Effects of nanocrystallization on the corrosion behavior of 309 stainless steel

Nanocrystallized (NC) 309 stainless steel (309SS) coating has been fabricated on glass substrate by DC magnetron sputtering. The coating, with an average grain size less than 50 nm, had ferritic (bcc) structure rather than the austenitic (fcc) structure of the bulk steel. The electrochemical corrosion behavior of the NC coating and the bulk steel in solutions of 0.25 M Na₂SO₄ + 0.05 M H₂SO₄ and 0.5 M NaCl + 0.05 M H₂SO₄ was investigated by using potentiodynamic polarization, potentiostatic polarization and AC impedance techniques. The results showed that the corrosion behavior of the NC 309SS coating and 309SS bulk steel depended on the composition of the solutions. In the Na₂SO₄ solution there was only a little difference between the corrosion resistance of the passive films on the NC coating and the bulk steel. However, in the solution with chloride ions, the localized corrosion resistance of 309SS was greatly enhanced by nanocrystallization due to the formation of a compact and stable passive film on the NC coating. The electronic structure of the passive film formed on the NC coating and on the bulk steel was analyzed by means of capacitance measurements, and a corrosion mechanism is proposed.     

Corrosion of carbon steel in sodium methanoate solutions

The behaviour of steel electrodes in sodium methanoate solutions was studied by coupling electrochemical techniques (voltammetry, OCP vs. time) with in situ micro-Raman spectroscopy analyses of the corrosion products. The polarisation curves depended strongly on the methanoate concentration. For the smallest concentration (10⁻³ mol L⁻¹), the current density increased regularly with the applied potential. So the behaviour of the electrode was typical of an active material. In contrast, for the largest concentration (10⁻¹ mol L⁻¹), the curves obtained were typical of a passive material. Methanoate ions favoured growth and stability of a passive oxide film more likely by adsorbing on its surface. The polarisation curve obtained for the intermediate concentration (10⁻² mol L⁻¹) was unusual and testified of an imperfect passivation of the steel surface. Finally, steel electrodes were left at the open circuit potential in the methanoate solutions. In any case, the passivity was rapidly lost and a general corrosion of the surface took place. In situ Raman spectroscopy analyses at the early stage of the corrosion process demonstrated that the first product to form was a green rust, GR(HCOO⁻). It was oxidised later into γ-FeOOH (lepidocrocite) by dissolved O₂. The process is then typical of what is usually observed in neutral or alkaline media, whatever the anions present and responsible of the GR formation. A new and detailed characterisation of GR(HCOO⁻) by X-ray diffraction was performed and a crystal structure is proposed.     

Effects of NO2 ions on localised corrosion of steel in NaHCO3 + NaCl electrolytes

Pitting corrosion of carbon steel electrodes in 0.1 mol L⁻¹ NaHCO₃ + 0.02 mol L⁻¹ NaCl solutions was induced by anodic polarisation. The evolution of the breakdown potential E_b with NO₂⁻ concentration was investigated by linear voltammetry. E_b increased from −15 ± 5 mV/SCE for [NO₂⁻] = 0 up to 400 ± 50 mV/SCE for [NO₂⁻] = 0.1 mol L⁻¹. During anodic polarisation at potentials comprised between E_b([NO₂⁻] = 0) and E_b([NO₂⁻] ≠ 0), the behaviour of the whole electrode surface, followed by chronoamperometry, was compared to the behaviour of one single pit, followed via scanning vibrating electrode technique (SVET). Addition of a NaNO₂ solution after the beginning of the polarisation led to a rapid repassivation of pre-existing well-grown pits. In situ micro-Raman spectroscopy was then used to identify the corrosion products forming inside the pits. The first species to be detected in the presence of NO₂⁻ were mainly dissolved Fe(III) species, more likely [Fe^III(H₂O)₆]³⁺ complexes. Iron(II) carbonate FeCO₃, siderite, and carbonated green rust GR(CO₃²⁻) were also detected in the active pits, as in the absence of nitrite. But they were accompanied by maghemite γ-Fe₂O₃, a phase structurally similar to the passive film, that forms from the Fe(III) complexes. The Raman analyses then correlate with the SVET observations and confirm that the main effect of nitrite ions is to oxidize iron(II) into iron(III). The passive film would then form from the Fe(III) species still bound to the steel surface.     

