Effect of phosphine on acid corrosion of iron

The effect of the phosphine-concentration (10^?8–10^?3 M) in strong acid solutions (pH = 0) on the corrosion rate of pure iron and Fe- 0.12% P alloy has been investigated. It was shown that at low concentrations (10^?7–10^?6M), phosphine accelerates the corrosion of iron, whereas at adequately high concentrations (10^?5–10^?3 M) it has inhibitory properties. In the presence of PH₃ the cathodic reduction of hydrogen ions is strongly accelerated. At the same time, the increase in PH₃ concentration causes an increase of surface coverage by PH₃ molecules, which results in the inhibition of both partial electrode processes.     

The corrosion behavior of two-phase Ni-Dy alloys in a eutectic (Li,K)2CO3 mixture at 650 °C

The corrosion behavior of binary Ni-Dy alloys containing 1, 3 and 5 wt.% Dy in a eutectic (0.62Li,0.38K)₂CO₃ mixture at 650 °C has been investigated. The alloys are two-phase composed of α-Ni solid solution and intermetallic compound Dy₂Ni₁₇. The experimental results indicate that the corrosion rate of the three Ni-Dy alloys is significantly lower than that of pure Ni, forming a scale mainly composed of NiO together with a small amount of dysprosium oxides. The weight gain of the alloys changes little with the content of dysprosium, while the thickness of the scale decreases with increasing content of dysprosium. The presence of Dy can promote the lithiation process of NiO during immersion in the melt. The corrosion mechanism of the alloys is also discussed.     

Intergranular corrosion of iron-phosphorus alloys in nitrate solutions

The corrosion of Fe-P alloys with 0.003–2.5 wt.%P has been studied in hot nitrate solutions. The current-potential curves show an extended active range and increased corrosion currents for high phosphorus alloys. The alloy with 2.5P does not passivate at all. The other alloys have been tested for intergranular corrosion in the passive range at 1,000 mV(SHE). The intergranular attack increases with increasing phosphorus content and can be correlated to its grain boundary concentration. The grain boundary concentrations had been established by equilibrium segregation at temperatures between 400 and 80°C and were determined by Auger electron spectroscopy (AES). The presence of phosphorus in solid solution and segregated to the grain boundaries prevents the formation of a passive layer on iron, if its local concentration is higher than ca. 2–3 at.%.     

Electrochemical behaviour of amorphous and nanoquasicrystalline Zr-Pd and Zr-Pt alloys in different environments

The corrosion behaviour of melt spun amorphous and nanoquasicrystalline Zr₇₀Pd₃₀ and Zr₈₀Pt₂₀ alloy ribbons has been investigated using potentiodynamic polarization study in NaCl, H₂SO₄ and NaOH solutions at different concentrations. The amorphous and nanoquasicrystalline alloys show better corrosion resistance than Zr in all the solutions studied. Both the alloys are susceptible to chloride attack and pitting has been observed. Complete passivation has been observed in H₂SO₄, while gradual break down of passivating layer occurs in NaOH. In general, nanoquasicrystalline state in both the alloys shows better corrosion resistance than amorphous state in all the solutions studied.     

High temperature Corrosion behaviour of iron aluminides and iron-aluminium-chromium alloys

The high temperature corrosion of different iron aluminides and iron-aluminium-chromium alloys containing between 6 and 17 wt% aluminium, 2 and 10 wt% chromium and additions of mischmetal has been investigated in air as well as in carburising, chlorinating and sulphidising environments. It was found that all alloys showed excellent corrosion resistance to both oxidation in air and carburisation in CH₄/H₂ up to at least 1100°C and to sulphidation in SO₂/air up to at least 850°C. In these environments the corrosion behaviour is not influenced by the concentrations of aluminium and chromium. In oxygen deficient H₂S-atmospheres, however, the corrosion behaviour depends sensitively on the aluminium and chromium concentration. At least 12 wt% aluminium in chromium-free alloys or 10 wt% aluminium in alloys containing 10 wt% chromium are required to provide sulphidation resistance at 550°C. The chlorination resistance of iron-aluminium-chromium alloys is low due to their formation of volatile aluminium chlorides.     

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.     

