Application of polyaniline to galvanic anodic protection on stainless steel in H2SO4 solutions

A stand-alone polyaniline (PANI) film electrode was made and then applied to protect 1Cr13 stainless steel (a type of stainless steel typically used in China, 0–15% C, 13% Cr) from corrosion in highly acidic solution, up to 5 M H₂SO₄. The stand-alone PANI electrode and 1Cr13 were coupled to study their galvanic interactions. PANI is a cathode while 1Cr13 is an anodic. The results indicate that PANI with a certain area can quickly passivate the stainless steel and effectively maintain the steady passive state for a long period of time. This specific method of PANI preventing 1Cr13 from corrosion is called galvanic anodic protection (GAP). Moreover, the efficiency, mechanism, and security of GAP were discussed. The results demonstrate that the stand-alone PANI electrode may have a potential application to galvanic anodic protection on stainless steel in highly corrosive H₂SO₄ solutions.     

Electrochemical kinetics of the high entropy alloys in aqueous environments—a comparison with type 304 stainless steel

A high entropy alloy (HEA) is a multi-component alloy containing several major alloying elements, which has a high degree of atomic disorder that leads to various unique magnetic, mechanical, and electrochemical properties. It is known that one HEA, evaluated previously, is more resistant to general corrosion than type 304 stainless steel (304s), both in H₂SO₄ and in NaCl solutions at room temperature, but pitting corrosion resistance of the HEA is less than that of 304s.The anodic polarization curves determined for the HEA in aqueous environments showed that the general corrosion resistance of both HEA and 304s decreases as the temperature increases above room temperature. The decrease in the corrosion resistance of the HEA with an increase in temperature is less in NaCl than in H₂SO₄. The general corrosion rate for the HEA is lower than that for 304s in H₂SO₄, but higher than that for 304s in NaCl. The activation energies are: 94.06 kJ/mole for the HEA and 219.97 kJ/mole for 304s in 1 N H₂SO₄, and 310.43 kJ/mole for the HEA and 343.18 kJ/mole for 304s in 1 M NaCl. In addition, it was observed that concentration polarization occurred in cathodic reduction processes in deaerated 1 M NaCl at various temperatures.The polarization curves for the HEA and 304s indicated there is mixed control in 1 N H₂SO₄ and anodic control in 1 M NaCl based on the assumption that the surface chemical compositions of the two alloys are similar to their bulk chemical compositions. However, the decrease in the cathodic current with time for the HEA at more negative applied potentials is attributed to the high hydrogen overvoltage. In addition, the weak endothermic reaction revealed by differential scanning calorimetry (DSC) analysis showed that only small amounts of Cu-rich phases are segregated in the interdendritic phases; and the wide range of temperatures over which the endothermic reaction occurs indicates the formation of pro-eutectoid phases.     

Microstructure and electrochemical properties of high entropy alloys—a comparison with type-304 stainless steel

High entropy alloys (HEAs) are a newly developed family of multi-component glassy alloys composed of several major alloying elements, such as copper, nickel, aluminum, cobalt, chromium, iron, silicon, titanium, etc. The HEA studied had a nearly amorphous structure as proven by X-ray diffraction (XRD), selected area diffraction (SAD), and differential scanning calorimetry (DSC) analysis. The dendritic phase was composed mainly of a non-crystalline phase with a little body centered cubic (BCC) structure whereas the interdendritic phase had an amorphous structure containing small amounts of nano-scale precipitates. The HEA had a high degree of atomic disorder with mechanical properties comparable to that of glass and it was therefore hard but brittle. Its hardness (Hv860) was much higher than that of type-304 stainless steel (Hv265). The anodic polarization curves of the HEA, obtained in aqueous solutions of NaCl and H₂SO₄, clearly indicated that the general corrosion resistance of the HEA at ambient temperature (∼25 °C) is superior to that of 304S, irrespective of the concentration of electrolyte in the range 0.1-1 M. On the other hand, the HEA’s resistance to pitting corrosion in a Cl⁻ environment is inferior to that of 304S, as indicated by a lower pitting potential and a narrower passive region for the HEA. Tests in 1 N sulfuric acid containing different concentrations of chloride ions showed that the HEA has least resistance to general corrosion at a chloride ion concentration of 0.5 M (close to the concentration in seawater). The lack of hysteresis in cyclic polarization tests confirmed that the HEA—like 304S—is not susceptible to pitting corrosion in chloride-free 1 N H₂SO₄.     

