Effect of Substrate Preparation and the Conversion Coating on the Corrosion Resistance in Ringer's Solution of 304L Stainless Steel Coated with Alumina Film

Abstract

This work aims to determine the best substrate preparation and conversion coatings that may enhance the anti-corrosive performance of alumina film/stainless steel substrate systems in Ringer’s solution. Boehmite, silica, silica/boehmite, and boehmite/silica conversion coatings were deposited on sanded or sandblasted 304L stainless steel samples, and the best results were further covered with alumina film for a long-time exposure evaluation. The best anti-corrosive performances for each substrate preparation were observed for the sandblasted sample covered with silica/boehmite conversion coating (JSB500) and the sanded sample covered with silica (LS500), which showed the highest global resistance (R_g) values (828 Ω.cm² and 1837 Ω.cm², respectively) and the smallest values of global capacitance, C_g (2.19 X 10^-6 F.cm^-2 and 3.37 X 10^-9 F.cm^-2, respectively). After covering with the alumina film and evaluating for 216 h in the corrosive medium, it was noted an increase in the R_g values for both systems (JSBA500 and LSBA500), likely due to the presence of a corrosion products layer. The presence of iron oxides in these conversion coatings led to a more stable alumina film. Comparing the selected coating systems, the LSBA500 exhibited an improved performance, reaching an increase of 85% in R_g and a decrease of 94% in C_g.

Keywords:
Corrosion; Conversion coatings; Roughness; Surface preparation

1. Introduction

AISI 304L stainless steel is known for its high corrosion resistance and mechanical strength¹,². Also, its cost can be considered low compared to other resistant materials³. These characteristics make this material a usual option in manufacturing surgical devices, such as prostheses, bone fixation, syringes, medical scissors, tweezers, and scalpels⁴-⁶. Its corrosion resistance is mainly associated with the formation of a thin passive film of oxide on the steel surface. However, the presence of an aggressive environment containing chloride ions and dissolved oxygen may affect the stability of this passive film, leading to corrosion processes.

Body fluids present complex chemical compositions and can be considered significantly aggressive media compared to metallic materials. Although most body fluids have a pH of 7.4 and a temperature of approximately 37 °C, the presence of dissolved oxygen and a chloride concentration of around 0.9% w/v NaCl make them slightly less corrosive than seawater⁶,⁷. Therefore, when stainless steels are used in biological environments, the body fluids may attack the passive oxide film, leading to different forms of localized corrosion, such as pitting, crevice, and stress corrosion cracking. Also, as stainless steels have high chromium and nickel concentrations in their composition, the release of metallic ions in the tissue due to these corrosion processes, may lead to adverse reactions for the patients, such as blood coagulation and cases of chronic inflammation.

Based on these aspects, improving the existing solutions to use stainless steel in biological environments is necessary. Heating treatments were proposed to enhance the intergranular corrosion resistance of AISI 304L and AISI 316 steels⁸. Also, it was shown that the 2205 duplex steel presented an improved anti-corrosive performance in phosphate-buffer saline solution (PBS)⁶. Chemical surface treatments, including passivation, electropolishing, and acid cleaning, were also used for improving the corrosion-resistance properties of AISI 316L and 304L in Hank’s solution, showing improved anti-corrosive performance for the passivated and electropolished samples⁹.

Using a protective coating may also increase the corrosion resistance of the stainless steel and promote more excellent compatibility with the surrounding human tissues, which is also indispensable³,¹⁰,¹¹. Organic coatings can be used to enhance the antibacterial response of stainless steel orthopedic implants¹². Composite coatings of chitosan and silver nanoparticles were applied to coat AISI 304L for orthopedic implants using galvanic deposition, showing improved corrosive protection in a simulated body fluid at 37 ^oC¹³. Bioceramics coatings, such as zirconia, titania, alumina, and hydroxyapatite, also present the desirable characteristics to improve the anti-corrosive ability of the AISI 304L in body fluids.

Alumina coating stands out because of its inertia and excellent mechanical resistance¹⁴, and it is used mainly for orthopedic purposes¹⁵. Several methodologies can produce this coating, leading to different coating microstructures¹⁶. Sol-gel dip coating is one of the most used methodologies to produce alumina coating onto metallic substrates¹⁷-¹⁹. The main advantages of using sol-gel dip coating are the coating homogeneity and greater control of the film thickness and porosity²⁰. Furthermore, this methodology provides the opportunity to cover substrates with different shapes and sizes²¹ and different natures, such as glass, plastics, metals, and ceramics²².

The coating properties lie mainly in the substrate pretreatment step, which is carried out to increase the strength of the bonds between the substrate and the coating material²³. The adhesion of the alumina coating is dependent on the substrate preparation²⁴ since the increase in the surface roughness improves the substrate’s wettability²⁵. The application of conversion coatings as a pretreatment step is another procedure used to improve the adhesion of the alumina film to the substrate and, consequently, increase its resistance to corrosion²⁶,²⁷. The kind of conversion coating and the heating treatment temperature also affect the anti-corrosive performance of alumina coating/substrate systems in aggressive saline media²⁶,²⁸.

The phosphate-buffered saline (PBS), Hanks’s, and Ringer’s solutions are the most used aggressive media employed to simulate the body fluids⁶,⁹,²⁹. Ringer's solution is isotonic (pH ~6.5) and is frequently used in in vitro experiments on organs or tissues³⁰, which makes it adequate for the electrochemical evaluation of biomaterials³¹. Therefore, this work aims to evaluate the influence of substrate preparation and the kind of inorganic conversion coating produced by sol-gel dip coating on the corrosion resistance in Ringer’s solution of alumina film deposited on converted AISI 304L stainless steel surface.

