Investigation into the corrosion rate and features of the samples made of nanostructured aluminum alloy in the H<Subscript>2</Subscript>S-containing medium

The corrosion rate was determined and corrosion damage features of the samples made of AK4-1 aluminum alloy were investigated in the NACE solution containing hydrogen sulfide (H₂S). The alloy was investigated in the ultra-fine-grain state compared with the coarse-grain state formed after standard treatment T6 (quenching + ageing). The alloy was nanostructured by equichannel angular pressing (ECAP). It is shown that the alloy corrosion rate after ECAP is higher than after T6 treatment by a factor of 1.9. Herewith, the total corrosion takes place in alloy after ECAP, while pitting corrosion is also observed after T6 treatment in addition to the total corrosion. The corrosion effect strongly affects surface-roughness parameters of the samples made of AK4-1 alloy after ECAP compared with the samples after T6 treatment.     

Combined effect of chloride and pH on corrosion resistance of Cu-10Ni alloy

Cu-10Ni alloy is a standard heat exchanger material for saltwater (including seawater) applications owing to its excellent thermal conductivity and corrosion resistance. The excellent corrosion resistance is due to formation of Cu₂O film, which accords protection. However, when varying amount of H⁺, OH^? and Cl^? ions are present, CuCl₂ ^? may also form. The CuCl₂ ^? is not protective as Cu₂O and hence corrosion resistance may be affected. Present paper investigates combined effect of chloride ion and pH on corrosion resistance of Cu-10Ni alloy. Cathodic and anodic polarization test results are presented for saltwater containing various amounts of NaCl at pH 6 and 8. It is found that, i_corr increases with increasing chloride concentration. The results are discussed using dissolution mechanisms, semi-conducting behaviour of Cu₂O film and deterioration of the film in presence of chloride ions.     

Effect of Superficially Applied Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Coating on High-Temperature Corrosion Behavior of Ni-Base Superalloys

Inhibitors and oxide additives have been investigated with varying success to control high-temperature corrosion. Effect of Y₂O₃ on high-temperature corrosion of Superni 718 and Superni 601 superalloys was investigated in the Na₂SO₄-60 pct V₂O₅ environment at 1173 K (900 °C) for 50 cycles. Y₂O₃ was applied as a coating on the surfaces of the specimens. Superni 601 was found to have better corrosion resistance in comparison with Superni 718 in the Na₂SO₄-60 pct V₂O₅ environment. The Y₂O₃ superficial coating was successful in decreasing the reaction rate for both the superalloys. In the oxide scale of the alloy Superni 601, Y and V were observed to coexist, thereby indicating the formation of a protective YVO₄ phase. There was a distinct presence of a protective Cr₂O₃-rich layer just above the substrate/scale interface in the alloy. Whereas Cr₂O₃ was present with Fe and Ni in the scale of Superni 718. Y₂O₃ seemed to be contributing to better adhesion of the scale, as comparatively lesser spalling was noticed in the presence of Y₂O₃.     

Stress corrosion cracking of pressure vessel steels in high-temperature caustic aluminate solutions

Stress corrosion cracking (SCC) behavior of three kinds of low alloy pressure vessel steels in high-temperature (200 °C to 300 °C) caustic aluminate (AIO_-2) solutions has been studied by slow strain rate tests (SSRT). The results indicate that these pressure vessel steels are susceptible to SCC in caustic aluminate solution and that the SCC susceptibility increases with increasing temperature between 200 °C to 300 °C. Sulfide content and stringered sulfide inclusions severely and anisotrop-ically affect the caustic SCC of these low alloy steels. The inclusions in the rare-earth-treated steel are predominantly globular rare-earth sulfides or oxysulfides, resulting in improved transverse prop-erties. The effect of inclusions on SCC behavior correlates with the projected area of inclusions perunit volume at the crack tip,A _v , on the plane perpendicular to the tensile direction. The susceptibility to SCC increases with increasingA _v .     

Corrosion behaviour of Al1.3CrFeNi chemically complex alloy

In this study, corrosion behaviour of double-phase Al_1.3CrFeNi chemically complex alloy was investigated, including hot corrosion and electrochemical corrosion. Hot corrosion behaviour of Al_1.3CrFeNi alloy was explored in molten 75 wt-% Na₂SO₄?+?25 wt-% NaCl salt. The result revealed that the corrosion kinetic curve of Al_1.3CrFeNi alloy followed the exponential rate rule through mass loss measurement. In addition, it prevented that the formed corrosion layer had obvious stratification including external granular Al₂O₃ and inner porous Cr₂O₃ when corrosion time was up to 100?h. Besides, it should be noted that the Al_1.3CrFeNi alloy was sensitive to the molten salt containing chlorine, which makes the alloy surface leave voids and bring about acceleration of corrosion. Meanwhile, electrochemical corrosion resistance of Al_1.3CrFeNi alloy in NaCl solution with different concentrations (0.6, 1.0 and 2.0?mol?L^?1) was investigated at room temperature. The results revealed that Al_1.3CrFeNi alloy showed superior corrosion resistance in NaCl solution due to the existence of Al and Cr which aid the formation of protective oxide layer.     

