainless steel in chloride-containing aqueous environments. This paper describes the corrosion behavior of stainless steel as a result of grain boundary etching to achieve hydrophobicity. Potentiostatic polarization on stainless steel 316L in a nitric acid solution at an anodic potential of 1.3 V vs. saturated calomel electrode (SCE) results in a grain boundary etched structure and a Cr- and Mo-rich passive ﬁlm as conﬁrmed by scanning electron microscopy and X-ray photoelectron spectroscopy. This modiﬁed stainless steel 316L surface exhibits enhanced corrosion resistance to a 0.6 M sodium chloride solution. Speciﬁcally, potentiodynamic polarization studies indicate that the breakdown potential increases and the sample-to-sample variability decreases. The modiﬁed surfaces show a narrow range of breakdown potentials (0.96 to 1.05 V vs. SCE) compared to as-received stainless steel 316L (0.32 to 0.86 V vs. SCE).

Due to the ubiquitous use of metals in infrastructure, production and manufacturing, transportation, and utilities, corrosion results in compromised safety and recurring repair costs. 1 One of the most commonly employed approaches to control corrosion is the use of corrosion resistant alloys. 1 Stainless steels (SSs), which are defined as steel alloys containing at least 10.5% chromium content by mass, are the most frequently used materials in aqueous environments because of their enhanced corrosion resistance relative to carbon steels. 2 Chromium participates in the formation of a stable passive film on SSs that protects the bulk metal against corrosion. 3 However, SSs still suffer from localized corrosion when exposed to chloride-containing aqueous environments, including seawater 4,5 and bleach plants associated with pulp and paper industries. 6 Strategies to improve the localized corrosion resistance of SSs include enrichment of Cr and Mo at the SS surfaces and removal of surface inhomogeneities. 7 These SS surface treatments include mechanical polishing, 8,9 passivation, 7,9-11 and electro-polishing. 8,[12][13][14] Mechanical polishing results in Cr-rich SS surfaces. 9 Surface passivation of SS using a nitric acid solution removes surface inhomogeneities and enhances the formation of a Cr-rich passive film. 7 Electro-polishing is a technique to control the surface finish of a metal by anodic electrochemical dissolution to yield a smooth metal surface. Electro-polishing removes surface inhomogeneities from the SS surface and simultaneously forms Cr-and Mo-rich passive films. Electropolished SS surfaces show higher localized corrosion resistance than mechanically polished and passivated SS surfaces. 14 Chromium oxide/hydroxide (Cr 2 O 3 /Cr(OH) 3 ) in the passive film hinders movement of cations into the electrolyte and thereby delays local breakdown of the passive film. 9 The Mo component also stabilizes the passive film. 15 In contrast, MnS inclusions are preferred sites for pit corrosion initiation. 5,[16][17][18][19][20] Fouling by marine organisms on submerged SS surfaces increases the probability of localized corrosion on SS surfaces due to metabolic activity of the bio-organisms. [21][22][23] Traditionally, coatings of antifouling materials have been employed to solve these problems, but antifouling agents can kill and destroy marine organisms and thus cause environmental concerns. 24 Recently, many studies have been devoted to the development of non-toxic strategies to prevent biofouling by mimicking surface topographies of marine species that naturally resist * Electrochemical Society Fellow. z E-mail: dennis.hess@chbe.gatech.edu biofouling such as sharks, 25,26 shells, [27][28][29] and crabs. 28,30 The combination of microscale topography and surface hydrophobicity is known to effectively reduce the fouling of marine organisms. 31,32 The SS surface treatments described above enhance localized corrosion resistance but generate smooth, hydrophilic surfaces. Therefore, it is of great interest to develop micro-structured, hydrophobic SS surfaces with resistance against localized corrosion for maritime applications. Grain boundary etching is a common metallographic technique used to delineate the grain structure of metals and metal alloys, thereby allowing analysis of size and orientation of grains. 33 Its application to modify SS surfaces for specific performance enhancement instead of metallography is rare: one report applies grain boundary etching to a drug-eluting SS stent surface to increase the surface area and, as a result, the drug loading. 34 Recently, we demonstrated that potentiostatic polarization in a nitric acid solution can be used to create/control surface structures on stainless steel 316L (SS316L), which enabled tunable water wettability. 35 The grain structure of SS316L surfaces was accentuated by selective grain boundary etching and the resulting roughness yielded a hydrophobic surface. This etching process provided the appropriate length scales of surface roughness for excellent wetting control. In the current study, we report the localized corrosion behavior of the hydrophobic, grain boundary etched SS316L and compare the results to as-received SS316L, as well as electro-polished SS316L, which is known to possess improved localized corrosion resistance, albeit with hydrophilic properties.
