Effect of surface treatments based on self‐assembling molecules and cerium coatings on the AA3003 alloy corrosion resistance

This work presents the effects of a cerium conversion coating, self‐assembling molecules (SAM) treatment, and a combination of these two treatments on the corrosion resistance of the AA3003 alloy. The results were compared to that of a conversion coating treatment with hexavalent chromium (Cr VI). The corrosion resistance of the surface‐treated AA3003 alloy samples was investigated by electrochemical impedance spectroscopy and anodic polarization curves in 0.5 mol/L sodium sulfate solution with pH adjusted to 4.0. The results showed that the SAM treatment offered better corrosion protection for the AA3003 alloy than that provided by cerium conversion coating. The combination of cerium conversion and SAM treatments improved the corrosion resistance of the AA3003 alloy due to SAM adsorption on the alloy substrate exposed at the defects in the cerium conversion layer.     

Effect of Mn/Mo incorporated oxide layer on the corrosion behavior of AA 2024 alloy

The oxide layer formed over AA 2024 using 10 wt.% H₂SO₄ (plain oxide, PO) was modified by Mn/Mo oxyanions (permanganate/molybdate modified oxide, PMMO) as an alternative to Cr(VI) ions to enhance the corrosion resistance. The corrosion current density values obtained for PMMO was found to be 2.8% and 1.4% of hydrothermal treated oxide (HTO) and PO respectively after 168 h immersion in 3.5% NaCl solution. The electrochemical studies showed the higher barrier layer resistance for PMMO. The improved corrosion behavior of PMMO was observed based on the damage function calculated. Similar observations were confirmed by continuous salt spray test.     

Effects of duty ratio on properties of micro-arc film on Ti–3Zr–2Sn–3Mo–25Nb

Porous titanium oxide layers, which are important features for improving the biological activity of Ti implants with bone tissues, have been obtained through the technique of micro-arc oxidation (MAO). Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to evaluate the micromorphology and crystalline structure of these oxide films, and the chemical compositions were measured by electron dispersive X-ray spectroscopy (EDS). TiO₂ layers presented the crystalline phases of rutile and anatase. During the micro-arc oxidation treatment, Ca and P ions were incorporated into the oxide layer, and incorporation of Ca and P with the Ca/P content (%) of around 1.38 is similar to that in the human body. Nb₂O₅ was also identified in the treatment samples. The corrosion resistance was evaluated using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation curves. In the electrochemical corrosion tests, the treated samples presented lower values of corrosion current density than untreated Ti, indicating a better corrosion resistance. Diffusion phenomena were present in the process of corrosion.     

Lanthanide salts as thermal conversion coatings on silicon carbide reinforced AA5A06 aluminium metal matrix composite

In this study, cerium and lanthanum chloride binary mixtures conversion coatings were investigated as corrosion inhibitors for silicon carbide particulates reinforced AA5A06 aluminium composite and AA5A06 aluminium alloy. Electrochemical impedance spectroscopy and potentiodynamic polarization tests revealed that cerium (III) chloride additions markedly improved the pitting corrosion resistance as compared to lanthanum (III) chloride additions, with maximum increase noted for samples treated with 750 ppm cerium (III) + 250 ppm lanthanum (III) chlorides. Scanning electron microscopy, energy dispersion spectroscopy and X‐ray photoelectron spectroscopy examinations revealed the existence of a heterogeneous layer formed by precipitation of cerium/lanthanum oxide/hydroxide on the cathodic intermetallics and an aluminium oxide film on the rest of the metal matrix. Improved corrosion resistance was attained for SiC particulates reinforced aluminium composite by means of the cerium/lanthanum conversion treatment, due to the presence of mainly cerium (IV) species as a result of oxidation of cerium (III), which provides auto‐protection properties.     

