Anticorrosion Behaviour of SS304 Microgroove Surfaces in Saline Water

Abstract

The 304 Stainless Steel (SS304) is severely affected by salt water corrosion due to its high surface wettability. By reducing its surface wettability, its corrosion can be reduced. To achieve this, topographical modification of the steel surface is an effective route. In this work, SS304 flat surfaces were topographically modified into microgrooves (ridge width 250 μm to 500 μm, groove width 200 μm, width ratio = ridge width/groove width >1). Wire cut electrical discharge machining was used to fabricate the microgrooves. Long-term wetting characteristics and long-term corrosion behaviour of flat surface and microgrooves were studied. The influence of the nature of wetting of the tested surfaces on their corrosion behaviour was examined. The sessile drop method and potentiodynamic polarization tests in sodium chloride (3.5 wt. % NaCl) solution (intermittent and continuous exposures for 168 h) were studied to characterize their wetting and corrosion behaviours, respectively. Topographical modification imparted long-term hydrophobicity and, as a consequence, long-term anticorrosion ability of the steel surface. Micropatterning reduced the corrosion rate by two orders of magnitude due to reduction in interfacial contact area with the corrosive fluid via composite wetting, i.e., solid–liquid–air interface. Microgrooves showed corrosion inhibition efficiency ≥88%, upon long-term exposure to NaCl solution. By comparing the wetting and corrosion behaviours of the microgrooves with those of the previously studied microgrooves (ridge width/groove width <1), it was found that the surface roughness of their ridges strongly influences their wetting and corrosion properties.

1. Introduction

Topographical modification of surfaces is relatively a new approach in the field of corrosion prevention, compared to coating, plating, painting, nhibitor, cathodic protection and microstructure modification [1,2]. Topographical modification involves creating geometrical patterns on metallic surfaces that induce antiwetting ability and consequently alleviate liquid mediated corrosion. Patterns are multi-functional. They can be utilized for more than one purpose in combination with their anticorrosion functionality, such as antifriction, antifouling, antifrosting/icing, drag reduction etc. [3,4,5].

Micropatterns can be incorporated in situ/post-manufacturing stage of components via chemical etching [6], machining methods such as laser techniques [7,8,9,10], focused ion beam method [11], electrochemical process [12], mechanical machining, e.g., grinding [13], micromachining, e.g., wire electrical discharge machining [14], ultrasonic elliptical vibration cutting [15], etc. With the advancement in additive manufacturing technology, metallic components can be printed and finished with micropatterns on their surfaces in a one-step process. As an example, Figure 1 shows 316L micropatterns printed by the binder jetting (BJ) additive manufacturing technology [16]. Such printing of micropatterns gives the capability to realize anticorrosion surfaces on components without additional machining/processing.

Topographical modification provides several advantages, such as ease of making patterns due to their geometrical nature, well-established processes to make patterns, absence of interface and complex chemistry etc. Topographical modifications via geometrical patterning of metallic surfaces can effectively reduce liquid mediated corrosion, and examples include: laser generated micropillars on aluminium surfaces [7], laser ablated micro-holes on nickel surfaces [8], micro-holes on biomedical devices created by focused ion beam [11], laser textured SS316L surfaces [9], microchannels fabricated by chemical etching on SS304 surfaces [6] and laser treatment of Al 7075 aluminium alloy to form microgrooves with hierarchical roughness [10]. However, these studies are restricted to one-time testing and do not report on the long-term corrosion behaviour of the topographically modified surfaces, which is important for application purposes.

In this work, 304 stainless steel (SS304) flat surfaces were topographically modified into microgrooves. Long-term wetting characteristics and long-term corrosion behaviour of the modified surfaces were studied. The aim of the investigation was to examine the efficacy of geometrical patterns to impart long-term anticorrosion ability. SS304 is a widely used engineering material with application in mechanical/architectural structures, food processing equipment, biomedical, automobile and aerospace sectors [17]. Although the steel has good resistance to chemicals due to chromium, it is severely affected by aggressive chloride ions in salt water, which is aggravated due to its high surface wettability [18,19]. Hence, reducing wettability of the steel surface is a solution to reduce the severity of salt water corrosion. Thereby, the utility and performance of the steel can be extended in humid/marine environments.