XPS analysis of the passive film formed on austenitic stainless steel coated with conductive polymer

Improvement of the passivation behavior of Type 304 austenitic stainless steel (SS) by coating with conductive polymers (CPs), like polyaniline (PANI) and poly(o-phenylenediamine) (PoPD), followed by exposure in an acid solution has been demonstrated. The passive films formed on SSs (after peeling off the polymer layer) are compared with those formed during anodic polarization under the same exposure condition. The passive films beneath the CPs are thicker and less hydrated than those formed on uncoated stainless steel. The polymer layer enhances the enrichment of chromium and nickel in the entire passive oxide, forming a more protective film than that formed during anodic polarization. The elemental distribution within the passive film is different in the two modes of passivation. The type of the polymer influences on the composition of the passive film. The best passivation is obtained by PoPD, with the passive film resulting in significant resistance of the SS to pitting corrosion in the 3% NaCl solution. The oxide film of this steel is characterized, in its inner and outer layers, by the highest ratio of Cr(OH)₃/Cr₂O₃ and the lowest content of iron species.     

Effect of Mo and Mn additions on the corrosion behaviour of AISI 304 and 316 stainless steels in H2SO4

The work addresses the influence of Mn and Mo additions on corrosion resistance of AISI 304 and 316 stainless steels in 30 wt.% H₂SO₄ at 25 and 50 °C. Corrosion mechanism was determined by gravimetric tests, DC polarization measurements and electrochemical impedance spectroscopy (EIS). The morphology and nature of the reaction products formed on the material surface were analysed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Reduction of temperature from 50 to 25 °C drastically decreased the corrosion rate of AISI 304 and 316 stainless steels in sulphuric acid solution. Mn additions did not affect significantly the general corrosion resistance due to its low ability to form insoluble compounds in acid medium. Meanwhile, the formation of molybdenum insoluble oxides enhanced the corrosion performance.     

The corrosion behaviour of Fe-15Mn-7Si-9Cr-5Ni shape memory alloy

The corrosion behaviour of Fe-15Mn-7Si-9Cr-5Ni (mass%) shape memory alloy at 25 °C in 0.5 M H₂SO₄ and 3.5% NaCl solutions has been studied using potentiodynamic polarization and electrochemical impedance techniques. Three different microstructures viz., single-phase γ, γ-δ and γ-Fe₅Ni₃Si₂, were produced by heat-treating the alloy in different equilibrium phase fields. The corrosion behaviour in 0.5 M H₂SO₄ solution is almost same for all three microstructures, barring a slight difference in the passivation range. Although, the passivation current in 0.5 M H₂SO₄, is in the same range as that of SS 304, the critical current required for onset of passivation is almost three orders higher and the passivation range is much shorter. In 3.5% NaCl solution the corrosion behaviour of all three microstructures of the Fe-15Mn-7Si-9Cr-5Ni shape memory alloy was that of general dissolution without passivity or localized attack (pitting). The best corrosion resistance in both H₂SO₄ and NaCl solutions is shown by the single-phase γ microstructure.     

Passivity of stainless steel in sulphuric acid under chemical oxidation

The main purpose of the present work was to study the passivity of stainless steel in sulphuric acid under chemical oxidation with H₂O₂ solution. Potentiodynamic polarisation and open circuit potential (OCP) measurements indicated that H₂O₂ facilitates the increase of OCP of stainless steel which shifts from active region into passive region. The X-ray photoelectron spectroscopy results suggested that a passive film, composed of oxyhydroxides, Cr₂O₃, Cr(VI) species, NiO, and sulphate (FeSO₄, Cr₂(SO₄)₃·xH₂O), is achieved after H₂SO₄–H₂O₂ passivation. The passive mechanism under H₂O₂ oxidation was discussed and the corrosion resistance of passive film was compared with that of the passive films produced by HNO₃ passivity and H₂SO₄ potentiostatic passivity. The results of electrochemical impedance spectroscopy, cyclic voltammetry, and anodic polarisation experiments confirmed that the chemical oxidation with H₂O₂ solution is capable of improving the corrosion resistance of stainless steel significantly and the passive film is more stable than those produced by other passivating methods.     