Erosion–corrosion characteristic of nano‐particulates reinforced Ni–Cr–Mo–Cu surface alloying layer in acidic flow and acidic slurry flow

In order to improve the corrosion and erosion–corrosion resistance of 316L stainless steel in engineering application, two kinds of composite alloying layers were prepared by a duplex treatment, consisting of Ni/nano‐SiC and Ni/nano‐SiO₂ predeposited by brush plating, respectively, and a subsequent surface alloying with Ni–Cr–Mo–Cu by double glow process. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were performed on the two kinds of composite alloying layer using 10 wt% HCl solution to assess the corrosion behavior. Erosion–corrosion tests were carried out by erosion–corrosion test rig in acidic flow and acidic slurry flow for test time of 20 h at four different rotational speeds. Results of electrochemical tests indicated that the corrosion resistance of composite alloying layer with brush plating Ni/nano‐SiO₂ particles interlayer approximated to that of single Ni‐based alloying layer, whereas the corrosion resistance of the composite alloying layer with brush plating Ni/nano‐SiC particles interlayer was apparently inferior to that of Ni‐based alloying layer in 10 wt% HCl solution at static state. Under the conditions of acidic flow and acidic slurry flow, the mass losses of tested samples increased with increase in the time of erosion–corrosion tests and the rotational speeds of samples. The mass losses of composite alloying layer with brush plating Ni/nano‐SiO₂ particles interlayer were lower than that of single Ni‐based alloying layer at all rotational speeds, except at 1.88 m/s in acidic flow. The mass losses of composite alloying layer with brush plating Ni/nano‐SiC particles interlayer were higher than that of single Ni‐based alloying layer at all rotational speeds, but were obviously lower than that of AISI 316L stainless steel. The influences of second phase on the corrosion and erosion–corrosion of the two kinds of composite alloying layer were discussed in this paper.     

Cerium‐based thermal conversion treatments on silicon carbide reinforced 2009 aluminum alloy composites

The conversion coatings on SiC_p/2009 aluminum (Al) composites and 2009 Al alloy were obtained by immersing the samples into the cerium (Ce)‐based solutions with varying the chemical concentration (CeCl₃·7H₂O, NaCl), solution pH/temperature, immersing time, and drying temperature. The corrosion inhibition mechanism and kinetics were studied by using the electrochemical techniques and surface analyses. Potentiodynamic polarization tests showed that coatings improved corrosion resistance as compared to untreated samples and environmental factors played important role in the formation of conversion layer. Scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), and X‐ray photoelectron spectroscopy (XPS) analysis revealed that relatively high inhibition efficiency for composite with 25 vol% SiC_p was due to the existence of rather high surface area of cathodic intermetallics/SiC_p, which support to deposit Ce oxides/hydroxides on the entire surface.     

Study of the oxide scale integrity on Ni‐base alloy CMSX‐4 during isothermal and thermal‐cycling exposure

The excellent isothermal high‐temperature corrosion resistance of the single crystalline Ni‐base superalloy CMSX‐4, which is due to external alumina formation, is substantially reduced under thermal‐cycling conditions. Particularly, tapered edges exhibit a high cracking and spallation rate leading to premature onset of breakaway oxidation in combination with strong internal corrosion attack. To characterize these effects, specimens with different geometries were prepared, i.e., rectangular‐shaped and wedge‐shaped specimens. Measurements under isothermal and thermal‐cycling conditions at a temperature of 1100°C in air were carried out using a self‐designed thermobalance. By using wedge‐shaped specimens the conditions for spallation, Al depletion and eventually, breakaway oxidation in combination with internal corrosion (Al₂O₃, TiN and AlN) could be correlated with the change in thickness of the specimen and the duration of the high‐temperature dwell time. As compared to rectangular‐shaped specimens, wedge geometries revealed a much higher susceptibility to oxide spallation and consequently, a higher rate of aluminium depletion. An evaluation of the surface fraction affected by oxide spallation and an analysis of the spalled oxidation products were carried out in order to determine the role of dwell time duration as well as the effect of the number of thermal cycles.     

Composite cerium oxide/titanium oxide thin films for corrosion protection of AZ91D magnesium alloy via sol–gel process

Anti‐corrosive composite cerium oxide/titanium oxide (CeO₂/TiO₂) thin films were successfully prepared on an AZ91D magnesium alloy substrate by applying cerium oxide (CeO₂) thin films as the inner layer with a sol–gel process. Composition and surface morphology of the thin films were analyzed using X‐ray diffraction (XRD) and scanning electron microscope (SEM). XRD showed that the composite films consisted of cerianite and anatase phases. The wettability of the thin films was evaluated by water contact angles measurements. Potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) tests were used to evaluate the corrosion behavior of the bare substrate and coated samples in 3.5 wt% sodium chloride solution (3.5 wt% NaCl). The results demonstrated that titanium oxide (TiO₂) thin film mainly dominated the corrosion resistance of samples and the composite films with excellent hydrophilicity could significantly improve the corrosion resistance of AZ91D magnesium alloy.     