2. Materials and Methods

As-received SS304 steel surfaces were polished using silicon carbide abrasive papers of different grades (80, 120, 220, 400, 600, 1000, 2000 grit size), followed by diamond polishing. The surfaces were cleaned with acetone to remove debris. Average surface roughness of the polished surfaces was 0.015 µm, as measured using a 3D optical profilometer (WYKO NT1100, Champaign, IL, USA). These surfaces were taken as the reference surface, herewith referred to as SS304 flat surface. SS304 flat surface and topographically modified SS304 flat surface in the form of microgrooves were taken as the test surfaces.

Wettability test and corrosion test were conducted for the surfaces in three different ways:

(1)

Wettability test, followed by corrosion test.

(2)

Intermittent exposure to sodium chloride (NaCl) solution for 168 h (wettability measured at an interval of every 24 h), followed by corrosion test.

(3)

Continuous exposure to NaCl solution for 168 h (wettability measured at an interval of every 24 h), followed by corrosion test.

2.1. SS304 Microgroove Surfaces

Microgrooves (MG) with constant height (100 µm), constant groove width (200 µm) and varying ridge width (250 µm, 300 µm, 400 µm and 500 µm) were machined on SS304 flat surfaces by wire cut electrical discharge machining (WEDM). A tungsten electrode of 0.18 mm diameter was used as the slicing wire. The spark gap between the wire and workpieces was maintained at 0.02 mm. Set process parameters were: peak current 3 A, pulse on time 15 µs, pulse off time 4 µs, gap voltage 100 V and cutting speed 2.23 mm/min.

Geometrical parameters of microgrooves, namely: ridge width (W_r), groove width (W_g) and groove depth (D_g) are illustrated in Figure 2a. The width ratio (WR = W_r/W_g) of MG was kept >1, such that W_r > W_g. Nomenclature and geometrical parameters of MG are mentioned in

Table 1. Nomenclature and geometrical parameters of MG.

Surface	Ridge Width (Wr) (µm)	Groove Width (Wg) (µm)	Groove Depth (Dg) (µm)	Width Ratio WR = Wr/Wg
MG1	250	200	100	1.25
MG2	300	200	100	1.5
MG3	400	200	100	2
MG4	500	200	100	2.5

. A scanning electron microscope (SEM) image of MG1 is shown in Figure 2b. An optical microscope image of a ridge surface of MG1 is shown in Figure 2c. Average surface roughness on the ridges of MG was 4.85 µm, as measured using a 3D optical profilometer (WYKO NT1100, Champaign, IL, USA). Surface roughness on the ridges is due to the WEDM machining process [20].

2.2. Wettability Test

Surfaces were cleaned with acetone, followed by distilled water and dried. Water contact angles on the surfaces were measured by a sessile drop method using an optical contact angle measuring system (Data Physics OCA15EC, Filderstadt, Germany). Distilled water droplets of volume 5 µL were placed on the surfaces. Images of water droplets were taken in directions perpendicular and parallel to the direction of MG, as shown in Figure 3a. Ten measurements of water contact angles were taken in both the perpendicular direction (θ_⊥) and parallel direction (θ_Π). Average values of the contact angles are reported (error ± 2°).

2.3. Electrochemical Corrosion Test

Potentiodynamic polarization tests were conducted for the surfaces at room temperature (27 °C) in saline water (3.5 wt. % NaCl solution), using an electrochemical work station (Autolab, Metrohm, The Netherlands). The SS304’s flat surface and MG were taken as the working electrode, Ag/AgCl as the reference electrode and platinum as the counter electrode. The area of the surfaces exposed to the corrosive fluid was 1 cm². Prior to the corrosion test, the surfaces were immersed in 3.5 wt. % NaCl solution for 60 min to obtain steady state open circuit potential. For each surface, the potential range selected for the test was ±0.1 V relative to its open circuit potential. Potentiodynamic polarization curves were generated at the scan rate of 1 mV/s. From the linear polarization resistance curves, corrosion potential (E_corr) and corrosion current density (i_corr) were determined. Corrosion rate was estimated using the Stern–Geary relation [21]. Surfaces after corrosion test were examined using a field-emission scanning electron microscope (FEI-Quanta-FEG 250, Hillsboro, OR, USA).