Electrochemical impedance spectroscopy and corrosion behaviour of Al2O3-Ni nano composite coatings

In this paper, the results on the electrochemical impedance spectroscopy and corrosion properties of electrodeposited nanostructured Al₂O₃-Ni composite coatings are presented. The nanocomposite coatings were obtained by codeposition of alumina nanoparticles (13 nm) with nickel during plating process. The coating thickness was 50 μm on steel support and an average of nano Al₂O₃ particles inside of coatings at 15 vol.% was present. The structure of the coatings was investigated by scanning electron microscopy (SEM). It has been found that the codeposition of Al₂O₃ particles with nickel disturbs the nickel coating's regular surface structure. The electrochemical behavior of the coatings in the corrosive solutions was investigated by polarization potentiodynamic and electrochemical impedance spectroscopy methods. As electrochemical test solutions 0.5 M sodium chloride and 0.5 M potassium sulphate were used in a three electrode open cell. The corrosion potential is shifted to more negative values for nanostructured coatings in 0.5 M sodium chloride. The polarization resistance in 0.5 M sodium chloride decreases in 24 h, but after that increases slowly. In 0.5 M potassium sulphate solution the polarization resistance decreases after 2 h and after 30 h of immersion the polarization resistance is higher than that of the beginning value. The corrosion rate calculated by polarization potentiodynamic curves obtained after 30 min from immersion in solution is smaller for nanostructured coatings in 0.5 M potassium sulphate (4.74 μm/year) and a little bit bigger in 0.5 M sodium chloride (5.03 μm/year).     

Influence of the micro-addition of Mo on glass forming ability and corrosion resistance of Cu-based bulk metallic glasses

(Cu₄₇Zr₁₁Ti₃₄Ni₈)_100−xMo_x bulk metallic glasses (BMGs) with x = 0, 1 and 2 at.% and a bulk metallic glass matrix composite with x = 5 at.% were successfully prepared by water-cooled copper mold casting. The effect of the addition of a small amount of Mo on the glass forming ability (GFA), thermal properties of the base alloy (i.e. x = 0) were investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and differential thermal analyzer (DTA). It is found that the addition of appropriate amount of Mo can enhance the GFA of the Cu-based BMG, as indicated by the increase in the reduced glass transition temperature T_rg (=T_g/T_l) and the parameter γ (=T_x/(T_g + T_l)) with the increase of Mo. On the other hand, the corrosion resistance of the Cu-based BMGs with different Mo contents was examined by electrochemical polarization and weight loss measurement in 1 mol/L H₂SO₄ and 1 mol/L NaOH solutions, respectively. It is found that the corrosion resistance of Cu-based BMGs increased with increasing Mo content with the lowest corrosion rate of (0.9 ± 0.2) × 10⁻³ mm/year in 1 mol/L H₂SO₄ solution and (0.3 ± 0.1) × 10⁻³ mm/year in 1 mol/L NaOH solution, respectively, for the BMG containing 2 at.% Mo. X-ray photoelectron spectroscopy (XPS) results revealed that the improvement of corrosion resistance of Cu-based BMG containing appropriate amount of Mo originated from the enrichment of ZrO₂ and TiO₂, but depletion in Cu- or Ni-oxides in the passive films formed during electrochemical polarization. Finally, the galvanostatic-step measurement was performed to investigate the kinetics of the formation of passive films on the BMG surfaces. It is demonstrated that the addition of an appropriate amount of Mo can effectively improve the stability and uniformity of the passive film. The role of Mo addition on the glass forming ability and corrosion behavior is discussed.     

