Protective Efficiency of ZrO₂/Chitosan “Sandwich” Coatings on Galvanized Low-Carbon Steel

Abstract

Enhanced corrosion efficiency of low-carbon steel was achieved by newly developed hybrid multilayers, composed of low-carbon steel coated with an electrodeposited zinc sublayer (1 µm), a chitosan (CS) middle layer and ZrO₂ coating by the sol–gel method (top-layer). The middle chitosan layer was obtained by dipping galvanized steel substrate in 3% tartatic acid water solution of medium molecular-weight chitosan, composed of β-(1–4)-linked D-glucosamine and N-acetyl-D-glucosamine with a deacetylation degree of about 75–85% (CS). The substrates were dipped into CS solution and withdrawn at a rate of 30 mm/min. One part of the samples with the CS layer was dried at room temperature for 2 weeks, and another part at 100 °C for 1 h, respectively. After CS deposition treatment, the substrates were dipped into an isopropanol sol of zirconium butoxide with small quantity of polyethylene glycol (PEG400). The dipping-drying cycles of the ZrO₂ coatings were repeated three times. After the third cycle, the final structures were treated at 180 °C. The samples were denoted as T25, which consists of the CS middle layer, and dried at RT and T100 with the CS middle layer treated at 100 °C, respectively. The samples were characterized by means of differential thermal analysis (DTA-TG), XRD analyses, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Hydrophobicity properties were evaluated by measuring the contact angle with a ramé-hart automated goniometer. Two electrochemical tests—potentiodynamic polarization technique (PD) and electrochemical impedance spectroscopy (EIS)—have been used to determine the corrosion resistance and protective ability of the coatings in a 5% NaCl solution. The results obtained by both methods revealed that the applied “sandwich” multilayer systems demonstrate sacrificial character and will hopefully protect the steel substrate in corrosion medium containing chloride ions as corrosion activators. The newly obtained hybrid multilayer coating systems have dense structure and a hydrophobic nature. They demonstrated positive effects on the corrosion behavior at conditions of external polarization independent of their various characteristics: morphology, grain sizes, surface roughness and contact angle. They extend the service life of galvanized steel in a chloride-containing corrosion medium due to their amorphous structure, hydrophobic surface and the combination of the positive features of both the chitosan middle layer and the zirconia top layer.

1. Introduction

Steels are basic traditional materials used in transport and in the construction of architectural and infrastructural sites (buildings, bridges, railway facilities, etc.). Generally, stainless steels distinguish with high corrosion resistance and mechanical stability, but they are highly expensive. In that sense, their application must also be realized in the light of financial possibilities. One way to provide good corrosion protection at an affordable price is the application of low-carbon or low-alloyed steels with an additional protective layer or system. For example, a possible approach to protect the respective steel products is the galvanizing of low-carbon steel by various methods. The practical role of zinc is as a sacrificial coating, as it is destroyed firstly by corrosion attack, forming a protective surface layer, the latter slowing down the demolition rate. However, the resistance of the zinc coating decreases strongly in media containing aggressive ions (chlorine, sulphate, etc.), such as in marine environments. The protective effect of the zinc can be significantly increased by additional surface treatment to form various types of protective films—conversion, sol–gel, etc. [1,2,3,4,5,6].

In recent years, the application of non-toxic oxide coatings on metal surfaces is a highly promising approach for their effective protection against corrosion. Researchers are focused on studying the effect of different types of inorganic coatings such as TiO₂, ZrO₂, SiO₂ and CeO₂ on steels to increase their corrosion resistance. These coatings have several significant advantages such as good mechanical properties (strength, hardness, wear resistance, etc.), chemical and thermal inertness, high dielectric constant and coefficient of thermal expansion, which reduces the possibility of cracks during the heat treatment. In addition, they are biocompatible and therefore have wide application in medicine (as bioimplants, etc.) [7].

