超音速大气等离子喷涂TiB2-SiC涂层的抗氧化性及耐熔盐腐蚀性能

采用超音速大气等离子喷涂制备全包覆TiB₂-SiC涂层,研究了TiB₂-SiC涂层在400和800 ℃的氧化性能,并探究其氧化机理。对TiB₂-SiC涂层在900 ℃下的抗铝熔盐腐蚀性能进行研究,并探讨其耐熔盐腐蚀机理。结果表明,超音速大气等离子喷涂制备的TiB₂-SiC涂层具有良好的抗氧化性,在400 ℃的氧化速率常数为1.92×10^-5 mg²·cm^-4·s^-1,在800 ℃的氧化速率常数为1.82×10^-4 mg²·cm^-4·s^-1。超音速大气等离子喷涂制备的TiB₂-SiC涂层在900 ℃下具有良好的抗熔盐腐蚀性能,熔盐腐蚀后TiB₂-SiC涂层都保持致密结构,未发生涂层的开裂及剥落。     

Korrosion von silber in neutralen alkalisulfatschmelzen

The corrosion of silver in the ternary eutectic of (mole %) 78 Li₂SO₄, 13·5 K₂SO₄ and 8·5 Na₂SO₄ at 625°C is proportional to the oxygen pressure above the melt. The corrosion current density of completely immersed specimens is 1·4 × 16^?6 A cm^?2 in the unstirred melt under a 100% oxygen atmosphere. The corrosion rate is increased by stirring.Specimens incompletely immersed in the melt are corroded at a much higher rate than fully immersed specimens. The specimen surface protruding from the melt is covered by a thin film of the molten material. Local cells are formed where O₂ is preferentially reduced on the surface above the melt, and silver is preferentially dissolved from the immersed portion of the surface. The corrosion current density is controlled by the ratio of areas inside and outside the melt.     

The stability of passive film growth on copper in anaerobic sulphide solutions

The corrosion behaviour of oxygen-free copper in anoxic 0.1?M NaCl?+?2?×?10^?4?M Na₂S·9H₂O solution (pH?=?9.0) was investigated under potentiostatic polarisation for different times. Electrochemical methods, including electrochemical impedance spectroscopy, Mott–Schottky analysis, localised electrochemical impedance spectroscopy (LEIS) and scanning electron microscopy observations, were conducted. The results indicated that the corrosion resistance of oxygen-free copper decreased with increasing applied potential, whereas it increased with increasing polarisation time. The passive film growth kinetics obeyed a logarithmic law (lnD?=?alnt?+?b, where D is the layer thickness, b is a constant taken as the initial growth rate, t is the polarisation time and a is the time exponent). Subsequent to the formation of a compact and coherent passive film, the thicker the film was, the more difficult for ion to migrate, which further resulted in a slower film growth rate. The passive film displayed p-type semiconductor behaviour and the acceptor density (cation vacancy) was approximately 10²² to 10²³?cm^?3. The LEIS results showed that the passive film achieved relative stability after 24?h of immersion under natural conditions, which was longer than the duration of potentiostatic polarisation (4?h at ?0.6?V_SCE).     

Use of SECM to compare corrosion resistance of DIN W. Nr. 1·4460 high N and AISI 316L austenitic stainless steels in physiological solutions

Abstract

The pitting corrosion resistance of DIN W. Nr. 1·4460 stainless steel (SS) with high amounts of nitrogen (0·87%) was evaluated to be used for medical implants. The SS pitting corrosion resistance was tested in a minimum essential medium at 37°C by electrochemical impedance spectroscopy and potentiodynamic polarisation curves and in a 0·1 mol L^?1 NaCl solution at 25°C, by scanning electrochemical microscopy. This last technique measures the concentration of chemical species released by corrosion processes. The potential of an ultramicroelectrode was set to amperometrically detect the Fe²⁺ ions released at the anodic areas and also the depletion of oxygen due to the cathodic reactions in the vicinity of the cathodic areas. The AISI 316L stainless steel was also tested for comparison reasons. The results showed that the DIN W. Nr. 1·4460 with 0·87% nitrogen presents higher pitting corrosion resistance than the AISI 316L SS, being a potential candidate for biomaterial applications.     

