In vitro electrochemical investigations of advanced stainless steels for applications as orthopaedic implants

Potentiodynamic anodic polarization experiments on advanced stainless steels (SS), such as nitrogenbearing type 316L and 317L SS, were carried out in Hank’s solution (8 g NaCl, 0.14 g CaCl₂, 0.4 g KC1, 0.35 g NaHCO₃, 1 g glucose, 0.1 g NaH₂PO₄, 0.1 g MgCl₂, 0.06 g Na₂HPO₄ 2H₂O, 0.06 g MgSO₄ 7H₂O/1000 mL) in order to assess the pitting and crevice corrosion resistance. The results showed a significant improvement in the pitting and crevice corrosion resistance than the commonly used type 316L stainless steel implant material. The corrosion resistance was higher in austenitic stainless steels containing higher amounts of nitrogen. The pit-protection potential for nitrogen-bearing stainless steels was more noble than the corrosion potential indicating the higher repassivation tendency of actively growing pits in these alloys. The accelerated leaching study conducted for the above alloys showed very little tendency for leaching of metal ions, such as iron, chromium, and nickel, at different impressed potentials. This may be due to the enrichment of nitrogen and molybdenum at the passive film and metal interface, which could have impeded the releasing of metal ions through passive film.     

Aluminium-Thiourea Mixtures as Corrosion Inhibitors for Mild Steel in 1·0% NaCl

Abstract

The inhibitive properties of mixtures containing aluminium salts and thiourea (so called aluminium-thiourea inhibitors) have already been investigated in relation to their inhibitive properties towards mild steel corrosion in distilled water and in water of high specific salinity (1·0% Na₂SO₄. This paper reports laboratory test data obtained during further investigations of aluminium-thiourea inhibitors in another corrosive medium, 1·0% NaCl at 20°c and under stagnant conditions. It was found that the corrosion rate of mild steel in sodium chloride solutions can be decreased markedly (by ~ 80%) if aluminium ions are used in the form of a mixture of sulphates and oxychlorides and in conjunction with thiourea, cationic surfactants and reducing agents (SnCl₂). Any change in the composition of the aluminium salts in the mixture, keeping a constant Al³⁺ concentration (for instance a substitution of sulphates/oxychlorides by sulphates/chlorides or chlorides/oxychlorides) reduces the inhibitive efficiency of the formulations tested. The author fails to find a satisfactory explanation of the very specific inhibitive behaviour of aluminium-thiourea mixtures in 1·0% NaCl.     

The Hot Corrosion of Fe-Mn-Al–C Alloy with NaCl/Na2SO4 Coating Mixtures at 750°C

The high-temperature corrosion behavior of Fe-30.1Mn-9.7Al-0.77C alloy initially coated with 2 mg/cm² NaCl/Na₂SO₄ (100/0, 75/25, 50/50, 25/75 and 0/100 wt.%) deposits has been studied at 750°C in air. The result shows that weight-gain kinetics in simple oxidation reveals a steady-state parabolic rate law after 3 hr, while the kinetics with salt deposits all display multi-stage growth rates. The corrosion morphology of the alloy with 100% Na₂SO₄ coating is similar to that of simple oxidation. NaCl acts as the predominant corrosion species for Fe-Mn-Al-C alloy, inhibiting the formation of a protective oxide scale. For the alloy coated with over 50% NaCl in salts, NaCl induces selective oxidation of manganese and results in the formation of secondary ferrite in the alloy substrate as well as void-layers with different densities of voids layer by layer in the secondary-ferrite zone.     

Na2SO4- and NaCl-Induced hot corrosion of six nickel-base superalloys

The short-time hot-corrosion behavior of six industrial nickel-base superalloys was investigated with static deposits of Na₂SO₄ or NaCl or both in still air. The oxidation kinetics and scale morphologies were measured with traditional laboratory techniques-thermobalance, metallography, electron microprobe, and x-ray analyses. Susceptibility to hot corrosion was found to be correlated to the type of scale produced during simple oxidation. Alloys forming an A1₂O₃ scale were found to be susceptible to Na₂SO₄ deposits, independent of their chromium content. The quantity of Na₂SO₄ deposit dictated the nature of the attack and, under certain conditions, the refractory element alloy additions appeared to play an essential role. Alloys containing Cr₂O₃ or TiO₂ in the simple oxidation scale proved to be sensitive to NaCl attack. Again, the severity of the attack within the susceptible alloy group was not related to the chromium or titanium content. Although less intensive than the Na₂SO₄ -induced hot corrosion, NaCl contaminations provoked extensive spalling. All of the hotcorrosion types encountered in this study were interpreted in the light of existing theories.Supported by the Délégation Générale à la Recherche Scientifique et Technique.     

