Sterling,Ringwood, and Greenwood

The growing new research area being developed along the New York-New Jersey border at Sterling Forest recalls metallurgical history that was made in this same region. For it was here that ironmaking was centered from colonial days until after the Civil War. This is an important chapter to be added to historical ironmaking articles featured in earlier issues of Journal of Metals.     

Low-temperature oxidation

Low-temperature oxidation is a reaction, occurring at or below room temperature, between a solid and a gas. It usually involves the combination of oxygen with metals, and it has the greatest commercial impact in the presence of moisture, as in corrosion. Cabrera and Mott put forward a theory of low-temperature oxidation, based on the assumption that cation migration occurs under the influence of a potential built up across the growing oxide film. Recent experimental results require that this theory be expanded to explain recent observations such as anion migration during oxide growth and the transition from the initial chemisorbed monolayer to a bulk, threedimensional oxide. The additional ideas put forward in the present paper may be summarized as follows. Low-temperature oxidation is controlled by the nature of the oxide; whether it is a network former or a modifier. A period of fast, linear oxidation is followed by a slow logarithmic reaction whose rate, in turn, can increase if the oxide film crystallizes to form grain boundaries. The initial fast oxidation is a continuation of the chemisorption process. Place exchange (anions and cations interchanging positions) occurs when the energy due to the image force of an oxygen ion is greater than the bond energy holding the ion in place. A stable film forms when this bond energy is greater than the image force energy. The oxygen ions formed on the oxide surface then set up a potential across the film. This potential provides the driving force for continued reaction. Oxide growth during this later stage is a slow, logarithmic process. A barrier to ion transport exists at the gas-oxide interface in the case of anion migration and at the metal-oxide interface in the case of cation migration. In both cases, the field built up across the oxide lowers the barrier sufficiently so that ion migration can occur. Network modifiers allow cation migration. The reaction rate is sensitive to crystallographic orientation of the metal, but not to oxygen pressure. A constant voltage is maintained across the film, so that the Cabrera-Mott theory explains the logarithmic kinetics. Network-forming oxides allow onion migration. The number of anions, and hence, the rate of reaction, is sensitive to oxygen pressure, but not crystallographic orientation of the metal substrate. Since the potential is a result of the mobile anions, the film tends to grow under constant field. The logarithmic kinetics then must be explained by an increasing activation energy for ion transport, as proposed by Eley and Wilkinson. The logarithmic growth rate can be increased by the presence of water vapor if the water introduces dangling bonds into an oxide network structure. Crystallization of the oxide film also increases its rate of growth and results in the formation of oxide islands.     

Thermal Transformation of Transition Aluminas

JOM - This paper records conventional (CTEM) and scanning (STEM) microstructural examination of four transition aluminas (γ, δ, χ and κ) prepared from beohmite and...     

Microstructural Investigation of Protective and Non-Protective Oxides on 11% Chromium Steel

FIB, SEM and STEM/EDX were used to investigate X20 stainless-steel samples exposed to dry O₂, or O₂ containing 40% H₂O, with a flow velocity of 0.5 cm/s or 5 cm/s, for 168 hr or 336 hr at 600°C. Thin protective Cr-rich (Cr,Fe)₂O₃ was maintained on the samples exposed to dry O₂, even after 336 hr, and on the sample exposed to O₂/H₂O mixture with the low-flow velocity (0.5 cm/s) for 168 hr. The oxide scale formed in the latter environment contained less Cr, due to Cr loss through CrO₂(OH)₂ evaporation. Breakaway oxidation occurred on the samples exposed in high-gas-flow velocity for shorter time (168 hr) or in low-gas-flow velocity (0.5 cm/s) for longer time (336 hr). The breakaway scales featured a two-layered structure: an outward-growing oxide “island” consisting of almost pure hematite (α-Fe₂O₃), and an inward-growing oxide “crater” consisting of (Cr,Fe)₃O₄. The transition from a thin protective (Cr,Fe)₂O₃ scale to a non-protective thick scale on this martensitic/ferritic steel originated locally and was followed by rapid oxide growth, resulting in a thick scale that covered the whole sample surface.     

Application of optical transmission interferometry for in-situ structural investigations of titanium dioxide sputter-deposited coatings

Titanium dioxide coatings (from 0.1 to 1.5 μm thick) have been dc sputter-deposited on glass slides from titanium targets in various Ar-O₂ reactive gas mixtures. Deposition rate and optical properties were controlled in-situ by optical transmission interferometry (OTI) with an optical fibre located behind the glass substrate in order to perform a real-time control of transmittance of the growing film. Thus, it is possible to determine in-situ the optical indices (n, k) and the thickness of the as-deposited film by using a simple simulation, developed on Matlab software. The optical properties of the films were investigated in relation to their structure, which depends on the sputtering conditions adopted. In particular, the effects of the sputtering pressure (working pressure and oxygen partial pressure), the discharge power and the substrate location into the reactor are investigated in detail. Films structure is assessed by standard grazing incidence X-ray diffraction (XRD).     

Improving the heat transfer of nanofluids and nanolubricants with carbon nanotubes

Low thermal conductivity is a primary limitation in the development of energyefficient heat transfer fluids required in many industrial and commercial applications. To overcome this limitation, a new class of heat transfer fluids was developed by suspending nanoparticles and carbon nanotubes in these fluids. The resulting heat transfer nanofluids and nanolubricants possess significantly higher thermal conductivity compared to unfilled liquids. Three types of heat transfer nanofluids and nanolubricants, each containing controlled fractions of single-wall carbon nanotubes, multiwall carbon nanotubes, vapor grown carbon fibers, and amorphous carbon have been developed for multifunctional applications, based on their enhanced heat transfer and lubricity properties. For more information, contact F.D.S. Marquis, South Dakota School of Mines and Technology, Department of Materials and Metallurgical Engineering, Rapid City, SD 57701; (605) 394-1283; fax (605) 394-3369; e-mail fernand.marquis@sdsmt.edu.     

High temperature oxidation of CoMn alloys

The oxidation of cobalt-manganese alloys in the range 0–45 wt%Mn corresponding to the stability of the α-phase of cobalt has been studied in the range 750–950°C as an example of binary alloys producing solid solution scales. The alloys oxidize according to a parabolic rate law with a rate constant intermediate between those of the pure metals. The scales were formed mainly by a CoOMnO solid solution with a spinel phase present either as dispersed particles or a continuous layer in the cubic oxide, particularly for the Mn-rich alloys: some degree of internal oxidation has also been observed. The scale properties show a time dependence at constant temperature which is unusual in these systems and is probably related to the presence of the spinel phase. The internal oxidation and the structure and composition of the external scale are discussed with reference to the phase diagram for this system and to the recent theories for the growth of solid solution scales.