Influence of nanocrystallization on passive behavior of Ni-based superalloy in acidic solutions

The electrochemical corrosion behaviors of Ni-based superalloy nanocrystalline coating (NC) fabricated by a magnetron sputtering technique have been investigated in comparison with cast alloy in 0.25 M Na₂SO₄ + 0.05 M H₂SO₄ and 0.5 M NaCl + 0.05 M H₂SO₄ solution, respectively. Compared with cast alloy, the NC coating had a little higher passive current density in Na₂SO₄ acidic solution, while it had superior resistance to pitting corrosion in NaCl acidic solution. The semiconductive type of passive film of the NC coating was p-type in both acidic solutions, while, that of cast alloy changed from p-type in Na₂SO₄ acidic solution to n-type in NaCl acidic solution. XPS results indicated that Cr₂O₃ was the main component for the passive films of the NC coating as well as those of the cast alloy. No chloride ion was found in the passive film of NC coating while it was in the passive film of cast alloy. The chloride ions adsorbing on the surface of cast alloy incorporated into the passive film, which induced the formation of n-type oxide film. The nanocrystallization of Ni-base superalloy obviously weakened the adsorption of chloride ions on surface, which decreased the susceptibility of pitting corrosion in acidic solution.     

The improvement of the corrosion resistance of 309 stainless steel in the transpassive region by nano-crystallization

The effect of nano-crystallization on the corrosion behavior of 309 stainless steel in the transpassive region was investigated in 0.5 M Na₂SO₄ (pH 2) solution. Three parts defined as transpassive dissolution, secondary passivity and oxygen evolution can be observed in the transpassive potential region of the anodic polarization curves. In the whole transpassive region the nano-crystalline coating has a smaller corrosion current density than the bulk steel, which indicates the transpassive dissolution rate is decreased by nano-crystallization. In addition, there is an obvious difference in the surface micrographs of the two materials at transpassive potentials, as scanning electron microscopy (SEM) shows. The electron probe microanalysis (EPMA) reveals that nano-crystallization improves the homogeneity of Cr on the surface. Mott-Schottky plots displays that the carrier density of the oxide film in the transpassive region is decreased by nano-crystallization. Thus, the interfacial reactions are decelerated and the corrosion resistance of the stainless steel in the transpassive region has been greatly improved by nano-crystallization.     

Influence of grain size on the corrosion behavior of a Ni-based superalloy nanocrystalline coating in NaCl acidic solution

The effects of grain size on the electrochemical corrosion behavior of a Ni-based superalloy nanocrystalline (NC) coating fabricated by a magnetron sputtering technique, has been investigated in 0.5 M NaCl + 0.05 M H₂SO₄ solution. Coatings with grain sizes 10 nm, 50 nm and 100 nm were fabricated on glass and the superalloy substrates. The results indicate that a passive film with porous property, n-type semiconductive property and incorporation of chloride ions formed on the NC coating with 100 nm grain size, which increased the susceptibility to pitting corrosion. The NC coatings with 10 nm and 50 nm grain size formed compact, non-porous and p-type passive films without chloride ions, which improved resistance to pitting corrosion. The smaller grain size of the material decrease the amount of chloride ions adsorbed on the surface and promoted the formation of compact passive film, which significantly increased the material's resistance to pitting corrosion in acidic solution.     

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.     

Electrochemical behaviour of high nitrogen bearing stainless steel in acidic chloride solution: Effects of oxygen, acid concentration and surface roughness

The effects of oxygen, H₂SO₄ concentration and surface roughness on the electrochemical behaviour of high nitrogen bearing stainless steel (HNS) in acidified 0.5 M NaCl solution were investigated using potentiodynamic polarization method. The results revealed three corrosion potentials indicating an unstable system. The number of the potentials was influenced by H₂SO₄ concentration. Three potentials existed above 0.011 M H₂SO₄, two between 0.011 and 0.0065 M H₂SO₄ and one below 0.0065 M H₂SO₄. Oxygen increased the number of corrosion potential at 0.005 M H₂SO₄ + 0.5 M NaCl solution while surface roughness had no noticeable influence on the number of potentials but increased the values of the corrosion potentials and passivation current densities with increase in the surface roughness. The corrosion mechanism has been discussed using the ideal polarization curve models.     