Corrosion protection studies of CeO2–TiO2 nanocomposite coatings on mild steel

Nanocomposite coatings have evolved as corrosion-resistant materials to protect metals and alloys in various environments. The need for development of corrosion-resistant materials for mild steel in marine environment is still in demand. The CeO₂–TiO₂ nanocomposite powders were produced via hydrothermal synthesis and the corrosion resistance behaviour of the nanocomposite coatings were evaluated in 3.5% NaCl solution using Tafel polarisation and electrochemical impedance spectroscopy techniques. The trends of open-circuit potential curves provided clear evidence that the incorporation of CeO₂ in TiO₂ nanostructures is beneficial, as it introduces potential shift towards noble positive potential for nanocomposite coatings. Also, the corrosion resistance was enhanced with increase in the CeO₂ content in TiO₂ nanocomposite coatings. Almost 22 times decrease in the corrosion current densities of mild steel were attained for 15?wt-% CeO₂–TiO₂, which demonstrated the advantage of CeO₂–TiO₂ nanocomposite coatings for corrosion protection of mild steel.     

Morphological Development of Subscale Formation in Fe–Cr–(Ni) Alloys with Chloride and Sulfates Coating

Three types of stainless steel (430, 304, and 310) with a coating of NaCl, NaCl/AlCl₃, or NaCl/Al₂(SO₄)₃ are exposed at 750 and 850°C. Results show that NaCl has a major effect on corrosion and sulfur plays an important role in intergranular corrosion. After high-temperature exposure with a 100% NaCl coating, the morphologies of alloys 304 and 310 show typical uniform subscale attack the depths of attack increasing with temperature, while alloy 430 showed a planar attack. Alloy 310 has the highest chromium content and has the least metal loss. After high-temperature exposure with a NaCl/AlCl₃ coating, the corrosion morphologies and depths of attack are similar to those associated with an NaCl coating, but only voids are larger in the subscale. When coated with NaCl/Al₂(SO₄)₃, the alloys are attacked simultaneously by sulfur and chlorine at 750°C, resulting in a typical sulfur-attack intergranular corrosion. However, as the temperature increases to 850°C, the corrosion morphology changes to a uniform subscale attack.     

In search of functionality for efficient inhibition of mild steel corrosion both in HCl and H2SO4

Abstract

While the organic ammonium compounds are known to be effective against mild steel corrosion in HCl, these cationic salts are not as effective in sulphuric acid media. A new series of tertiary diallylamines containing C₁₂, C₁₄, C₁₆ and C₁₈ alkyl hydrophobes, N-hexadecylpyrrolidine and their corresponding ammonium salts were found to be among a rare class of inhibitors in arresting mild steel corrosion in HCl as well as in H₂SO₄ media effectively. While all the inhibitor molecules exhibited excellent inhibition efficiencies (%IEs) (～99%) in 1M HCl, ammonium salts containing alkyne moiety at 400 ppm provided 93–97%IE even in 7·7M HCl. The inhibitors (400 ppm) containing diallyl moiety demonstrated the %IEs in the range 80–97% in 0·5M H₂SO₄. The interactions of the π electrons in the diallyl moiety with the low lying vacant d-orbitals of iron help the inhibitor molecules to undergo adsorption and interfere with the anodic dissolution process.     

The role of NaCl in the hot-corrosion behavior of Nimonic alloy 90

The influence of sodium chloride on the hot-corrosion behavior of Nimonic alloy 90 has been investigated by employing the half-immersion, crucible test. Nimonic 90 samples were hot corroded in the presence of NaCl between 700–900°C. The results showed that the weight-loss plots with both time and temperature were linear indicating the catastrophic nature of attack. An examination of the corroded samples by XRD, XRF, EPMA, SEM, and chemical analysis indicated that as the corrosion time increased, an increase in the depletion of alloying constituents like Cr, Al, Ti, and Co took place with a resultant enrichment of nickel on the alloy surface. The formation of CoCl₂ and Na₂CrO₄ was observed in all the tests. A few experiments were carried out in the presence of Na₂SO₄ and in a 1% NaCl mixture, in order to see the influence of NaCl on Na₂SO₄. The results indicated that Na₂SO₄ is innocuous when compared with NaCl. However, the severe attack was observed in the presence of the 1% NaCl mixture between 700–800°C, i.e., above the eutectic temperature and the m.p. of NaCl (800°C). The corrosion was minimum, when the salt mixture existed in the molten state. All the corroded samples were magnetic in nature. The role of NaCl on the hot-corrosion behavior of Nimonic 90 has been discussed in the light of the above crucible-test investigations.     