Corrosion behavior of UCX,KHR35, and KHR45 alloys in molten nitrates

The corrosion behavior of three high-Ni, high-Cr alloys, that is, UCX, KHR35, and KHR45 alloys, in a mixture of 60% NaNO₃–40% KNO₃ at 600°C has been evaluated by using weight loss tests, potentiodynamic polarization curves, and electrochemical impedance spectroscopy measurements. Cr contents ranged between 23.25 and 43.2 wt.%, whereas Ni ranged between 36.6 and 50.3 wt.%. For comparison, the same studies were performed on 304-type stainless steel (304SS). Tests were complemented with detailed scanning electronic microscope and X-ray diffraction studies. Results showed that that the three high-Ni, high-Cr alloys had lower weight loss than that for 304SS. Polarization tests indicated the formation of a passive layer in all cases. Electrochemical impedance spectroscopy data have shown that the corrosion mechanism for all the alloys was charge transfer from the alloy to the molten salt. Finally, X-ray patterns showed the presence of Cr₂O₃ in all tested alloys, which is responsible for the observed passive behavior and their corrosion resistance.     

Inhibition mechanism of S-containing additives towards intergranular corrosion of a sensitized stainless steel

The comparison of the polarization curves recorded on both pure Fe, Cr, Ni, and Fe-18Cr-8Ni, Fe-8Cr-8Ni alloys, and Fe-10Ni, Fe-17Cr alloys in 1 N H₂SO₄ at 70°C clarifies the mechanism of inhibition of IG corrosion on sensitized AISI 304 SS by S-containing additives. These additives stimulate the anodic dissolution process of Fe-18Cr-8Ni alloy and inhibit this process on Fe-8Cr-8Ni alloy. The anodic behaviour of Fe-18Cr-8Ni is similar to that of pure chromium, while the behaviour of Fe-8Cr-8Ni is similar to that of pure iron.     

Accelerated corrosion of five commercial steels under a ZnCl2-KCl deposit in a reducing environment typical of waste gasification at 673-773 K

The corrosion of five Fe-Cr commercial steels containing 0-18 wt.% Cr at 673-773 K has been studied in a reducing H₂-HCl-CO₂ atmosphere under a ZnCl₂-KCl deposit typical of waste gasification environments. In comparison with the behavior of the same steels in a similar gas mixture without salt deposit, all steels suffered from accelerated corrosion induced by the salt and formed porous scales with poor adherence to the underlying steels. Some Cl was detected close to the steels/scale interface, indicating that Cl-containing species were able to go through the scale down to the metal matrix. Even though the corrosion rates generally decreased with increasing Cr content, the high-Cr stainless steel SS304 was still unable to provide a good corrosion resistance against the ZnCl₂-KCl deposit. The reaction mechanisms are discussed on the basis of thermodynamic considerations and of the “active oxidation” model.     

Corrosion assessment of nitric acid grade austenitic stainless steels

The corrosion resistance of three indigenous nitric acid grade (NAG) type 304L stainless steel (SS), designated as 304L1, 304L2 and 304L3 and two commercial NAG SS designated as Uranus-16 similar to 304L composition and Uranus-65 similar to type 310L SS were carried out in nitric acid media. Electrochemical measurements and surface film analysis were performed to evaluate the corrosion resistance and passive film property in 6 N and 11.5 N HNO₃ media. The results in 6 N HNO₃ show that the indigenous NAG 304L SS and Uranus-65 alloy exhibited similar and higher corrosion resistance with lower passive current density compared to Uranus-16 alloy. In higher concentration of 11.5 N HNO₃, transpassive potential of all the NAG SS shows a similar range, except for Uranus-16 alloy. Optical micrographs of all the NAG SS revealed changes in microstructure after polarization in 6 N and 11.5 N HNO₃ with corrosion attacks at the grain boundaries. Frequency response of the AC impedance of all the NAG SS showed a single semicircle arc. Higher polarization resistance (R_P) and lower capacitance value (CPE-T) revealing higher film stability for indigenous NAG type 304L SS and Uranus-65 alloy. Uranus-16 alloy exhibited the lowest R_P value in both the nitric acid concentration. Auger electron spectroscopy (AES) study in 6 N and 11.5 N HNO₃ revealed that the passive films were mainly composed of Cr₂O₃ and Fe₂O₃ for all the alloys. The corrosion resistance of different NAG SS to HNO₃ corrosion and its relation to compositional variations of the NAG alloys are discussed in this paper.     