2. Experimental Methodology

2.1. Substrate preparation

Two different methodologies were used to prepare the surface of the AISI 304L stainless steel samples (4.0 cm²). Some samples were abraded in silicon carbide paper ranging from 120-600 grit, while others were sandblasted using glass beads. The average roughness (R_a) of the sanded or sandblasted substrates was measured using a Mitutoyo SJ-210 roughness meter, following ISO 4287-1997, in three different regions of the sample. Immediately before the deposition, the sanded and sandblasted samples were cleaned with acetone in ultrasound for five minutes and then activated in 4 mol L⁻¹ KOH solution for 5 min at 75 °C²⁶,²⁸.

2.2. Production of the solutions/sols

2.2.1. Silica sol-gel - SiO₂

The silane precursor used in this work was tetraethyl orthosilicate (TEOS), while ethanol (EtOH) and acetic acid were the solvent and the catalyst used in the hydrolysis reaction, respectively. Using a molar ratio of TEOS:H₂O:EtOH was 4:90.5:5.5³², concentrated acetic acid was added slowly under stirring until pH 2.5²¹,³³. The solution was stirred until a homogenous suspension was obtained and kept standing for 24 h³⁴.

2.2.2. Boehmite sol-gel - AlO(OH)

Boehmite Disperal P2, kindly provided by Sasol®, was used to prepare a 0.4 mol L^-1 boehmite solution in water³⁵. The solution was stirred for six hours and kept standing for 24 h³⁴. Using boehmite powder in an aqueous sol reduces the reaction step and contributes to a lower generation of waste and liquid effluents.

2.2.3. Alumina sol – Al₂O₃

The alumina sol was prepared using the following molar rates: water:aluminum isopropoxide (the precursor) = 20; ethanol (the solvent):precursor = 5; and solvent:acetic acid (the catalyst) = 2.5. The sol was initially kept at 70 ºC under stirring during the first 4 h²¹ and then maintained at room temperature during the following 14 h of intermittent mixing²⁶,²⁸.

2.3. Production of the conversion coatings

The conversion coatings were deposited using dip coating equipment (MARCONI). Based on the literature data²⁶,²⁸, this equipment was configured with the parameters deposition time (t = 60 s) and withdrawal speed (v = 100 mm/min) for producing the boehmite or the silica monolayer coatings. In the case of multilayered conversion coatings (silica/boehmite or boehmite/silica), the substrate was dipped in the first sol (silica, for the silica/boehmite film, or boehmite, for the boehmite/silica film) during the same deposition time and removed using the same withdrawal speed earlier mentioned. The xerogel film was dried at room temperature for 2 min and then dipped in the second sol, forming double-layer films, one over the other.

Independent of whether monolayered or multilayered films were formed, all the xerogel films deposited on steel substrates were heat treated in a muffle furnace at 500 °C for 2 h²⁶,²⁸. The descriptions for the conversion coatings are presented in Table 1.

Table 1
Sample identification.

Sample	Substrate preparation	Conversion coating
J-SS	Sandblasted	-
JB500	Sandblasted	Boehmite
JS500	Sandblasted	Silica
JSB500	Sandblasted	Silica/Boehmite
JBS500	Sandblasted	Boehmite/Silica
L-SS	Sanded	-
LB500	Sanded	Boehmite
LS500	Sanded	Silica
LSB500	Sanded	Silica/Boehmite
LBS500	Sanded	Boehmite/Silica

2.4. Production of the alumina coatings

The conversion coatings deposited on sanded or sandblasted substrates that presented the best anti-corrosive performance were immersed in alumina sol for 60 s, withdrawn at a controlled speed of 100 mm min^-1, and dried at room temperature for 2 min. This procedure was repeated one more time to form a thicker coating. The final coated samples were then heated in a muffle furnace at 500 °C for 30 min²⁷.

2.5. Surface Characterization

The thicknesses of the conversion coatings were measured using a magnetic induction probe (Dualscope), according to DIN EN ISO 2178, at five different regions of the coated substrate. The morphology of these converted surfaces was analyzed by scanning electron microscopy (SEM) at 15 kV, using secondary electrons mode with 2000 X magnification, on a JEOL microscope.

Contact angles (CA) of converted stainless-steel surfaces were measured by a tensiometer (DATAPHYSICS OEC 15-EC) at room temperature. The volume of the alumina sol drop was 5 μL, and the droplet's shape was recorded by a direct reading goniometer telescope after 20 s at 25 °C. The angles between the baseline of the drop and the tangent at the drop boundary were calculated using the image analysis system.

Additionally, the average roughness (R_a) of the different conversion coatings deposited on both sanded and sandblasted substrates was measured using a Mitutoyo SJ-210 roughness meter, following ISO 4287-1997, in three different regions of the sample.

The conversion coatings that presented the best anti-corrosive performances were analyzed by X-ray diffraction (XRD) analysis using a Bruker D8 diffractometer and kα- Cu radiation. The 2θ scanning ranged from 10º to 90º, with a step of 0.020ºs^-1.

After covering them with alumina films, the surfaces of the selected conversion coatings were also evaluated by scanning electron microscopy (SEM) at 15 kV and secondary electrons mode using the same JEOL microscope. A magnification of 5000 X was used for these samples.

2.6. Electrochemical characterization

The conversion coatings were embedded in water-resistant resin to permit only 1 cm² of a delimited surface of the coated substrate to be exposed to the electrolyte. These samples were used as the working electrodes for the electrochemical experiments in a three-electrode electrochemical cell. In this cell, the reference electrode was a saturated calomel electrode, and the counter electrode was a platinum spiral. The electrochemical characterizations were performed in duplicate, at 25 °C, in a Ringer’s solution, aiming to simulate body fluids²⁹. The composition of this solution is presented in Table 2³⁶.