Effects of cold work and sensitization treatment on the corrosion resistance of high nitrogen stainless steel in chloride solutions

The effects of cold work and sensitization treatment on the microstructure and corrosion resistance of a nickel-free high nitrogen stainless steel (HNSS) in 0.5 M H₂SO₄ + 0.5 M NaCl, 3.5% NaCl and 0.5 M NaOH + 0.5 M NaCl solutions have been investigated by microscopic observations, electrochemical tests and surface chemical analysis. Cold work introduced a high defect density into the matrix, resulting in a less protective passive film as well as reduced corrosion resistance for heavily cold worked HNSS in a 3.5% NaCl solution. No obvious degradation in corrosion resistance took place in a 0.5 M H₂SO₄ + 0.5 M NaCl solution, possibly due to the stability of the passive film in this solution. Sensitized HNSSs showed reduced corrosion resistance with increasing cold work level in both 3.5% NaCl and 0.5 M H₂SO₄ + 0.5 M NaCl solutions due to a reduction in the anti-corrosion elements in the matrix during the cold work-accelerated precipitation process. The cold work and sensitization treatment had no influence on the corrosion resistance of the HNSS in the 0.5 M NaOH + 0.5 M NaCl solution even though the property of the passive film changed. The effects of cold work and sensitization treatment on the characteristics of passive films formed in the three solutions are discussed.     

Improvement of the electrochemical behavior of steel surfaces using a TiN[BCN/BN]n/c-BN multilayer system

The aim of this work is to improve the electrochemical behavior of AISI 4140 steel substrates by using a TiN[BCN/BN]_n/c-BN multilayer system as a protective coating. We grew TiN[BCN/BN]_n/c-BN multilayers via reactive r.f. magnetron sputtering technique, systematically varying the length period (Λ) and the bilayer number (n), maintaining constant the total thickness of the coating and all other growth parameters. The coatings were characterized by FTIR spectroscopy that showed bands associated to h-BN bonds, and c-BN stretching vibrations centered at 1385 cm^− 1 and 1005 cm^− 1, respectively. Film composition was studied via X-ray photoelectron spectroscopy where typical signals for C1s, N1s and B1s are shown. The electrochemical properties were studied by electrochemical impedance spectroscopy and Tafel curves. In this work, the maximum corrosion resistance for the coating with (Λ) equal to 80 nm was obtained, corresponding to n = 25 bilayers. The polarization resistance and corrosion rate were around 10.1 kOhm cm² and 0.22 mm/year; these values were 83 and 15 times higher, respectively, than uncoated AISI 4140 steel substrate (0.66 kOhm cm² and 18.51 mm/year). Optical microscopy was used for surface analysis after corrosive attack. The improvement of the electrochemical behavior of the AISI 4140 coated with this TiN[BCN/BN]_n/c-BN multilayer system can be attributed to the presence of several interfaces that offer resistance to diffusion of Cl⁻ of the electrolyte toward the steel surface.     

Corrosion protection of carbon steel by thin films of poly(3-alkyl thiophenes) in 0.5 M H2SO4

Poly(3-octyl thiophene) (P3OT) and poly(3-hexylthiophene) (P3HT) dissolved in toluene were deposited onto 1018 carbon steel and corroded in 0.5 M H₂SO₄. P3OT and P3HT films were chemically deposited by drop casting onto 1018-type carbon steel with two surface finishing, i.e. abraded with 600-emery paper and with alumina (Al₂O₃) particles of 1.0 μm in diameter (mirror finish). Their corrosion resistance was estimated by using potentiodynamic polarization curves, linear polarization resistance (LPR), and electrochemical impedance spectroscopy, EIS, techniques. In all cases, polymeric films protected the substrate against corrosion, but the protection was improved if the surface was polished with Al₂O₃ particles of 1.0 μm in diameter. The polymer which gave the best protection was P3HT because the amount of defects was much lower than that for the P3OT films. The polymers did not act only as a barrier layer against aggressive environment, but they improved the passive film properties by decreasing the critical current necessary to passivate the substrate, increasing the pitting potential and broadening the passive interval.     

Local electrochemical impedance measurements on inclusion-containing stainless steels using microcapillary-based techniques

MnS inclusions are good precursor sites for pitting corrosion of stainless steel. The objective of this paper was to quantify the passive properties of resulfurized stainless steel after immersion in chloride media. This was done by combining microcapillary techniques with electrochemical impedance spectroscopy and numerical analysis (specific equivalent circuit). It was shown that sulfur species produced in the electrolyte during the dissolution of inclusions react with the native passive film to CrS and FeSO₄. Local electrochemical impedance spectroscopy measurements provided data describing the behaviour of the affected matrix at the microscale. For example, the value of the charge transfer and migration of point defects resistance decreases from 51,700 Ω cm², in sites free of any metallurgical heterogeneity down to 12,200 Ω cm², in sites containing a high density of inclusions. It was also shown that the integrity of the microcapillary can be altered by the presence of high quantity of sulfur in the electrolyte. Local impedance data allowed the detection of such problems.     