The effect of cysteine on the corrosion of 304L stainless steel in sulphuric acid

The effect of cysteine on the corrosion of 304L stainless steel in 1 mol l⁻¹ H₂SO₄ was studied using open-circuit potential measurements, anodic polarization curves, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). All the electrochemical measurements obtained in the presence of low cysteine concentration (10⁻⁶-10⁻⁵ mol l⁻¹) presented the same behaviour as those obtained in the absence of cysteine, a passivated steel surface. However, for higher cysteine concentrations (10⁻⁴-10⁻² mol l⁻¹), a different behaviour was observed: the corrosion potential stabilized at a more negative value; an active region was observed in the anodic polarization curves and the electrochemical impedance diagrams showed an inductive loop at lower frequencies and a much lower polarization resistance. These results show that the presence of cysteine at high concentration turns the surface of 304L stainless steel electrochemically active, probably dissolving the passivation layer and promoting the stainless steel anodic dissolution. SEM experiments performed after immersion experiments at corrosion potential were in good agreement with the electrochemical results.     

1Cr18Mn14N双相不锈钢在腐蚀介质中的抗空蚀性能

利用磁致伸缩空蚀实验机研究了1Cr18Mn14N双相不锈钢在3%NaCl和05 mol/L HCl溶液中的空蚀行为.结果表明：在3%NaCl溶液中，低硬度的Cr18Mn14N双相不锈钢的抗空蚀性能优于高硬度的水轮机常规用材0Cr13Ni5Mo.1Cr18Mn14N双相不锈钢的空蚀破坏首先在铁素体相发生，铁素体相的失效方式为脆性失效，而奥氏体相是延性失效.奥氏体相区由滑移和孪生引起的塑性变形耗散了空泡溃灭产生的冲击能量，从而提高1Cr18Mn14N双相不锈钢的抗空蚀性能.在05 mol/L HCl溶液中，1Cr18Mn14N的抗空蚀性能不如0Cr13Ni5Mo，结果与3%NaCl溶液中的正好相反，这是由于阳极溶解和氢共同作用的结果.     

Electrochemical impedance spectroscopy for the detection of stress corrosion cracks in aqueous corrosion systems at ambient and high temperature

Electrochemical impedance spectroscopy (EIS) has been used as a tool to detect stress corrosion cracking in a stainless steel sample exposed to an aqueous environment at ambient and high temperature. A model has been developed to describe the impedance of a cracked surface. This model could distinguish between a flat electrode surface and an electrode surface with cracks. To test this technique, three case studies were considered: (1) Slow Strain Rate Tests (SSRT) on sensitised Type 304 SS specimen in a 5 N H₂SO₄ + 0.1 M NaCl solution at room temperature, (2) constant load tests on Type 304 SS in a boiling (±110 °C) acidified sodium chloride solution, and (3) Slow Strain Rate Tests (SSRT) on Type 304 SS in an oxygen containing solution of 0.01 M Na₂SO₄ at 300 °C.EIS measurements were always performed simultaneously on two identical samples: one stressed with a SSRT or a constant load test and one free of stress. Kramers Kronig Transformations have been used to validate the experimental data obtained with the EIS measurements. It was shown that the phase shift between the two samples could be related to the stress corrosion cracking process. Analysis of the fracture surfaces confirmed that stress corrosion cracks were formed in all three cases.     

Influence of Cu and Sn content in the corrosion of AISI 304 and 316 stainless steels in H2SO4

This paper addresses the influence of Cu and Sn addition on the corrosion resistance of AISI 304 and 316 stainless steels in 30 wt% H₂SO₄ at 25 and 50 °C. The corrosion process was evaluated by gravimetric tests, DC measurements and electrochemical impedance spectroscopy (EIS). The corrosion products were analysed by SEM, X-ray mapping and XPS before and after accelerated tests. The behaviour of both AISI 304 and 316 stainless steels in sulphuric acid solution was greatly improved by increasing Cu concentration and the synergic effect of Cu and Sn. Addition of Sn increased corrosion resistance, but less than addition of copper.     

Corrosion behavior of Ti(C,N)-Ni/Cr cermets in H2SO4 solution

The effects of partial substitution of Ni with Cr on the microstructure and corrosion resistance of Ti(C,N)-based cermets in H₂SO₄ solution were investigated in this paper. The results showed that partial substitution of Ni with Cr had a minor effect on the microstructure, whereas the hardness of the Ti(C,N)-based cermets could be improved for dissolution of Cr in Ni binder. The corrosion behavior of Ti(C,N)-based cermets with different Cr content in 0.2 mol/L H₂SO₄ solution was also studied via potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and corrosion test. The tested Ti(C,N)-Ni/Cr cermets had three typical passive regions. The current of the first passive region was reduced and the passive range was enlarged with the increase of Cr content in binder attributed to the synergistic effect of Ti-based and Cr-based passive layers. But the remaining passive regions were pseudopassive regions. The EIS results also demonstrated that the impendence of the reaction rose with the increase of Cr content in binder. Moreover, the corrosion resistance of cermets in H₂SO₄ solution was improved remarkably by Cr dissolving in the binder, which gave rise to the enhanced passivation ability.     

