Influence of chromizing treatment on the corrosion behavior of AISI 316 stainless steel in supercritical water oxidation

SCWO, sometimes referred to as hydrothermal waste processing, uses the solvating traits of water in its supercritical condition to effectively destroy liquid organic wastes. One major problem in the supercritical water oxidation process is corrosion, because all metallic tubes in the process are exposed to high temperature and high pressure as well as severe corrosive species such as Cl⁻, F⁻, S²⁻, and O²⁻. The presence of Cl⁻ when the pH of a solution is very low and the solution has excess oxygen causes active corrosion and metal loss by metal-chloride and/or oxychloride formation. This study performed a chromizing treatment on 316 stainless steel and immersion tests in supercritical water. Weight change of chromized steels and untreated steels was measured, and the chemical state and composition of oxide films on 316 stainless steel were investigated. On the basis of SCWO tests using distilled water, the oxide layer was found to be very thin and homogeneous and weight gain was observed regardless of testing temperature, while the chromizing treatment slightly reduced weight gain. In the case of SCWO tests using salt water, weight loss was observed regardless of testing temperature and its corrosion mode was pitting by chloride ion, while chromizing treatment greatly decreased the corrosion rate.     

The Kinetics and Mechanism of Thermal Oxidation of Electrolessly Deposited Ni–B and Ni–W–B Alloys

The kinetics and mechanism of thermal oxidation of electrolessly reduced Ni–B and Ni–W–B alloys are studied by precision thermogravimetry and x-ray photoelectron spectroscopy. It is shown that the oxidation is mainly determined by properties of boron oxides. At temperatures over 300°C, the oxidation is accompanied by a sublimation of boron oxides formed. Traces of water and hydrogen in freshly deposited alloys affect significantly both the kinetics of the alloys oxidation and sublimation of the oxides. The vitreous oxide film well adsorbs water vapor and can be readily removed from the sample surface.     

Process Requirements for Cost-Effective Precision Grinding

Costs in precision cylindrical grinding are compared for different abrasives, machines and grinding conditions. The analysis is for repeated batch production. Account is taken of machine cost and abrasive cost. Cost comparisons were based on extensive trials to assess re-dress life against workpiece quality requirements. Experiments show that different workpiece materials require different strategies to reduce costs. Easy-to-grind AISI 52100 and difficult-to-grind Inconel 718 materials were ground at conventional speeds and at high speeds. It is shown that wheel speed affects production rate through acceptable values of re-dress life, removal rate and dwell time. Advantages were gained using vitrified CBN at conventional speed and at high speed. For both materials, vitrified CBN wheels used at high speed, gave better quality at lower cost than conventional abrasives. Wheel costs became negligible and labour costs greatly reduced. Re-dress life trials, usually neglected, are shown to be essential to reduce costs and maintain quality [1].     

Adaptive Control of Supply Chains: Building blocks and tools of an agent-based simulation framework

An adaptive control concept for supply chains is presented. Its background is a complex supply chain scenario originating from the machine building domain with a focus primarily on the development of an agentbased method concerning the adaptive coordination of customer orders along the supply chain. It aims at flexibly handling disturbances in relation to the re-allocation of alternative suppliers to ensure a timely and accurate fulfilment of customer orders. The researched and described building blocks and tools originate from artificial intelligence, decision theory and operations management, which have been implemented in an agentbased simulation framework.     

Effects of thermal and mechanical treatments on a titanium-based conversion coating for aluminium alloys

The influence of mechanical deformation and heating on the protective performance of a chromium-free conversion treatment for aluminium alloys has been assessed by polarisation and AC impedance measurements. Protection is unlikely to be seriously compromised by typical production operations.     

Streaming potential measurements on stainless steels surfaces: evidence of a gel-like layer at the steel/electrolyte interface