Hot corrosion of NiCr alloys in SO2+O2 atmospheres—I. Corrosion kinetics

The corrosion of Ni, Cr and NiCr (0·1–50 at.%) has been studied in the temperature range 600–1000°C using a microgravimetric apparatus with a liquid seal. The corrosion rate for 0·1 and 1% CrNi alloys was higher than that for Ni under similar conditions. Higher proportions of Cr decreased the corrosion rate considerably. All the alloys passed through a maximum corrosion rate between 700 and 800°C in maximum p_SO3 conditions. In situ visual observations were made to view nodular, dendritic and needle-growths and the liquid surface during corrosion. Au and Pt markers were used in marker studies. Electron probe and X-ray diffraction analyses indicated that the corrosion product was mainly NiO, with some Cr₂O₃, Ni₂S₃ and CrS.     

Microstructure,Phase Occurrence,and Corrosion Behavior of As-Solidified and As-Annealed Al-Pd Alloys

In the present work, we studied the microstructure, phase constitution, and corrosion performance of Al₈₈Pd₁₂, Al₇₇Pd₂₃, Al₇₂Pd₂₈, and Al₆₇Pd₃₃ alloys (metal concentrations are given in at.%). The alloys were prepared by repeated arc melting of Al and Pd granules in argon atmosphere. The as-solidified samples were further annealed at 700 °C for 500 h. The microstructure and phase constitution of the as-solidified and as-annealed alloys were studied by scanning electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray diffraction. The alloys were found to consist of (Al), ε _n (~ Al₃Pd), and δ (Al₃Pd₂) in various fractions. The corrosion testing of the alloys was performed in aqueous NaCl (0.6 M) using a standard 3-electrode cell monitored by potentiostat. The corrosion current densities and corrosion potentials were determined by Tafel extrapolation. The corrosion potentials of the alloys were found between ? 763 and ? 841 mV versus Ag/AgCl. An active alloy dissolution has been observed, and it has been found that (Al) was excavated, whereas Al in ε _n was de-alloyed. The effects of bulk chemical composition, phase occurrence and microstructure on the corrosion behavior are evaluated. The local nobilities of ε _n and δ are discussed. Finally, the conclusions about the alloy’s corrosion resistance in saline solutions are provided.     

Synthesis and Characterization of Functionally Gradient Ni-ZrO<Subscript>2</Subscript> Composite Coating

For the first time, functionally ZrO₂ content graded Ni-ZrO₂ composite coating has been successfully co-electrodeposited from a bath with gradually increasing stirring rate. For this, different composite coatings were electroplated in the same bath with different stirring rates to find the optimum stirring rate in which the maximum content with uniform distribution of ZrO₂ particles in the coating can be achieved. To produce ZrO₂ content graded Ni-ZrO₂ composite coating, the stirring rate was continuously increased from 0 to optimum value. By increasing of ZrO₂ particles content, the microhardness increases from interface towards the surface of the coating. The results of wear resistance measurements, Electrochemical impedance spectroscopy and potentiodynamic polarization test revealed that wear and corrosion resistances of functionally graded Ni-ZrO₂ (FGNZ) is higher than that of ordinary Ni-ZrO₂ (ONZ) composite coating. This result has been attributed to lower mechanical mismatch between coating and substrate in the functionally graded composite coating with respect to the uniformly distributed one.     

Corrosion behaviour of nanocrystalline Cu90Ni10 alloy in neutral solution containing chlorides

The corrosion behaviour of nanostructured Cu₉₀Ni₁₀ alloys in neutral solution containing different chloride concentrations has been investigated. The passivity of the alloy and the influence of the crystal structure have been examined. A critical chloride concentration for the passive behaviour of the alloys has been determined. The comparison between nanostructured and coarse grained Cu₉₀Ni₁₀ alloys shows a decrease in the protective properties of the passive layer in the nanostructured alloy. Some possible reasons to explain this behaviour have been discussed.     

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.     