Corrosion behavior of Cr/Cu-coated Mg alloy (AZ91D) in 0.1 M H2SO4 with different concentrations of NaCl

An electroplating process was proposed for obtaining a protective Cr/Cu deposit on the two-phase Mg alloy AZ91D. The corrosion behavior of Cu-covered and Cr/Cu-covered AZ91D specimens was studied electrochemically in 0.1 M H₂SO₄ with different NaCl concentrations. Experimental results showed that the corrosion resistance of an AZ91D specimen improved significantly after Cr/Cu electrodeposition. The corrosion resistance of Cr/Cu-covered AZ91D decreased with increasing NaCl concentration in 0.1 M H₂SO₄ solution. After immersion in a 0.1 M H₂SO₄ with a NaCl-content above 3.5 wt.%, the surface of Cr/Cu-covered AZ91D suffered a few blisters. Cracks through the Cr deposit provided active pathways for corrosion of the Cu and the AZ91D substrate. Formation of blisters on the Cr/Cu-covered AZ91D surface was confirmed based on the results of an open-circuit potential test, which detected an obvious potential drop from noble to active potentials.     

Acoustic emission during pitting corrosion of 304 stainless steel

Acoustic emission (AE) during pitting corrosion of 304 stainless steel (304 SS) in H₂SO₄ solutions with different pH values and Cl⁻ concentrations was studied. Two types of AE signals are detected in all solutions. Each type of signals is characterized by AE parameters (rise time, counts number, duration and amplitude) and waveform carefully. It is believed that the hydrogen bubbles evolution inside the pits is the AE source.     

Corrosion and mechanical properties of duplex-treated 301 stainless steel

Plasma nitriding is a widely used technique for increasing the surface hardness of stainless steels, and consequently, for improving their tribological properties. It is also used to create an interface between soft stainless steel substrates and hard coatings to improve adhesion. This paper reports on the mechanical and corrosion properties of AISI301 stainless steel (SS) after a duplex treatment consisting of plasma nitriding followed by deposition of Cr bond coat and CrSiN top layer by magnetron sputtering. Mechanical properties of the deposited films, such as hardness (H) and reduced Young's modulus (E_r), were measured using depth-sensing indentation. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were carried out to evaluate resistance to localized and to general corrosion, respectively. The corrosion behavior has been correlated with the microstructure and composition of the surface layers, determined by complementary characterization techniques, including XRD, SEM, and EDS. The CrSiN layers exhibited an H value of 24 GPa, whereas the nitrided layer was shown to present a gradual increase of H from 5 GPa (in the nitrogen-free SS matrix) to almost 14 GPa at the surface. The electrochemical measurements showed that the nitriding temperature is a critical parameter for defining the corrosion properties of the duplex-treated SS. At a relatively high temperature (723 K), the nitrided layer exhibited poor corrosion resistance due to the precipitation of chromium nitride compounds and the depletion of Cr in the iron matrix. This, in turn, leads to poor corrosion performance of the duplex-treated SS since pores and defects in the CrSiN film were potential sites for pitting. At relatively low nitriding temperature (573 K), the nitrided interface exhibited excellent corrosion resistance due to the formation of a compound-free diffusion layer. This is found to favor passivation of the material at the electrode/electrolyte interface of the duplex-treated SS.     

Galvanic corrosion between the constituent phases in duplex stainless steel

The exclusive single-phase with the exact chemical composition of the constituent phase in 2205 duplex stainless steel (DSS) could be prepared using selective dissolution method. The respective electrochemical behavior of each constituent phase could then be measured. The experimental results showed that the two distinct peaks in the active-to-passive transition region of the polarization curve determined in 2 M H₂SO₄ + 0.5 M HCl mixed solution were actually corresponded to the respective anodic peaks of the single austenite and ferrite phases. A polarity reversion was found between austenite and ferrite phases in mixed H₂SO₄ + HCl solution and HNO₃ solution. Galvanic current measurements also revealed that austenite was anode in HNO₃ solution, but became cathode when exposed in 2 M H₂SO₄ + 0.5 M HCl mixed solution.