Oxidation Behavior of TiAl<Subscript>3</Subscript>/Al Composite Coating on Orthorhombic-Ti<Subscript>2</Subscript>AlNb Based Alloy at Different Temperatures

This paper reports the oxidation behavior of TiAl₃/Al composite coating deposited by cold spray. The substrate alloy was orthorhombic-Ti-22Al-26Nb (at.%). The oxidation kinetics of the coating was tested at 650, 800, and 950 °C, respectively. The parabolic rate constant for the coating oxidized at 650 °C was k _p = 7.2 × 10⁻² mg·cm⁻²·h^−1/2 for the tested 1200 h. For the coating oxidized at 800 °C, the oxidation kinetics could be separated into two stages with k _p value of 39.8 × 10⁻² mg·cm⁻²·h^−1/2 for the initial 910 h and 17.7 × 10⁻² mg·cm⁻²·h^−1/2 for the stage thereafter. For the coating oxidized at 950 °C, the oxidation kinetics can be separated into three stages with k _p of 136.9 × 10⁻² mg·cm⁻²·h^−1/2 in the first 100 h, followed by 26.9 × 10⁻² mg·cm⁻²·h^−1/2 from 100 to 310 h, and 11.8 × 10⁻² mg·cm⁻²·h^−1/2 from 310 to 1098 h. XRD, SEM, and EPMA were used to study the microstructure of the coating. The results indicated that the oxidation took place throughout the entire coating instead of only at the surface. The aluminum phase in the composite coating was soon oxidized to Al₂O₃ in all tested cases. The aluminum in TiAl₃ phase was depleted gradually and oxidized to Al₂O₃ along with the degradation of TiAl₃ to TiAl₂ and TiAl as the temperature increased and time proceeded. AlTi₂N was also a typical oxidation product at temperature higher than 800 °C. The experimental results also indicated that the protection of the coating was attributed greatly to the interlayer formed between the coating and the substrate.     

Studies on magnesium and its alloys in battery electrolytes

Abstract

Studies on the anode efficiency of magnesium and its alloys in battery electrolytes such as magnesium perchlorate, magnesium bromide, and magnesium chloride solutions were made by galvanostatic polarisation and corrosion rate measurements. The open circuit potential and corrosion rate of magnesium increase when lead is present in the alloy. The corrosion rates were observed to follow the order Mg> AP65 (Mg?(6·0?7·0) Al?(4·4?S·0)Pb) > AZ61(Mg?(5·8?7·2)Al?(0·5?1·5)Zn) > AZ31(Mg?(2·5?3·5)Al?(0·6?1·4)Zn) in all three electrolytes. Anode efficiency increased with increasing current density up to 40 mA cm^?2, but decreased above this level. Galvanostatic polarisation results indicated that corrosion of magnesium and its alloys in these electrolytes occurs under cathodic control. Of the tested alloys, AZ31 and AZ61 were found to be most suitable in respect of corrosion rate and anode efficiency in 2M Mg(ClO₄)₂ solution.     

Study of molybdate–phytic acid passivation on galvanised steel

Abstract

A combined molybdate–phytic acid passivation treatment on galvanised steel is studied in this paper. Both copper decoration drop tests and immersion mass loss tests indicated that the combined molybdate–phytic acid passivated samples showed better corrosion resistance than that of samples treated separately by molybdate or phytic acid. The electrochemical behaviours of the passivated samples were investigated using potentiodynamic polarisation and electrochemical impendence spectra in 0·5 mol L^?1 NaCl solution and showed corrosion current densities of 4·95×10^?5, 5·10×10^?5 and 1·19×10^?5 A cm^?2 respectively. Electrochemical impendence spectra also indicated that molybdate–phytic passivated samples exhibited better corrosion resistance. The mechanism of synergetic effect for molybdate–phytic acid passivation on galvanised steel is discussed in brief.     

Bestimmung der Abtragungsraten von Zirconium,Tantal und der Legierung Tantal-40Niob in fluoridhaltiger azeotroper Salpetersäure mit Hilfe der Radiotracer-Methode