高温防护涂层的熔盐热腐蚀研究进展

高温热腐蚀是热元件主要失效形式之一,Na₂SO₄和NaCl熔盐会加速高温下的热腐蚀,甚至导致灾难性事故发生。本文就Na₂SO₄和/或NaCl熔盐引起的热腐蚀进行了讨论,其中Na₂SO₄是主要的腐蚀反应物,详细介绍了2种典型的热腐蚀行为和性能特点。重点介绍了几种热腐蚀模型和机理,以及Na₂SO₄、NaCl、Na₂SO₄+NaCl熔盐的反应公式和腐蚀机理。根据目前的研究状况来看,制备防护涂层是缓解热腐蚀的最佳途径,总结了近年来MCrAlY涂层、NiAl涂层、热障涂层和新型涂层的发展情况,并探讨了进一步提高涂层耐腐蚀性能的方法。最后,展望了防护涂层的未来发展方向。     

Anion effects on the stress corrosion cracking behaviour of aluminium alloys

The stress corrosion cracking behaviour of plate material of the aluminium alloys 2024‐T351, 8090‐T8171, 7475‐T651, and 7075‐T7351 was investigated performing constant load tests. Short transverse tensile specimens were permanently immersed in aerated aqueous 0.6 M Na₂Cl solutions with additions of Na₂SO₄, NaNO₃, NaHCO₃, NH₄HCO₃, Na₂HPO₄, Na₂SO₃ or Na₂CO₃. The concentration of the added salts was 0.06 M. The applied stress was 100 MPa, except with 7075‐T7351 specimens, which were loaded at 300 MPa. Environment induced failure was not observed in neutral 0.6 M NaCl solution. The various salts added promoted intergranular stress corrosion cracking with the alloys 2024‐T351, 8090‐T8171, and 7475‐T651. Threshold stresses were generally below 100 MPa. For 8090‐T8171 exposed to chloride containing electrolytes with additions of sulfate, hydrogen phosphate, or sulfite, threshold stresses were approximately 100 MPa or higher. Similar results were obtained for 7475‐T651 plate when immersed in chloride‐hydrogen phosphate and chloride‐carbonate solutions. Alloy 7075‐T7351 was resistant against intergranular stress corrosion cracking. Specimens suffered pitting corrosion during immersion in the corrosive environments. Failure observed with 7075‐T7351, in particular when exposed to the chloride‐nitrate solution, was associated with reduction of cross‐sectional area due to pitting and transgranular stress corrosion cracking.     

Long term corrosion of X70 steel and iron in humid supercritical CO2 with SO2 and O2 impurities

Abstract

The corrosion of X70 steel and iron in supercritical CO₂/SO₂/O₂/H₂O environment were investigated after a 454 h exposure. Optical microscopy was applied to observe the morphology of etch pits and synthesise the three-dimensional morphology. X-ray diffraction and X-ray photoelectron spectroscopy were employed to detect the composition of product scales. Experimental results verified that the localised corrosion occurred on the X70 steel sample under corrosion product deposits. Ferrous sulphate, sulphur and iron sulphide were detected as the corrosion products.     

Aluminizing and boroaluminizing treatments of Mar-M247 and their effect on hot corrosion resistance in Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>−NaCl molten salt

The effect of surface modifications of Mar-M247 superalloy on hot corrosion resistance was examined in Na₂SO₄−NaCl molten salt. The Mar-M247 was aluminized and boroaluminized by pack cementation in Ar and underwent a cyclic hot corrosion test in Na₂SO₄−NaCl molten salt. The XRD results showed that a Ni₂Al₃ phase was formed between the aluminized layer and the substrate when the surface modification temperature was below 1273 K. However, a NiAl phase formed when the temperature was above 1273 K. The intensity of the XRD peak in the NiAl phase increased after post heat treatment. Hot corrosion resistance increased for the specimens containing NiAl rather than Ni₂Al₃ phase. The ductile NiAl phase suppressed the potential for crack initiation during thermal cycling. Post heat treatment increased the corrosion resistance of the aluminized layer for Mar-M247, which underwent surface modification at 1273 K and above. In the boroaluminized Mar-M247 specimens, corrosion resistance decreased as a result of the blocking of outward diffusion of Cr by boron and decreased cohesion between the oxide scale and the aluminized layer during thermal cycling.     

