The effect of phosphorus-containing additives in fuels on automobile exhaust valve corrosion

It is suggested that the burning of exhaust valves involves the participation of sodium sulphate, arising from salt ingested with the air and sulphur in the fuel. Phosphate salts, such as lead phosphate which arises from lead anti-knock additives and phosphorus ignition control additives, may increase this accelerated attack. To test the roles of the various possible salt species the effect of mixtures of sodium sulphate and lead phosphate, sodium sulphate and sodium phosphate, lead sulphate and lead phosphate, and sodium sulphate and phosphorus pentoxide on the corrosion of a stainless steel, Fe-18% Cr, the cobalt-base alloy X-40 and the nickel-base alloy Nimonic 115 in a crucible test have been studied.The results show that the presence of sulphates markedly enhances corrosion, with internal sulphides being formed: in alloys containing nickel a liquid sulphide can be formed resulting in accelerated attack. The presence of phosphorus enhances the attack of sodium sulphate: for these alloys pure sodium sulphate is not particularly aggressive, but the addition of phosphate salts resulted in heavy corrosion.Phosphorus-rich mixtures appeared to be capable of producing a type of corrosion involving direct participation of phosphorus, forming phosphides or phosphates of the metals.The presence of sodium is not essential for heavy attack. Lead sulphate-lead phosphate mixtures produced heavy corrosion of all the alloys studied.     

A corrosion investigation of solder candidates for high-temperature applications

The step-soldering approach is being employed in the multi-chip module technology. High-lead-containing alloys are among the solders currently being used in this approach. Au-Sn and Au-Ge based candidate alloys have been proposed as alternative solders for this application. In this work, a corrosion investigation was carried out on potential ternary lead-free candidate alloys based on these binary alloys for high-temperature applications. These promising ternary candidate alloys were determined by the CALPHAD approach based on the solidification criterion and the nature of the phases predicted in the bulk solder. This work reveals that the Au-Sn-based candidate alloys close to the eutectic composition (20 wt.% Sn) are more corrosion resistant than the Au-Ge-based ones.     

Protection against high temperature corrosion with laser welded claddings – Applied and tested on exhaust valve discs of large diesel engines burning heavy fuel oil

In close co‐operation between Wärtsilä Switzerland and BIAS, exhaust valves of two‐stroke diesel engines were clad by laser welding with functional coatings to improve the corrosion behaviour and extend the life cycle of these exhaust valves. The coatings consisted of different nickel‐based alloys. The investigations were carried out in a period of three years. The cladding tests and experimental investigations were carried out at BIAS, whereas Wärtsilä Switzerland organised the suitable valves and a seagoing ship for the testing. The inspections of the coatings on board and the evaluations of the results after valve dismounting were carried out jointly. In the preliminary stages of the study, the process window of various corrosion‐resistant materials were determined and the most promising ones were applied on Nimonic 80A valve discs. The laser‐clad valves were installed for test runs in a modern container ship. Depending on the process parameters and the corrosion resistant materials used the laser generated coatings were inspected after every 1000 h of service and evaluated comparatively. The results of this study are presented.     

Sulfate-induced high-temperature corrosion of nickel

Corrosion of high-purity nickel with deposits of sodium sulfate has been studied at 900°C in O₂+4.2 % SO₂ at gas pressures ranging from 700 to 0.5 Torr. Corrosion rates in the gas mixture are faster than in oxygen or sulfur dioxide alone. At the higher pressures the initial reaction is parabolic, but subsides after extended reaction. The parabolic reaction is concluded to comprise a diffusion-controlled transport of oxygen atoms and sulfur dioxide molecules through the molten sulfate layer with dissolved nickel oxide. Ni-S liquid solution is formed at the metal surface during the reaction. At reduced gas pressures the initial reaction is nonparabolic (linear and subsequent gradually increasing rate) prior to the parabolic stage. After extended reaction at sufficiently low partial pressure of SO₂ or reaction in 1 atm O₂ significant amounts of Ni-S liquid solution cannot be detected at the metal surface below the sulfate melt. The reaction mechanism is illustrated and discussed in terms of the stability diagrams of the Na-O-S and Ni-O-S systems.     