Electrochemical properties of iron covered by CVD‐silicon and silicon‐organic molecules

The adhesion of polymeric coatings on iron and steel is interesting especially for industrial applications. Lacquers are important because of their optical properties but also because they protect the metallic surface e.g. from corrosion. The new concept of using adhesion promoters between the polymeric coating and the metallic surface was introduced recently. But the adhesion of polymeric coatings is a difficult task on iron since the surface must be oxide free. This problem can be avoided by covering the iron surface with a thin film of silicon, silicides and SiO₂. Silane molecules chemisorb on top of it and act as adhesion promoters. The silicon reacts with the iron in a CVD process and the silane via a self‐assembly process. We describe the electrochemical properties and the stability of the iron covered by silicon and by silicon‐organic molecules. Impedance and current‐voltage measurements are used for this. The influence of oxygen and nitrogen solved in the electrolyte is described. Also the time dependence of the stability of the surface layer was measured.     

Synthesis and passivation behavior of electroless Ni–P‐CNT composite coating

Carbon nanotubes (CNTs) have high chemical stability, unique hollow nanotube structure, and are believed to be ideal materials for fabricating composites. In this study, Ni–P and Ni–P‐CNT composite coatings were fabricated by electroless plating. Scanning electron microscopy was used to characterize the coatings. The corrosion behavior of Ni–P and Ni–P‐CNT coated samples were evaluated by polarization curves and electrochemical impedance spectroscopy in 3.5 wt% NaCl and 0.1 M H₂SO₄ aqueous solutions at room temperature. The results indicated that incorporation of CNTs in the coating significantly increased corrosion resistance. The role of CNTs in improvement of corrosion resistance of the coating was also discussed.     

拉应力条件下微弧氧化膜对铝合金腐蚀及电化学行为的影响

为研究在拉应力条件下微弧氧化膜对铝合金腐蚀及电化学行为的影响,采用恒载荷应力环在3.5% NaCl溶液中研究了经微弧氧化（MAO）处理后的7050铝合金（AA7050）应力腐蚀行为。用原位电化学阻抗谱（EIS）的方法评价在拉应力条件下,膜层的腐蚀破坏随浸泡时间的变化,并建立了相应的等效电路模型。结果表明,在3.5% NaCl溶液中,微弧氧化膜在有无拉应力的条件下都可以提高AA7050的耐蚀性和减少AA7050的塑性损失。在400 MPa拉应力条件下,微弧氧化膜的阻抗在应力腐蚀的过程中呈现出先减小后增大,再减小最后趋于稳定的规律;另外,AA7050在有拉应力的条件下,拉应力会促进基体的点蚀形核,提高腐蚀速率,微弧氧化膜的疏松层在拉应力的作用下会失去对基体的保护作用。     

Evaluation of different sealing methods for anodized aluminum‐silicon carbide (Al/SiC) composites using EIS and SEM techniques

Electrochemical impedance spectroscopy (EIS) and the scanning electron microscope (SEM) have been used in an investigation of the effectiveness of various sealing methods that can be used to improve the corrosion resistance of an anodized aluminum‐silicon carbide (Al/SiC) composite. Anodic oxide films were grown on Al7075‐T6 and the Al/SiC composite by sulfuric acid anodizing and sealing in a cold saturated solution of nickel acetate. Other samples were sealed using the traditional method of boiling water or hot nickel acetate for comparison. The results revealed a uniform anodized layer on Al7075‐T6 that resisted pitting corrosion for more than 2 weeks exposure to NaCl, whereas a cracked oxide film with variations in thickness was observed on the composite material. Pit initiation occurred in less than 5 days on the anodized Al/SiC that was sealed in the hot solutions. This study suggests that the traditional hot sealing methods did not provide sufficient corrosion protection for aluminum metal–matrix composites (MMCs) because the reinforcing SiC particles deteriorated the surface film structure. However, this defective film can be repaired by nickel hydrate precipitation during cold sealing or by applying a thick polyurethane coating.     

