Graphite-Enhanced TiO2 Nanoparticles Protect Duplex Stainless Steel against Acidic Corrosion

INTRODUCTION 
Stainless steels are an essential group of engineering alloys that have been used in a wide range of things, from household appliances to parts for spacecraft. Because of their austenitic and ferritic grains, duplex stainless steels (DSSs) can be very useful [1,2]. DSSs have higher toughness and weldability than ferritic ones [3–5]. DSSs are more resistant to pitting and stress corrosion cracking than austenitic grades [6]. As a result, they are widely utilized in the chemical, petrochemical, culinary, electricity, transportation, pulp and paper, and oil refinery industries. DSSs have higher toughness and weldability than ferritic ones [7,8]. DSSs are more resistant to pitting and stress corrosion cracking than austenitic grades [9]. DSSs have strong corrosion resistance due to their high Cr content combined with significant additions of Mo, Ni, and N. Chromium adds to stainless steel corrosion resistance by producing protective Cr-oxide or hydroxide in the passive coating [10-12]. Due to the creation of a passive protective coating, stainless steels have a wide range of industrial uses due to their corrosion resistance [13]. Chromium and other major alloying elements are to blame for this film. As sulfate anions are present, they destabilize at sites or regions of inclusions, impurities, grain boundaries, and other faults that cause localized corrosion [14]. Because of chromium and other essential alloying elements, this film exists. It destabilizes in the presence of aggressive sulfate anions, particularly at inclusions, impurities, grain boundaries, and other imperfections that cause localized corrosion. Sulfuric acid, the world’s most essential and frequently used industrial chemical, is the primary source of corrosive sulfates [15,16]. Its corrosivity to stainless steel alloys varies according to concentration and alloy type [17]. The majority of sulfuric acid encountered is in diluted proportions for various chemical processes, including mineral processing, petroleum production, and water treatment [16,18]. Alloy metals like nickel and chromium can be added to steel to increase corrosion and oxidation resistance, creating high-alloy steel like duplex stainless steel [19,20]. The use of corrosion-resistant materials is desirable in most circumstances where cost is considered [21]. Ferrous metals have long been protected against corrosion by applying corrosion inhibitors [22]. Electron-sharing and film-forming activity is the fundamental processes of inorganic corrosion inhibitors [23]. Organic coatings are commonly utilized in industry to protect metals from corrosion [24]. Ceramic coatings are also preferred over metal coatings because they are more resistant to oxidation, corrosion, erosion, and wear than metals in high-temperature conditions [25]. Nanoparticles are employed to improve hybrid sol-gel coatings’ corrosion resistance and mechanical qualities [26-28]. Nanoparticles reduce fractures and porosity in nanocomposite coatings [29]. Nanoparticles can be introduced as a powder to coat materials or created during the creation of sol-gel coatings [30]. As a result, in this work, to improve the corrosion resistance of duplex stainless steel, we decided to take advantage of the beneficial features of nanoparticles, which were discovered through previous research. The DSSs were coated with titanium nanoparticles containing graphite (TiO2/graphite) using a negative electrodeposition technique at a constant temperature. Electrochemical methods such as open circuit potential (OCP), and potentiodynamic (PD) were used to study the protection of DSSs at a constant temperature (298.15 K). Two surface morphology techniques, field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectroscopy (EDS), were used to identify the nature of the coated surface. As a result, X-ray diffraction (XRD) was utilized in conjunction with the other tests, as it is a vital non-destructive technique for determining the alloy’s protective film composition.

MATERIALS AND METHODS
Table 1 lists the components of the DSS samples. The following dimensions and thickness (2 cm x 2 cm) were used to build a piece (1 cm). Following polishing with emery sheets of varying diameters, all specimens were subsequently lubricated with diamond paste applied to an ultra-thin soft cloth. Sulfuric acid in deionized water saturated with carbon dioxide was utilized as an electrolyte solution to evaluate the protective effects of TiO2 / graphite on the DSS electrode surface.
A TiO2/graphite layer was electrodeposited on the DSS using a glass cell with a volume of 200 cm3 and a DSSs electrode sandwiched between two parallel titanium metal electrodes. The electrodes were separated by 0.3 cm. The electrolyte, which contained 1.5 g/L TiO2 and 0.5 g/L graphite, was agitated using a magnetic stirrer. The negative electrodeposition was performed for 2.5 minutes at a continuous DC voltage of 150 V, with the electrolyte held at 333.15 K. The samples were air-dried for 24 hours at room temperature before being stored in a desiccator until further testing. The corrosion electrochemical cell was coupled to a potentiostat device to determine comprehensive electrochemical parameters. FESEM (ZEISS Gemini, Germany, LTD. Company) was used to characterize specimens. The compositions of the chemical components were determined using EDS spectra. The XRD patterns were captured using the Bruker D6 Advanced diffraction system and a Cu radiation source.