An electrochemical study on the effect of pitting inhibition in weakly alkaline solution by copper addition in pure iron

The effects of copper on the corrosion rafe of steel have been investigated in order to propose the application of scraps as material of reinforcing steels in concrete structures. The corrosion behaviors of pure iron and several Fe−Cu alloys were examined by conducting potentio-dynamic and galvanostatic polarization in pH 10, 12.5 and 13.5 Ca(OH)₂ solutions added to several chloride concentrations. Values of the pitting potential (E _p) of pure iron were plotted at negative potential. In contrast to pure iron, theE _p values of Fe−Cu alloys showed much more positive values. The polarization curves in weakly alkaline solutions containing chloride ions indicated that the addition of copper up to 3 mass% significantly improved the corrosion resistance of steel.     

Hot Corrosion of Ti–Re Alloys Fabricated by Selective Laser Melting

Selective laser melting (SLM) is an additive manufacturing process that enables novel alloy production by combining metals with significantly different physical properties. In this paper, the hot corrosion behavior of Ti–Re alloys fabricated by SLM was studied in a mixture of Na₂SO₄ and NaCl salts at 600 °C. The morphology and composition of the corrosion products were characterized by scanning electron microscopy with energy-dispersive X-ray spectroscopy and X-ray diffraction to understand the degradation mechanisms. It has been shown that the hot corrosion resistance of Ti–Re alloys was influenced by the chemical inhomogeneity of the oxide scale resulting from the presence of rhenium particles undissolved during the SLM process.     

Fe-based soft magnetic amorphous alloys with high saturation magnetization above 1.5 T and high corrosion resistance

Fe-rich amorphous alloys with minor-addition of Cr and/or Nb were examined with the aim of developing Fe-based amorphous alloys exhibiting simultaneously high saturation magnetization above 1.5 T and good corrosion properties. Fe₈₂Cr₂B₈P₄Si₃C and Fe₈₂NbB₉P₄Si₃C amorphous alloys were found to exhibit high saturation magnetizations of 1.49 T and 1.57 T, respectively, and rather good corrosion resistance in 3.5 mass% NaCl solution at 298 K. The minor-addition of Cr or Nb enables the formation of amorphous alloy particles without harmful oxide layer by water atomization process which makes these alloys suitable for applications as soft magnetic core materials. The addition of 1 at% Nb improved the corrosion resistance through the increase in E_corr value, which makes easy to reach passive state, and the suppression of pitting corrosion. Besides, it has been proved that the simultaneous addition of Nb and Cr has an effect on forming protective passive film.     

Influence of purification process on incipient material corrosion in molten FLiNaK salts

Abstract

This study investigated the incipient corrosion characteristics on Ni based and Mo based alloys at 600°C in FLiNaK molten salts regarding to different purification processes of FLiNaK molten salts. Ni based Hastelloys-N alloys exhibited relatively better corrosion resistances, while Mo based TZM alloys showed the poorest corrosion resistance. The mass loss of the tested alloys was strongly dependent on purification process of the FLiNaK molten salts; through preheating and stirring the FLiNaK salts at higher temperature during the purification process, impurities and residual moisture of the FLiNaK salts could be effectively removed, leading to the improvement of mass loss of the tested alloys. The results of microstructural characterisation revealed that Hastelloy-N tended to exhibit intergranular corrosion, whereas grains of Hastelloy-B3 were also attacked by the corrosive medium in addition to grain boundaries. The results of EDX mapping showed that both Cr and Mo were found to be dissolved into FLiNaK salts for Hastelloy-N and Hastelloy-B3. The Mo based TZM alloys exhibited general corrosion pattern.     

The high-temperature corrosion of Fe-Nb alloys in a H2/H2O/H2S gas mixture

The corrosion of Fe-Nb alloys containing up to 40 wt.% Nb has been studied over the temperature range 600–980°C in a mixed gas of constant composition having sulfur and oxygen pressures ranging from 10^–8 to 10^–4 atm. and from 10^–27 to 10^–18 atm., respectively. All alloys were two-phase, consisting of an Fe-rich solid solution and an intermetallic compound, Fe₂Nb. The scales formed on the Fe-Nb alloys were duplex, consisting of an outer layer of iron sulfide (FeS) and an inner complex layer of Fe_xNbS₂(FeNb₂S₄ or FeNb₃S₆), FeS and unreacted Fe₂Nb. No oxides were detected at any temperature. The addition of Nb reduced the corrosion rate. The corrosion kinetics of Fe-Nb alloys followed the parabolic rate law, regardless of alloy composition and temperature. Platinum markers, attached to the original alloy surfaces, were always located at the interface between the inner and outer scales.     

