The hot corrosion of directionally solidified Ni-Cr-Nb-Al (γ/γ′-δ) eutectic alloys

The hot-corrosion resistance of two /- eutectic alloys, Ni-23.1 Nb-4.4 Al and Ni-19.7Nb-6Cr-2.5 Al, has been studied using three techniques: (1) a salt-coated test, in which the specimens are coated with a thin layer of Na₂SO₄ and oxidized; (2) the Dean rig test, in which salt is evaporated into the gas stream and allowed to condense continuously on the specimen; and (3) a sulfidation/oxidation test, in which the specimens were presulfidized for a short time in a H₂/H₂S mixture and then oxidized. The salt-coated test suggested that these alloys are generally quite resistant to sodium-sulfateinduced corrosion, although localized severe attack was sometimes encountered. The morphology of the corrosion products suggested that the mechanism of the attack was sulfidation/oxidation, and that salt fluxing mechanisms were relatively unimportant. In the Dean rig test it was shown that sodium chloride greatly increased the severity of attack, as has been shown earlier for alumina-forming superalloys.     

Reinterpretation of Type II Hot Corrosion of Co-Base Alloys Incorporating Synergistic Fluxing

The components of gas-turbine engines operating in marine environments are highly susceptible to hot corrosion, which is typically classified as Type II (650–750 °C) and Type I (900–950 °C) hot-corrosion attack. Even though hot-corrosion has been widely investigated in the last 50 years, several critical questions remain unanswered and new ones have emerged based on recent observations that, in part, are associated with the increasing complexity of the alloy systems and the sulfate-deposit chemistries. The present work is focused on the Type II hot-corrosion mechanism for Co-base alloys. Observations for a CoCrAlY model alloy (isothermally exposed at 700 and 800 °C under different atmospheres, including: air and O₂ with 100 and 1000 ppm SO₂) suggest the rapid dissolution of Co (as Co-oxide) is not the controlling factor in the degradation mechanism, as was proposed by Luthra, since the γ-phase which is richer in Co, is not attacked as significantly as the Al-rich β-phase. To the contrary, it is suggested that Al (and Cr) is (are) the element(s) which is (are) removed first. A modified interpretation of the Type II hot-corrosion mechanism is proposed, which is based on the synergistic fluxing model developed by Hwang and Rapp.     

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.     

A study on the localized corrosion of cobalt in bicarbonate solutions containing halide ions

The localized attack of cobalt in bicarbonate aqueous solutions containing halide ions was investigated using electrochemical techniques, scanning electron microscopy, UV-visible and Raman spectroscopies. Rotating disc and rotating ring-disc electrodes were used to determine the effect of bicarbonate concentration, solution pH, nature and concentration of the halide ions, convection and potential sweep rate on the corrosion processes. These parameters were found to play a key role on the localized attack induced by halide ions by influencing the production of a Co(HCO₃)₂ precipitate on the pit surface. Potentiostatically generated cobalt oxide films (CoO and Co₃O₄) were found to be efficient to reduce pitting corrosion of cobalt.     

An electrochemical study of the effect of Li on the corrosion behavior of Ni3Al intermetallic alloy in molten (Li + K) carbonate

A study of the effect of lithium content (1, 3 and 5 wt.%) and heat treatment (400 °C during 144 h) on the corrosion behavior of Ni₃Al alloy has been carried out in a 62 mol.%Li₂CO₃-38 mol.%K₂CO₃ mixture at 650 °C using electrochemical techniques. Employed electrochemical techniques included potentiodynamic polarization curves, linear polarization resistance, LPR, electrochemical impedance spectroscopy, EIS, and electrochemical noise, measurements EN. Results have shown that the alloys exhibited an active-passive behavior regardless of the heat treatment. For alloys without heat treatment, the most corrosion resistant was the Ni₃Al base alloy, but when they were heat treated, the most corrosion resistant was the alloy containing 3%Li. EIS results showed that for short immersion tests, the corrosion process was under diffusion control, but for longer exposure times, the presence of a protective scale was evident. All the alloys were highly susceptible to a localized type of corrosion according to EN measurements and supported by SEM micrographs.     

Effect of Al2O3 reinforcement and precipitates on corrosion behaviour of 2618 and 6061 aluminium MMCs

ABSTRACT

The corrosion resistance features of two different Metal Matrix Composites based on 6061 and 2618 aluminium alloys reinforced by 20% Al₂O₃ particles by stir casting process, were studied in 3.5% NaCl and compared. The composites and their respective base alloys were characterised in terms of microstructure by optical and scanning electron microscopy and in terms of corrosion resistance by polarisation curves and electrochemical impedance. Results show how the presence of Al₂O₃ has different effects depending on matrix composition. In 6061 MMC, Al₂O₃ promotes the formation of Mg₂Si which has a cathodic behaviour altering the pitting susceptibility of the composite. In 2618 MMC, the high content of Cu induces a strong uniform attack. The subsequent increase in the Open Circuit Potential caused a pitting attack.     