Table 2
Ringer’s solution composition.

Chemical	Quantity (g.L-1)
NaCl	6.80
KCl	0.40
CaCl2	0.20
MgSO4 7H2O	0.20
NaH2PO4 H2O	0.14
NaHCO3	2.20
Glucose	1.00

The electrochemical impedance spectroscopy (EIS) experiments were performed using a potentiostat/galvanostat AUTOLAB PGSTAT 302N at the open circuit potential (OCP) after 180 min for stabilization. The sinusoidal potential perturbation was applied with an amplitude of 10 mV and a frequency range from 10^-3 Hz to 10⁵ Hz. The conversion coatings that showed the best anti-corrosive performance for each substrate preparation were covered with alumina film and analyzed by EIS tests under the same conditions described earlier during 216 h of exposure to the Ringer's solution.

The result data were simulated and adjusted in both cases by METROHM NOVA 1.10 software using the equivalent circuits shown in Figure 1. The simulation adjustment was considered suitable for an error value smaller than 2%³⁷.

Figure 1
Equivalent circuits used to simulate the EIS data.

3. Results and Discussion

3.1. Conversion coatings’ surface characterization

Figure 2 shows the surface morphology of the boehmite, silica, boehmite/silica, and silica/boehmite conversion coatings on the sanded and sandblasted stainless-steel substrates. At the same time, Table 3 exhibits the average thickness and roughness values obtained for these coatings. For comparison, images of the bare sanded and sandblasted substrates, as well as their thickness and roughness average values, are also presented in Figure 2 and Table 3, respectively.

Figure 2
Surface morphology of the converted coatings produced on sanded and sandblasted substrates. The samples are presented as identified in Table 1. As comparison, images of a (A) sanded and a (B) sandblasted substrate are shown. (C) LB500; (D) JB500; (E) LS500; (F) JS500; (G) LBS500; (H) JBS500; (I) LSB500; (J) JSB500. Magnification: 2000 X.

Table 3
Average roughness (R_a) and thickness (h) of conversion coatings deposited on stainless steel substrates with different preparations.

Conversion coating	Sanded (L)			Sandblasted (J)
	Ra (μm)	Ra increase (%)	h (μm)	Ra (μm)	Ra increase (%)	h (μm)
SS	0.068 ± 0.019	-	-	2.160 ± 0.272	-	-
B500	0.249 ± 0.075	266	0.925 ± 0.020	2.484 ± 0.181	15.0	1.226 ± 0.200
S500	0.278 ± 0.094	309	0.568 ± 0.011	2.280 ± 0.132	5.54	0.985 ± 0.019
SB500	0.232 ± 0.014	240	1.180 ± 0.025	2.612 ± 0.270	20.9	2.513 ± 0.008
BS500	0.242 ± 0.072	255	1.008 ± 0.045	2.541 ± 0.303	17.6	1.502 ± 0.060

It is possible to observe that the sandblasted sample (Figure 2B) exhibited more surface irregularities than the sanded surface (Figure 2A), suggesting that the sample preparation affected the morphology of the substrate. Table 3 confirms these results, showing that the roughness values of the sandblasted steel surface (Ra) are higher than those of the sanded substrates.

All the conversion coatings formed onto sanded substrates (Figures 2C, 2E, 2G, and 2I) present smooth and apparently thin surfaces since it is possible to observe scratches formed in the substrate sanding process. Also, no significant differences between the studied conversion coatings can be perceived. The average thickness values (Table 3) confirm this assumption since the conversion coatings’ thicknesses are smaller than or near 1.0 μm. The thickest coatings were obtained for the multilayered conversion coatings, while the smallest one was verified for the silica conversion coating. On the other hand, rough, irregular, and apparently thicker coatings can be noted when the conversion coatings were produced on sandblasted substrates (Figures 2D, 2F, 2H, and 2J). Table 3 ratifies this observation, although the silica conversion coating produced on a sandblasted substrate also exhibited an average thickness smaller than 1.0 μm. The highest thickness values were observed for the multilayered coatings, mainly the silica/boehmite conversion coating (2.513 ±0.008 μm).

Amiriafshar et al.³⁸ indicated that the degree of surface roughness contributed to the uniformity of a Zn electrodeposited coating. Although another deposition process was used in the present work, the differences in the substrate morphology and roughness, observed in Figures 2A and 2B, may have affected the uniformity of the conversion coatings deposited on them using sol-gel dip coating. Therefore, the R_a values of the converted sanded surfaces are smaller than the converted sandblasted ones (Table 3). Nonetheless, it is noted in this table that the converted sanded surfaces exhibited higher average R_a values than the bare substrate, independent of the kind of conversion coatings used. The highest variations were verified when the sanded substrate was covered with silica (309%) or boehmite (266%) conversion coatings.

Considering the sandblasted converted samples, the R_a variation was smaller than the values observed for the sanded ones, as the roughness value of the substrate was already high (2.160 ± 0.272 μm). This initial high average R_a values could be responsible for the morphologies of the sandblasted samples, leading to irregular and rough surfaces, as shown in Figures 2D, 2F, 2H, and 2J. All conversion coatings increased the average R_a values compared to the bare sandblasted substrate. The highest R_a variations were verified for the multilayered samples JSB500 (20.9%) and JBS500 (17.6%), as already suggested in the micrographs of Figures 2H and 2J.