Einfluß der β/α-Umwandlung und der ZrV2-Ausscheidung bei Zirkonium-Vanadium-Legierungen auf das Korrosionsverhalten im Sattdampf

Influence of the β-α-transformation and the ZrV₂-precipitation on the corrosion behaviour of ZrV alloys in saturated steam The corrosion behaviour of ZrV alloys with vanadium contents up to 3.5% exhibits a pronounced dependence an heat treatment. In saturated steam of 350° C the weight increase is highest for as-quenched samples. Oxidation follows a linear kinetic law, but already after 5 days bubbles and cracks appear in the oxide layer. Specimens annealed at 450° C exhibit a corrosion behaviour improving with increasing annealing duration. The same is true with annealing at 600° C. The specimens annealed at 750° C exhibited poorer corrosion resistance, but the influence of the duration of treatment was not very pronounced in this case. The Kinetic law is identical for worked and non-worked specimens of the same structure. Non-worked specimens, however, are characterized by increased corrosion susceptibility because of the precipitation of impurities at the primary grain boundaries in the as-cast condition; break-away sets in at an earlier stage in these cases. The external characteristic feature of increasing corrosion is an increasing yellowing of the oxide layer which points to increasing incorporation of variation ions in the oxide layer.     

Effect of the cerium (III) chloride heptahydrate on the corrosion inhibition of aluminum alloy

In the present work, the corrosion protection of aluminum alloy AA2024-T3 has been studied in NaCl solution, with and without the addition of cerium (III) chloride heptahydrate. The corrosion inhibitor efficiency after immersion into 10 mM NaCl, with or without 3 mM of CeCl₃·7H₂O at 20°C, 40°C, and 60°C was investigated. The performed quantitative tests include electrochemical techniques, such as the method of quasipotentiostatic polarization (Tafel extrapolation), cyclic polarization, and electrochemical impedance s pectroscopy to determine corrosion rate (v_corr), inhibition efficiency (η %), protective ability (γ), degree of coverage (ϑ), and pitting nucleation resistance. The samples were analyzed with scanning electron microscopy and energy dispersive X-ray analysis to evaluate and characterize the precipitates formed on the surface of aluminum samples and to determine dominant type of corrosion. The formation of Ce³⁺ precipitates occurred on cathodic intermetallic sites and the surface, in general, resulting in improved corrosion resistance. Tested cerium (III) chloride heptahydrate proved to be an effective inorganic corrosion inhibitor for AA2024-T3 in chloride solution, which, by the action of cerium ions, reduced corrosion on the surface of the studied aluminum alloy.     

The effect of surface treatment on the corrosion behavior of a pure Ta sheet in an equimolar NaCl–KCl melt at 850°C in air. Part 1: Laser surface melting

The corrosion behavior of pure Ta and laser surface melting–treated Ta sheets was investigated in an equimolar NaCl–KCl melt at 850°C in air. The results show that the predominant factors involved in the consumption of materials were reactions that form sodium‐containing oxides and Ta₂O₅. High corrosion stress led to slip of the untreated Ta surface grains, further resulting in preferential corrosion along slip bands. The slip of surface grains substantially deteriorated the bonding between the corrosion scale and matrix, thus leading to rapid degradation of the corrosion scale. The melt layer consisted of a thin Ta₂O₅ film, and the inner dendrite layer demonstrated self‐healing characteristics during corrosion and presented a better hot corrosion resistance than the untreated Ta owing to the excellent bonding at the scale and melt layer interface.     

Effect of thermomechanical treatment on the corrosion of AA5083

Corrosion resistance degradation induced by the presence of β-phase (Al₃Mg₂) limits the performance of aluminum alloy 5083 for some critical applications. The effect of thermomechanical treatment on the corrosion of AA5083 is presented in this paper. The samples subjected to the thermomechanical treatment showed superior corrosion resistance compared to the as-received samples. The effects of grain boundary character distribution, grain shape, texture, and precipitates on corrosion are discussed based on experimental observations. Special boundaries are expected to have a weak effect on the overall corrosion resistance of the material compared to the other factors.     