Corrosion inhibition of mild steel by natural product compound

Corrosion inhibition of mild steel in H₃PO₄ containing chloride or sulphate ions have been studied using different electrochemical techniques. The corrosion and hydrogen evolution of mild steel alloy in 2 M H₃PO₄ acid containing 0.5 M NaCl can be effectively inhibited by addition of natural product compound, Thymol (IPMP), of different concentrations. However, in 2 M H₃PO₄ containing 0.5 M Na₂SO₄ corrosion cannot be effectively inhibited. The results of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements confirm the synergistic effects which describe the increase in the effectiveness of a corrosion inhibitor in the presence of Cl⁻ ions in the corrosive medium. At any temperature, an increase in it leads to an increase of the corrosion rate and hydrogen evolution on mild steel. Polarization and EIS results are in good agreement with each other. The obtained results were confirmed by surface examination using scanning electron microscope.     

Effects of nitrogen on the passivation of nickel-free high nitrogen and manganese stainless steels in acidic chloride solutions

The role of nitrogen on the passivation of nickel-free high nitrogen and manganese stainless steels was investigated in 0.5 M H₂SO₄, 3.5% NaCl and 0.5 M H₂SO₄ + 0.5 M NaCl solutions using potentiodynamic polarization, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy techniques. The passive film stability was enhanced in 0.5 M H₂SO₄ and the pitting resistance was improved in 3.5% NaCl solution by more nitrogen addition. The influence of nitrogen extended the whole anodic polarization region in 0.5 M H₂SO₄ + 0.5 M NaCl solution, as demonstrated by the enhanced dissolution resistance, promoted adsorption and passivation process, improved film protection and pitting resistance with increasing nitrogen content. Possible mechanisms relating to the role of nitrogen in different potential regions were discussed.     

Effects of phosphate species on localised corrosion of steel in NaHCO3 + NaCl electrolytes

Pitting corrosion of carbon steel electrodes in 0.1 M NaHCO₃ + 0.02 M NaCl solutions was induced by anodic polarisation. The evolution of the breakdown potential E_b with the phosphate concentration was investigated by linear voltammetry. E_b increased from −15 ± 5 mV/SCE for [HPO₄²⁻] = 0 to 180 ± 40 mV/SCE for [HPO₄²⁻] = 0.02 mol L⁻¹. During anodic polarisation (E = 50 mV/SCE), the behaviour of the whole electrode surface, followed by chronoamperometry, was compared to the behaviour of one single pit, followed via the scanning vibrating electrode technique (SVET). The addition of a Na₂HPO₄ solution after the beginning of the polarisation did not lead to the repassivation of pre-existing well-grown pits. The corrosion products forming in the pits were identified in situ by micro-Raman spectroscopy. They depended on the phosphate concentration. For [HPO₄²⁻] = 0.004 mol L⁻¹, siderite FeCO₃ was detected first. It was oxidised later into carbonated green rust GR(CO₃²⁻) by dissolved O₂. The beginning of the process is therefore similar to that observed in the absence of phosphate. Finally, GR(CO₃²⁻) was oxidised into ferrihydrite, the most poorly ordered form of Fe(III) oxides and oxyhydroxides. Phosphate species, adsorbing on the nuclei of FeOOH, inhibited their growth and crystallisation. For [HPO₄²⁻] = 0.02 mol L⁻¹, siderite was accompanied by an amorphous precursor of vivianite, Fe₂(PO₄)₃·8H₂O. This shows that, in any case, phosphate species interact strongly with the iron species produced by the dissolution of steel.     

The corrosion behavior of Fe-10Cr nanocrystalline coating

Fe-10Cr nanocrystalline (nc) coatings with a grain size of 20-30 nm were synthesized on glass substrates by magnetron sputtering. The corrosion behavior was investigated in 0.05 mol/L H₂SO₄ + 0.25 mol/L Na₂SO₄ and 0.05 mol/L H₂SO₄ + 0.5 mol/L NaCl solution by polarization curves, EIS and Mott-Schottky analysis. The results showed that compared to Fe-10Cr cast alloy, the active dissolution of the coating was accelerated; the passive film contained more Cr and therefore the coating was easier to passivate. The passive films formed on Fe-10Cr nc and cast alloy exhibited n-type semiconducting behavior in acidic solutions without Cl⁻ and p-type semiconducting behavior in acidic solutions with Cl⁻. The lower breakdown potential for both materials in the solution with Cl⁻ is related to the p-type passive film formed on them. For Fe-10Cr nc, lower donor density and increased Cr content were responsible for the chemical stability of the passive film.     