It is expected that the greater the roughness of the conversion coating, the greater the adhesion of the alumina film deposited on it, contributing to the production of improved anti-corrosive coating systems³⁹. Therefore, to obtain a better inference about the adhesion of the alumina sol to the different conversion coating surfaces studied, the contact angle of an alumina sol drop on the converted stainless steel substrates was measured. The images of the drop and the contact angle values are shown in Figure 3 for the conversion coatings produced on sanded or sandblasted substrates, respectively. The results obtained for the bare substrates are also presented for comparison.

Figure 3
Images of sessile drop for alumina sol on sandblasted and sanded stainless steel covered with the conversion coatings. The samples are presented as identified in Table 1. (A) J-SS; (B) JB500; (C) JS500; (D) JSB500; (E) JBS500; (F) L-SS (G) LB500; (H) LS500; (I) LSB500; (J) LBS500.

The contact angle (θ) is related to the work of adhesion (W_a), as shown in Equation 1, where γ_L is the free surface energy obtained from the liquid⁴⁰. Thus, the smaller the contact angle, the higher the adhesion.

$W_a={\gamma }_L\left(1+cos\theta \right)$ (1)

It is possible to note that the bare sandblasted substrate (Figure 3A) presents a lower alumina wettability than the sanded substrate (Figure 3F). This effect can be related to the higher surface area and roughness verified for the sandblasted sample, leading to a scattering of the alumina sol droplet. Thus, the present result indicates that the sandblasted surface is chemically more active than the sanded one⁴⁰, which could enhance the alumina adhesion.

However, different contact angle responses are verified when the converted surfaces are evaluated, depending on the substrate preparation and the conversion coating used. It was expected that converted sandblasted surfaces would result in smaller contact angle values than the converted sanded ones since a rough surface favors the penetration of liquid between its cavities, increasing the spread of the drop over the surface⁴¹,⁴². Nevertheless, the results shown in Figure 3 indicate that the conversion coatings produced onto sanded surfaces led to smaller contact angle values. This result could be related to the higher roughness variation verified for the converted sanded samples (Table 3).

Compared to the bare sandblasted substrate (Figure 3A), the contact angles of the boehmite and the silica/boehmite conversion coatings (Figures 3B and 3D, respectively) decreased, indicating that the presence of these conversion coatings may improve the adhesion of the alumina film. This result may be associated with the alumina sol interaction with the hydroxyl groups on the surface of these conversion coatings since the conversion of boehmite to alumina is not completed after the heat treatment²⁶.

On the other hand, an increase in the contact angles was observed when silica or boehmite/silica conversion coatings covered the sandblasted substrate (Figures 3C and 3E, respectively), decreasing the wettability for the alumina sol. Therefore, when silica xerogel was deposited as a monolayer or as the outer layer in a multilayered conversion coating onto a sandblasted substrate, it exhibited a lower adhesion with the alumina coating than the bare substrate. Okido et al.⁴³ and Lei et al.⁴⁴ have proposed that Si-O-Zn or Si-O-Mg bindings could have been formed during the condensation and heating temperature stages used to prepare silica and silane coatings, respectively, which decreased the interaction between the conversion coating and the testing solution. In the present case, the observed effect could be related to the formation of Si-O-Fe bindings or interactions between the silica and the hydroxyl groups in the bottom boehmite layer, leading to a decrease in the alumina film adhesion and the anti-corrosive performance of the whole coating system prepared under these conditions. Nonetheless, further experiments are needed to reach a conclusion about this topic.

Considering the converted sanded sample, however, all the conversion coatings presented lower contact angle values than that observed for the bare sanded substrate (Figure 3F), suggesting that the presence of the studied conversion coatings on a sanded surface would favor the adhesion of the final alumina film. Although few differences could be noted in the contact angles of these converted surfaces, it seems that the multilayered coatings favored a decrease in the contact angle value. The smallest one was verified for the boehmite/silica coating (Figure 3J).

Besides the improvement in the alumina final film adhesion to the substrate, it is also expected that the conversion coatings studied in this work may enhance the barrier effect of the final coating systems. Thus, high-thickness coatings, showing few defects, could contribute to achieving this effect. Based on the results in Figure 2 and Table 3, the coatings produced on sandblasted substrates exhibited higher thickness values than those prepared on sanded ones. However, the irregular surfaces observed for these sandblasted samples could also lead to defects that would permit the substrate to be attacked by the electrolyte. Nonetheless, the decrease in the alumina wettability observed in some of the rough and all of the smooth converted surfaces (Figure 3) may improve the adhesion of the alumina film deposited on it, contributing to upgrading the anti-corrosive coating systems. Therefore, these results suggest that the conversion coatings tested in this work may enhance the corrosion protection of alumina films.

3.2. Conversion coatings/substrate systems’ electrochemical evaluation

The EIS diagrams obtained for the conversion coatings deposited on sanded and sandblasted substrates are shown in Figure 4. Relating the increase in the diameter of the semicircle with the increase in corrosion resistance, the Nyquist diagrams of the systems prepared on sanded substrates (Figure 4A) show that the capacitive loops of all samples (LB500, LS500, LSB500, and LBS500) exhibited a larger diameter than that observed for the substrate. This result indicates that, regardless of the conversion coating deposited on sanded stainless steel substrates, all showed improved anti-corrosive performance in Ringer's solution. The highest increase was verified for the sample covered with the silica conversion coating (LS500). This sample also presented the highest impedance modulus (|Z|) at low frequency (approximately 5 X 10^-3 Hz), as shown in Figure 4B. The phase diagrams of these samples (Figure 4C) exhibit two time constants, which could be related to a surface film at high frequencies and the substrate at lower frequency values.

Figure 4
Electrochemical evaluation of conversion coatings produced on sanded (A, B, and C) and sandblasted (D, E, and F) substrates in the Ringer’s solution. (A, D) Nyquist diagram; (B, E) Bode modulus and (C, F) phase diagrams.

