The effect of recrystallization on the oxide scale composition and growth of whiskers during the oxidation of pure iron

The kinetics of oxidation, the composition of oxide scales, and the morphology of scales were markedly affected by recrystallization of the iron substrate at 500°C. There was little difference in behavior over the temperature range of 600 to 900°C. Whiskers of Fe₂O₃ formed on the outer surface during oxidation at 800 and 900C, the orientation of the whiskers being either [001] or [110] surface, depending upon the batch of the iron. Different batches of iron from the same supplier exhibited variable oxidation behavior which has been attributed to variations in the thermal stability of the crystal texture in the substrates.     

The growth of oxide platelets on nickel in pure oxygen. I. Morphology and oxidation kinetics

A very specific oxide morphology consisting in rounded oxide platelets a few microns in diameter has been obtained on pure nickel. The main experimental factors in the achievement of such a controlled morphology have been thoroughly investigated. Platelets are produced in the temperature range from 600 to 850°C and for oxidation times varying from a hundred hours to a very few minutes. Platelets could not be grown on very pure nickel. Scanning electron microscopy has been used extensively, and the corresponding observations are reported in the first part of this study together with thermogravimetric results.     

The growth of oxide platelets on nickel in pure oxygen. II. Surface analyses and growth mechanism

The structural properties of NiO platelets emerging from a primary oxide layer by oxidation of pretreated nickels in pure oxygen between 650 and 800° C have been investigated in relation with the initial metallic layers and the primary oxide. Surface composition and segregation of impurities were also studied by X-ray photoelectron spectroscopy and Auger electron spectroscopy. Textural properties and structural orientation of both the primary oxide layer and the platelets were analyzed by X-ray diffraction and transmission electron microscopy. Platelets grew along {111} planes, leading to elliptical or semicircular bicrystals. The driving force for the present type of growth originates from the stresses accumulated in the thin, primary oxide layer. Impurities do not directly interact with this growth, but enable a specific grain structure to be developed thereby favoring platelet growth.     

The oxidation of nimonic 80A in oxygen at reduced pressure

The oxidation of Nimonic 80A in the temperature range 1048–1143K in oxygen at pressures in the range 2 × 10^?7 to 1·5 × 10^?4 torr has been studied by weight gain measurements. Initially the reaction proceeds according to a linear rate law and subsequently follows a parabolic law. These observations are discussed in terms of the formation of two titanium oxides on the metal surface.Metallographic examination of the oxidized metal showed grain boundary attack in the metal substrate. The reaction product found in the metal grain-boundaries was a mixed oxide of aluminium and titanium.     

Effect of oxygen partial pressure on the high-temperature oxidation of CVD SiC

The oxidation behavior of chemically vapor-deposited silicon carbide (CVD SiC) was studied at 1670-2010 K in O₂-Ar and CO₂-Ar. The oxidation kinetics in O₂-Ar was parabolic or linear parabolic, and was parabolic in CO₂-Ar. The activation energy for the parabolic rate constants (k_p) was 210-220 kJ/mol in O₂-Ar, and was 290-300 kJ/mol in CO₂-Ar. The oxygen partial pressure (P_O2) dependence of k_p was expressed as k_p∝ (P_O2)ⁿ, where n=0.08-0.13 and 0.37-0.53 in O₂-Ar and CO₂-Ar, respectively. Bubbles were formed at more than 1985 K and P_O2>5 kPa. The bubble formation temperature decreased with decreasing P_O2 at P_O2<5 kPa.     

Observations of nodule growth during the oxidation of pure binary iron-aluminum alloys

Oxidation studies were carried out in oxygen at 800°C, on a series of pure binary iron-based alloys with between 1.9 and 9.8 wt. % aluminum. The results are presented in conjunction with the existing literature and these permit the development of a classification of scale morphologies based on alloy composition. Alloys with less than about 2.4 wt. % aluminum form bulky stratified scales composed of Fe₂O₃ and Fe₃O₄ with FeAl₂O₄ and Al₂O₃ at the scale-metal interface. Alloys with between 2.4 and 6.9 wt. % form an external Al₂O₃ scale but this is interspersed with iron oxide nodules that penetrate the alloy substrate. Only alloys with greater than 6.9 wt. % aluminum form completely protective Al₂O₃ scales. Models based on oxide nucleation are presented for the growth of bulky scales and also the iron oxide nodules.     

The effect of oxygen partial pressure on the filiform corrosion of organic coated iron

An in-situ time-lapse optical microscopy study, using a novel dual-compartment cell is used to gain mechanistic understanding of filiform corrosion (FFC) affecting an organic-coated iron surface. The apparatus allows independent control of environments surrounding the filament head and tail regions. When oxygen-containing environments surround the filament head, an anterior cathodic arc is formed, constraining the filament head electrolyte. When oxygen is removed from the vicinity of the head, FFC propagation rates remain unchanged, although the constraining arc is not present. Maximum propagation is observed when oxygen is available for transport through the filament tail to the rear of the head.     