The radiotracer technique as a means to investigate the corrosion of zirconium, tantalum, and a Ta-40Nb alloy in fluoride containing azeotropic nitric acid Zirconium and tantalum as well as the tantalum 40% niobium alloy are of considerable technical importance due to their high corrosion resistance against numerous corrosive media. With respect to corrosion testing in analytically pure azeotropic nitric acid in the temperature range between 20 and 121°C, corrosion rates were determined for zirconium: 7 · 10^?6 to 5 · 10^?4 mm/y, for tantalum: 10^?8 to 4 · 10^?6 mm/y, and for the Ta-40Nb alloy: 2 · 10^?7 to 8 · 10^?6 mm/y [1]. These corrosion rates will be markedly increased by adding small amounts of fluorides or by fluoride impurities. The radiotracer method after neutron activation was applied to determine the corrosion rates in azeotropic fluoride containing nitric acid. Even minute additions of fluorides strongly affect the corrosion resistance of zirconium. In the range between 0.15 and 10 ppm F^? and at a temperature of 108°C, corrosion rates between 5.3 · 10^?3 and 3.1 mm/y were measured. It was impossible to establish a limit for the fluoride concentration, below which the corrosion rate of zirconium will not be adversely influenced. The corrosion rates of tantalum and the Ta-40Nb alloy are considerably increasing above a fluoride concentration of 10 ppm. The highest corrosion rates measured were between 8.4 · 10^?3 mm/y at 50°C/280 ppm F^? and 1.4 · 10^?2 mm/y at 110°C/320 ppm F^?. Within the range of this investigation, the corrosion resistance of tantalum was higher than that of the Ta-40Nb alloy by one order of magnitude. The corrosion resistance of zirconium and tantalum was not influenced by any treatment of the samples before testing.     

The influence of carbonaceous surface impurities on excrescence initiation in the rimming steel/CO2/CO system

The influence of surface carbon contamination in determining the sites of excrescence growth has been studied, for the reaction of rimming steel in carbon dioxide containing 1·5%CO + 1000 ppm H₂O + 10 ppm CH₄ at 17·25 × 10⁶ N m^?2 and 450°C. Patterns visible in the oxide layer in the early stages of oxidation can be attributed to hydrocarbon impurities remaining on surfaces not rigorously cleaned. The impurity can accumulate at specific areas on the surface and can then accelerate oxidation such that excrescences start growing early in the normally protective stage. Surface cleaning procedures and their limitations for corrosion studies are discussed.     

Microstructure and corrosion resistance of tantalum after nitrogen ion implantation

This paper investigates the effect of nitrogen ion implantation on surface structure as well as resistance against tantalum corrosion. Bulk Ta surface was implanted with 30?keV nitrogen ions at a temperature of 100°C with doses between 1?×?10¹⁷ and 1?×?10¹⁸?ions/cm². The implanted samples were characterised by atomic force microscopy, X-ray diffraction analyses and the corrosion test to identify structural, compositional and electrochemical changes at various doses. The experimental results indicate the formation of hexagonal tantalum nitride (TaN_0.43), in addition to the fact that by increasing the ion dose, nitrogen atoms occupy more interstitial spaces in the target crystal, a case which can significantly improve corrosion resistance. The maximum extent in the improvement of the micro hardness was 75% and the reduction in the corrosion current was 83%. According to scanning electronic microscopy and corrosion results, in the dose of 1?×?10¹⁸?ions/cm² the highest corrosion resistance was received against the H₂SO₄ corroding media.     

Characterisation and corrosion resistance of Zn–Mn coatings electrodeposited from acidic chloride bath

Zn–Mn alloy coatings were galvanostatically electrodeposited from an acidic chloride bath. Effects of deposition current density, pH and temperature on surface morphology, microstructure and corrosion resistance of Zn–Mn coatings were studied. The coatings deposited at 10, 50 and 100 mA cm^?2 had a single η-Zn phase structure. However, a dual phase structure of η-Zn and ?-Zn–Mn with higher Mn content was found for the coatings deposited at 200 mA cm^?2. The dual structure degraded the corrosion resistance of the coatings. The highest corrosion resistance was achieved for the Zn–Mn coating deposited at 100 mA cm^?2, pH 4·9 and 25°C. This coating contained 4·1 wt-%Mn and showed a unique surface morphology consisting of randomly arranged packs of very thin platelets, laid perpendicular to the surface and provided a high compactness deficient free structure.     

Effects of applied potential and solution temperature on the pitting corrosion of pure aluminium in sulphate ion-containing chloride solution