Figure 4D shows that all converted sandblasted samples also presented capacitive loops higher than the substrate, suggesting that these conversion coatings also improved the anti-corrosive resistance of a sandblasted stainless steel substrate in Ringer’s solution. Under these conditions, the highest semicircle was verified for the substrate covered with silica/boehmite conversion coating (JSB500). The modulus diagram in Figure 4E agrees with this result, as this system also presented the highest |Z| value at low frequency. However, the complexity of the coating systems can be noted by the phase diagrams (Figure 4F), as broad phase angles are observed, likely related to different and superimposed phase constants.

The data obtained from these diagrams were simulated using the equivalent circuits shown in Figures 1A and 1B, and the simulated results can be seen in Table 4. The circuit used in each case was chosen based on the phase diagrams (Figures 4C and 4F) and the assumptions previously made when they were discussed.

Table 4
Simulated results obtained from the EIS data shown in Figure 4.

SAMPLES	Ro	R1	N1	R3	N2	Rg	C1	C3	Cg	Circuit	χ2
	Ω	kΩ cm2		kΩ cm2		kΩ cm2	F cm-2	F cm-2	F cm-2
L-SS	35.6	9.15	0.711	8.15	0.784	17.3	5.82 x 10-6	9.67 x 10-7	8.29 x 10-7	[R(RQ)(RQ)]	0.0021
LB500	36.5	13.9	0.806	981	0.804	995	2.14 x 10-6	9.67 x 10-7	6.66 x 10-7	[R(RQ)(RQ)]	0.0019
LS500	30.4	15.8	0.803	1821	0.755	1837	1.17 x 10-4	3.38 x 10-9	3.37 x 10-9	[R(RQ)(RQ)]	0.0022
LSB500	34.4	9.87	0.796	383	0.713	393	1.69 x 10-4	1.72 x 10-7	1.72 x 10-7	[R(RQ)(RQ)]	0.0016
LBS500	71.1	9.65	0.822	143	0.554	153	4.92 x 10-6	4.90 x 10-7	4.46 x 10-7	[R(RQ)(RQ)]	0.0014
J-SS	27.6	10.2	0.807	-	-	10.2	1.19 x 10-4	-	1.19 x 10-4	R(RQ)	0.0034
JB500	34.4	9.95	0.700	159	0.808	68.2	9.03 x 10-6	3.16 x 10-5	7.55 x 10-6	[R(RQ)(RQ)]	0.0011
JS500	22.7	9.21	0.758	152	0.774	61.0	1.53 x 10-4	9.04 x 10-5	5.69 x 10-5	[R(RQ)(RQ)]	0.0009
JSB500	25.5	10.0	0.701	818	0.704	828	2.97 x 10-5	2.36 x 10-6	2.19 x 10-6	[R(RQ)(RQ)]	0.0027
JBS500	20.7	10.6	0.706	134	0.792	44.1	2.90 x 10-5	1.47 x 10-5	9.77 x 10-6	[R(RQ)(RQ)]	0.0016

The circuit in Figure 1A was used only to simulate the sandblasted substrate and is represented in Table 4 as [R(RQ)], where Q is the admittance of the constant phase element (CPE). The two series time constants shown in Figure 1B are related to the presence of a compact film on the surface of the steel substrates (R₃//CPE₃) and the substrate itself (R₁//CPE₁). This kind of circuit is represented in Table 4 as [R(RQ)(RQ)]⁴³. In these circuits, R_o represents the ohmic resistance, R₁ is the charge transfer resistance, and R₃ is the oxide layer (in the case of the sanded substrate) or the compact conversion coating resistance. R_g, the global resistance of the coating/substrate system containing the conversion coating or the oxide layer and the substrate resistances (R₁ and R₃, respectively), was calculated by Equation 2.

$R_g=R_1+R_3$ (2)

In the circuits of Figures 1A and 1B, CPE₁ and CPE₃ are the pseudocapacitances (or the constant phase element) of the substrate and the conversion coating or oxide layer, respectively. The C₁ and C₃ capacitances, related to the double-layer capacitance and the conversion coating or oxide layer capacitance, respectively, were calculated based on CPE₁ and CPE₃ using Equation 3⁴⁵-⁴⁷. N_i defines the equivalence degree of the constant phase elements for the capacitive component in this equation. The global capacitance of the protective coating/substrate system (C_g), considering the conversion coating and the substrate capacitances, was also obtained (Equation 4) to permit a better evaluation of the wholly capacitive effect.

$C_i=CP{E}_i{}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$N_i$}\right.}\times {\left(\frac{R_o{R}_i}{R_o+R_i}\right)}^{\left(\frac{1}{N_i}-1\right)}$ (3)

$\frac{1}{C_g}=\sum \frac{1}{C_i}$ (4)

It is possible to observe in Table 4 that all coated samples produced on sanded and sandblasted substrates presented higher R_g values compared to those exhibited by the uncoated sanded or sandblasted substrates, respectively, indicating that these coatings increased the barrier effect of the sanded stainless steel in a Ringer’s solution. However, the R_g values of the sanded or sandblasted substrates were smaller than the R_ct obtained for an as-received stainless steel sample in the same medium (29.6 kΩ.cm²)⁴⁸. Preparing a sanded or a sandblasted substrate involves removing the oxide layer (mainly chromium and iron oxides) present in the stainless steel samples, decreasing the corrosion protection. Also, this process may not be completely efficient; it may lead to a smooth surface partially covered by a thin oxide film layer (mainly when the sanding process was used) or an extremely rough surface (for the sandblasted ones). In both cases, it may contribute to the presence of anodic and cathodic areas on the substrate surface and adhesion problems of the conversion coatings on the substrate.