It can be concluded that the deposition of selected oxide layers [8,9,10,11,12,13,14,15,16,17,18] mainly on stainless steels is the subject of intensive research worldwide. To the best of our knowledge there is practically no information (or the latter is rather scarce) available in the world scientific literature on the deposition of such layers as a top coating on galvanized steel.

In addition, multilayer coatings have been recently increasingly used as anti-corrosion coatings due to the fact that they provide better protective properties compared with single-layer (one-component) coatings [19,20,21]. This configuration has a combined effect of their advantages, as the surface layer, which should protect the base metal, should have increased corrosion resistance. Our previous investigations have revealed that the applying of multi-layer configurations is a more promising approach with respect to single-oxide coatings [7]. Recently, organic–inorganic composite (hybrid) films have received great attention in the field of corrosion protection due to the combination of useful properties of both organic and inorganic components. The most common methods to produce these hybrid films are, in particular by thin-film deposition [20] or by incorporation or surface decoration of an organic matrix with inorganic fillers [22,23].

Chitosan (CS) is the most plentiful natural polymer next to cellulose, which is derived from the polysaccharide chitin [24]. CS is a promising material for corrosion-protective coatings due to its low cost and specific solubility. Another highly useful feature of CS layers is a good adhesion to both metallic substrate and sol–gel coatings. Several works have proved that chitosan can be used as a polymer reservoir for different corrosion inhibitors. Additionally, its matrix could be modified either in the bulk or interface. The possibility of using inhibitor-loaded chitosan systems to protect metallic substrates, either as components in multi-layer coatings systems, or as free-standing films was revealed [25,26,27,28,29,30,31]. It was also found that CaP/chitosan coating effectively protected the AZ91D magnesium alloy from corroding. A few works are devoted to the preparation of composite coatings based on chitosan with TiO₂ and ZnO, using various techniques. CS possesses hydroxyl, amine and carbonyl functional groups, which can serve as chelation sites of various metal ions. When CS is dissolved in a dilute acidic medium, the amine groups become protonated, and the ensuing positive charges confer polyelectrolyte-like behavior to the macromolecule. Therefore, it is necessary to choose a weak acid as the solvent, such as tartaric acid (TA). TA contains abundant carboxyl groups (–COOH), which can react with the amino groups in CS and form a stable double-chelating network [30].

Due to the above positive qualities of both components (chitosan and ZrO₂), it would be interesting to investigate the anticorrosion behavior of new “sandwich” type structures on galvanized steel, covered by the organic middle layer, composed by chitosan, dissolved in TA water solution and inorganic ZrO₂ sol–gel coating as the top layer. The aim is to study how the treatment temperature of the chitosan layer influences the physicochemical parameters of the CS-ZrO₂ on galvanized steel “sandwich” structures and the relationship with the corrosion resistance of the galvanized steel.

2. Materials and Methods

2.1. Chemicals and Sample Types

The following chemicals were used: Zirconium butoxide (Zr(C₄H₉O)₄—Alfa Aesar), medium molecular-weight chitosan, composed of β-(1–4)-linked D-glucosamine and N-acetyl-D-glucosamine with a deacetylation degree of about 75–85% (Sigma Aldrich, Saint Louis, MO, USA). The value of the molecular weight of the chitosan is 190–310 kDa for medium molecular weight chitosan (MMW).

Low-carbon steel substrates with composition (in weight %) C 0.05–0.12; S ≤ 0.04; P ≤ 0.35; Mn 0.25–0.5; Cr ≤ 0.1; Si ≤ 0.03; Ni ≤ 0.3; Cu ≤ 0.3; As ≤ 0.08; Fe—balance (manufacturer Metalsnab, Sofia, Bulgaria) and sizes 3 cm × 1 cm × 0.1 cm were galvanically coated with a layer of ordinary zinc with a thickness of 1 µm from an electrolytic bath at electrodeposition conditions: current density 2 A dm⁻², pH value ~5, temperature range 20–25 °C, metallurgical Zn anodes. The composition of the bath was: ZnSO₄·7H₂O—150 g·dm⁻³; (NH₄)₂Cl—30 g·dm⁻³; H₃BO₃—30 g·dm⁻³.