2.4. Long-Term Exposures to Saline Water

To evaluate the corrosion behaviour of surfaces over long-term duration, the surfaces were exposed to saline water (3.5 wt. % NaCl solution), under two different conditions: (i) intermittent exposure by drop method for 168 h and (ii) continuous exposure by dip method for 168 h. In the intermittent exposure test, using a droplet generator, saline water droplets at the rate of one droplet per second were dropped on the surfaces that were kept inclined at 45°. This method is used in industrial weathering tests [22]. Static water contact angles (θ_⊥, θ_Π) were measured at an interval of every 24 h of intermittent exposure, until 168 h. In the continuous exposure test, the surfaces were dipped in saline water for 168 h, and static water contact angles (θ_⊥, θ_Π) were measured at an interval of every 24 h of continuous exposure, until 168 h. After an interval of every 24 h, prior to measuring the contact angles, the surfaces were dried in open atmospheric conditions for 30 min. After the long-term exposures, i.e., after intermittent exposure for 168 h and after continuous exposure for 168 h, corrosion test was conducted for the surfaces as described in Section 2.3.

3. Results and Discussion

3.1. Wetting Behaviour

Wettability of a surface is characterised by the contact angle a water droplet makes on its surface. Measured contact angles (CA) prior to long-term exposures of MG as a function of their width ratio (WR = W_r/W_g) are shown in Figure 4. The flat surface is hydrophilic with θ ≈ 77.7° and MG are hydrophobic (θ_⊥ and θ_Π > 90°). CA values decrease with the increase in the width ratio. WR of MG is >1. Hence, the increase in WR value corresponds to the increase in the ridge width (W_r), as the groove width (W_g) is constant. CA of the surfaces measured before, during and after intermittent and continuous exposures to NaCl solution as a function of the exposure time (t) are shown in Figure 5. CA values of surfaces decrease with the increase in the exposure time. The decrease is very noticeable for the flat surface when compared to those of MG. Defining θ_r as the magnitude of reduction in CA due to long-term exposures (i.e., θ_r = CA before long-term exposure − CA after long-term exposure), it is seen that θ_r values of MG are lower than those of the flat surface (

Table 2. Contact angle of surfaces prior to long-term exposures to 3.5 wt. % NaCl solution. θ_r values (θ_r = CA before long-term exposure − CA after long-term exposure) of surfaces: (I) after intermittent exposure and (II) after continuous exposure.

Surface	Width Ratio WR = Wr/Wg	Contact Angle (°) Prior to Long-Term Exposures		θr (°)
				I		II
		θ⊥	θΠ	∆θ⊥	∆θΠ	∆θ⊥	∆θΠ
Flat		77.7		8.6		9.5
MG1	1.25	137.6	117.2	2.2	2.1	4.1	4.0
MG2	1.50	130.2	109.4	2.4	2.2	4.4	4.7
MG3	2.00	121.0	102.0	2.5	2.5	4.9	5.0
MG4	2.50	113.2	94.3	2.6	3.5	5.1	5.2

). Wettability of the flat surface increases rapidly upon long-term exposures than those of MG.

Anisotropic wetting is a characteristic of unidirectional surface topographies. Due to their anisotropic topography, MG have CA values, such that their θ_⊥ > θ_Π. Water droplets tend to spread along the groove direction as the energy barrier for spreading is lower than that in the direction across the grooves, giving rise to wetting anisotropy [23]. Anisotropic angle, θ_a (θ_a = θ_⊥ − θ_Π), is an indicator of directional wettability of unidirectional anisotropic surfaces. θ_a depends on groove width and groove depth [15,24]. MG have the same groove width (200 µm) and same groove depth (100 µm). Hence, their θ_a values are same ≈18° ± 1°. θ_a is zero for the flat surface, which means a water droplet spreads on its surface uniformly in all directions.

Wetting of topographically modified surfaces occurs in two distinct states, namely: (a) Wenzel state, wherein water droplets completely wet their surface features and (b) Cassie–Baxter state, wherein water droplets wet the top of their surface features, with air trapped in between their surface features, beneath the water droplets (composite wetting, i.e., solid-liquid-air interface) [23]. As seen from Figure 3, the wetting of MG is in the Cassie–Baxter state (θ_CB). According to the Cassie–Baxter model, CA of a surface is dependent on f, the solid fraction (f = actual surface area/planar surface area) of the surface (Equation (1), [24]).