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.     

Corrosion resistant low temperature carburized SS 316 as bipolar plate material for PEMFC application

One of the current challenges for application of PEM fuel cell is to find corrosion resistant, electrically conductive, light weight, cost competitive bipolar plate material. Low temperature carburization (LTC) of stainless steels is a novel, patented process by Swagelok Company. This paper addresses the corrosion resistance characteristics of LTC SS 316 for polymer electrolyte membrane fuel cell (PEMFC) bipolar plate applications. Corrosion properties of this material were studied using potentiodynamic and potentiostatic tests in simulated (1 M H₂SO₄ + 2 ppm HF, 0.5 M H₂SO₄, pH: 4.0, and 5% HCl + 5% Na₂SO₄) PEMFC conditions. LTC SS 316 showed excellent corrosion resistance in these conditions compared to SS 316. The mechanism of anodic dissolution and general corrosion of LTC SS 316 was observed to be similar to SS 316 however the extent of LTC SS 316 corrosion was less. LTC SS 316 showed corrosion currents well below 16 μA cm⁻² in anodic and cathodic atmospheres under potentiostatic conditions. The potentiostatic current rapidly falls to ∼4.0 and ∼1.5 μA cm⁻² under anodic and cathodic conditions, respectively. LTC SS 316 was observed to form a thinner oxide layer as compared to SS 316 after 24 h of potentiostatic testing. Moreover LTC SS 316 lowered the interfacial contact resistance by approximately 24% as compared to SS 316 after corrosion testing. Hence this study clearly states the performance advantage of using LTC SS 316 as bipolar plate material as compared to conventional materials.     

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 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.     

Enrichment of chromium-content in passive layers on stainless steel coated with polyaniline

Enrichment of chromium-content in passive layers on a 304 stainless steel (SS) by coating polyaniline (PANI) has been confirmed by means of electrochemical and X-ray photoelectron spectroscope (XPS) techniques and thus prepared oxide films demonstrated to be highly stable in aggressive environment. PANI films are deposited on SS from a sulfuric acid solution with aniline monomer. The PANI-coated SS is found to maintain the passive state in a deaerated 1 M H₂SO₄ at 45 °C for several weeks. The passive state of SS remains in the same solution for several days even after the PANI films were completely removed. The passive layers underlying the PANI are certified to have higher chromium-content compared with the air-formed and anodically-formed oxide films on SS. It is suggested that PANI films enhance the passivation of SS by increasing the Cr₂O₃ content in the passive layer and by preventing the chromic oxide from directly interacting with aggressive solution.     

Effect of surface nanocrystallization on corrosion and corrosion inhibition of low carbon steel: Synergistic effect of methionine and iodide ion

The corrosion properties of a nanocrystalline low carbon steel coating (SNCLCS) fabricated on a low carbon steel substrate by magnetron sputtering and the bulk steel (BLCS) were investigated in aerated 0.5 M H₂SO₄ solution by EIS and polarization techniques. The corrosion inhibiting effect of methionine and synergistic KI additives was also studied. The results show that both specimens underwent active corrosion with no transition to passivation in the potential range studied. Surface nanocrystallization was observed to increase the corrosion susceptibility of low carbon steel, leading to a decrease in interfacial impedance and an increase in the kinetics of the anodic reaction. Methionine inhibited the corrosion of both specimens with comparable inhibition efficiencies and iodide ions synergistically increased the inhibition efficiency. This synergistic effect was, however, more pronounced for BLCS. This has been discussed vis-à-vis the more positive surface charge on BLCS compared to SNCLCS.     