Effects of applied potential and solution temperatureT _s on the pitting corrosion of pure aluminium (Al) were investigated in 0.01 M NaCl solutions containing various sulphate (SO₄ ^2-) ion concentrations using a potentiodynamic polarisation experiment, the potentiostatic current transient technique, ac impedance spectroscopy and atomic force microscopy (AFM). The potentiodynamic polarisation curves showed a rise in the pitting potentialE _pir values and a simultaneous increase in anodic current density at potentials much higher than theE _pit value as the SO42~ ion concentration increases. This implies that (SO₄ ^2-) ions impede pit initiation at potentials belowE _pit but enhance pit growth aboveE _pit. This was confirmed from the larger pit growth rate parameterb values of pure Al exposed to (SO₄ ^2-) ion-containing chloride solutions during the abrading action than those exposed to (SO₄ ^2-) ion-free chloride solution. Furthermore, at 7_s=25°C, the charge densityQ values for the Al metal dissolution in the presence of (SO₄ ^2-) ions were smaller than the value in its absence. By contrast, as validated by the capacitance values and the AFM images of the re-anodized specimens, an enhanced metal dissolution was observed in (SO₄ ^2-) ion-containing chloride solutions at 7_s=60° and 80°C. From the experimental findings, it is suggested that (SO₄ ^2-) ions act as inhibitors of pitting corrosion on pure Al belowE _pit and at 7_s=25°C, whereas they act as promoters at 7_s=60 ° and 80°C. This originates from the accelerated dissolution of the bare metal extensively exposed to the temperature-sensitive Cl ion attack, which occurs at potentials aboveE _pit     

The atmospheric corrosion of unprotected carbon steel – a comparison between field study and laboratory test

A comparison is made between the atmospheric corrosion of unprotected carbon steel in outdoor exposures and results from laboratory tests. The atmospheric corrosion in the outdoor exposures is expressed as a function of certain atmospheric parameters (e.g. SO₂ concentration). In the laboratory tests the atmosphere consisted of air containing known amounts of SO₂ and water vapour. The experiments were carried out in such a way that both supply and deposition of SO₂ was known. There was good agreement between the empirical formula and the results from the laboratory tests provided that the supply of SO₂ in the laboratory test was below ca. 0.5 μg · h^?1 · CM^?2.     

The influence of sodium chloride on the atmospheric corrosion of steel

The influence of sodium chloride on the atmospheric corrosion of mild steel has been studied. Samples covered with sodium chloride crystals (8 m?g NaCl · cm^?2) have been exposed to an SO₂-free atmosphere at different relative humidities. The combined effect of sodium chloride crystals on a steel surface and sulphur dioxide in the atmosphere has been investigated at an SO₂-supply of 1 m?g SO₂ · cm^–2 · h^–1 (1 ppm SO₂). The corrosion attack was followed by periodic weighings. The corrosion products formed have been studied using the SEM-technique. On exposure of samples with sodium chloride in an SO₂-free atmosphere the extent of corrosion increased with increasing relative humidity from 58% to 90%, interrupted by a sharp minimum at about 87% relative humidity. The SEM-studies showed that tower shaped corrosion products were formed at a high relative humidity while filiform corrosion appeared when the relative humidity was lowered. At 90% relative humidity more corrosion was observed with clean steel samples and an SO₂-supply of 1 m?g ·cm^–2 ·h^–1 than with sodium chloride crystals on the surfaces (8 m?g NaCl ·cm^–2) in the absence of SO₂. In the combined influence of sodium chloride on the steel surfaces and sulphur dioxide in the atmosphere a synergic effect was noticed at 90% relative humidity. At 70% relative humidity no influence of an SO₂-supply of 1 m?g SO₂ ·cm^–2 ·h^–1 on the corrosion of steel samples with sodium chloride crystals on their surfaces could be observed.     

Inhibiting Effect of some Monosaccharides on the Corrosion of Copper in Nitric Acid

Abstract

A study has been made of the inhibitive efficiency of some monosaccharides (glucose, fructose, mannose, galactose, in the concentration range 10^?3M–1·0M) on copper corrosion in HNO₃ (0·01, 0·1 and 1·0 M) at temperatures of 15°, 25° and 35°c. It was found that the structures of the various saccharides (pyranosic or furanosic) affected neither the efficiency nor the mechanism of inhibition. Inhibition was practically zero at concentrations of the monosaccharides between 10^?3 and 10^?1 M, whereas, at 1·0 M, 65–70% inhibition was reached in 0·01 and 0·1 M HNO₃, and 50–60% in 1·0 M HNO₃.

In 1·0 M HNO₃ the saccharides act as cathodic inhibitors (reducing HNO₂ present in the HNO₃ solutions), whereas in 0·1 and 0·01 M HNO₃ they also acted as anodic inhibitors through theformation of an organometallic compound with Cu²⁺, which gave slight protection to the metal surface.     