2.2. Preparation of Chitosan Solution

Chitosan solution was prepared by dispersing chitosan (0.5 g) in 3% water solution of tartaric acid and stirring with a magnetic stirrer (manufacturer, city and country) for 2 weeks. After ultrasonic cleaning of the substrates (zinc layer with a thickness of 1 µm on steel substrate) in ethanol for 10 min, they were immersed each one in the CS solution, and after 30 s were withdrawn at a rate of 30 mm/min. The samples were divided into 2 groups—for the first, the CS layer was dried at room temperature (RT) for 1 week and for the second group, the deposits with the CS layer were treated thermally in air first at 50 °C for 0.5 h, after which the temperature rises to 100 °C for 1 h.

2.3. Preparation of ZrO₂ Coatings on the CS Underlayer

Zirconium butoxide (47.2 mL) was diluted with 45 mL isopropanol and small amount of acetyl acetone (8.6 mL) and acetic acid (5.6 mL) as complexing agents. It was added 0.4 mL nitric acid and 0.5 mL polyethylene glycol (PEG400). The final solution was diluted up to 0.2 M. The solution was stirred for 2 h in order to complete the reaction between alkoxide and acetic acid. The obtained solution possessed a yellowish-transparent color. The previously obtained series samples of galvanized steel, coated with the CS layer, were immersed into Zr-based precursor solution; after the withdrawing (rate of 30 mm/min), they were dried at room temperature (RT) and subsequently at 100 °C for 0.5 h. The final treatment of the structures was carried out at 180 °C for 1 h. The deposition and treatment of the ZrO₂ coating were repeated 3 times. The final hybrid multilayer systems were denoted as: 1/. The sample with CS, dried at RT—as T25; 2/. The sample with CS, thermally treated at 100 °C—as T100.

2.4. Differential Thermal (DTA)/Thermogravimetric (TG) Analyses

The differential thermal analysis was accomplished on a combined DTA–TG apparatus LAB-SYSEVO 1600 manufactured by the SETARAM Company (Lyon, France). Synthetic air was used as the carrier gas: flow rate of 20 mL min⁻¹; heating rate 10 °C/min; and probe mass is 25 mg. The analyses were carried out proceeding within the interval from 25 °C to 250 °C. A corundum crucible and a Pt/Pt–Rh thermocouple were used.

2.5. AFM Studies

The surface topography was evaluated by means of atomic force microscope (AFM) (NanoScopeV system, Bruker Inc., Billerica, MA, USA) operating in tapping mode in air. The scanning rate was set at 1 Hz. Subsequently, all the images were flattened by means of the Nanoscope software. AFM permits evaluation of the roughness of the sample surface and its standard deviation (R_q). The roughness analysis gives also the value R_a.

Images from three independent locations of the samples were taken for reproducibility purposes. From the applied roughness analysis, statistical values according to the relative heights of each pixel in a particular AFM image were calculated [31]. The roughness analysis gives the value R_a, which is an arithmetic average of the absolute values Z_j of the surface height deviations measured from the mean plane, i.e.,

$$R_a=\frac{1}{N}{\displaystyle \sum }_{i=1}^N\left|Z_i\right|$$

(1)

while the image R_q is the root mean square average of height deviations taken from the mean image data plane, expressed as

$$R_q=\sqrt{\frac{1}{N}{\displaystyle \sum }_{i=1}^N{Z}_i^2}$$

(2)

2.6. XPS Studies

The measurements were carried out on AXIS Supra electron-spectrometer (Kratos Analitycal Ltd., Stretford, England) using achromatic AlKα radiation with a photon energy of 1486.6 eV and charge neutralisation system. The binding energies (BE) were determined with an accuracy of ±0.1 eV, using the C1s line at 284.6 eV (adsorbed hydrocarbons). Using the commercially available data processing software of Kratos Analytical Ltd., the concentrations of the different chemical elements (in atomic %) were calculated by normalizing the areas of the photoelectron peaks to their relative sensitivity factors.