The effect of aluminum content on the corrosion behavior of Fe-Al alloys in reducing environments at 700 °C

The high-temperature corrosion behavior of monolithic Fe-Al alloys, with 0 to 20 wt pct Al, was investigated at 700 °C in a reducing atmosphere (p(S₂) = 10⁻⁴ atm, p(O₂) = 10⁻²⁵ atm) for up to 100 hours. Postexposure characterization of the corrosion reaction products consisted of surface and cross-sectional microscopy, in combination with energy dispersive spectroscopy, electron probe microanalysis, and quantitative image analysis. From the kinetic data, three stages of corrosion behavior (i.e., inhibition, breakdown, and steady state) were found with the observance and/or duration of each stage directly related to the aluminum content of the alloy. The first stage, labeled the inhibition stage, was characterized by low weight gains and the absence of rapid degradation of the alloy. Typically observed for compositions with 10 to 20 wt pct Al, protection was afforded due to the development of a thin, continuous alumina scale. For alloys with 7.5 wt pct A1, the ability to maintain the initially formed alumina scale was not observed, resulting in the breakdown stage. Localized corrosion product nodules, containing iron sulfide (Fe_1-x S) and the spinel-type tau phase (FeAl₂S₄), developed through the alumina scale due to sulfur short-circuit diffusion. These growths were accompanied by relatively high corrosion rates. Further decreasing the aluminum content to 5 wt pct and below lead to the formation of a continuous sulfide scale whose growth was controlled by iron and sulfur diffusion through the previously formed product. The alloy wastage rates in the steady-state stage were relatively high when compared to the previous two regions.     

Corrosion Behavior of Alloy 625 in PbSO4-Pb3O4-PbCl2-ZnO-10?Wt?Pct CdO Molten Salt Medium

Corrosion behavior and degradation mechanisms of alloy 625 under a 47.288 PbSO₄-12.776 Pb₃O₄-6.844PbCl₂-23.108ZnO-10CdO (wt pct) molten salt mixture under air atmosphere were studied at 873?K, 973?K, and 1073?K (600?°C, 700?°C, and 800?°C). Electrochemical impedance spectroscopy (EIS), open circuit potential (OCP) measurements, and potentiodynamic polarization techniques were used to evaluate the degradation mechanisms and characterize the corrosion behavior of the alloy. Morphology, chemical composition, and phase structure of the corrosion products and surface layers of the corroded specimens were studied by scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) and X-ray map analyses. Results confirmed that during the exposure of alloy 625 to the molten salt, chromium was mainly dissolved through an active oxidation process as CrO₃, Cr₂O₃, and CrNbO₄, while nickel dissolved only as NiO in the system. Formation of a porous and nonprotective oxide layer with low resistance is responsible for the weak protective properties of the barrier layer at high temperatures of 973?K and 1073?K (700?°C and 800?°C). There were two kinds of attack for INCONEL 625, including general surface corrosion and pitting. Pitting corrosion occurred due to the breakdown of the initial oxide layer by molten salt dissolution of the oxide or oxide cracking.     

Effects of transient carbon deposition on the sodium sulfate-induced corrosion of nickel-base alloys

The role of solid carbon in accelerating the Na₂SO₄-induced corrosion of nickel-base alloys at high temperature has been investigated. Corrosion rates of salt-coated IN 738 alloy in an oxidizing atmosphere containing 0.1 pct SO₂ were much increased when the alloy was initially exposed to a low concentration of methane for a short time at 1273 K (1000°C). Crucible tests carried out with Ni-5 Al, Ni-30 Al, Ni-20Cr-15 Al and IN 738 specimens immersed in Na₂SO₄-carbon mixtures showed that corrosion of these alloys was dramatically enhanced if the carbon persisted long enough to form a Na₂S phase by reaction with the Na₂SO₄. In an oxidizing environment a graphitic carbon had a more pernicious effect on alloy corrosion resistance than a finely dispersed carbon black which burnt away more readily. Corrosion rates in air were also accelerated in the presence of carbonized crude oil residue such as might be formed as a result of incomplete fuel combustion in a gas turbine.     