Electrochemical monitoring of high-temperature molten-salt corrosion

Hot molten-salt corrosion can cause serious metal degradation in boiler plant, incinerators, and furnaces. In this research, electrochemical-impedance and electrochemical-noise techniques have been evaluated for the monitoring of hot-corrosion processes in such plants. Tests have been carried out on Ni-1 % Co and Alloy 800, a commercial material of interest to operators of industrial plants, utilizing a bulk molten-salt environment and also a simulated combustion test, where thin films of molten salt were established on the alloy surfaces. Electrochemical-impedance and electrochemical-noise data were compared with the results of metallographic examination of the test alloys and showed reasonable correlation between the electrochemical data and the actual degradation processes. Current-noise analysis gives valuable information on the initiation stages of hot corrosion, while impedance measurements can detect propagation or rapid corrosion of the base metal. This preliminary work indicated that the electrochemical techniques show considerable promise as instruments for the monitoring of high-temperature corrosion processes.     

Numerical modeling of high-temperature corrosion processes

Numerical modeling of the diffusional transport associated with high-temperature corrosion processes is reviewed. These corrosion processes include external scale formation and internal subscale formation during oxidation, coating degradation by oxidation and substrate interdiffusion, carburization, sulfidation and nitridation. The studies that are reviewed cover such complexities as concentration-dependent diffusivities, cross-term effects in ternary alloys, and internal precipitation where several compounds of the same element may form (e.g., carbides of Cr) or several compounds exist simultaneously (e.g., carbides containing varying amounts of Ni, Cr, Fe or Mo). In addition, the studies involve a variety of boundary conditions that vary with time and temperature. Finite-difference (F-D) techniques have been applied almost exclusively to model either the solute or corrodant transport in each of these studies. Hence, the paper first reviews the use of F-D techniques to develop solutions to the diffusion equations with various boundary conditions appropriate to high-temperature corrosion processes. The bulk of the paper then reviews various F-D modeling studies of diffusional transport associated with high-temperature corrosion.     

690合金腐蚀疲劳裂纹扩展研究

基于DCPD方法测量了690合金在室温空气、325℃空气和325℃除氧超纯水中的疲劳裂纹扩展速率。试验结果采用Priddle模型进行拟合分析,得到690合金在3种条件下疲劳裂纹扩展的门槛应力强度因子幅值ΔK_(th)和失稳断裂应力强度因子Kc。结果表明,高温水环境下,疲劳裂纹萌生和扩展加快,这可以用滑移-溶解机理解释;高温下,材料强度下降,ΔK_(th)和Kc也下降,高温加速了材料的疲劳断裂。SEM断口形貌表明,空气中的疲劳断口观察到明显的滑移台阶,疲劳破坏形式为穿晶断裂;高温水下的疲劳断口则同时出现穿晶和沿晶开裂,为混合型断口特征。     

汽车排气管件内高压成形数值模拟及试验

利用有限元分析软件,对汽车排气管件的压弯和内高压成形过程进行了数值模拟,研究了压弯成形过程中管坯在不同弯曲变形量下的壁厚分布和内高压成形过程中压力加载路径对成形零件壁厚的影响。基于有限元模拟结果,进行了压弯成形和内高压成形实验。结果表明,压弯成形和内高压成形弯曲叠加处最易破裂;通过优化加载路径可避免缺陷,获得壁厚均匀性较好的成形零件;成形实验结果与数值模拟结果一致。     