Electrochemical stability of TiO2 nanotubes with different diameters in artificial saliva

Recently, titanium and titanium alloys with nanotube layers by anodizing process have gained great interests as surgical implant materials. In this present paper, their electrochemical stability of self-organized TiO₂ nanotubue layers prepared by anodization of pure Ti in 0.5 wt.% hydrofluoric acid has been investigated in simulated biological environment by use of open-circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests. The electrochemical testing results indicate that the nanotubular Ti with the diameter of TiO₂ nanotube lower than 86 nm shows a better corrosion resistance in artificial saliva than that of the mechanically polished Ti. Moreover, the electrochemical stability of Ti nanotubes 22 to 59 nm in diameter is improved but that of Ti nanotubes larger than 86 nm decreases. Besides, the corrosion attack of the nanotubular Ti is shown by the collapse of TiO₂ nanotubue layer. The results suggest that the electrochemical corrosion behavior of nanotubular Ti in artificial saliva is related to the diameter of the nanotubes and thickness of the barrier layer.     

Evolution of the impedance diagrams of 8090 Alloy in saline solution with low aggressiveness

Aircraft industry has focused in the search for new materials with high performance properties such as composite materials and aluminium alloys. During the last decade a new generation of aluminium-lithium alloys have been developed due to their low density and high elastic modulus. From corrosion viewpoint most of the work published on these alloys are based on atmospheric exposure tests and accelerated laboratory tests. Recently, an increasing interest has been paid in the use of electrochemical techniques on aluminium alloys due to their main advantages: determining of corrosion kinetics in real time and knowledge of the corrosion mechanism. This paper attempts to establish a correlation between the evolution of the impedance diagrams and the process of the attack undergone by a commercial 8090 T8171 alloy in sodium chloride solutions. During the first 100 h of immersion the electrochemical impedance spectroscopy reveals that the corrosion process is under charge transfer resistance control. After this time, the technique showed clearly the role of the adsorbed ions from the electrolyte, which promotes an intergranular attack on the alloy.     

The sensitization of ferritic steels containing less than 12% Cr

The susceptibility of a range of ferritic Cr Mo steels to sensitization during welding has been investigated. No localized attack occurred in any of the standard test solutions used to assess susceptibility to intergranular corrosion. However potentiostatic etching in 16 wt%H₂SO₄ at 18°C produced narrow trenching close to the heat affected zone-parent metal interface in the 12%Cr 1%Mo and 9%Cr 1%Mo steels. The thermal history and the metallurgical structure of the attacked region have been examined. The effect of post weld heat treatment on the occurrence of localized attack has been studied and correlated with hardness changes. Analytical and electrochemical measurements have been used to test the various theories proposed to explain intergranular corrosion in ferritic materials. Finally the implications of these findings are considered.     

Intergranular corrosion of 8090 Al–Li: interpretation by electrochemical impedance spectroscopy

Abstract

Aluminium–lithium alloys are widely used in the aircraft industry because of their low density and high elastic modulus. The Al–Li alloy 8090 has been well characterised from a mechanical viewpoint, but its corrosion mechanism has not been studied so completely. Recent work has focused on the use of electrochemical techniques to detect the early stages of localised corrosion and to assess the rate of corrosion taking place at specific locations in aircraft. Electrochemical impedance spectroscopy (EIS) has been used to measure and monitor the localised corrosion susceptibility of this Al–Li alloy, which is currently being used in aerospace applications, in solutions of 1M NaCl ++ 10 mL H₂O₂ (30 wt-%) at neutral pH. During the corrosion process, there is an increase in surface inhomogeneity which causes deviations from the ideal behaviour proposed by Randles. A tendency to behave as a porous electrode is evidenced in the Nyquist plot by the presence of a straight segment in the high frequency range. This response is the result of preferential dissolution at grain boundaries, which leads to the development of deep intergranular attack and promotes a non-homogenous signal distribution. At low frequencies a capacitive arc appears that may be related to the chloride adsorption involved in alloy dissolution.     

Corrosion behaviour of Nimonic–75 and Ni–19.4 Cr–1.25 Al in NaCl and Na2SO4

The corrosion behaviour of Nimonic-75 and Ni-Cr-Al alloys has been investigated in NaCl and Na₂SO₄ by employing the half-immersion crucible test. Weight loss results obtained as a function of temperature (700–800°C) and concentration of NaCl in the mixture (1–30 wt%) show that the highest attack occurs in Na₂SO₄-3% NaCl mixture around 750°C, for Nimonic-75 and 800°C for Ni-Cr-Al, resulting in blister formation for both the alloys. The results also show that the addition of Al to Ni-Cr alloy drastically enhances corrosion. Pre-heating the Ni-Cr-Al alloy to recrystallization temperature for 2 to 4 h fails to indicate any inhibiting effect on catastrophic corrosion. The results strongly suggest that it is not only the concentration of NaCl in the mixture and the temperature of the salt mixture but also the volume fraction of liquid which plays a predominant role in the enhancement of corrosion attack.     