Cosθ_CB = f (Cosθ₀ + 1) − 1

(1)

where, θ_CB is the CA in Cassie–Baxter state, f is the solid fraction and θ₀ is the CA of flat surface. For MG, the solid fraction was estimated as W_r/(W_r + W_g). Solid fraction values of MG are: 0.55, 0.60, 0.66 and 0.71 for MG1, MG2, MG3 and MG4, respectively. f = 1 for the flat surface.

The solid fraction parameter is the surface area available for wetting by water droplets. Therefore, it is seen that CA of MG decreases with the increase in their width ratio (WR = W_r/W_g), in particular with the increase in their ridge width (W_r), as their groove width (W_g) is constant. Decrease in W_r corresponds to decrease in the surface area available for wetting, which in turn increases the CA (θ_⊥, θ_Π). Hence, amongst the microgrooves, MG1 that has the smallest WR of 1.25 and smallest f value of 0.55 has the highest CA values (θ_⊥ 137.6° and θ_Π 117.2° before long-term exposures; θ_⊥ 135.4° and θ_Π 115.1° after intermittent exposure; θ_⊥ 133.5° and θ_Π 113.2° after continuous exposure). MG4 that has the largest WR of 2.5 and largest f value of 0.71, amongst the microgrooves, has the lowest CA values (θ_⊥ 113.2° and θ_Π 94.3° before long-term exposures; θ_⊥ 110.6° and θ_Π 90.8° after intermittent exposure; θ_⊥ 108.1° and θ_Π 89.1° after continuous exposure). The flat surface has f = 1, and has the lowest CA values of all the surfaces (77.7° before long-term exposures; 69.1° after intermittent exposure; 68.2° after continuous exposure). Wetting of a surface by a fluid directly depends on the area exposed by the surface to the fluid, i.e., the area of the surface that comes in contact with the fluid. Considering θ_r values of surfaces (Table 2), the hydrophobic MG have antiwetting ability over long-term exposures, as indicated by their lower θ_r values. In contrast, the hydrophilic flat surface is prone to severe wetting over long-term exposures, as indicated by its higher θ_r values.

3.2. Corrosion Behaviour

Corrosion potential (E_corr), corrosion current density (i_corr) and corrosion rate (CR) of the surfaces before long-term exposures are listed in

Table 3. Corrosion potential (E_corr), corrosion current density (i_corr) and corrosion rate (CR) of surfaces, prior to long-term exposures to 3.5 wt. % NaCl solution.

Surface	Prior to Long-Term Exposures
	Ecorr (V)	icorr (A/cm2)	CR × 10−4 (mm/year)
Flat Surface	−0.5121	11.16 × 10−6	166
MG1	−0.0374	2.10 × 10−8	1.7
MG2	−0.0451	2.46 × 10−8	2.1
MG3	−0.1078	2.65 × 10−8	2.4
MG4	−0.1165	2.85 × 10−8	2.9

. Their values after long-term exposures are listed in

Table 4. Corrosion potential (E_corr), corrosion current density (i_corr) and corrosion rate of surfaces, after long-term exposures to 3.5 wt. % NaCl solution.

Surface	After Intermittent Exposure			After Continuous Exposure
	Ecorr (V)	icorr (A/cm2)	CR × 10−4 (mm/year)	Ecorr (V)	icorr (A/cm2)	CR × 10−4 (mm/year)
Flat	−0.2703	26.60 × 10−6	311	−0.2789	26.90 × 10−6	313
MG1	−0.0454	2.12 × 10−8	2.2	−0.0639	2.18 × 10−8	2.7
MG2	−0.0766	2.58 × 10−8	2.5	−0.0889	2.66 × 10−8	3.0
MG3	−0.1174	2.87 × 10−8	2.7	−0.1202	2.92 × 10−8	3.1
MG4	−0.1789	3.16 × 10−8	3.2	−0.1834	3.23 × 10−8	3.7

. Tafel plots of the flat surface and MG1 are shown in Figure 6. From Table 3 and Table 4, it is seen that when compared to the flat surface, MG have (i) higher E_corr values, (ii) lower i_corr values and (iii) CR values that are lower by two orders of magnitude.