Electric charges at the surface of a passive stainless steel are generally considered as concentrated either in the passive film itself, or at the metal/passive film interface, or in the electrical double layer at the film solution interface. Rest potential time dependence after immersion of a passive surface in aqueous electrolytes suggests however that slow processes occur in the onset of the surface charge. Specific experiments, such as streaming potential measurements and electrochemical impedance spectroscopy in a thin electrolyte cell, were carried out for understanding better this phenomenon. An AISI 304 type austenitic stainless steel with polished or bright annealed surface finishes was immersed in NaCl aqueous solutions with various pH and chloride concentrations. The streaming potential time evolution shows two steps: a first rapid one (∼2 min) is attributed to the onset of the surface charge. The second step is much slower (approximately several hours) and possibly due to an interphase layer between the passive film and the solution. Following this idea, the whole kinetics is controlled by cation migration across the interphase when the pH is larger than the isoelectric pH (pH_iep), while chloride ions are incorporated in the interphase when pH < pH_iep. Impedance measurements allow determining both the kinetics of charge transport and the thin cell conductivity. When glass is used as reference material for the cell walls instead of stainless steel, the Nyquist plots show a high-frequency response. For stainless steel cell walls, a low-frequency response is observed, attributed to a slow charge reorganisation inside the interphase layer. The charge distribution at metal/electrolyte interface is discussed in terms of a gel-like layer which possibly takes place at the passive film/electrolyte interface.     

Effects of Minor Elements on the Cyclic-Oxidation Behavior of Commercial Fe-Base 800-Series Alloys

The cyclic-oxidation behavior of commercial iron-base 800-series chromia-scale-forming alloys was studied at 1000°C in still air. Each thermal cycle consisted of one day at temperature followed by air cooling to room temperature. Two different types of 800-series alloys were studied (800 and 800HT), with each type being supplied by two different producers. Although the alloys were of similar chemical composition, their variations in minor-constituent contents were apparently sufficiently different to cause marked differences in oxidation behavior. Compared to the 800 alloys, the higher-strength 800HT alloys generally performed the worst, undergoing rapid weight-loss kinetics after an initial period of relatively protective oxidation. The time required for the commencement of rapid weight-loss kinetics depended significantly on the producer of the 800HT alloy. In general, however, the 800HT alloys developed protrusion of Ti- and Cr-rich oxides into the alloy along the alloy/scale interface, which promoted spallation and poorer cyclic-oxidation resistance. The formation of Si-rich oxide particles in the vicinity of the alloy/scale interface could improve cyclic-oxidation resistance; however, higher Si contents would cause internal Si-rich oxide formation and promote the formation of Ti- and Cr-rich oxide protrusions at the alloy/scale interface. Good cyclic-oxidation resistance requires optimization of the minor-element constituents, Si, Ti, and Al in particular.     

Wear behaviour of thermal flame sprayed FeCr coatings on plain carbon steel substrate

The principle aim of this study is to investigate the wear behaviour of FeCr coatings on Ni-based bond deposited plain carbon steel substrate for several applications in power generation plants. For this purpose, FeCr and Ni-based powders were sprayed on plain carbon steel substrates using a thermal flame spray technique. Fabricated layers were characterized by using a X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), microhardness and surface roughness testers. FeCr coatings were subjected to sliding wear against AISI 303 stainless steel counter bodies under dry and acidic environments. A pin-on-plate type of apparatus was used with normal loads of 49 and 101 N and sliding speed of 1 Hz. XRD results revealed that FeCr, Fe, Cr, Fe–Cr–Ni, γ-Fe₂O₃ and Fe₃O₄ phases are exist in the coating. In addition, some inhomogenities such as oxides, porosity, cracks, unmelted particles and inclusions were observed by SEM. The surface morphologies of FeCr samples after wear experiments were examined by SEM and EDS. It was found that friction coefficients of the coatings in dry condition are higher than that in acidic environment.     

Austenite stability in Fe–Mn–Si-based shape memory alloys

The stability of austenite in a number of Fe–Mn–Si-based shape memory alloys has been investigated. It was found that a grain boundary precipitate of BCC structure is formed over a wide range of alloy compositions and heat treatment temperatures. This grain boundary phase has been identified as the chi (χ) phase. Although up to 3 vol.% of the grain boundary precipitate was generated by isothermal aging in the range 500–800 °C, it was found not to markedly affect the mechanical properties or the shape memory effect. Nano-indentation indicated that the hardness and strength of the parent and precipitate phase are very similar, as are their compositions.     

Phase composition and hardening of steels of the Fe-Cr-Ni-Co-Mo system with martensite-austenite structure

The phase composition and mechanical properties of maraging steels of the Fe-Cr-Ni-Co-Mo system are studied as a function of the alloying and of the temperatures of quenching and aging. The intermetallic phases strengthening martensite in different aging stages are determined. The degree of the hardening and the variation of the impact toughness at cryogenic temperatures are compared for steels with different structures (martensite and martensite-austenite) in the stages of maximum hardening and overaging. The effect of retained and reverted austenite on the resistance to crack propagation under impact loading is determined for steels with martensite of a different nature and amount of hardening phases. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 32–37, April, 2007.