In addition to this point, the deposition of the conversion coatings by sol-gel dip coating on a sanded substrate is generally difficult due to its low surface energy (lower roughness), leading to smoother and thinner films, as observed in Table 3 and Figure 2. These features may likely facilitate the electrolyte access to the substrate. On the other hand, it was expected that the high roughness (Table 3) and the irregular image presented by the sandblasted substrate (Figure 2B) enhanced the deposition of the conversion coatings, producing thicker and more homogeneous films that could improve the barrier effect of these systems. Although these characteristics were obtained, defects can also be observed in the morphological images of Figure 2, which could have permitted the substrate to be attacked by the corrosive medium.

Considering the converted sanded samples, the highest R_g and lowest C_g values were verified for the LS500 sample, which have shown the smallest thickness value among all sanded samples (Table 3). This result is likely related to the morphological homogeneity presented by this conversion coating (Figure 2E), which may have led to a smaller surface area exposed to the corrosive medium than the other samples, increasing the global resistance and reducing the global capacitance. Among the converted sandblasted samples, the best anti-corrosive performance was observed for the JSB500 sample, which also presented the highest thickness value (Table 3). In this case, the increase in the conversion coating thickness enhanced the barrier effect of the system, as expected.

Based on these results and the low contact angles with the alumina sol drop presented by LS500 and the JSB500 samples (Figure 3), these samples were selected to be covered with the final alumina film and electrochemically evaluated for long exposure periods in a Ringer’s solution.

3.3. Alumina film/selected conversion coatings/substrate systems’ characterization

3.3.1. Phase analysis

The diffraction patterns obtained for the conversion coatings selected to be covered with alumina film are shown in Figure 5. The main significant difference observed in the structural composition of the two selected coatings is the presence of alumina and non-converted boehmite in the JSB500 sample and the absence of these compounds in the LS500. In both cases, it is possible to note the presence of diffraction peaks referring to silica, independent of the surface treatment or sol used. CrO, Fe₂O_3, and Fe₃O₄ were also detected on both surfaces, resulting from chromium and iron's thermal oxidation (from the stainless steel substrate) and the chemical reactions between the substrate and the sols used when the xerogel film was formed. The presence of these iron and chromium oxides contributes to the anti-corrosive performance of the system²⁶-²⁸,⁴⁴ by filling possible pores with the formed oxides and facilitating the formation of alumina in a more adherent and stable phase. Diffraction lines from the substrate (Fe and Fe-Cr) can also be noted.

Figure 5
Phase analysis of the selected conversion coatings.

3.3.2. Morphological evaluation

The surface morphology of the LS500 and the JSB500 samples covered with alumina film (from now on, denominated LSA500 and the JSBA500 samples) are presented in Figure 6. The main characteristics already observed for the converted coatings can also be noted here, indicating that the roughness of the converted coating affected the morphology of the whole coating system. The LSA500 sample (Figure 6A) still shows a smooth surface, although some corrugation and small cracks can be noted. On the other hand, the JSBA500 sample exhibits a rougher surface with few defects. These morphologies may affect the anti-corrosive performance of these coating systems in the studied corrosive environment, mainly after a long time of exposure.

Figure 6
Morphological evaluation of the selected alumina coating systems. (A) LSA500; (B) JSBA500. Magnification: 5000 X.

3.3.3. Long-time electrochemical evaluation of the alumina film/conversion coating/stainless steel substrate systems

The JSBA500 and LSA500 systems were electrochemically evaluated during 216 h of exposure in the Ringer’s solution using EIS. The results comparing the t = 3 h (180 min for OCP stabilization) and t = 216 h is shown in Figure 7.

Figure 7
Long-term electrochemical evaluation of the selected alumina coating systems in the Ringer’s solution. (A, B, and C) LSA500; (D, E, and F) JSB500. (A, D) Nyquist diagram; (B, E) Bode modulus and (C, F) phase diagrams.

Independent of the substrate preparation and the kind of conversion coating used, it is interesting to note an increase in the capacitive loop of both systems when the exposure time increased (Figures 7A and 7D for the LSA500 and JSBA500 systems, respectively), indicating an increase in the anti-corrosive ability of the coating systems. This behavior could be related to the presence of corrosion products in the interface substrate/conversion coating generated by the attack of the electrolyte through the pores and defects of the coating system. These corrosion products are likely composed of iron and/or chromium oxides, which may have filled the coating system's pores and reduced the substrate's corrosion process as the exposure time increased.

Although this difference in the electrochemical performance of the systems is not very clear in the modulus diagrams (Figures 7B and 7E for the LSA500 and JSBA500 systems, respectively), another time constant is present the phase diagrams obtained after 216 h of exposure time (Figures 7C and 7F for the LSA500 and JSBA500 systems, respectively). This new time constant could be related to forming a corrosion product layer on the interface between the substrate and the conversion coating covered by alumina film as the exposure time increases. As earlier proposed, these corrosion products could have filled the pores present in the coatings, improving corrosion protection. Additionally, the layer they formed could help avoid further corrosion processes, enhancing the anti-corrosive properties of the whole coating system.