CR values of MG, before and after long-term exposures as a function of their width ratio (WR), are plotted in Figure 7. Their CR value increases with the increase in WR, and in magnitude it is in the order: after continuous exposure > after intermittent exposure > prior to long-term exposures.

As seen from Figure 4, Figure 5 and Figure 7, with the increase in WR, CA values decrease, whereas CR values increase for MG. Therefore, CR decreases with the increase in CA (i.e., decrease in f, the solid fraction), indicating that surface wettability determines the corrosion rate. Wettability of a surface depends on the area available for wetting (i.e., f, the solid fraction value) by corrosive fluid. Hence, amongst the microgrooves, MG1 that has the highest CA values has the lowest CR at all the test conditions, and vice versa. Amongst all the surfaces, the flat surface has the lowest CA and hence has the highest CR at all the test conditions. On a comparative note, for the flat surface, CA reduced by 11% and 12.2% upon intermittent and continuous exposures, respectively and correspondingly its CR value increased by 87.3% and 88.5% with respect to its CR value prior to its long-term exposures. In contrast, for MG1, CA (θ_⊥, θ_Π) reduced by 1.59%, 1.79% and 2.9%, 3.4% upon intermittent and continuous exposures, respectively, and correspondingly its CR increased by 29.4%.and 58.82% with respect to its CR value prior to its long-term exposure. As observed in the present work, the dependence of CR on CA such that the pattern with highest CA has lowest CR has been reported earlier for other metallic patterns [6,7,9,11].

Corrosion of SS304 occurs by pitting in NaCl solution [25]. Although passive film forms on the steel surface, it is prone to localized breakdown in the presence of aggressive chloride ions and causes pitting [25]. Chloride ions penetrate and move through passive film, and eventually as they reach metal/film interface they cause film breakdown and the film gets detached from surface [25,26]. In the absence of passive film, the new surface becomes exposed from underneath, which accelerates corrosion. Also, pitting corrosion accelerates due to its autocatalytic nature [27].

In the present work, the flat surface exposes its entire surface area to the corrosive fluid (Figure 8) and so experiences severe corrosion, whereas in the case of MG, the corrosive fluid wets and corrodes only the top of their ridges, due to their composite wetting [28,29,30] (Figure 8). Air trapped in their grooves resists the entry of the corrosive fluid and prevents electrochemical activity [6,7,9,10].

Figure 9 shows corrosion pits on the flat surface and MG1 after corrosion test. These are metastable pits that usually form at low potentials [31]. In stainless steels, during the early stage of corrosion, chloride ions damage the passive film and promote the nucleation of metastable pits [31]. Stainless steels repassivate upon metastable pit formation [31,32]. With increase in potential, eventually, stable pits form that break down the passive film. The potential at which stable pits form is higher than the passivation potential [33]. Usually, stable pits are larger in size (e.g., 50 µm diameter; seen on 304L austenitic steel in 3.5% NaCl [25]) than metastable pits [31,32,33]. Reports [7,9] mention that the pitting potential (E_pit, the potential above which stable pits form) was higher for the metallic patterns than their flat counterpart surfaces and that the pattern with the highest CA showed the highest E_pit value [7].

Reduction in CA values of surfaces due to long-term exposures to NaCl solution (Table 2) could be attributed to the occurrence of pitting on their surfaces. Corrosion pits are holes that are relatively small in size [25,27] and can actively sustain capillarity [34]. Hence, formation of corrosion pits on surfaces during their long-term exposures to NaCl solution increases their surface wettability causing a reduction in their CA values and as a consequence their CR values increase (Figure 7). Therefore, it is observed that their CA values are in the order: after continuous exposure < after intermittent exposure < prior to long-term exposures to NaCl solution, and their CR values are in the order: after continuous exposure > after intermittent exposure > prior to long-term exposures to NaCl solution.

Corrosion inhibition efficiency (η) of MG was estimated by using Equation (2) [35].