Der Einfluß der intermetallischen Phase ZrV2 auf die Oxidations- und Korrosionseigenschaften von Zirkonium-Vanadium- Legierungen

The influence of the intermetallic ZrV₂ phase on the oxidation and corrosion behaviour of zirkonium vanadium alloys The good oxidation resistance of zirkonium and some of its alloys is due to the formation of an oxide layer characterized by good adhesion. Since, however, cracks are formed in such a layer upon prolonged oxidation the corrosion rate can be rather high in the later stages. An improvement of the properties of the oxide layer may be obtained by providing for the incorporation into the scale of atoms able to prevent oxygen diffusion. In the case of vanadium, however, no protective effect is obtained and the corrosion rate even increases as the vanadium content is increased. This phenomenon is due to the poor solubility of vanadium in the zirconium so that the incorporation of vanadium in the scale is inhibited during the initial stages of corrosion. The corrosion mechanism is the same in saturated vapour and NaCl-solution: there are ZrV₂-Particles which are not attacked and are embedded in a largely destroyed zirkonium matrix.     

Electrochemical evaluation of Ti‐13Nb‐13Zr,Ti‐6Al‐4V and Ti‐6Al‐7Nb alloys for biomedical application by long‐term immersion tests

In this investigation the electrochemical behaviour of the Ti‐13Nb‐13Zr, Ti‐6Al‐4V and Ti‐6Al‐7Nb alloys, for application as implant materials was evaluated in Hanks' solution by electrochemical techniques. The alloys were immersed in this solution for 410 days and periodically they were tested by electrochemical impedance spectroscopy. At the end of this period, polarization curves of the three titanium alloys were obtained. The electrochemical impedance experimental results were interpreted using an equivalent electrical circuit that simulates a duplex structure oxide composed of an inner compact layer, here called barrier layer, and an outer and porous layer. The results indicated that all the alloys present a very high corrosion resistance in the electrolyte used, typical of passive alloys, and that the corrosion resistance is mainly due to the barrier layer. The passive like behaviour was maintained during the whole period of test.     

Novel anti-corrosion nano-sized vanadia-based thin films prepared by sol–gel method for aluminum alloys

Chromate conversion coatings have been widely applied for the corrosion protection of aluminum alloys. However, the waste containing Cr6+ has many limitations due to the environmental consideration and health hazards. Vanadates are among the proposed alternatives to chromating. Series of specimens were prepared under the following conditions: (a) as polished, (b) directly treated with vanadia, (c) etched, (d) oxide thickened, and (e) etching followed by oxide thickening. After surface preparation, the specimens were dipped in vanadia solution prepared via sol gel method. Electrochemical impedance spectroscopy (EIS) and polarization measurements have been used to evaluate the coating performance in 3.5% NaCl. The optimum conditions under which vanadia treatments can provide good corrosion protection to the aluminum substrate were determined. The surface morphologies of the treated samples were investigated using SEM and EDS. Optical microscope was also used to investigate the occurrence of pitting corrosion. The surface preparation prior to vanadia treatment was found to have a marked effect on the corrosion protection of AA6061 T6. Generally, vanadia treatments improve the corrosion resistance due to formation of highly protective vanadium oxides. According to the EIS and polarization measurements, a combination between etching and oxide thickening prior to vanadia treatment plays an important role on the corrosion protection mechanism.     

Interaction of 12-aminododecanoic acid with a carbon steel surface: Towards the development of ‘green’ corrosion inhibitors

The inhibiting effect of 12-aminododecanoic acid (AA) on corrosion of carbon steel (CS) in CO₂-saturated hydrochloric acid was investigated. It was found that AA acts as a mixed-type inhibitor, yielding a maximum inhibition efficiency of 98.1 ± 0.1%. The mechanism of its corrosion inhibition is by formation of a self-assembled monolayer (SAM), which presents a tight hydrophobic barrier imposed by the (-CH₂)₁₁ chain. In-situ PM-IRRAS measurements revealed that the SAM is amorphous. The SAM formation process was found to be spontaneous and reversible. The corresponding standard Gibbs energy of AA adsorption on CS was calculated to be −28 kJ mol⁻¹.