Localized Corrosion Behavior of Al-Si-Mg Alloys Used for Fabrication of Aluminum Matrix Composites

The relationship between microstructure and localized corrosion behavior in neutral aerated chloride solutions was investigated with SEM/EDAX, conventional electrochemical techniques, and with scanning Kelvin probe force microscopy (SKPFM) for two custom-made alloys with Si/Mg molar ratios of 0.12 and 0.49. In this order, Al₃Fe, Al₃Mg₂, and Mg₂Si intermetallics were identified in the first alloy and Al(FeMn)Si and Mg₂Si particles in the second one. Anodic polarization curves and corrosion morphology showed that the alloy with higher Si/Mg molar ratio exhibited a better corrosion performance and evidence was shown that it had a more corrosion-resistant passive film. The corrosion process for both alloys in aerated 0.1 M NaCl solutions was localized around the Fe-rich intermetallics. They acted as local cathodes and produced dissolution of the aluminum matrix surrounding such particles. Mg₂Si and Al₃Mg₂ exhibited anodic behavior. SKPFM was successfully used to map the Volta potential distribution of main intermetallics. The localized corrosion behavior was correlated with a large Volta potential difference between the Fe-rich intermetallics and the matrix. After immersion in the chloride solution, such Volta potential difference decreased.     

Relation between corrosion behavior and microstructure of Mg-Zn-Y alloys prepared by rapid solidification at various cooling rates

The relation between corrosion resistance and microstructure of Mg-Zn-Y alloys with a long period stacking ordered (LPSO) phase has been investigated. In order to clarify the influence of microstructure evolution by rapid solidification on the occurrence of localized corrosion such as filiform corrosion, several Mg_97.25Zn_0.75Y₂ (at. %) alloys with different cooling rates were fabricated by the gravity casting, copper mould injection casting and melt-spinning techniques and their corrosion behavior and microstructures were examined by the salt immersion tests, electrochemical measurements, XRD and TEM. When the cooling rate was less than 3 × 10⁴ K s⁻¹, filiform corrosion propagated in the early stage of salt immersion test, due to formation of a massive block-shaped LPSO phase during casting. On the other hand, when the cooling rate was increased up to 3 × 10⁴ K s⁻¹, rapidly solidified (RS) alloys exhibited excellent corrosion resistance because of grain refinement and formation of a supersaturated single-phase solid solution. Large-sized Mg_97.25Zn_0.75Y₂ alloys fabricated by consolidation of the RS ribbons also exhibited excellent corrosion resistance with passivity. Enhancement of microstructural and electrochemical homogeneities in the Mg-Zn-Y alloys by rapid solidification techniques results in the passivity of substrate materials.     

Oxidation and hot corrosion of nickel-based alloys containing molybdenum

The oxidation of Ni-15% CrMo alloys has been studied at 900°C in flowing and static oxygen atmospheres. In flowing atmospheres, molybdenum has no effect: all the alloys oxidize in a protective manner. However, in static atmospheres the oxidation rate of alloys with > 3% Mo eventually accelerates, and catastrophic destruction of the alloy takes place. Under these circumstances a molybdenum-rich oxide layer is detected adjacent to the alloy.When specimens are coated with Na₂SO₄ prior to oxidation, alloys containing > 3% Mo again suffer catastrophic degradation, in either flowing or static atmospheres, and again a molybdenum-rich oxide layer is observed. This suggests that the principal role of the salt coating is to prevent the escape of MoO₃ to the atmosphere.The morphology of the attack in the rapid propagation region is very similar to that obtained in pre-sulphidation/oxidation experiments in the absence of salt and that particular aspect of the reaction is not greatly affected by molybdenum; the aluminium content is more important in determining the nature of the propagation.Attack similar to that exhibited by molybdenum-containing alloys can be obtained with Ni-15%Cr binary alloys in the presence of MoO₃ vapour in the atmosphere, and this might suggest that the MoO₃ reacted with the Na₂SO₄ to produce an acid (SO₃-rich) salt, leading to acidic fluxing. However, very similar types of attack were obtained when Na₂MoO₄ was added to the Na₂SO₄, and this should not have affected the acidity of the salt at all.These experiments suggest that acidic fluxing may not be important in the hot corrosion of alloys of this type (molybdenum-containing) and that when catastrophic corrosion is observed, its initiation is probably due to the formation of a molybdenum-rich oxide layer, molten during the reaction. There appears to be a threshold molybdenum content below which attack does not occur, and this seems insensitive to an increase in the chromium content from 15 to 25%.     