η (%) = [(i_{corr (f)} − i_{corr (MG)})/i_{corr (f)}] × 100

(2)

where, i_{corr (f)} is the current density of the flat surface and i_{corr (MG)} is that of a MG. η values of MG before and after long-term exposures to NaCl solution are listed in

Table 5. Corrosion inhibition efficiency (η) of MG (Equation (2)).

Surface	Corrosion Inhibition Efficiency (η), %
	Prior to Long-Term Exposures	After Intermittent Exposure	After Continuous Exposure
MG1	81	92	93
MG2	78	90	90
MG3	76	89	89
MG4	75	88	88

. MG are efficient in inhibiting saline water corrosion over the long-time duration with η values ≥88%.

3.3. Factors Influencing Wetting and Corrosion Behaviour of Microgrooves

For hydrophobic micropatterns that follow the Cassie–Baxter model, the solid fraction (f) influences their surface wetting and consequently determines their corrosion. Along with the solid fraction, the surface roughness on their ridges also influences their surface wetting and corrosion rate. To elucidate this fact, results from a previous work on SS304 microgrooves (GS) with width ratio (WR) < 1 [6] and the results from the present work on MG (WR > 1), are compared. W_r, W_g, WR and solid fraction (f) values of GS are given in

Table 6. Nomenclature, geometrical parameters, width ratio, water contact angles and f values (Cassie-Baxter Equation (1)) of SS304 microgrooves with WR < 1. (data from [6]).

Surface	Ridge Width (Wr) (µm)	Groove Width (Wg) (µm)	Width Ratio WR = Wr/Wg	Contact Angle (o)		f
				θ⊥	θΠ
Flat				77.7
GS-1	55.70	138.50	0.40	139.6	111.2	0.28
GS-2	111.53	140.61	0.79	133.8	108.9	0.44
GS-3	118.27	141.76	0.83	133.2	107.0	0.46

[6]. Optical profilometer images of GS-1 and MG1 are shown in Figure 10. GS have the same groove depth (Dg ≈ 58 μm), almost similar groove width (W_g ≈ 140 µm, −2/+1 µm), but varying ridge width (W_r) [6].

CA (θ_⊥, θ_Π) and CR of GS and MG as a function of their width ratio (WR = W_r/W_g) are shown in Figure 11. Solid fraction (f) values of GS and MG are mentioned near their respective CR data points. MG have larger f values when compared to those of GS. Hence, it could be expected for MG to have (i) lower CA values than those of GS (blue arrow: expected decreasing trend of θ_⊥ values of MG compared to those of GS; red arrow: expected decreasing trend of θ_Π values of MG compared to those of GS) and (ii) higher CR values than those of GS (black arrow: expected increasing trend of CR values of MG compared to those of GS). However, the CA values and CR values of MG do not follow the expected trends; rather, the measured CA values are higher and the measured CR values are lower than their respective expected trends, as can be seen from Figure 11. The discrepancy between the expected trends and the measured values of CA and CR of MG is due to the higher surface roughness of MG on their ridges. Average surface roughness on the ridges of MG is 4.85 µm, which is two orders higher than the roughness value on the ridges of GS (0.013 µm, [6]). GS were fabricated by a chemical etching process [6], whereas MG were fabricated by WEDM. Micro-craters form on MG ridge surfaces (Figure 2c), due to electric sparks during the electrical discharge machining process, which gives rise to higher surface roughness [20,36,37]. It should be noted here that the theoretical estimation of the solid fraction (f) of the microgrooves includes ridge width (W_r) and not the surface roughness on their ridges. Hence, although the solid fraction values of MG are higher than those of GS according to the theoretical estimates, it is reasonable to suggest that the solid fraction values of MG are overestimated, as the surface roughness is not taken into account. Rougher surfaces increase CA and reduce wettability, which in turn reduces corrosion [38,39,40]. Also, groove depth (D_g) influences CA values, such that taller grooves often demonstrate increased contact angles [14]. D_g of GS is 58 µm [6], whereas that of MG is 100 µm (Figure 10). Thus, the taller grooves and rougher ridge surfaces of MG further enhance their CA values and reduce their wettability, which contributes towards lowering their CR values (Figure 11). In other words, if MG had the same D_g and same surface roughness on their ridges as those of GS, their CA values would have been lower and thereby their CR values would have been higher than those observed in the present work, owing to their larger solid fraction (f) values than those of GS. It is notable that amongst MG, the CA and CR depend on their solid fraction. Similarly, amongst GS microgrooves, the CA and CR depend on their solid fraction [6].