Evaluation of the corrosion resistance of Ni-Co-B coatings in simulated PEMFC environment

The corrosion resistance behavior of Ni-Co-B coated carbon steel, Al 6061 alloy and 304 stainless steel was evaluated in simulated proton exchange membrane fuel cell (PEMFC) environment. The phase structure of the NiCoB based alloy was determined by Rietveld analysis. The PEMFC environment was constituted of 0.5 M H₂SO₄ at 60 °C and the evaluation techniques employed included potentiodynamic polarization, linear polarization resistance, open circuit potential measurements and electrochemical impedance spectroscopy. The results showed that in all cases the corrosion resistance of the Ni-Co-B coating was higher than that of the uncoated alloys; about two orders of magnitude with respect to carbon steel and an order of magnitude compared to 304 stainless steel. Except for the uncoated 304 type stainless steel, the polarization curves for the coated specimens did not exhibit a passive region but only anodic dissolution. The corrosion potential value, E_corr, was always nobler for the coated samples than for the uncoated specimens. This was true for the stainless steel in the passive region, but in the active state for the carbon steel and Al 6061 alloy. The corrosion of the underlying alloy occurred due to filtering of the solution through coating defects like microcracks, pinholes, etc. During the filtering process the E_corr value of the coating decreased slowly until it reached a steady state value, close to the E_corr value of the underlying alloy.     

A new corrosion protection coating with polyaniline-TiO2 composite for steel

Organic coating strategies for corrosion protection with inherently conducting polymers have become important because of restriction on the use of heavy metals and chromates in coatings due to their environmental problems. This work presents the synthesis of polyaniline-TiO₂ composites (PTC) and the corrosion protection behaviour of PTC containing coating on steel. PTC was prepared by chemical oxidation of aniline and TiO₂ by ammonium persulfate in phosphoric acid medium. The PTC was characterized by FTIR, XRD and SEM techniques. Suitable coating with PTC was formed on steel using acrylic resin. Using electrochemical impedance spectroscopy, the PTC containing coating's behaviour in 3% NaCl immersion test and salt spray test has been found out. Results indicate that the coating containing PTC is able to maintain the potential of steel in passive region due to its redox property. The resistance of the coating containing PTC was more than 10⁷ Ω cm² in 3% NaCl solution after 60 days and 10⁹ Ω cm² in the salt spray test of 35 days. But the resistance of the TiO₂ containing coating was found to be less than 10⁴ Ω cm² in both the cases. The high performance of PTC containing coating is attributed to the passivation of steel by polyaniline.     

Corrosion behavior of reinforcing steel in simulated concrete pore solutions: A scanning micro-reference electrode study

The scanning micro-reference electrode (SMRE) technique was used to study the corrosion behavior of reinforcing steel in simulated concrete pore (SCP) solutions with different pH values. The early stage as well as the propagation of the localized corrosion of reinforcing steel in different solutions was explored. The results indicated that the potential distribution on the reinforcing steel surface changed in homeostasis and the steel remained passive in the pure simulated concrete pore solution. The solution pH had a significant effect on the localized corrosion of reinforcing steel, and the critical pH value for localized corrosion of reinforcing steel in SCP solutions was between 11.46 and 11.31.     

Acetyl thiourea chitosan as an eco-friendly inhibitor for mild steel in sulphuric acid medium

Acetyl thiourea chitosan polymer (ATUCS) has been synthesized and evaluated as corrosion inhibitor. The electrochemical behavior of mild steel in naturally aerated 0.5 M H₂SO₄ acid containing different concentrations of ATUCS has been studied by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) measurements and surface examination via scanning electron microscope (SEM) technique. The results of EIS showed that the resistance (R_t) increases slightly with increasing immersion time indicating a slight decrease in corrosion rate of the steel with time. Also, the corrosion rate increases with either increasing temperature or decreasing the polymer concentration as observed by polarization technique. Electrochemical impedance spectroscopy measurements under open-circuit conditions confirmed well polarization results. ATUCS has shown very good inhibition efficiency (IE) in 0.5 M sulphuric acid solution reaches to 94.5% for 0.76 mM concentration. IE of this compound has been found to vary with the concentration of the polymer solution, immersion time and temperature.     