A passivation mechanism of doped polyaniline on 410 stainless steel in deaerated H2SO4 solution

To investigate the role of polyaniline (PANI) in the corrosion protection of stainless steel (SS) in oxygen-deficient acidic solution, a separate doped PANI film electrode on a glass substrate was prepared and the test solution (1 M H₂SO₄) was purged with high-purity N₂ until dissolved oxygen level decreased more than two orders of magnitude. In this deaerated 1 M H₂SO₄ solution, the galvanic coupling interaction between the separate PANI film electrode and 410 SS was studied. Results reveal that the separate PANI film can passivate the 410 SS steadily for a long period of time. A variety of experimental methods including potentiodynamic measurement, potentiostatic (current-time) examination and X-ray photoelectron spectroscopy (XPS) are used to explore the mechanism by which the separate PANI film passivated the galvanic coupling SS in the deaerated sulfuric solution. These studies show that passivation is achieved because PANI film provides a large critical current at the early stage of coupling and a persistent passive current by its electrochemical dedoping/re-doping equilibrium activity with the acidic environment at the subsequent stage of coupling.     

Styrene butadiene co-polymer based conducting polymer composite as an effective corrosion protective coating

Electroactive conducting polymer composite coatings of polyaniline (PANI) are electrosynthesized on styrene–butadiene rubber (SBR) coated stainless steel electrode by potentiostatic method using aqueous H₂SO₄ as supporting electrolyte. The protective behaviour of these coatings in different corrosion media (3.5% NaCl and 0.5 M HCl) is investigated using Tafel polarization curves, open circuit potential measurements and electrochemical impedance spectroscopy. The results reveal that SBR/PANI composite coating is much better in corrosion protection than simple PANI coating. The corrosion potential of composite films shifts to more noble values indicating that SBR/PANI composite coating act as an effective corrosion protective layer.     

Influence of micro-structure on corrosion behavior of a Ni-based superalloy in 3.5% NaCl

The electrochemical corrosion behavior of a Ni-based superalloy with polycrystalline (cast alloy), single-crystalline (SC (2 0 0)) and nanocrystalline (NC) micro-structures has been studied in 3.5% NaCl solution. The results indicated that among the three materials, the corrosion resistance increased in the order cast alloy < SC (2 0 0) alloy < NC coating. XPS analyses revealed that the composition of passive film on the three materials was different. In addition to Cr₂O₃ and TiO₂, some NiO was detected in the passive film on the cast alloy, little in that of the SC (2 0 0) alloy and none in that of the NC coating. The double-log plots showed the compact property of the passive film formed on the samples also varied in the order cast alloy < SC (2 0 0) alloy < NC coating, with the cast alloy displaying the worst compact property and the NC coating the best. The micro-structure influenced both the composition of passive film as well as the initial growth of passive film, which determined the compact property of the film and resulted in the observed differences in the corrosion behavior of the three materials.     

Poly(N-ethylaniline) coatings on 304 stainless steel for corrosion protection in aqueous HCl and NaCl solutions

Poly(N-ethylaniline) (PNEA) coatings were grown by potentiodynamic synthesis technique on 304 stainless steel (SS) alloy from 0.1 M of N-ethylaniline (NEA) in 0.3 M oxalic acid solution. Characterization of adhesive and electroactive PNEA coatings was carried out by cyclic voltammetry, FT-IR spectroscopy and scanning electron microscopy (SEM) techniques. The protective properties of PNEA coatings on SS were elucidated using linear anodic potentiodynamic polarization, Tafel and electrochemical impedance spectroscopy (EIS) test techniques, in highly aggressive 0.5 M HCl and 0.5 M NaCl solutions. Linear anodic potentiodynamic polarization test results proved that PNEA coating improved the degree of protection against pitting corrosion in HCl and NaCl solutions. Tafel test results showed that PNEA coating appears to enhancement protection for SS in 0.5 M NaCl and 0.5 M HCl solutions. However, according to long-term EIS results, PNEA coating is better for the protection of SS electrodes during the long immersion period in NaCl compared to that in HCl medium.     