Thermodynamics of sulfur in the BaO-MnO-SiO2 flux system

The sulfide capacity of the molten BaO-MnO-SiO₂ system was determined by measuring the sulfur partition ratio between the oxide and an Ag-S alloy in a controlled CO-CO₂-Ar gas mixture at 1573 K for two purposes. One is to develop highly basic fluxes which can extensively desulfurize manganese-based alloys, and the other is to examine the effect of BaO addition on thermodynamic properties of sulfur in the MnO-SiO₂ melts from which MnS precipitates functioning as the nucleus for subsequent γ -α transformation of steel. The sulfide capacity of the BaO-MnO-SiO₂ system strongly depends on the composition, and MnO has been found to enhance sulfide capacity to a greater extent than BaO at less basic compositions.     

Intermetallic diffusion coatings for enhanced hot-salt oxidation resistance of nitrogen-containing austenitic stainless steels

This article presents the preparation, characterization, and hot-salt oxidation behavior of nitrogen-containing type 316L stainless steel (SS), surface modified with intermetallic coatings. Three different types of intermetallic coating systems, containing aluminum, titanium, and titanium/aluminum multilayers, were formed by diffusion annealing of type 316L austenitic SS containing 0.015, 0.1, 0.2, and 0.56 pct nitrogen. Analysis by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and secondary ion mass spectroscopy (SIMS) confirmed the formation of various intermetallic phases such as AIN, Al₁₃Fe₄, FeAl₂, FeTi, Ti₂N, and Ti₃Al in the coatings. Hot salt oxidation behavior of the uncoated and surface-modified stainless steels was assessed by periodic monitoring of the weight changes of NaCl salt-applied alloys kept in an air furnace at 1023 K up to 250 hours. The oxide scales formed were examined by XRD and stereomicroscopy. Among the various surface modifications investigated in the present study, the results indicate that the titanium-modified alloys show the best hot-salt oxidation resistance with the formation of an adherent, protective, thin, and continuous oxide layer. Among the four N-containing alloys investigated, the titanium and Ti/Al multilayer modified 0.56 pct N alloy showed the best hot-salt oxidation resistance as compared to uncoated alloys. The slower corrosion kinetics and adherent scale morphology indicate that the surface-modified titanium intermetallic coatings could provide high-temperature service applications up to 1073 K, particularly in chloride containing atmospheres, for austenitic stainless steels.     

Corrosion protection of magnesium alloys anode by cerium-based anodization coating in magnesium-ait battery

CeN₃O₉·6H₂O(0.5,1.0,1.5,and 2.0 g/L) was added into an 8.0% NaCl electrolyte solution to investigate this electrolyte for use in a Mg-air battery.The effects of the amount of CeN₃O₉-6H₂O on the corrosion resistance of an AZ31 Mg alloy anode and battery performance were investigated using microstructure,electrochemical(dynamic potential polarization method and electrochemical impedance spectroscopy),and battery measurements.The re ...     

Effects of Different Boron Compounds on the Corrosion Resistance of Andalusite-Based Low-Cement Castables in Contact with Molten Al Alloy

Interfacial reactions between Al alloy and andalusite low-cement castables (LCCs) containing 5 wt pct B₂O₃, B₄C, and BN were analyzed at 1123 K and 1433 K (850 °C and 1160 °C) using the Alcoa cup test. The results showed that the addition of boron-containing materials led to the formation of aluminoborate (9Al₂O₃.2B₂O₃) and glassy phase containing boron in the prefiring temperature (1373 K [1100 °C]), which consequently improved the corrosion resistance of the refractories. The high heat of formation of the aluminoborate phase (which increased its stability to reactions with molten Al alloy) and the low solubility of boron in molten Al were the major factors that contributed to the improvement in the corrosion resistance of B-doped samples.     