The correlation between the properties of the passive film on 304L steel and its susceptibility to SCC in chloride media

Certain physico-chemical properties of passive films formed in hot concentrated NaCl, CaCl₂ and MgCl₂ on 304L steel were investigated in order to elucidate their effect on susceptibility to SCC. The resistance to pitting corrosion and suceptibility to SCC was found to be correlated. The resistance to pitting corrosion is discussed on the basis of composition and surface condition of passive films determined by AES methods and the dielectric response of the metal-passive film-electrolyte system as a function of frequency.     

The corrosion of Nb-modified Ti3Al in a combustion gas with and without Na2SO4−NaCl deposits at 600–800°C

The corrosion behavior of a Nb-modified Ti₃Al intermetallic compound containing 11 at.% Nb in a simulated combustion gas with and without deposits of a Na₂SO₄–NaCl mixture was examined at 600–800°C for times up to four days. In the absence of salt deposits the corrosion rates were rather low and increased only slightly with temperature, producing very thin scales of mixed oxides of Ti, Al, and Nb without sulfides. The presence of the salt deposits produced higher weight gains during an initial stage of one to two days at 600 and 700°C, after which the reaction stopped. A more important and longlasting effect was observed instead at 800°C, when the kinetics of hot corrosion became nearly linear. The scales formed by hot corrosion were complex mixtures of various corrosion products at all temperatures and showed a porous outer region containing a mixture of unreacted salts with oxides (mainly TiO₂), an intermediate region of a mixture of variable composition of oxides of the three metals, and a TiO₂-rich layer beneath it. At 800°C the scales tended to form a thin, discontinuous Al₂O₃-rich layer in the middle and contained an additional innermost region presenting a large concentration of sulfur, very likely as Nb and Ti sulfides. The high rate of hot corrosion at 800°C is attributed to the appearance of sulfides in the inner region of the scale and to a more efficient scale fluxing.     

The effect of K2SO4 additive in Na2SO4 deposits on low temperature hot corrosion of iron-aluminum alloys

To understand the effect of K₂SO₄ additive in an Na₂SO₄ deposit on low temperature hot corrosion, the corrosion behavior of Fe-Al alloys induced by Na₂SO₄+K₂SO₄ was compared to that by Na₂SO₄ alone, and sulfation of Fe₂O₃ in the presence of either Na₂SO₄ or Na₂SO₄+K₂SO₄ was studied. It was found that K₂SO₄ additive promoted the low temperature hot corrosion, but did not change the corrosion-mechanism. Experimental results refuted the prior suggestions that the accelerated hot corrosion resulted either from the formation of K₃Fe(SO₄)₃ or from the stimulation of sulfation of Fe₃O₃. The earlier formation of the eutectic melt caused the accelerated hot corrosion, or in other words, the K₂SO₄ additive shortened the induction stage of hot corrosion.     

The influence of CO2, AlCl3 · 6H2O, MgCl2 · 6H2O, Na2SO4 and NaCl on the atmospheric corrosion of aluminum

The influence of salt deposits on the atmospheric corrosion of high purity Al (99.999%) was studied in the laboratory. Four chloride and sulfate-containing salts, NaCl, Na₂SO₄, AlCl₃ · 6H₂O and MgCl₂ · 6H₂O were investigated. The samples were exposed to purified humid air with careful control of the relative humidity (95%), temperature (22.0 °C), and air flow. The concentration of CO₂ was 350 ppm or <1 ppm and the exposure time was four weeks. Under the experimental conditions all four salts formed aqueous solutions on the metal surface. Mass gain and metal loss results are reported. The corroded surfaces were studied by ESEM, OM, AES and FEG/SEM equipped with EDX. The corrosion products were analyzed by gravimetry, IC and grazing incidence XRD. In the absence of CO₂, the corrosivity of the chloride salts studied increases in the order MgCl₂ · 6H₂O < AlCl₃ · 6H₂O < NaCl. Sodium chloride is very corrosive in this environment because the sodium ion supports the development of high pH in the cathodic areas, resulting in alkaline dissolution of the alumina passive film and rapid general corrosion. The low corrosivity of MgCl₂ · 6H₂O is explained by the inability of Mg²⁺ to support high pH values in the cathodic areas. In the presence of carbon dioxide, the corrosion induced by the salts studied exhibit similar rates. Carbon dioxide strongly inhibits aluminum corrosion in the presence of AlCl₃ · 6H₂O and especially, NaCl, while it is slightly corrosive in the presence of MgCl₂ · 6H₂O. The corrosion effects of CO₂ are explained in terms of its acidic properties and by the precipitation of carbonates. In the absence of CO₂, Na₂SO₄ is less corrosive than NaCl. This is explained by the lower solubility of aluminum hydroxy sulfates in comparison to the chlorides. The average corrosion rate in the presence of CO₂ is the same for both salts. The main difference is that sulfate is less efficient than chloride in causing pitting of aluminum in neutral or acidic media.     