Mechanism of Degradation of Titanium Alloy IMI 834 and Its Protection Under Hot Corrosion Conditions

The excellent combination of high-temperature strength and lightweight properties makes titanium-base alloys attractive for high-temperature applications in aircraft engines. However, more hot corrosion of titanium alloys is a life-limiting factor, particularly when aircraft fly at low altitudes across the sea. In the present paper, an attempt has been made to understand the degradation mechanism of titanium alloy, IMI 834 under hot corrosion conditions at elevated temperatures. The hot corrosion studies were carried out by determining weight loss at different temperatures and in salts of pure Na₂SO₄, 90% Na₂SO₄+10% NaCl and 90% Na₂SO₄+5% NaCl+5% V₂O₅. Subsequently, the rate constants were evaluated. The depth of attack due to hot corrosion was compared with oxidation data. Finally, the degradation mechanism of the titanium alloy that leads to degradation of mechanical properties in aggressive environments has been discussed and suitable coatings suggested to enhance the operational life of engines by effectively preventing both oxidation and hot corrosion.     

Study of aging and electrochemical behaviour of Al?Li?Cu?Mg alloys

Aging study of 1441 and 8090 Al? Li? Cu? Mg alloys exhibited characteristic precipitation hardening phenomena. Potentiodynamic polarisation studies carried out on various tempers of the 1441 and 8090 Al? Li? Cu? Mg alloys in 3.5% NaCl, with a small amount of H₂O₂, and in 3.5% NaCl solution with pH 10 and 12 showed the shifting of open circuit potential (OCP) towards more negative potential and higher corrosion rate, with the increase of aging time. The OCP value has shifted anodically with addition of H₂O₂ in 3.5% NaCl solution. Further, passivity phenomenon has been observed for all the alloy tempers in 3.5% NaCl solution at pH 10 and 12. The observation of OCP shift towards more negative potential and the higher corrosion rate with the increase of aging time has been attributed to the presence of higher amounts of anodic δ (AlLi), S′ (Al₂CuMg) and T₁ (Al₂CuLi) phases, studied by TEM, XRD and DSC methods.     

Evaluation of Cyclic Oxidation and Hot Corrosion Behavior of HVOF-Sprayed WC-Co/NiCrAlY Coating

Corrosion of metallic structural materials at an elevated temperature in complex multicomponent gas environments are potential problems in many fossil energy systems, especially those using coal as a feedstock. Combating these problems involves a number of approaches, one of which is the use of protective coatings. The high velocity oxy fuel (HVOF) process has been used to deposit WC-Co/NiCrAlY composite powder on two types of Fe-based alloys. Thermocyclic oxidation behavior of coated alloys was investigated in the static air as well as in molten salt (Na₂SO₄-60%V₂O₅) environment at 700 °C for 50 cycles. The thermogravimetric technique was used to approximate the kinetics of oxidation. WC-Co/NiCrAlY coatings showed a lower oxidation rate in comparison to uncoated alloys. The oxidation resistance of WC-Co/NiCrAlY coatings can be ascribed to the oxide layer of Al₂O₃ and Cr₂O₃ formed on the outermost surface. Coated alloys extend a protective oxide scale composed of oxides of Ni and Cr that are known to impart resistance to the hot corrosion in the molten salt environment.     

Corrosion of Ni-Ti alloys in the molten (Li,K)2CO3 eutectic mixture

The corrosion of pure Ni and of binary Ni-Ti alloys containing 5, 10, and 15 wt.% Ti respectively in molten (0.62Li,0.38K)₂CO₃ at 650°C under air has been studied. The corrosion of the single-phase Ni-5Ti alloy was slower than that of pure Ni, forming an external scale composed of NiO and TiO₂. The two-phase Ni-10Ti and Ni-15Ti alloys underwent much faster corrosion than pure Ni, producing an external scale containing NiO and TiO₂, and a thick internal oxidation zone of titanium mainly involving the intermetallic compound TiNi₃ in the original alloys. The rates of growth of the external scales for the Ni-Ti alloys were reduced with the increase of their titanium content, while the internal oxidation was significantly enhanced. The corrosion mechanism of the alloys is also discussed.