Corrosion of 310 stainless steel in H2- H2O- H2S gas mixtures: Studies at constant temperature and fixed oxygen potential

Corrosion of SAE 310 stainless steel in H₂-H₂O-H₂S gas mixtures was studied at a constant temperature of 1150 K. Reactive gas mixtures were chosen to yield a constant oxygen potential of approximately 6 × 10^-13 Nm^-2 and sulfur potentials ranging from 0.19 × 10^-2 Nm^-2 to 33 × 10^-2 Nm^-2. The kinetics of corrosion were determined using a thermobalance, and the scales were analyzed using metallography, scanning electron microscopy, and energy dispersive X-ray analysis. Two corrosion regimes, which were dependent on sulfur potential, were identified. At high sulfur potentials (P _{S 2} ± 2.7 × 10^-2 Nm^-2) the corrosion rates were high, the kinetics obeyed a linear rate equation, and the scales consisted mainly of sulfide phases similar to those observed from pure sulfidation. At low sulfur potentials (P _{S 2} ± 0.19 × 10^-2 Nm^-2) the corrosion rates were low, the kinetics obeyed a parabolic rate equation, and scales consisted mainly of oxide phases. Thermochemical diagrams for the Fe-Cr-S-O, Fe-Ni-S-O, Cr-Ni-S-O, and Si-Cr-S-O systems were constructed, and the experimental results are discussed in relation to these diagrams. Based on this comparison, reasonable corrosion mechanisms were developed. At high sulfur potentials, oxide and sulfide phases initially nucleate as separate islands. Overgrowth of the oxide by the sulfide occurs and an exchange reaction governs the corrosion process. Preoxidation at low oxygen potentials and 1150 K is beneficial in suppressing sulfidation at high sulfur potentials. Formerly a Senior Scientist with Materials and Molecular Research Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720.     

Precipitate Rafting in a Polycrystalline Superalloy During Compression Creep

Rafting is an industrially and scientifically important phenomenon for precipitate-strengthened alloys utilized at high temperatures. Although this phenomenon is observed in polycrystalline alloys as well, the literature lacks scientific work on rafting in polycrystals. Scientific work is usually conducted on single-crystal superalloys. Being one of the many polycrystalline nickel-base superalloys, IN738LC has a good high-temperature strength and hot corrosion resistance. Coherency strains between the FCC gamma matrix (γ)- and L1₂ gamma prime (γ′)-precipitate phase particles mainly provide the high-temperature strength in IN738LC. Conical IN738LC specimens have been aged under compression for various times [24, 192, 480, and 960 hours at 1223 K (950 °C) and 12, 24, 192, and 480 hours at 1323 K (1050 °C)] in order to observe the morphological evolution of the γ′ precipitate microstructure. Dislocations play a determining role in morphological changes. Fingerprints of matrix dislocations in the form of indentations on γ′ precipitates have been identified by scanning electron microscope. Precipitate morphology has become more complex through dissolution/merging as temperature, aging time, and stress have increased. The precipitate morphology has evolved toward rafting at appropriate strain, temperature, and time. Localized slip bands have marked the beginning of rafting. The rafts have been observed at around a 45 deg angle away from the load direction. For higher stress positions, there is a trend toward N-type rafting which is expected of a positive misfit alloy under compression. Rafts eventually have collapsed due to severe creep deformation.     

Investigation of the Effect of Magnesium on the Microstructure and Mechanical Properties of NiTi Shape Memory Alloy Prepared by Self-Propagating High-Temperature Synthesis

This work aims to describe the effect of magnesium on the microstructure, phase composition, amount of undesirable Ti₂Ni phase, martensitic transformation, mechanical properties, and corrosion resistance of NiTi alloy. To minimize the quantity of Ti₂Ni phase, we use the magnesium as an element with high affinity to oxygen, because this phase is stabilized by oxygen. Various quantities of magnesium (1, 3, and 5 wt pct) were tested. Self-propagating high-temperature synthesis (SHS) was used as a production method of the alloys. The samples prepared by SHS were pulverized by a vibrating mill, and the obtained powders were used for consolidation by means of spark plasma sintering. Results showed a significant reduction of the content of undesirable Ti₂Ni phase by the addition of magnesium. Further, magnesium increased corrosion resistance and yield strength.

     

The role of microstructure in localized corrosion of magnesium alloys

The article presents new findings on the influence of microstructural changes on corrosion behavior of magnesium alloy AZ91 in chloride solution, with a particular attention to the role of the β phase (Mg₁₇Al₁₂) and the surrounding Al-rich-α area. The as-cast alloy was subjected to solutionizing and aging heat treatments, in order to incorporate variation in morphology and distribution of the intermetallic β phase and the surrounding Al-rich-α (also known as eutectic α). Although previous workers have ascribed the higher corrosion resistance of a fine-grained alloy to the formation of the finely distributed β phase, the present work suggests that it is the ratio of the β phase to the Al-rich-α that governs the localized corrosion of the aged alloy. Corrosion characteristics were investigated by immersion and electrochemical tests. Surface microtopography, optical microscopy, and scanning electron microscopy (SEM) were employed to characterize the localized corrosion.     