RESULTS AND DISCUSSION 
As a function of immersion time, the fluctuation can be used to determine the DSS coating in OCP. Fig. 1 illustrates the evolution of the open circuit potential with time for DSS in 1 M H2SO4 solution without and with coated protection. The curves demonstrate that the TiO2/graphite protective layer significantly alters the temporal behaviour of the OCP. It is determined that it is a cathodic displacement of DSS in the absence of protection. However, when the cathodic (active) direction is protected by a TiO2/graphite layer, the potential change in the cathodic (active) direction is more pronounced. OCP curve profiles with TiO2/graphite coating exhibit typical anodic tendencies. On the other hand, the OCP values stayed steady in the latter case, moving slightly into the 1000s. This could indicate that a protective covering attaches to the surface of the DSS.
The polarization curves of an uncoated DSS and a TiO2/graphite coated DSS are shown in           Fig. 2. The corrosion potential (Ecorr), the current density value (icorr), the anodic Tafel constant (βa), the cathodic Tafel constant (βc), and the corrosion rate (CR) are all determined using these curves. The output of this calculation is summarized in Table 2. It was discovered that the coated specimen had a lower current density than the untreated DSS specimen. The coated sample’s corrosion potential is changed from cathodic to anodic. This result indicates the coating’s corrosive medium resilience. The following calculation can determine the percentage of protection efficiency based on corrosion current density measurements [31] (%PE).

icorr, and i(coating)corr are the corrosion current densities values in uncoated and coated, respectively.
Coating type TiO2/graphite significantly reduced the corrosion current densities, which decreased from 4.3 milliamps per cm2 for the uncoated DSS to 0.957 milliamps per cm2 for the coated DSS. Because the coating layer has a lower corrosion rate (CR) than the untreated surface (437.5 mpy), the coating layer has a lower corrosion rate (CR). As previously stated, the corrosion current density of the TiO2/graphite coated surface is smaller than that of the uncoated surface, and the percent PE is higher on the covered surface than on the uncoated surface. TiO2/graphite coatings provided better corrosion protection due to the nanoparticles and charged graphite particles adhering to the electrode surface and insulating it from corrosive chemicals [32] such as chloride ions, hydrogen gas and oxygen.
A thin TiO2/graphite coating applied to the DSS surface in 1 M sulfuric acid solution lowered the cathodic and anodic slopes, implying that a thin protective film on the alloy surface influenced the hydrogen generating mechanisms. As previously stated, it is evident that the hydrogen evolution reaction could be controlled, and the mechanism of the proton discharge reaction [33] varied according to the protective approach used.
The X-ray diffraction peaks of the protective layer’s porous alloy surface and crystalline phases were recorded and are summarized in Table 3. The strong peaks at 43.620, 50.520, and 74.650, respectively, can be attributed to the diffraction of (111), (200), and (220) Miller planes, which are characteristic of NiFe elements [34] in DSS alloys with a face-centered cubic (fcc) structure, as illustrated in Fig. 3. Additionally, two weak peaks arise at two values of 44.560 and 64.870, corresponding to the iron diffraction planes (110) and (200). The products have an fcc-type NiFe matrix within a DSS alloy based on fcc-type Ni, with Fe atoms substituting for some Ni. 
A significant concentration of iron atoms in the NiFe matrix is visible in the pattern, indicating that the porous material is composed mostly of iron. The XRD patterns of TiO2/graphite are depicted in Fig. 3. The diffraction peaks confirmed the thin crystallinity and purity of the coating layers, which showed that there were no diffraction peaks associated with any impurities in the coating layers. 
As seen in Table 3, the TiO2/graphite coating layers contributed to the high diffraction peaks at 27.580 and 36.170 associated with titanium dioxide’s rutile [35-36] phase. The absence of graphite’s diffraction peak may be attributable to its integration with TiO2 at ≈ 270 or the reduced graphite concentration in the TiO2/graphite composite. Additionally, as shown in Table 3, the XRD pattern exhibits the same diffraction peaks as the DSS surface but varying intensities. TiO2/graphite has an average particle diameter of 23.06 nm. The findings revealed additional differences in the particle sizes of the protective layer formed on the DSS surface by the electrodeposition methodology, indicating that earlier electrochemical approaches revealed a noticeable shift in protection. An EDX spectrum of the uncoated DSS is shown in Fig. 4, along with FESEM images and mapping spectra. Image A shows further cracking layers formed due to corrosion, suggesting that the metal has suffered substantial surface damage due to its breakdown. As illustrated in Fig. 5, the TiO2/graphite layer that has been created on the DSS surface protects it from corrosion (image A). 
It demonstrates that the coatings formed on the DSS alloy surface are more homogeneous and dense than those formed on other surfaces. Because of the density of the deposited layer, there are no cracks or separation of the coatings with obvious superficial cracks. This is due to the high quality of the coating applied. All of these findings are in agreement with the electrochemical measurements that were taken during the corrosion tests.
The EDS elemental analysis is shown in Fig. B (Fig. 5), which indicates a higher Ti content related to the type of coating layer. Besides, the oxygen and carbon contents were 23.6% and 7.6%, respectively. The iron element content changed from 46.8% to 2.7% in the presence of a TiO2/graphite coating layer. Besides, the elements were detected by the mapping distribution analysis in the uncoated DSS and TiO2/graphite related to the contents of components, as shown in image C in Figs. 4 and 5.

CONCLUSION 
This work reveals that TiO2/graphite is an excellent coating effectively formed on a DSS specimen using the electrodeposition method and exhibits excellent corrosion resistance in sulfuric acid solution (1M) saturated with carbon dioxide. Electrochemical tests found that coating the DSS surface with TiO2 and graphite particles reduced corrosion current densities, resulting in higher polarization resistance in corroded acidic solutions. It takes less time to generate a homogeneous, compact, and adherent TiO2/graphite coating at high voltage. Electrochemical experiments were utilized to determine the protective layer coating’s indirect resistance. Current density and corrosion potential are notably different for the TiO2/graphite layers than for the DSS sample. The OCP results agree with the potentiodynamic measurements. This study reveals that the TiO2/graphite coating layer has high corrosion resistance and might be used to protect duplex stainless steel against corrosion in a 1 M H2SO4 solution. FESEM confirms these results, corroborated by EDX and mapping examinations and XRD investigations.

ACKNOWLEDGMENT
We thank the Iraqi cement plant in Al-Najaf for its support in our inquiry. The authors thank the University of Al-Qadisiyah for its cooperation and enabling some measurements.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.