2.7. Contact Angle Measurements

Contact angle measurements were performed with the ramé-hart automated goniometer model 290 with DROPimage advanced v2.4 (Succasunna, Roxbury, NJ, USA) at room temperature. Water drops of 2–5 μL were formed and deposited by the ramé-hart automatic dispensing system The corrosion resistance of the samples was estimated according to EN ISO10289/2006). [Bulgarian Institute for Standardization, https://bds-bg.org/bg/project/show/bds:proj:50398 (accessed on 12 September 2010).] In order not to disturb the integrity of the test coating, the samples were washed several times with distilled water and dried before weighting.

2.8. Electrochemical Tests—Potentiodynamic Polarization (PDP) and Electrochemical Impedance Spectroscopy (EIS) Investigations

In order to characterize the corrosion behavior of both sample types, two electrochemical methods have been applied—potentiodynamic (anodic and cathodic) polarization (PDP) and electrochemical impedance spectroscopy (EIS). For these experiments, a tri-electrode electrochemical glass cell with a volume of 300 mL was used. Reference electrode was saturated calomel electrode (SCE) and a platinum plate serves as a polarizing one. Both electrochemical investigations were realized by “PAR VersaStat 4” unit (Princeton Applied Research, Oak Ridge, TN, USA). PDP curves were carried out at a selected low potential scan rate—1 mV/s. EIS measurements were realized in the interval 100 kHz–10 mHz. The equipment used for potentiodynamic investigations was according to [29,30].

2.9. Reproducibility

All electrochemical tests have been realized in a selected model corrosive medium of 5% NaCl solution. The reproducibility of the tests was an average of 5 samples per sample type.

3. Results

3.1. Differential Thermal (DTA)/Thermogravimetric (TG) and XRD Analyses

The DTA curves of chitosan have not revealed any peaks up to 200 °C. The change in the TG curve of the sample was observed at about 100 °C interpreted as desorption of water molecules, adsorbed on the top oxide coating as well as evaporation of water in the inner polymer and indicates about 8% mass loss. Similar results regarding the thermal behavior of chitosan were proved by other researchers [24,32]. The second stage is due to the dehydration of the zirconium organic precursor at about 190 °C with 1% mass loss [6].

According to the DTA results we have chosen the temperature of the final thermal treatment of our samples to be 180 °C.

Additional XRD analyses (Figure 1b) revealed only the main peaks, which belong to metallic Fe and Zn. In both types of structures, the crystallographic phase peaks of ZrO₂ were not recorded, i.e., they had an X-ray amorphous structure.

3.2. AFM Studies

The AFM method was used for more precise examination on the surface in the nanometric scale. It can be seen that the coating of T25 is dense, without any visible pores (Figure 2). The surface has a grained structure, having numerous grains with various sizes. The freshly prepared T100 coating after treatment at 100 °C is also relatively dense but possesses a more irregular structure than the T25 sample (Figure 2). As can be expected, the surface roughness of this sample is higher than that of T25 coatings (

Table 1. AFM analyses of the roughness of the coatings.

Roughness (nm)	Galvanized Steel	T25	T100
	Scan 5 µm	Scan 5 µm	Scan 5 µm
Rq	66	119	230
Ra	52	95.7	178

).

Table 1 represents the Ra and Rq values for all freshly prepared coatings and the substrate (galvanized steel). The results obtained are due to the so-called “sintering effect”: after thermal treatment the particles size increases and proceeds toward agglomeration of the ions. The closer contact between the particles leads to decreased surface roughness.