The problem of sulfur in high-temperature corrosion

The role of defect and transport properties of transition metal sulfides on the kinetics and mechanism of high-temperature sulfide corrosion of metals and alloys is discussed. It has been shown, that due to the very high concentration of defects in common metal sulfides, not only pure metals but also conventional high-temperature alloys (chromia and alumina formers) undergo very rapid degradation in highly sulfidizing environments. Refractory metals, on the other hand, are highly resistant to sulfide corrosion, their sulfidation rates being comparable with the oxidation rate of chromium. Pioneering work of Douglasset al. has shown that alloying of common metals by niobium or molybdenum, and in particular combined alloying by molybdenumand aluminum, dramatically decreases the sulfidation rate. A novel Fe–30Mo–9Al alloy has been proved to be highly resistant to sulfide corrosion, its sulfidation rate being comparable with that of pure molybdenum. Even better resistance to highly-sulfidizing environments show new amorphous Al–Mo and Al–Mo–Si alloys, these materials also being simultaneously oxidation resistant. Thus, new prospects have been created for the development of a new generation of coating materials, resistant to multicomponent sulfidizing-oxidizing atmospheres, often encountered in many branches of modern technology.     

Enhanced corrosion protection by powder coatings on hot rolled steel (HRS) for high-temperature applications

Hot rolled steel (HRS) has long been a major product to the motor industry for bodywork, as galvanised steel (zinc coatings), and it is widely used in building and as tinplate (including tin and chrome oxide coatings) for food, etc. can-making. Among more specialist uses its moderate production costs makes it suitable for the manufacturing process. In this study, newly developed corrosion protection enhanced silicone based powder coating was evaluated on hot rolled steel (HRS). The powder coating in this study was developed to withstand high temperatures up to 550°C without any degradation. In the study silicone resin was incorporated with fillers, corrosion inhibitors and fibres at various compositions to achieve maximum heat resistance and improved corrosion resistance. Protective behaviours of the differently formulated powder paints were investigated before and after heat exposure. The developed paint system was evaluated using various test methods such as heat resistance, salt spray, Electrochemical Impedance Spectroscopy (EIS), adhesion test and microstructure study using SEM. The evaluation of test results revealed that powder coating combining zinc dust particles and mineral fibres have better performance due to better corrosion and adhesion stability in the corrosive environment before and after heat exposure up to 550°C.     

On the mechanism of high-temperature sulphur corrosion of binary alloys

Taking into account the phase composition and morphology of the scales three groups of alloys may be distinguished. The first of them contains alloys forming monophase scales which constitute solid solutions of sulphides of both alloy components (AgCu type). The mte of corrosion of these alloys is a monotonous function of their composition, and the mechanism of scale formation depends on the geometrical configuration of the reaction system. The second group includes alloys the components of which form sulphides of limited mutual solubility or sulphides having spinel structures (FeCr type); the scale may then be a mono- or double phase one and the exact structure is a function of the concentration of the alloying element. The metals belonging to the third group form mutually immiscible sulphides (CuZn type). The scales form double layers, the external layer being made up by sulphides of the base metal, while the internal layer is a heterophase mixture of the sulphides of both alloy components. The kinetics and the mechanism of the corrosion of these alloys is largely independent from the atmosphere (elemental sulphur or H₂? H₂S mixture).     

Stress analysis for the high-temperature gaseous corrosion of metal tubes

Stress analysis for the high-temperature gaseous corrosion of metal tubes is presented. The mechanical stresses arising from the difference in thermal expansion coefficients and volumes of a metal and its oxide can result in cracking of the oxide scale and in an increase of the oxidation rate. The model interprets the different behavior of oxide scale growths on outer versus inner tube surfaces. The oxidation behavior of tantalum tubes at temperatures of 825 °C and 930 °C is calculated based on this analysis. Results of the calculations are in good agreement with observed experimental data.     

On the high-temperature corrosion of Fe-Cr alloys in sulfur vapor

The mechanism of sulfidation of Fe-Cr alloys ranging from 8 to 97 wt.% chromium was determined from studies of scale structures, surface morphologies of scale, and reaction kinetics. Although the kinetics of sulfidation were quite similar to those previously determined by Mrowec et al., the structures in the present work were different, being triplex in nature. The growth mechanism of each layer was determined, and the overall sulfidation behavior was compared to the oxidation behavior. Many similarities between the two corrosion processes were observed.     