Corrosion inhibition of mild steel by alkylimidazolium ionic liquids in hydrochloric acid

The acid corrosion inhibition process of mild steel in 1 M HCl by 1-butyl-3-methylimidazolium chlorides (BMIC) and 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM]HSO₄) has been investigated using electrochemical impedance, potentiodynamic polarization and weight loss measurements. Potentiodynamic polarization studies indicate the studied inhibitors are mixed type inhibitors. For both inhibitors, the inhibition efficiency increased with increase in the concentration of the inhibitor and the effectiveness of the two inhibitors are in the order [BMIM]HSO₄ > BMIC. The adsorption of the inhibitors on mild steel surface obeyed the Langmuir's adsorption isotherm. The effect of temperature on the corrosion behavior in the presence of 5 × 10⁻³ M of inhibitors was studied in the temperature range of 303-333 K. The associated activation energy of corrosion and other thermodynamic parameters such as enthalpy of activation (ΔH), entropy of activation (ΔS), adsorption equilibrium constant (K_ads) and standard free energy of adsorption (ΔG_ads) were calculated to elaborate the mechanism of corrosion inhibition.     

Effect of Al content on the corrosion behavior of Mg-Al alloys in aqueous solutions of different pH

The effect of systematic increase of Al content on the electrochemical behavior of the Mg-Al alloys in aqueous solutions of different pH was investigated. Different electrochemical methods such as open-circuit potential measurements, polarization techniques and electrochemical impedance spectroscopy, EIS, were used to investigate the electrochemical behavior of the alloys in aqueous solutions. The results have shown that Mg-5Al is easily corroded due to the microgalvanic effect between α-phase and β-phase, its corrosion rate is even higher than that of Mg itself. The increase of Al content increases the corrosion resistance of the alloy due to the formation of the β-phase (Mg₁₇Al₁₂) together with the Mg α-phase. The ranking of the corrosion rate of these alloys was Mg-5Al > Mg > Mg-10Al ≅ Mg-15Al. The corrosion rates of the alloys in acidic solutions are pronouncedly high compared to those measured in neutral or basic solutions. The impedance measurements are in consistence with the polarization techniques and the impedance data were fitted to theoretical data obtained according to an equivalent circuit model describing the electrode/electrolyte interface.     

Corrosion behavior of austenitic stainless steels as a function of pH for use as bipolar plates in polymer electrolyte membrane fuel cells

Stainless steels (types 304 and 310S) were employed as bipolar plates for polymer electrolyte membrane fuel cells. For the cell operation, the decayed cell voltage was approximately 22 mV for the type 310S stainless steel after 1000 h operation, while that for type 304 stainless steel was about 46 mV. Corrosion products appeared on the cathode side bipolar plate for the type 304 stainless steel, while trace of corrosion was barely detected for type 310S stainless steel. In order to follow the pH on the bipolar plates during fuel cell operation, polarization tests were carried out for the type 310S stainless steel in synthetic solutions (0.05 M SO₄²⁻ (pH 1.2-5.5) + 2 ppm F⁻) as a function of pH (1.2-5.5) at 353 K. We also examined the contact resistance between the stainless steel and carbon diffusion layer before and after polarization. X-ray photoelectron spectroscopic (XPS) analyses were carried out for comparison of the surface states of the steels after the polarization tests and cell operation. In the synthetic solutions with lower pHs (≤3.3), the films were thinner and were mainly composed by enriched with chromium oxide. Whereas, they mainly consisted of relatively thick iron oxide when the solution pH was higher (≥4.3). XPS analyses for the bipolar plate of type 310S stainless steel on cathode side after cell operation demonstrated pH gradient on the plate, that is, the thicker iron-rich surfaces presented relatively higher pH from the gas inlet to center area, and the thinner chromium-rich surface appeared with lower pH around the gas outlet.