The EIS data were simulated using the equivalent circuits in Figures 1B and 1C. For the systems evaluated after 180 min of exposure in the corrosive medium (t = 3 h), independent of the substrate preparation, the circuit presented in Figure 1B was used. The two time constants in series (R₁//CPE₁ and R₃//CPE₃) are related to the presence of a compact coating formed by the alumina film + the conversion coating and the stainless steel substrate, respectively. In the circuit of Figure 1C, another time constant was included (R₂//CPE₂), representing the corrosion product formed when the exposure time to the corrosion medium increased. This circuit is represented in Table 5 as [R(RQ)(RQ)(RQ)]. In these cases, the global resistance (R_g) was also calculated by the summation of the individual resistances, as shown in Equation 2 (R₁ + R₃, for the systems assayed at t = 3 h; R₁ + R₂ + R₃, for longer exposure times). C₁, C₂, and C₃ capacitances, related to the double-layer, the corrosion product layer, and the alumina film + conversion coating capacitances, respectively, were calculated based on CPE₁, CPE₂, and CPE₃ using Equation 3⁴⁵-⁴⁷. The global capacitance of the protective coating/substrate system (C_g) was also obtained using Equation 4, considering the time the system was exposed to the corrosive medium. The R_g and C_g variations are presented in Figure 8, while the complete simulated results are exhibited in Table 5.

Table 5
Simulated results obtained from the EIS data of the selected systems at different exposure times in Ringer’s solution.

SAMPLES	Ro	R1	N1	R2	N2	R3	N3	Rg	C1	C2	C3	Cg	Circuit	χ2
	Ω	kΩ cm2		kΩ cm2		kΩ cm2		kΩ cm2	F cm-2	F cm-2	F cm-2	F cm-2
LSA500 3H	36.3	11.3	0.653	-	-	6.46	0.975	17.8	1.64 x 10-6	-	8.58 x 10-6	1.38 x 10-6	[R(RQ)(RQ)]	0.0019
LSA500 24H	38.0	5.55	0.816	0.370	0.642	146	0.615	152	5.92 x 10-6	7.52 x 10-10	1.31 x 10-5	7.52 x 10-10	[R(RQ)(RQ)(RQ)]	0.0018
LSA500 48H	48.1	4.46	0.642	0.203	0.853	178	0.656	183	4.90 x 10-7	2.66 x 10-7	2.92 x 10-5	1.73 x 10-7	[R(RQ)(RQ)(RQ)]	0.0038
LSA500 72H	49.3	3.43	0.565	0.389	0.838	183	0.620	187	1.30 x 10-7	1.56 x 10-7	2.18 x 10-5	7.07 x 10-8	[R(RQ)(RQ)(RQ)]	0.0054
LSA500 144H	54.1	2.38	0.667	0.658	0.851	234	0.598	237	9.47 x 10-7	9.80 x 10-8	1.63 x 10-5	8.88 x 10-8	[R(RQ)(RQ)(RQ)]	0.0064
LSA500 168H	56.6	2.69	0.663	0.680	0.860	138	0.642	141	1.01 x 10-6	9.73 x 10-8	3.37 x 10-5	8.85 x 10-8	[R(RQ)(RQ)(RQ)]	0.0016
LSA500 192H	52.6	2.30	0.674	0.677	0.859	219	0.604	222	9.46 x 10-7	9.28 x 10-8	1.78 x 10-5	8.41 x 10-8	[R(RQ)(RQ)(RQ)]	0.0072
LSA500 216H	53.2	2.41	0.649	0.662	0.851	120	0.648	123	1.03 x 10-6	9.36 x 10-8	3.76 x 10-5	8.56 x 10-8	[R(RQ)(RQ)(RQ)]	0.0070
JSBA500 3H	30.3	1.02	0.614	-	-	27.3	0.526	28.3	2.86 x 10-6	-	2.38 x 10-6	1.30 x 10-6	[R(RQ)(RQ)]	0.0041
JSBA500 24H	38.8	1.14	0.598	0.440	0.563	89.0	0.598	90.6	2.33 x 10-6	1.87 x 10-7	2.21 x 10-5	1.72 x 10-7	[R(RQ)(RQ)(RQ)]	0.0095
JSBA500 48H	50.1	0.932	0.599	0.422	0.618	78.6	0.651	80.0	3.22 x 10-6	2.89 x 10-6	5.20 x 10-5	1.48 x 10-6	[R(RQ)(RQ)(RQ)]	0.0046
JSBA500 72H	48.8	1.23	0.616	0.207	0.882	74.1	0.652	75.5	3.47 x 10-6	3.02 x 10-4	6.14 x 10-5	3.25 x 10-6	[R(RQ)(RQ)(RQ)]	0.0047
JSBA500 144H	55.4	0.945	0.663	0.436	0.599	90.5	0.663	91.9	4.64 x 10-5	3.34 x 10-6	7.74 x 10-5	3.00 x 10-6	[R(RQ)(RQ)(RQ)]	0.0050
JSBA500 168H	56.3	0.495	0.586	0.249	0.890	70.0	0.684	70.7	3.59 x 10-6	2.23 x 10-4	9.49 x 10-5	3.40 x 10-6	[R(RQ)(RQ)(RQ)]	0.0052
JSBA500 192H	58.9	1.57	0.606	0.277	0.876	60.6	0.772	62.4	2.20 x 10-6	2.41 x 10-4	2.63 x 10-5	2.17 x 10-6	[R(RQ)(RQ)(RQ)]	0.0060
JSBA500 216H	53.2	0.471	0.581	0.228	0.891	59.5	0.686	60.2	4.07 x 10-6	2.30 x 10-4	1.08 x 10-4	3.85 x 10-6	[R(RQ)(RQ)(RQ)]	0.0060

Figure 8
(A) R_g and (B) C_g variation observed for the selected alumina coating systems during 216 h of exposure in the Ringer’s solution.