赛车排气管制造工艺及模具设计

针对赛车排气管进行弯曲成型时易起皱、压扁、截面积变小的现象,分析产生这种曲陷的原因,提出解决问题的方法,并设计弯曲成型模具.     

真空排气阀在压铸模上的应用

介绍压铸生产中模具型腔内部残留的空气会导致铸件充型不良现象,运用CAE技术对铸件进行充型分析,根据分析结果在模具合适的位置安装真空排气阀,可以解决排气问题,提高零件成型的合格率。     

An investigation of chloride-substituted Schiff bases as corrosion inhibitors for steel

Polarization and impedance measurements were performed on steel in deaerated 5% HCl solution with and without Schiff base additives within the concentration range 1 × 10⁻⁴–5 × 10⁻³ mol/dm³. The Schiff base compounds used were salicylaldimine, R, N-(2-chlorophenyl)salicyaldimine, 2Cl-R, N-(3-chlorophenyl)salicyaldimine, 3Cl-R, and N-(4-chlorophenyl)salicyaldimine, 4Cl-R. It was found that when the concentration of the inhibitors were increased the inhibitor efficiencies, η, also increased with increasing surface coverage. The results indicated that the ortho-substituted (2Cl-R) compound had the highest inhibition efficiency. All the Schiff base additives studied obeyed the Langmuir isotherm.     

Mechanism of high-temperature sulphide corrosion of metals and alloys

Sulfidation appears to be considerably more suitable than oxidation for studying the high-temperature behaviour of metals. Sulfide scales are formed at considerably higher rates and at lower temperatures, in addition it is much easier to work with the radioactive sulfide isotope. The experiments can be conducted in sulfur vapour or in hydrogen/hydrogen sulfide mixtures; differences are due only to different sulfur partial pressures. Sulfide scale is formed on pure metals (e. g. Cu, Ag, Ni) as well as in binary or ternary alloys. (e. g. FeNi, NiCr, CuZn, CoCr, FeCrAl) with consistantly reproducible results. Irrespective of scale thickness and structure the reaction rates are always controlled by outward diffusion with the high defect concentration favouring high reaction rates. Since various metals tend to form low melting metal/metal sulfide eutectics, it is necessary to keep temperatures below the particular melting point. Contrary to inner oxidation there is no inner sulfidation. The protective action of sulfide scale is considerably inferior to that of oxide scale. According to the results of the present compilation no material has become available which combines sufficient scaling resistance with good high-temperature mechanical properties. Solution of this problem may perhaps be found by adding rare earth metals.     

Post-test corrosion analysis of high-temperature thermal energy storage capsules

Thermal energy storage capsules have been freeze-thaw cycled in vacuum at 1000 ± 100 K. The capsules were fabricated from Inconel 617^® (Inco Alloys International, Inc.) and contained eutectic fluoride mixtures of sodium, magnesium, lithium, and potassium. Samples that were thermal cycled for 20,000 and 30,000 hr were analyzed for corrosion effects. Radiography indicated neither flaws nor inhomogeneities, and there were no significant microstructural changes in the container alloy. The microstructural damage penetrating the inside surface was as deep as 120 m and that penetrating the outside surface was approximately 150 m. Microprobe results on the containers revealed a concentration gradient of alloying elements. The aluminum concentration was reduced from 1.34% in the original matrix to 0.4% at 10 m from the inside surface, and chromium was reduced from a nominal value of 23% to 10% at the outer surface. The depletion of aluminum and chromium from the outer surface was due to vacuum vaporization at elevated temperature. X-ray diffraction revealed the formation of possible protective films consisting of MgNaF₃ and MgF₂. The measured and theoretically predicted concentrations of aluminum and chromium were in good agreement. It was concluded that the corrosion process is a solid-state diffusion-dominated process and an expected lifetime of 5 to 7 years is a reasonable estimate.