3.3. XPS Investigations

The surface chemistry of the ZrO₂ layers were characterized by the XPS-method. XPS analysis presents that both types of films show similar surface characteristics. Figure 3 shows the high-resolution photoelectron signal of C1s and O1s. The C1s peaks are typical for composite layers and could be deconvoluted into several components, corresponding to the bonds: C–C, C–O and C=O. After deconvolution of the O1s peaks O=C and O–C bonds were registered (Figure 3). The binding energies of the C1s and O1s components are shown in

Table 2. XPS results of the deconvoluted components of C1s peak.

Binding Energy (eV)	Chemical Bond	Concentration (%)
C1s		T25
284.8	C-C	56.1
286.4	C-O	17.1
288.7	C=O	26.8
C1s		T100
284.8	C-C	47.0
286.3	C-O	18.5
288.6	C=O	34.5

and

Table 3. XPS results of the deconvoluted components of O1s peak.

Binding Energy (eV)	Chemical Bond	Concentration (%)
O1s		T25
529.9	O-Me	21.5
531.7	O=C	72.2
533.9	O-C	6.3
O1s		T100
529.8	O-Me	22.3
531.7	O=C	71.3
533.8	O-C	6.4

, respectively.

Figure 4 presents Zr3d core level spectra. The position of the peaks at 182.2 eV for Zr3d_5/2 and at 184.4 eV for Zr3d_3/2 and spin-orbit separation of ~2.2 eV between 3d_5/2 and 3d_3/2 lines are characteristic for ZrO₂. A Zn2p peaks are also observed, due to the Zn diffusion from the substrate.

3.4. Contact Angle Measurements

Figure 5 represents the photographs of water drops on the coatings surface. It is interesting to note that the chitosan layers possess a hydrophilic nature: CS layer, dried at RT has contact angle of 45.6°, while the same parameter of the sample treated at 100 °C is 41°. The zirconia layer clearly expressed the hydrophobic nature of the surface with a contact angle of −107°.Both types of “sandwich” multilayer coatings possess the hydrophobic nature of the surface (Figure 6). The water contact angles have close values and the average values for T25 and T100 coatings are 104° and 121°, respectively.

3.5. Potentiodynamic Investigations

Figure 7 demonstrates the results obtained based on the potentiodynamic polarization curves of the investigated samples in the model corrosive medium of 5% NaCl solution. For comparison, the PD curve of low-carbon steel is also presented. The electrochemical cell used is presented in Figure 8.

Some of the most important and characteristic electrochemical parameters such as corrosion potential (E_corr) values, corrosion current densities (I_corr) as well as the anodic current densities in the middle of the demonstrated passive zones (I_pass) from the PD curves of the investigated samples are presented in

Table 4. Electrochemical parameters according to the PD polarization curves.

Sample	Ecorr, V	Icorr, A/cm2	Ipass, A/cm2	mm y−1
T25	−1.10	9.4 × 10−6	5.2 × 10−5	0.141
T100	−1.11	1.3 × 10−5	1.8 × 10−5	0.196
Steel	−0.72	4.5 × 10−6	−	0.068

below. The corrosion rate has been recalculated according to corrosion rate conversion requirements as “mm y⁻¹”.

The electrochemical data obtained clearly shows that the corrosion potentials of both T25 and T100 samples are almost equal and their I_corr values are close. The course of the curves is also similar. However, the I_corr of T25 is lower compared with the T100 sample. Some differences appear also in the value of I_pass the latter being about 3 times higher for T25. In addition, the T25 sample demonstrates lower anodic current density in the zone of maximal anodic dissolution (in the potential area between −0.985 V and −0.865 V about 1.5 times compared with the sample T100. It is evident that the polarization curves and the corrosion potential values of both newly developed multilayers are placed more negative (with about 300 mV) direction, which is a clear sign for their protective and sacrificial nature toward the low-carbon steel substrate.

3.6. EIS Studies

The results obtained via EIS method for both T25 and T100 samples immediately after immersion and stabilization of the corrosion potential in the model media are presented in Figure 8. For comparison, the EIS curve of the low-carbon steel is also demonstrated. It is obvious that the polarization resistance (Rp) of the T100 sample is the greatest one (~37 ohm) followed by the T25 sample (~30 ohm) and steel sample (~6 ohm). These data are in general close to the results obtained by PDP curves and support the positive influence of the multilayer system on the corrosion resistance of low-carbon steel in that medium.