We investigated the corrosion behavior of these three different SS316L surfaces through potentiodynamic polarization in a neutral 0.6 M sodium chloride solution, which mimics a seawater environment. 36 Potentiodynamic polarization was performed up to potentials corresponding to the onset of localized corrosion. Water contact angle measurements were used to determine the water wettability of the SS316L surfaces. Surface structures of the different SS316L samples were analyzed by scanning electron microscopy (SEM). Chemical composition of the SS316L surfaces was obtained by X-ray photoelectron spectroscopy (XPS), which supplied insight into the relationship between changes in surface chemistry and corrosion behavior in 0.6 M sodium chloride solution induced by both grain boundary etching and electro-polishing.

Experimental
Materials.-Nitric acid (ACS reagent, 70%) was purchased from Sigma Aldrich. Sodium chloride (ACS reagent, ≥99.0%) was Sample preparation: potentiostatic polarization.-Two different sizes (2.5 × 1.5 × 0.05 cm 3 and 2.5 × 2.5 × 0.05 cm 3 ) of SS316L samples were cut from the as-received sheets using a water jet cutter. These two samples served as working and counter electrodes, respectively, for the potentiostatic polarization. Prior to potentiostatic polarization treatments, the SS316L substrates were washed with acetone, methanol, and isopropyl alcohol to remove surface organic contaminants, and the samples were air-dried at ambient temperature. Stainless steel wires were attached to the electrodes via spot welding to establish electrical connections. Insulating tape was employed to mask the working electrode, leaving an active area (0.13 cm 2 ) exposed for electrochemical treatments. Nitric acid (48% by weight) was used as the electrolyte. A saturated calomel electrode (SCE) served as the reference electrode in the three-electrode system. All potential values reported in this paper are relative to SCE. The distance between working and counter electrodes was maintained at 3 cm. A potentiostat (Gamry Reference 600) was used to perform the potentiostatic polarizations. After initial delays of 300 s at open circuit conditions, potentiostatic polarizations were performed at anodic potentials of 1.3 V and 2.4 V for 300 s at room temperature; the two potentials yielded grain boundary etched and electro-polished SS316L surfaces, respectively. 35 After the potentiostatic polarizations, the SS316L samples were removed from the electrochemical cell, rinsed with deionized water, and dried at room temperature for one day prior to characterization. Details about the experimental configuration for potentiostatic polarization have been reported previously. 35 Corrosion behavior: potentiodynamic polarization.-Corrosion resistance of the three SS316 samples was tested using potentiodynamic polarization in 0.6 M sodium chloride solution in an open jar with natural aeration. The as-received, grain boundary etched, and electro-polished SS316L samples were used as working electrodes, while platinum foil and the SCE served as counter and reference electrodes, respectively. Mechanical polishing of SS316L with 600 grit sandpaper is a known method to remove surface inhomogeneities of samples, thereby reducing sample to sample variation. However, mechanical polishing is a post-processing step that must be conducted on individual parts and affects the dimensions in a poorly controlled manner, which limits the applicability of mechanical polishing as surface finishing step in ultimate applications where corrosion resistance is critical. Therefore, we selected commercially available as-received SS316L as a reference point to compare corrosion behavior. After an initial delay of 1800 s at open circuit conditions, potentials were ramped in the anodic direction with a scan rate of 0.2 mV/s. Scans were performed from the open circuit potentials to the potential where the SS316L samples showed stable localized corrosion behavior, as indicated by a sudden increase in current density by more than two orders of magnitude. 37 All potentiodynamic polarization experiments were conducted at room temperature. After potentiodynamic polarization, the SS316L samples were removed from the electrochemical cell, rinsed with deionized water, and dried at room temperature prior to surface morphology characterization.