Electrochemical corrosion behavior of Mg-5Al-0.4Mn-xNd in NaCl solution

The electrochemical corrosion behavior of Mg-5Al-0.4Mn-xNd (x = 0, 1, 2 and 4 wt.%) alloys in 3.5% NaCl solution was investigated. The corrosion behavior of the alloys was assessed by open circuit potential measure, potentiodynamic polarization, and electrochemical impedance spectroscopy. The electrochemical results show the intermetallic precipitates with Nd behave as less noble cathodes in micro-galvanic corrosion and suppress the cathodic process. During corrosion, Al₂O₃ and Nd₂O₃, in proper ratio, is incorporated into the corrosion film, and enhances the corrosion resistance.     

An evaluation study of aluminide and chromoaluminide coatings on IN-100

Aluminide diffusion coatings are commonly used to protect aircraft gas turbine blades and vanes from oxidation and hot-corrosion attack. These coatings are based on NiAl intermetallic compound with other alloying elements like Cr and Ti either diffused from the superalloy substrate or incorporated in a separate coating step. The present investigation is mainly concerned with the development of both aluminide and chromoaluminide coatings on IN-100, a cast Ni-base superalloy. The coating structure and composition have been characterized and the cyclic oxidation and hot corrosion properties have been evaluated for the different types of coatings. The difference in the hot-corrosion properties between the aluminide and the chromoaluminide coatings has been rationalized in terms of the coating chemistry. The mode of coating degradation under hot-corrosion conditions has also been analyzed.     

Ni对Zr-Cu-Al系非晶合金在NaCl溶液中腐蚀性能的影响

利用动电位极化和电化学阻抗谱研究了Ni元素对Zr-Cu-Al系非晶合金在3.5wt.%NaCl中性溶液中的电化学腐蚀行为的影响规律。结果表明：含Ni元素的Zr₅₅Cu₃₀Ni₅Al₁₀比Zr₅₅Cu₃₅Al₁₀非晶合金在NaCl溶液中具有更优异的耐腐蚀性能。Zr-Cu-Al系非晶合金在NaCl溶液中发生点腐蚀,圆形腐蚀坑内布满泡沫状孔洞。通过腐蚀前后的元素分布对比,发现Zr-Cu-Al非晶合金在Cl^_-作用下合金元素选择性溶解。含Ni元素的非晶合金形成致密的钝化膜,抑制了金属元素的选择性溶解,从而提高了耐腐蚀性能。研究结果可为耐Cl^_-环境腐蚀非晶合金成分设计及其应用提供参考。     

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.     

Alloying effect of copper concentration on the localized corrosion of aluminum alloy for heat exchanger tube

This study examined the alloying effect of Cu content on the localized corrosion properties of Al alloy in synthetic acid rain containing 200 ppm of Cl- ion. In aluminum alloy tubes, a small amount of Cu is contained as the additive to improve the mechanical strength or as the impurity. The Cu-containing intermetallic compound, Al₂Cu can cause galvanic corrosion because it has more noble potential than Al matrix. Therefore aluminum tube could be penetrated by localized corrosion attack. The results were obtained from electrochemical test, scanning electron microscopy, and time of flight secondary ion mass spectrometry (ToF-SIMS) mapping. Severe localized corrosion was occurred on the Al-0.03 wt% Cu alloy. The negative effect of Cu on the pitting corrosion was attributed to the presence of the Al₂Cu precipitates.     

Effect of heat treatment on H2S corrosion of a micro-alloyed C–Mn steel

The H₂S corrosion resistance of a C–Mn pipeline steel with three different microstructures has been evaluated using electrochemical techniques with a 3% wt. NaCl solution at 50 °C. Microstructures included martensite, ferrite, and ferrite + bainite. Electrochemical techniques included potenthiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), linear polarization resistance (LPR), and electrochemical noise (EN) measurements. Most of the tests lasted 24 h. All techniques showed that the highest corrosion rate corresponded to the steel with a martensitic microstructure; up to one order of magnitude higher than the corrosion rate for steels with a ferritic + bainitiic microstructure, whereas the steel with the ferritic microstructure showed the lowest corrosion rate. EIS tests showed that the corrosion process was under charge transfer control, whereas EN results indicated that the three steels exhibited a clear tendency towards a localized type of corrosion. However, for longer immersion times, the steel with a martensitic microstructure tended to exhibit a mixture of uniform and localized attack. Results were discussed in terms of grain size, grain boundary energy, amount and distribution of particles found in each steel.