3.7. Cross-Sections

Figure 9 demonstrates the cross-sections of both samples investigated—T25 (up) and T100 (down), respectively, which look quite similar in morphology. The thickness of the whole “sandwich type” system is visible (the white border between the gray and the dark zones). It seems that this border is the zinc layer (1 µm thickness). The CS and ZrO₂ layers cannot be detected as separate parts since they have similar structure and are extremely thin. Due to these reasons, it is extremely difficult to register them and their areas.

It is also obvious that these cross-sections give only a general and approximate picture of the thicknesses and locations of the individual layers of the system. More details will be explored in our further work on these objects, looking for more appropriate methods for this purpose.

4. Discussion

The newly developed “sandwich” multilayer structures exhibit different textures and roughness, which depends on the type of chitosan treatment: basically, the structures with CS layers dried at RT possess finer structures of lower roughness than those of the thermally treated CS layer. The CS layer only has a clearly expressed hydrophilic surface irrespective of the mode of treatment (within the range varying from 41° to 46°).

After depositing the ZrO₂ layer, the surface of the final multilayer structure, whatever the type of the treatment of the CS sublayer, clearly expressed a hydrophobic character (contact angle above 100°). In our previous article, we have proved that in the case of ZrO₂-TiO₂ coatings the presence of amorphous dense structures with a hydrophobic nature are among the basic factors responsible for achieving a good corrosion stability [32]. Both coatings demonstrate their sacrificial character and will hopefully protect the steel substrate in the presence of chloride ions.

The results, obtained from PD and EIS measurements, have demonstrated high corrosion resistance (low corrosion current density, respectively) of the multilayer coatings in a corrosive medium containing chloride ions as corrosion activators, independent of their various characteristics: morphology, grain sizes, roughness and contact angle. Our choice of CS as a sublayer in the protective “sandwich” structure is due to its superior film-forming properties, strong adhesion to the metal and anticorrosion effectiveness. However, the relatively high affinity towards water of the layer makes it less efficient in the course of a prolonged stay in corrosive medium. For this purpose, chitosan-based coatings loaded with inhibitors have been studied in detail.

The application of hybrid organic–inorganic multilayer structure has a series of advantages compared with the application of single-oxide film. The final hybrid coating type possesses higher strength and increased resistance to peeling than the single coating. For this reason, the ZrO₂ layer was selected for deposition as an inorganic oxide coating onto the CS layer, which alters the hydrophilic character and high water permeability of the CS layer. Additionally, its higher corrosion stability serves as a barrier against corrosion processes in metal. The results from the electrochemical tests showed that, independent of temperature and time intervals of treatment, the corrosion resistance is high without any substantial differences. This study gives us the reason to consider the application of the CS layer as an important ingredient in hybrid organic–inorganic structures, manifesting high anti-corrosion efficiency.

5. Conclusions

The newly developed “sandwich”type structures on galvanized steel, covered by chitosan and the top layer of the ZrO₂ sol–gel coating showed relatively close protective efficiency and better expressed corrosion resistance in a model medium with chlorine ions as corrosion activators. The electrochemical tests demonstrated the sacrificial nature of the multilayer coating systems as well as better protective ability toward the galvanized low-carbon steel substrate. All of the samples possess a dense amorphous structure having a hydrophobic nature. Both structures demonstrate high anti-corrosion action without any substantial difference, independent of temperature and time intervals of treatment of the chitosan sublayer. The latter is highly important from a corrosion point of view due to its hydrophilic nature of the surface as well as the presence of different functional groups, ensuring good adhesion to both metallic substrate and sol–gel coatings. As a result, CS plays the role of a middle layer ensuring enhanced protective characteristics of the final “sandwich” structure.