Independent of the sample evaluated, Figure 8A shows an increase in the R_g values as the exposure time in Ringer’s solution increased. After 216 h of exposure, R_g increased 86% for the LSA500 sample and 53% for the JSBA500 sample. These results agree with the Nyquist diagrams shown in Figure 7 and indicate that the presence of the intermediate corrosion product layer observed since 24 h of exposure contributed to increase the corrosion protection of the systems. Although the LSA500 system presented a smaller R_g value than the JSBA500 after 3 h of exposure, Figure 8A and Table 5 also show that the anti-corrosive improvement observed in the LSA500 system for longer exposure times were consistently higher than that observed for the JSBA500. This result suggests that the substrate preparation and the kind of conversion coating used directly influenced the long-time corrosion resistance of the studied systems.

In addition to this result, it is essential to evaluate the C_g variation with the exposure time (Figure 8B). It is clearly seen a decrease in the C_g value after 24 h of exposure to the Ringer’s solution for both coating systems, confirming that the presence of the corrosion product enhanced their anti-corrosive performance. This C_g decrease was sharper for the LSA500 system, reaching 7.52 X 10^-10 F.cm^-2. However, after a small increase in C_g for 48 h of exposure, it is interesting to note that the rise in the exposure time led to different results depending on the system evaluated. While it was observed a decrease in the C_g value for the LSA500 system, remaining around 8 X 10^-8 F.cm^-2, an increasing trend was verified in this variable for the JSBA500, reaching 3.85 X 10^-6 F.cm^-2, an increase of 66%.

As C_g was calculated based on capacitances C₁, C₂, and C₃ (Equation 4), it is necessary to verify the behavior of these capacitances with the exposure time. Considering the values in Table 5, almost no variations could be noted in the C₁ values related to the double-layer capacitance, independent of the system evaluated. Although there was a slight increase in the values of the C₃ after 24 h of exposure, the capacitance of the compact ceramic coating of the LSA500 system showed few changes. On the other hand, there is an increasing trend of this capacitance value for the JSBA500 system, mainly for longer exposure times, likely suggesting a degradation of the coating and an increase in the exposed area. However, it is possible to note that the presence of the corrosion product layer directly influenced the C_g values, as the C_g decrease observed after 24 h of exposure (Figure 8B) is related to C₂, as well as the slight increase in the C₂ values after 48 h, also seen in Table 5. After this point, C₂ values decreased and became stable for the LSA500 system, while trended to increase for the JSBA500 system.

The contact angles (with an alumina sol drop) of the silica conversion coating on a sanded stainless steel substrate (LS500) and the silica/boehmite conversion coating on a sandblasted substrate (JSB500) were not significantly different (Figure 3). Nonetheless, the smaller value obtained for the LS500 sample (19.5^o against 21.3^o for the JSB500) may indicate a higher adhesion to the alumina coating when compared to the JSB500, leading to an increase in the barrier effect of the final coating system (LSA500 system). Additionally, this coating system exhibited a more regular morphology, showing few defects (Figure 6A), which could have contributed to decreasing the electrolyte attack to the substrate and the formation of a stable and probably protective corrosion product layer, reducing further corrosion processes and leading to high R_g and low C_g values.

Although both systems exhibited improved anti-corrosive performance when submitted to long-time exposure to the Ringer’s solution, the performance of the JSB500 system was inferior to the LSA500 system. The higher CA value verified for the JSB500 sample likely resulted in lower adhesion to the alumina film, and the electrolyte infiltration could have been facilitated. Also, an irregular surface with defects (Figure 6B) could have resulted in lower corrosion protection with time when the JSBA500 system was evaluated. In this case, the corrosion product layer may not be stable, leading to an increase in C₂ values at high exposure times. These characteristics probably resulted in lower R_g values and higher C_g values compared to the LSA500 system.

4. Conclusion

The effects of substrate preparation and the kind of conversion coating used on the anti-corrosive performance of alumina film on a stainless steel substrate in a Ringer’s solution were evaluated. It was verified that the substrate preparation directly affected the morphology of the conversion coatings and their roughness. Thus, sanded substrates produced smooth and less rough coatings, while sandblasted surfaces led to rougher and more irregular coatings. Also, the contact angle of these coatings’ surfaces with the alumina sol drop depended on the substrate preparation and the kind of conversion coatings used. For sandblasted surfaces, the smallest CA values were obtained for the boehmite and silica/boehmite conversion coatings (JB500 and JSB500, respectively), while low CA values (< 20^o) were verified for all sanded converted samples.

The EIS evaluation in Ringer’s solution showed that all sanded or sandblasted converted samples could enhance the barrier effect of the sanded or sandblasted stainless steel, respectively. The best anti-corrosive performances were verified for the LS500 and JSB500 samples among all sanded or sandblasted converted samples. After covering them with an alumina film, these coatings, containing chromium and iron oxides, were evaluated for long-time exposure in Ringer’s solution. Although both systems showed improvements in their anti-corrosive performance for higher exposure time, the LSA500 system, showing a smoother surface with few defects and a higher adherence between the conversion coating and the alumina film, exhibited an enhanced result, showing an R_g increase of 86% and a C_g decrease of 94% after 216 h of exposure to the Ringer’s solution.

5. Acknowledgments

The authors would like to thank the Carlos Chagas Filho Foundation for Research Support of Rio de Janeiro (FAPERJ), the Brazilian National Research Council (CNPq), the Postgraduation Support Program (PROAP), the Rio de Janeiro State University (UERJ) for the financial support. Camila S. Silva also thanks FAPERJ for the TCT scholarship. This study was financed in part by the "Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES)" - Finance Code 001.

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Publication Dates

• Publication in this collection
  10 May 2024
• Date of issue
  2024

History

• Received
  30 Jan 2024
• Reviewed
  30 Mar 2024
• Accepted
  02 Apr 2024