SS316L surface characterization.-Surface morphologies of the SS316L samples were characterized by SEM (Hitachi SEM SU8010, Japan) at 3 kV acceleration potential. A goniometer (Ramé-Hart 290) was used to measure water contact angles. Water contact angles were obtained by dispensing 4 μL deionized water droplets onto the SS316L samples at room temperature; images of droplets were captured with a CCD camera and analyzed via the goniometer software. Chemical composition of the SS316L samples was determined by XPS using a Thermo Fisher Scientific K-Alpha XPS with a 400 μm micro-focused monochromatic Al Kα X-ray source.

Results and Discussion
Surface modification of SS316L.-As shown in Figure 1, the surface structure of SS316L was modified by potentiostatic polarization. An applied anodic potential of 1.3 V results in a grain boundary etched SS316L surface with accentuated 5-20 μm intrinsic grain structures (Fig. 1b), while 2.4 V leads to a smooth electro-polished SS316L surface (Fig. 1c). The roughness features that are evident on the as-received SS316L (Fig. 1a) were created during the sheet manufacturing processes. Grain boundaries are preferential etching sites under certain potential conditions. 38,39 In this system, highly selective grain boundary etching is achieved at an applied potential of 1.3 V, which accentuates the intrinsic grain structures on the SS316L surface. Potentiostatic polarization at an anodic potential of 2.4 V shows little to no selectivity toward grain boundary etching, thereby generating an electro-polished SS316L surface. The effects of modifying the SS316L surfaces via potentiostatic polarization on corrosion behavior in 0.6 M sodium chloride solution and on wettability are discussed in the following sections.   Figure 2 shows anodic potentiodynamic polarization curves of as-received, grain boundary etched, and electro-polished SS316L samples. These curves display onset points of SS316L sample dissolution, passivity, and rapid increases in current density due to localized corrosion. 37,40 Since the SS316L samples were polarized in the anodic direction from open circuit conditions, the starting point of each curve represents the open circuit potential (E OC ). The sharp increase in current density at high potential (i.e. flat section of the graph) represents the onset of localized corrosion, and the potential at which this occurs is referred to as the breakdown potential (E BD ). 37 In order to capture the stochastic nature of localized corrosion, 41,42 eight samples were prepared for each SS316L surface and potentiodynamic polarization measurements were performed on all samples (Figs. 2a-2c). Representative potentiodynamic polarization curves for each substrate are displayed in Fig. 2d Figs. 2e and 2f. The asreceived SS316L samples exhibit spikes in current density in potential ranges from open circuit potentials to 0.2 V, while sharp increases in current density were observed in the potential range 0.32 V to 0.86 V; these increases represent metastable localized corrosion and stable localized corrosion, respectively (Fig. 2a). 40 In the potentiodynamic polarization curves of grain boundary etched and electro-polished SS316L samples, no characteristic metastable localized corrosion was observed; passivity was observed at potentials up to 0.9 V. Beyond the passivity region, stable localized corrosion occurred for the grain boundary etched and the electro-polished SS316L samples in fairly narrow potential ranges: 0.96 to 1.05 V for grain boundary etched, and 0.99 to 1.07 V for electro-polished samples (Figs. 2b, 2c). The lack of metastable localized corrosion and the high E BD values with a narrow distribution indicate superior localized corrosion resistance compared to the as-received SS316L sample (Fig 2e). In addition, the E OC values for the grain boundary etched and electro-polished SS316L samples were higher than for the as-received SS316L sample, which can be attributed to a higher rate of cathodic reaction on electrochemically treated SS316L samples (Fig. 2f). The corrosion behavior of SS316L mechanically ground with 600 grit paper was also tested in 0.6 M sodium chloride solution. Results demonstrate that the breakdown potential occurred in the range of that of as-received SS316L samples (Fig. S1a). In addition, we prepared mechanically ground and grain boundary etched SS316L samples and tested the corrosion behavior, which was consistent with potentiodynamic polarization curves of grain boundary etched SS316L without the mechanical grinding pretreatment (Fig. S1b).