Study on the corrosion behavior of reinforcing steel in simulated concrete pore solutions using in situ Raman spectroscopy assisted by electrochemical techniques

In situ Raman spectroscopy, electrochemical impedance spectroscopy (EIS) and polarization curves were used to study the corrosion behavior of reinforcing steel in simulated concrete pore (SCP) solutions (saturated Ca(OH)₂ solutions). Results indicated that the reinforcing steel remained passive in chloride-free SCP solutions. However, the anodic polarization curve of the steel did not exhibit a stable passive region in the SCP solution with 0.5 M NaCl, the corrosion current density exceeded 0.1 μA cm⁻², the steel surface was unstable with chloride attack and localized corrosion appeared on it with FeCO₃ and Fe₂O₃ as the main corrosion products.     

Effects of surfactants and their mixtures on inhibition of the corrosion process of ferritic stainless steel

The corrosion inhibition characteristics regarding mixtures of cationic/zwitterionic types of surfactant (Myristyltrimethylammonium bromide/Palmitylsulfo-betaineas), and non-ionic surfactant TRITON-X-405 mixed with 1 mM of KBr, as corrosion inhibitors for stainless steel (SS) (type X4Cr13) in aqueous solutions of 2 M H₂SO₄ were investigated using potentiodynamic polarisation measurements. The polarisation data showed that mixtures of the surfactants used in this study acted as mixed-type inhibitors, adsorbing on the stainless steel surface in agreement with the Flory-Huggins adsorption isotherm. The tensiometric results of this study suggest the existence of a second state of aggregation for zwitterionic/cationic surfactant mixtures. From these values of the free energy of adsorption, which in both mixtures decreased with respect to a single surfactant, we concluded that the adsorption in mixtures was stronger. The mixtures studied here showed good inhibition properties for ferritic stainless steel type X4Cr13 in 2 M H₂SO₄ solution.     

Electrochemical corrosion behavior of nanocrystallized bulk 304 stainless steel

By employing Mott-Schottky analysis in conjunction with the point defect model (PDM), we compared donor density and donor diffusion coefficients in the passive films formed on the surface of nanocrystallized bulk 304 stainless steel (NB304ss) and cast 304 stainless steel (304ss) in 0.05 mol/L H₂SO₄ + 0.25 mol/L Na₂SO₄ solution. The donor density at the metal/film interface of the NB304ss was lower than that at the metal film interface of the cast 304ss. Based on the Mott-Schottky analysis, an exponential relationship between donor density and formation potentials of the passive films on the NB304ss and the cast 304ss was built up. The results showed that the donor diffusion coefficients in the passive film formed on the surface of NB304ss was lower than that in the cast 304ss. The lower donor density and the lower diffusion coefficient restrained the electrochemical reaction in the passive film and improved the stability of the passive film. That is the reason why the passive film formed on the NB304ss was more protective.     

Hot corrosion behaviors of gas tunnel type plasma sprayed La2Zr2O7 thermal barrier coatings

Gas tunnel type plasma sprayed free-standing La₂Zr₂O₇ coating specimens with a thickness of 300-400 μm were prepared under optimized operating conditions and were subjected to hot corrosion test in the presence of corrosive impurities such as V₂O₅, Na₂SO₄, and Na₂SO₄ + V₂O₅ mixtures (60:40 wt%) at two different temperatures for duration of 5 h, i.e. 1000 and 1350 K for V₂O₅ and Na₂SO₄ + V₂O₅ mixtures, 1200 and 1350 K for Na₂SO₄. For temperatures at 1350 K, the reaction mechanism of V₂O₅ and the mixture of Na₂SO₄ + V₂O₅ are similar and LaVO₄ is formed as the corrosive product, which leads to massive phase transformation from pyrochlore to tetragonal and monoclinic phases. Microstructural observations from planar reaction zone (PRZ) and melt infiltrated reaction zone (MIRZ) reveals that the present La₂Zr₂O₇ coating exhibits good hot corrosion resistance in V₂O₅ environment and moderate for the mixture of Na₂SO₄ + V₂O₅, but is worst in Na₂SO₄ environment.     

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.