Role of thickness on electrochemical behaviour of Ni2Si coatings in NaCl solution

Abstract

Ni₂Si intermetallic coatings were deposited on a substrate of 420 stainless steel using a high velocity oxy-fuel (HVOF) process. The electrochemical corrosion behaviour of the coatings in 3.5% NaCl solution was investigated by means of Tafel polarisation tests and electrochemical impedance spectroscopy (EIS) measurements. Two coatings with different thicknesses, 55 and 115?μm, were investigated. The results showed that the corrosion rate of the thicker coating was lower than that of stainless steel by one order of magnitude. Local attack of the substrate was observed after the polarisation test of this coating, while the coating was still intact. The thinner coating and the stainless steel substrate showed similar corrosion rates. In this case, the substrate was severely attacked after the polarisation test. Two time constants were observed in the EIS spectra of both coatings which were related to charge transfer processes and pore resistance, respectively.     

Comparison of corrosion scales formed on KO8OSS and N8O steels in CO2/H2S environment

Abstract

Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) were applied to analyse the microstructure and composition of the corrosion scale formed on KO80SS and N80 tubes with carbon dioxide (CO₂) and hydrogen sulphide (SO₂). The corrosion scales of both KO80SS and N80 tubes were of the double layer structure, and not only uniform corrosion but also localised corrosion was observed. The crystal of the surface layer is laminar. The main phase in the outer layer is calcium carbonate (CaCO₃), and the inner scale consisted of iron carbonate (FeCO₃) for KO80SS steel and FeS_0·9 with a little amount of FeCO₃ for N80 steel respectively. Additionally, the electrochemical techniques were used to investigate the characteristics of the corrosion scales. The results indicated that the polarisation resistance R_p of KO80SS steel film was nobler than that of N80 steel film. Finally, the corrosion current I_corr of KO80SS steels was lower than that of N80 steels. Corrosion scale of KO80SS tube steels is more protective to the matrix than that of N80 tube steels.     

Effect of O2 on down-hole corrosion during air-assisted steam injection for heavy oil recovery

ABSTRACT

Air-assisted steam injection is used to enhance hydrocarbon recovery from heavy oil reservoirs. During this process, downhole tubular goods are subject to corrosion in a mixed oxygen-carbon dioxide-steam environment at temperature up to around 200°C causing a low-temperature oxidation phenomenon. Here we investigate the influence of the O₂/CO₂ ratio on the corrosion of P110 steel, a grade that is commonly used for oil well casings. Our findings show that corrosion is greatly influenced by the O₂ pressure but hardly at all by the CO₂ pressure. The maximum corrosion rate was in excess of 65?mm per year. However, alkalisation of the injected fluid, which promoted the formation of a protective magnetite scale, was found to greatly reduce corrosion.     

Effect of Na2SO4 and Water Vapor on the Corrosion of Ti3SiC2

The corrosion behavior of polycrystalline Ti₃SiC₂ was studied in the presence of Na₂SO₄ deposit and water vapor at 900°C and 1000°C. The mass gain per unit area of the samples superficially coated with Na₂SO₄ exposed to water vapor was slightly lower than that of the samples corroded without water vapor. The microstructure and composition of the scales were investigated by SEM/EDS and XRD. Pores were observed in the corroded sample surfaces. The main corrosion phases on the sample surface were identified by XRD as TiO₂, Na₂Si₂O₅ and Na₂TiO₃. After Ti₃SiC₂ corroded in the presence of the Na₂SO₄ deposit and water vapor, the scale had a three-layer microstructure, which was different from the duplex corrosion scale formed on Ti₃SiC₂ beneath the Na₂SO₄ film without water vapor. Because water vapor penetrated the corrosion layer and then reacted with SiO₂ to form volatile Si(OH)₄, an intermediate porous and TiO₂-enriched layer formed in the corrosion layer.