On the effect of environment on spontaneous growth of lead whiskers from commercial brasses at room temperature

In this paper we report on a detailed statistics-based study of the effect of various environments – air, argon, vacuum, oil and water – on the room temperature spontaneous nucleation and growth of lead whiskers from hacksaw-cut surfaces of three leaded commercial brasses. The samples were initially held in the various environments for 166 h, before being stored in ambient air for a total of 1126 h. The environment was found to have a strong effect. The highest whisker density, ∼30,000 cm⁻², was observed for the sample initially kept under a mechanical (∼1 Pa) vacuum; the lowest, ∼3000 cm⁻², was observed for the sample initially immersed in water; the densities of the others were in between. The samples held in water only grew whiskers when removed from the water. Once exposed to air, a few whiskers grew at an average rate that, at 0.9 nm s⁻¹, was the highest measured. When the samples were exposed to air, after the initial storage in the different environments, both nucleation and growth of the whiskers were accelerated, before ultimately ceasing to grow roughly 400 h after creating the surfaces from which the whiskers grew. These results are consistent with a scenario in which oxygen and/or moisture diffusion down the Pb/brass interfaces results in a volume expansion that provides the driving force for whisker growth. The results also indicate that the oxygen levels needed for whisker growth can be as low as a few parts per million.     

Effect of temperature on oxidation behavior of ternary Fe-15Cu-5Al alloy in pure oxygen

The effect of temperature on oxidation behavior of ternary Fe-15Cu-5Al （mole fraction, %） alloy in pure oxygen was studied. Fe-15Cu-5Al alloy presents an irregular high-temperature oxidation behavior between 700-1 000 ℃, though the kinetic curve at each temperature can be approximately considered as being composed of two quasi-parabolic stages. At 700 ℃ the alloy forms bulky stratified scales. On the contrary, at 800℃ the alloy forms an external protective Al2O3 layer. The trend of decrease of oxidation rate does not continue at elevated temperatures. Due to the high stress-growth effect at 900℃, the thin Al2O3 layer cannot completely prevent further oxidation of the alloy underneath. When the temperature rises to 1 000℃, the high stress-growth effect is more obvious and the alloy forms overgrown large nodules. Compared with the Fe-10Al binary alloy, the presence of 15 % Cu tends to greatly increase the flaw of the formed Al2O3 layer at elevated temperatures, resulting in a peculiar oxidation pattern of Fe- 15Cu-5Al alloy.     

Investigation of high-temperature oxidation of lanthanum hexaboride in oxygen and the effect of internal oxidation on the protective properties of the scale

The vacuum microbalance method, as well as petrographic and x-ray phase analysis were employed to investigate the oxidation of lanthanum hexaboride as a function of temperature (700–1200°C), oxygen pressure (0.1–740 Torr) and porosity of the sample. It has been established that LaB₆ is highly resistant to oxidation in oxygen up to a temperature of 1200°C. The protective properties of the scale are determined by the peculiarities of the process of internal oxidation, during which a lower lanthanum boride, LaB₄, was observed to be enveloped by the vitreous oxide phase, B₂O₃.     

High temperature oxidation of austenitic stainless steel in low oxygen pressure

The oxidation of unstabilized austenitic stainless steel (En 58A) at temperatures between 770–1070K in 10^?5Nm^?2 oxygen pressure is reported. The surface composition was determined during oxidation by Auger electron spectroscopy and measurements carried out when oxidation was complete were made using energy dispersive X-ray analysis and electron microprobe analysis. The influence on the subsequent oxidation characteristics of trace and minor elements present in the bulk alloy was found to be important.Initially the formation of surface oxide was retarded by the diffusion of sulphur from the bulk. Subsequently manganese diffused through the first formed oxide at a rate two orders of magnitude greater than chromium to form a major fraction of the surface oxide.Diffusion rates for sulphur through the bulk alloy and manganese and chromium through the oxide have been determined and a mechanism for the high temperature oxidation of unstabilized austenitic stainless steel in low oxygen pressures is proposed.     

The nucleation and growth of metastable pitting on pure iron

The nucleation and growth of metastable pitting on pure iron surface in NaNO₂ + NaCl solution were investigated by potentiodynamic tests. The current fluctuations appear as cluster and overlapped. The active sites for nucleation depend mainly on the surface geometry. The growth behaviour of pure iron is different from stainless steels and carbon steels. On pure iron surface, many pits pile up together to form huge damage area. Pits are shallow, do not grow deeply and pits array one by one along the direction of abrasion grooves. The growth of stable pitting is different from metastable pitting.     

High-temperature oxidation of tungsten boride in oxygen and the effect of scale evaporation

The oxidation of tungsten boride in oxygen at pressures from 1 to 740 Torr over the temperature range of 500 to 1200°C was studied by means of thermogravimetric, petrographic, and x-ray analyses. Kinetic parameters of the oxidation were interpreted on the basis of the Frantzevitch oxidation equation, characterizing the volatility of the phases formed, the emf of a high-temperature galvanic cell, having a gas cathode and a metal anode, and the values of the electrical resistance of the film.     