The above discussion highlights that solid fraction (f), surface roughness of ridges and groove depth (D_g) together determine the wettability of patterned surfaces (consequently their corrosion). Note that, natural antiwetting surfaces are very effective due to the combination of these features. As an example, rice leaves have taller microgrooves (height 200 μm), thinner ridge width (50 μm) and have micro-papilla (height 3–5 µm, width 5–8 µm) on their ridges that provide roughness and enhance water-repellence [41]. The roughness effect on the antiwetting characteristic becomes amplified as ridges become thinner. MG1 that has the smallest W_r amongst the MG thus has the highest CA, and thereby experiences the lowest CR.

3.4. Advantages of Topographical Modification

The prime advantage of topographical modification is the flexibility in designing micropatterns due to their geometrical nature. Other notable advantages are:

• Absence of complex chemistry and interface in micropatterns. Chemical modification approaches such as coatings, platings and paints create a chemical barrier on surfaces that prevent corrosive fluids from contacting metallic surfaces. In the topographical modification approach, fluids are in contact with metallic surfaces, but the area of contact of surfaces with fluids is greatly reduced. Although chemical modification approaches effectively reduce corrosion, they have several critical issues that can cause loss of anticorrosion performance. These issues need to be considered while formulating the main functionality of protection against corrosion, such as adhesion to surfaces due to the presence of interface, layer thickness, mechanical durability, presence of micro-defects and chemical complexity in coatings [42,43,44]; non-uniform thickness, brittleness and cracking in plating [45]; the presence of volatile organic compounds in paints [44]; and most inhibitors are hazardous and expensive [46]. Micropatterns are environmentally friendly and can be implemented over large surface areas. They can provide corrosion inhibition efficiency (η) similar to that by chemical modification of surfaces. For example, manganese-based thin films (thickness 190 nm) on SS304L when tested in 3.5 wt. % NaCl solution showed η ≈ 96% [47]. MG1 shows η ≥ 92% (Table 5), after long-term exposure to 3.5 wt. % NaCl solution.
• Random textures (i.e., random surface roughness) on metallic surfaces provide hydrophobicity and induce anticorrosion capability [38,39,40]. However, micropatterning of surfaces provides the capability to control/manipulate corrosion by geometrical design.
• Superhydrophobic micropatterns with ultra-antiwetting capability can be designed to mitigate liquid mediated corrosion [10,40]. Surface features of natural water-repellent plant leaves provide inspiration and insights to design such surfaces [10,40,41]. For example, surface features on rice leaves induce anisotropic wetting characteristics to the leaves surface (water contact angles: 151° across their length, 146° along their length) [41]. Hierarchical surface architecture of rice leaves imparts directional motion to water droplets falling on them and forces the water droplets to roll over along their length.
• Micropatterns have multi-functionality and can be utilized for more than one purpose in combination with their anticorrosion functionality [3,4,5].

4. Conclusions

The present investigation was aimed to examine the long-term antiwetting ability of topographically modified SS304 surfaces and their efficacy to impart long-term anticorrosion ability to the steel surface in saline water. The following are the main conclusions that can be drawn from the work:

• SS304 flat surface is severely affected by salt water corrosion due to its hydrophilic nature, i.e., high surface wettability.
• Microgrooves due to their hydrophobicity reduce surface wetting by the corrosive fluid, due to their smaller surface areas via the composite solid–liquid–air interface.
• Microgrooves have high corrosion inhibition efficiency (η ≥ 88%) in inhibiting saline water corrosion of SS304 surface over long duration.
• For hydrophobic micropatterns, along with their solid fraction value, the roughness on their ridges and their groove depth should also be considered for a better understanding of their surface wettability and corrosion behaviour.
• Surface wettability is an important driving factor for liquid mediated corrosion. By controlling surface wettability, liquid-mediated corrosion can be readily controlled.
• Topographical modification offers great flexibility and ease in mitigating liquid-mediated corrosion via design/selection of geometrical and surface parameters. It has immense research and applicative potential in terms of long-term anticorrosion capability.