High-Temperature Behavior of a High-Velocity Oxy-Fuel Sprayed Cr3C2-NiCr Coating

High-velocity oxy-fuel (HVOF) sprayed coatings have the potential to enhance the high-temperature oxidation, corrosion, and erosion-corrosion resistance of boiler steels. In the current work, 75?pct chromium carbide-25?pct (nickel-20?pct chromium) [Cr₃C₂-NiCr] coating was deposited on ASTM SA213-T22 boiler steel using the HVOF thermal spray process. High-temperature oxidation, hot corrosion, and erosion-corrosion behavior of the coated and bare steel was evaluated in the air, molten salt [Na₂SO₄-82?pct Fe₂(SO₄)₃], and actual boiler environments under cyclic conditions. Weight-change measurements were taken at the end of each cycle. Efforts were made to formulate the kinetics of the oxidation, corrosion, and erosion-corrosion. X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM)/energy dispersive spectroscopy (EDS) techniques were used to analyze the oxidation products. The coating was found to be intact and spallation free in all the environments of the study in general, whereas the bare steel suffered extensive spallation and a relatively higher rate of degradation. The coating was found to be useful to enhance the high-temperature resistance of the steel in all the three environments in this study.     

High-temperature environmental embrittlement of thermomechanically processed TiAl-based intermetallic alloys

Thermomechanically processed TiAl-based intermetallic alloys with various alloy compositions and microstructures were tensile tested in various environmental media, including air, water vapor, and a gas mixture of 5 vol pct, H₂ + Ar, as functions of temperature and strain rate. All the TiAl-based intermetallic alloys showed reduced tensile fracture stress (or elongation) in air, in water vapor, and in a gas mixture of 5 vol pct H₂ + Ar, not only at ambient temperature (RT ∼ 600 K), but also at high temperature, from 600 to 1000 K (and sometimes at temperatures higher than 1000 K). The high-temperature environmental embrittlement of TiAl-based intermetallic alloys depended upon the microstructure. The factors causing the high-temperature environmental embrittlement may include hydrogen atoms decomposed from water vapor (H₂O) or hydrogen gas (H₂), similar to those causing the low-temperature environmental embrittlement. Also, it is demonstrated that the oxidized scale is effective in reducing high-temperature environmental embrittlement.     

Microstructure and phase stability of INCONEL alloy 617

INCONEL alloy 617 (54 Ni, 22 Cr, 12.5 Co, 9 Mo, 1 Al, 0.07 C) is a solid-solution alloy with good corrosion resistance and an exceptional combination of high-temperature strength and oxidation resistance. A laboratory study was performed to determine the effects of long-time (215 to over 10,000 h) exposure to temperatures up to 2000°F (1093°C) on the microstructure and phase stability of the alloy. To investigate the strengthening response exhibited by the alloy during high-temperature exposure, microstructures were correlated with mechanical properties. The major phase present in the alloy after exposure to all temperatures from 1200 to 2000°F (649 to 1093°C) was found to be M₂₃C₆. The phase precipitated as discrete particles and remained stable at aü temperatures. No MC or M₆C carbides were found. A small amount of gamma prime was found in samples exposed at 1200°F (649°C) and 1400°F (760°C). A PHACOMP analysis indicated 0.63 pct gamma prime could form. No topological close-packed phases such as sigma, mu, and chi were found. Strengthening of the alloy during exposure to temperature was found to result primarily from the precipitation of M₂₃C₆. The phase provides effective strengthening because it precipitates in discrete particles and remains stable at temperatures to 2000°F (1093°C). The amount of gamma prime formed is not sufficient to cause appreciable hardening, but it does provide some strengthening at 1200 to 1400°F (649 to 760°C).     

Wear resistance and high-temperature compression strength of Fcc CuCoNiCrAl<Subscript>0.5</Subscript>Fe alloy with boron addition

This study discusses the wear resistance and high-temperature compression strength of CuCoNiCrAl_0.5Fe alloy with various amounts of boron addition. Experiments show that within the atomic ratio of boron addition from x=0 to x=1.0 in CuCoNiCrAl_0.5FeB _x (referred to as B-0 to B-1.0 alloys), the alloys are of fcc structure with boride precipitation. The volume fraction of borides increases with increasing boron addition. The corresponding hardness increases from HV 232 to HV 736. Wear resistance and high-temperature compression strength are significantly enhanced by the formation of boride. The alloys with boride are less tough. The superior wear resistance of B-1.0 alloy, which is even better than SUJ2 wear-resistant steel, indicates that the CuCoNiCrAl_0.5FeB _x alloys have potential applications as ambient- and high-temperature mold, tool, and structural materials.