Novel laser surface treatment approach to suppress sensitisation in modified type 316(N) stainless steel weld metal

Abstract

Welded components are subjected to solution annealing heat treatment for achieving full stress relief and restoration of mechanical properties and corrosion resistance. During such heat treatments, optimum cooling rate has to be selected because very slow cooling rate will result in sensitisation and susceptibility to intergranular corrosion whereas fast cooling will result in reintroduction of residual stress. For 316 LN stainless steel which is welded using modified E316-15 electrodes (0·045–0·055%C), critical cooling rate above which there is no risk of sensitisation is 75 K h^?1. This paper presents a novel laser surface treatment which suppresses sensitisation in weld metal, even at a slower cooling rate of 65 K h^?1. Experiments involving laser surface melting were carried out with 150 W average power pulsed Nd:YAG laser and 10 kW CO₂ laser, in both continuous wave and pulse modulated (100 Hz) modes. Best results were obtained when surface melting was performed with high frequency pulse modulated CO₂ laser beam. The processed weld metal remained unsensitised after solution annealing followed by slower rate of cooling at 65 K h^?1. Numerical simulation study was performed with ANSYS 7·0 software to understand the physical reason behind the difference in sensitisation behaviour of CO₂ laser melted specimens under continuous wave and high frequency pulse modulated conditions and the predictions were validated using results of electron backscattered diffraction studies. Weld metal specimens treated with high frequency pulse modulated CO₂ laser clearly showed evolution of fine grains near the fusion boundary region which enhanced sensitisation resistance.     

Influence of SO2, periodical wetting and corrosion products on the atmosperic corrosion of steel

Results from laboratory atmospheric corrosion tests on samples of a carbon and a low-alloy steel, with and without pre-exposure (1, 3 and 12 months) in an out-door atmosphere are reported. The laboratory exposure was performed in SO₂-containing (SO₂-supply: 0.4 μg SO₂ · cm⁻² · h⁻¹) and SO₂-free atmospheres at 90% relative humidity and 22.0° C. The results clearly demonstrate that the influence of atmospheric SO₂ on the instantaneous corrosion rate markedly de-decreases with increasing pre-exposure. Comparisons of corrosion rates calculated from out-door exposures and laboratory exposure tests with and without periodical wettings show that the atmospheric corrosion in the main proceeds during or following periods of atmospheric precipitation or condensation, i. e. under conditions of an apparantly wet rust layer. A synergistic effect of SO₂ and periodical wettings was established. Due to the leaching of sulphate ions from the rust layer, immersion sequences (to simulate rainfall) throughout resulted in lower corrosion rates than the simulation of dew by short spraying sequences. The corrosion of pre-exposed samples could be related to the amount of sulphate leached from the rust during immersions.     

Nickel electrodeposition from novel lactate bath

Abstract

A new electroplating bath for nickel deposition has been developed. Lactic acid was used as a complexing agent in the bath replacing boric acid. The effect of bath composition, current density, pH and temperature on the cathodic polarisation, cathodic current efficiency, morphology and structure of the deposit was carried out. Optimum conditions for producing sound and satisfactory nickel deposits were: 0·30M NiSO₄.7H₂O, 0·50–1·0M lactic acid and 0·3M Na₂SO₄ at pH=10, c.d.=0·98 to 9·80 mA cm^?2 and 25°C. The surface morphology of the deposited nickel was investigated using SEM. The crystal structure was examined by X-ray diffraction analysis. The results showed that the nickel deposits have a face centred cubic structure.     

Influence of hydrostatic pressure on pitting of aluminium in sea water

Abstract

The effect of hydrostatic pressure on the corrosion of aluminium in sea water oj pH 8·2 containing 7ppm dissolved oxygen has been investigated at 20°C. On increasing the pressure from 1 to 300 atm, the corrosion rate increased, an effect attributable to an increase in the rate of the anodic reaction while the cathodic rate remained unchanged. The observed increased susceptibility to pitting corrosion with increasing pressure was found to correlate with the presence of increasing quantities of SO^2?₄ and Cl^? ions in the oxidation layer. The introduction of electron acceptors such as SO^2?₄ and Cl^? into n type compounds such as aluminium oxides increases the density of lattice dejects and promotes breakdown oj the barrier provided by the oxidation layer.     

On the resistance of duplex steel to stress corrosion cracking

The paper gives the results of tests carried out into the stress corrosion cracking in duplex stainless steel (Type 02Cr22Ni5Mo3N, W.Nr. 1.4462); this grade is characterized by high resistance to intergranular corrosion, while resistance to stress corrosion cracking may be impaired by temperature and mode of loading. This behaviour requires special attention. The tests included testing in 35% MgCl₂ solution under constant load at 120°C, the drop evaporation test using a 0.1 mol · 1^? NaCl solution and the slow strain rate test in 35% MgCl₂ solution at 120°C.