Corrosion behavior of SS316L surfaces.-
The morphology of localized corrosion sites on SS316L samples after potentiodynamic polarization was also investigated using SEM (Fig. 3). The as-received SS316L samples showed formation of mouth pits with lacy pits around the mouth pit peripheries (Fig 3a). 43 The grain boundary etched and electro-polished SS316L samples showed localized corrosion in the form of pitting and crevice corrosion (Figs.  3b, 3c). Cavities formed inside the active area (see experimental details) of grain boundary etched and electro-polished SS316L surfaces represent pitting corrosion (Fig. 3b1, 3b2, and 3c1, 3c2), while cavities along the periphery of the active area are indicative of crevice corrosion (Fig. 3b3, c3). We hypothesize that the confined space between masking tape and SS316L surfaces along the periphery of the active area can act as an initiation site for crevice corrosion by limiting access of bulk electrolyte to the space. Further study is necessary to  (Figs. 4a,  4b). After potentiostatic polarization, the Mn content in the passive films decreases slightly (Fig. 4c). In contrast, no definitive changes in Fe content were observed in the passive films of SS316L samples after potentiostatic polarization (Fig. 4d). O1s XPS spectra show that the passive films on surface-modified SS316L samples possess more oxide than hydroxide components (Fig. 4e). Enrichment of Cr and Mo and the removal of Mn at SS surfaces are known to improve  the localized corrosion resistance of SSs. 9,15,20 Therefore, the improved corrosion resistance of the grain boundary etched and electropolished SS316L samples in 0.6 M sodium chloride solution can be attributed to the formation of superior passive films during potentiostatic polarization. Figure 5 shows the water wetting behavior of as-received, grain boundary etched, and electropolished SS316L samples. The static contact angle of 4 μL deionized water droplets on the as-received SS316L is 87.3 ± 4.5 • , while the grain boundary etched and the electro-polished SS316L samples show static contact angles of 135.7 ± 2.6 • and 81.0 ± 4.2 • , respectively. The increased water contact angle on the grain boundary etched SS316L surface can be attributed to the evolved microscale intrinsic grain structures which allow air to be trapped beneath the water droplet, thereby significantly enhancing the observed contact angle. 44,45 The electro-polished SS316L surface lacks microscale roughness and thus showed a similar water contact angle as the as-received SS316L surface. The grain boundary etched SS316L displayed hydrophobicity and microscale structure, which can prevent bio-fouling, thereby further reducing the probability of localized corrosion initiation in a maritime environment. [21][22][23]26,31,32 Preliminary investigations have indeed shown that the surface topography on SS316L achieved by potentiostatic polarization can significantly reduce the adhesion of bacteria compared to the surfaces of as-received and electro-polished SS316L. An in-depth study of the correlation between SS316L surface topography and biofouling is currently underway, but discussion of these results is outside the scope of the current paper.

Conclusions
Grain boundary etching has long been used in metallurgy as a metallographic method to reveal grain structure. Here we have shown that the process can also be used to simultaneously enhance wetting and corrosion properties of stainless steel. Specifically, we demonstrated a simple method to improve the localized corrosion resistance of SS316L surfaces in 0.6 M sodium chloride solution, while simultaneously achieving microscale topography and hydrophobicity. Grain boundary etched SS316L surfaces were created by potentiostatic polarization at the anodic potential of 1.3 V in nitric acid solution, leading to superior localized corrosion resistance with a narrow distribution of high breakdown potentials ranging from 0.96 to 1.05 V. On the other hand, as-received SS316L samples show relatively poor localized corrosion resistance with a wide range of low breakdown potential values from 0.32 to 0.86 V. This enhanced localized corrosion resistance of the grain boundary etched SS316L can be attributed to the formation of Cr-and Mo-rich passive film of SS316L during the potentiostatic polarization surface treatment. The corrosion resistance of the etched samples is similar to electropolished substrates, the current benchmark, but the grain boundary etched SS316L also displays microscale topography at the appropriate roughness length scale to yield a hydrophobic surface with a water contact angle of 135.7 ± 2.6 • . In contrast, electropolished SS316L is hydrophilic. The combination of microscale topography and hydrophobicity on grain boundary etched SS316L offers the potential to prevent biofouling in maritime environment and thus further deter the occurrence of localized corrosion.