Influence of water vapor on the oxidation behavior of aluminized coatings under low oxygen partial pressure

FeCrNi alloy after aluminizing was oxidized at 1000 °C in dry and humid (2.23 vol.% water) H₂. Experimental results showed that H₂ promotes the formation of θ alumina and its transformation to α alumina. The morphology of surface alumina coating does not change significantly, but the oxidation rate of the aluminized layer accelerates by the addition of water vapor. As a result, more cracks are found beneath the alumina layer when water vapor is present. The addition of water vapor seems having a favorable effect on the selective oxidation of Al and concentration of oxygen vacancy in the aluminized alloys.     

低氧分压下NiTi记忆合金氧化行为的研究

本文研究了NiTi形状记忆合金在H₂-H₂O气氛下400-700 °C的氧化行为。合金的氧化过程遵循立方规律,氧化激活能127.52 kJ/mol。氧化显著降低了试样表面的Ni含量。400 °C氧化的试样,其表面形貌与其他试样不同,并且氧化膜较薄,截面结构无法用SEM分析。500 °C、600 °C、700 °C氧化的试样,表面有两种形貌的氧化物,一种是颗粒状氧化物,另一种是晶须状氧化物。截面分析表明,氧化膜分为两层,上层由TiO₂构成,下层由Ni₃Ti构成,两层界面处有孔洞生成。     

Characterization of microarc oxidation coatings on pure titanium

The morphology, composition, and phase structure of the oxide coatings produced on the surface of pure titanium by alternating-current microarc discharge in aluminate solution were investigated by X-ray diffraction and scanning electron microscopy. The profiles of the hardness H and the elastic modulus E in the coatings were determined using a nanoindenta-tion method. The concentration distributions of Ti, Al, and O in the coating show that this coating over 30 μm thick contains two layers: an outer layer and an inner layer. The oxide coating is mainly composed of TiO2 rutile and Al2TiO5 compounds. During oxidation, the temperature in the microarc discharge channel was very high to make the local coating molten. From the surface to the interior of the coating, H and E increase gradually, and then reach maximum values of 9.78 GPa and 176 GPa respectively at a distance of 7μrn from the coating/titanium interface. They are also rather high near the interface.     

Microstructure and growth mechanism of tin whiskers on RESn3 compounds

An exclusive method was developed to prepare intact tin whiskers as transmission electron microscope specimens, and with this technique in situ observation of tin whisker growth from RESn₃ (RE = Nd, La, Ce) film specimen was first achieved. Electron irradiation was discovered to have an effect on the growth of a tin whisker through its root. Large quantities of tin whiskers with diameters from 20 nm to 10 μm and lengths ranging from 50 nm to 500 μm were formed at a growth rate of 0.1–1.8 nm s^?1 on the surface of RESn₃ compounds. Most (>85%) of these tin whiskers have preferred growth directions of 〈1 0 0〉, 〈0 0 1〉, 〈1 0 1〉 and 〈1 0 3〉, as determined by statistics. This kind of tin whisker is single-crystal β-Sn even if it has growth striations, steps and kinks, and no dislocations or twin or grain boundaries were observed within the whisker body. RESn₃ compounds undergo selective oxidation during whisker growth, and the oxidation provides continuous tin atoms for tin whisker growth until they are exhausted. The driving force for whisker growth is the compressive stress resulting from the restriction of the massive volume expansion (38–43%) during the oxidation by the surface RE(OH)₃ layer. Tin atoms diffuse and flow to feed the continuous growth of tin whiskers under a compressive stress gradient formed from the extrusion of tin atoms/clusters at weak points on the surface RE(OH)₃ layers. A growth model was proposed to discuss the characteristics and growth mechanism of tin whiskers from RESn₃ compounds.     

电解液成分对TA1纯钛等离子电解氧化陶瓷膜生长与组织的影响

钛合金长期植入体内依然存在磨损和腐蚀的问题,因此钛合金表面改性的目的就是提高其表面耐蚀性、耐磨性并赋予生物活性.本文在3种不同的电解液中,采用等离子电解氧化技术在TA1纯钛表面制备含有Ca、P的氧化钛多孔复合陶瓷层.研究了不同电解液中,改变工艺参数对纯钛表面陶瓷层平均厚度和生长机制的影响.采用扫描电镜和能谱分析了电流密度改变时,3种不同电解液中生成复合陶瓷膜的表截面形貌以及成分.结果表明,相同的电流密度,在电解液B中,陶瓷膜初始击穿电位最大,电解液C中陶瓷膜初始击穿电位值最小.正向电流为16 A/dm2时,在电解液A中陶瓷膜表面出现粉末状团聚物,在电解液B和C中,没有粉末状团聚物出现.无论是工艺参数还是电解液成分,对纯钛生物陶瓷膜的生长过程与组织结构都产生很大的影响,适当控制可制备所需的功能陶瓷膜.