Robert A. Rapp

Kinetics, Microstructures and Mechanism of Internal Oxidation - Its Effect and Prevention in High Temperature Alloy Oxidation

Abstract A review and evaluation of the role of internal oxidation in the oxidation of alloys is presented and five alloy types represented by Ag-In, Cu-Be, Ni-Cr, Nb-Zr and Cu-Zn-Al are considered...

Hot corrosion of materials: a fluxing mechanism?

Abstract Hot corrosion is the accelerated oxidation of a material at elevated temperature induced by a thin film of fused salt deposit. Fused Na 2 SO 4 , which is the dominant salt involved in hot corrosion, is an ionic conductor, so that the corrosion mechanism is certainly electrochemical in nature. Further, the acid/base nature of this oxyanion salt offers the possibility for the dissolution (fluxing) of the normally protective oxide scale. Non-protective precipitated oxide particles are often observed in the corrosion products. In this paper, the status of knowledge for the solubilities of oxides in fused Na 2 SO 4 is reviewed, and the effects of various influences on a fluxing mechanism are discussed. An evaluation of a “negative solubility gradient” as a criterion for continuing hot corrosion is made.

High temperature corrosion in energy systems

At high temperatures, particularly in response to the unique environments associated with the conversion or combustion of fossil fuels, further fundamental studies of alloy reactions with mixed gaseous oxidants are required. Thermodynamic, phase equilibria and diffusion data are lacking in part, and isotope and tracer studies have not been forthcoming. Corrosive thin films of salts and slags on the hardware of gas turbines, heat exchangers, fuel cells and batteries cause an accelerated “hot corrosion”. Thin film electrochemical studies for simple salts and alloys, and supporting thermodynamic studies (solubilities of solids and gases in salts), are required to understand the corrosion mechanism. The effects of several trace gaseous impurities (particularly chlorine) both on the growth and damaging of protective oxide scales and on the degradation of alloy mechanical properties should be studied. High resolution in situ scanning electron microscopy studies could prove fruitful in clarifying uncertain scale growth mechanisms. New protective coating compositions must be found for specific corrosive environments, and more reliable but less expensive coating methods are required. Factors critical to the adhesion of oxide scales (e.g. α-Al2O3 and Cr2O3) on alloys, and the effects of trace alloying elements or dispersed second phases on scale adherence, deserve further attention. The effect on gas-alloy attack of solid deposits, either reactive or relatively inert, should be examined. Electrochemical studies should be made of alloy corrosion in deep salt melts or slags, where the gaseous environment is remote from the alloy surface. The role of grain boundaries in corrosion product scales as short-circuit transport paths for the outward diffusion of metal and the inward ingress of oxygen, sulfur and carbon needs to be clarified. Erosion-corrosion interactions should be studied, with attempts to define the types of coatings that are most resistant to such conditions. Particularly in solar applications, the role of thermal cycling and cyclic stressing on high temperature scaling (corrosion fatigue) needs to be studied. New methods for the monitoring of the concentrations of corrosive components, particularly sulfur and chlorine, in gaseous and fused salt environments require development. The influences of temperature gradients and heat fluxes on material compatibility, redistribution of chemical components and properties of corrosion product layers need further study. High temperature corrosion-resistant alloys excluding the strategic metals chromium and cobalt need to be developed.

Mixed Conduction in Zr0.85 Ca0.15 O 1.85 and Th0.85 Y 0.15 O 1.925 Solid Electrolytes

The d‐c polarization technique according to Wagner was applied for the first time at elevated temperatures for the determination of σ and σ ⊕ in the and electrolytes over a range of oxygen activity and temperature. The applicability of the technique and an improved method for data analysis are demonstrated. Experimental difficulties and limitations are discussed. The total conductivity of was determined over a range of oxygen activity and temperature.

Whitney Award Lecture—1986: Chemistry and Electrochemistry of the Hot Corrosion of Metals

Abstract Metals and alloys may experience accelerated oxidation when their surfaces are coated by a thin film of fused salt in an oxidizing gas. This mode of attack is called hot corrosion, and the most usual or dominant salt involved is sodium sulfate because of its high thermodynamic stability. The corrosive oxyanion fused salts exhibit an acid-base chemistry and are usually ionically conducting electrolytes, so that the corrosion attack must exhibit an electrochemical mechanism with certain characteristics analogous to aqueous atmospheric corrosion. Hot corrosion may involve fluxing of the protective oxides as either acidic or basic solutes in the fused salt. The thermodynamic phase stability can be described by high-temperature Pourbaix-type diagrams, and these can be used to interpret the basicity-dependent solubilities. Measured solubilities for NiO, Co3O4, Al2O3, Fe oxides, SiO2, and Cr2O3 in fused Na2SO4 at 1200 K exhibit remarkable agreement with the expected behavior and permit the calculation o...

Chromia Scale Growth in Alloy Oxidation and the Reactive Element Effect

An analysis for the kinetics of scale growth involving simultaneous cation and anion diffusion, and for anion diffusion with blocked cation diffusion, is presented and compared to experimental data in the literature. The significant reduction in scaling kinetics for the growth of chromia on pure Cr and on Fe-, Ni-, or Co-base alloys is attributed to the elimination of cation diffusion by the blocking of the cationic reaction step at the metal/scale interface. Large highly charge reactive element (RE) ions segregate at the metal/scale interface and pin the misfit dislocations whose climb otherwise serves to create interstitial cations (or annihilate vacancies). Then scale growth must proceed by oxygen diffusion over anion vacancies, corresponding to the lateral climb of misorientation dislocation (reaction at monoatomic steps) at the metal/scale interface. This poisoned interface model provides an interpretation for the reactive element effect (REE) which is consistent with the four well-known REE characteristics.

Hot corrosion of materials: Fundamental studies

Hot corrosion is the accelerated oxidation of materials at elevated temperatures induced by a thin film of fused salt deposit. Because of its high thermodynamic stability in the mutual presence of sodium and sulfur impurities in an oxidizing gas, Na2SO4 is often found to be the dominant salt in the deposit. The corrosive oxyanion-fused salts are usually ionically conducting electrolytes that exhibit an acid/base chemistry, so that hot corrosion must occur by an electrochemical mechanism that may involve fluxing of the protective oxides. With the aid of high-temperature reference electrodes to quantify an acid/base scale, the solubilities for various metal oxides in fused Na2SO4 have been measured, and these show remarkable agreement with the theoretical expectations from the thermodynamic phase stability diagrams for the relevant Na-Metal-S-O systems. The solubilities of several oxides infused Na2SO4-NaVO3 salt solutions have also been measured and modeled. Such information is important both in evaluating the corrosion resistance of materials and in interpreting any oxide fluxing/reprecipitation mechanisms. Various electrochemical measurements have identified the S2O72− anion (dissolved SO3) as the oxidant that is reduced in the hot corrosion process. Electrochemical polarization studies have elucidated the corrosion reactions and clarified the corrosion kinetics of alloys. Mechanistic models for Type I and Type II hot corrosion are discussed briefly.

Displacement reactions in the solid state

Simple displacement reactions in the solid state are considered with the purpose of predict-ing the morphologies and the reaction rates from a knowledge of pertinent thermodynamic and diffusion data. The theoretical predictions are substantiated by experimental observa-tions for four reaction couples [Ni/Cu2O, Co/Cu2O, Fe/Cu2O, and Fe/NiO] reacted at 1000°C. Displacement reactions are classified according to the product morphology; layered and ag-gregate arrangements of the product phases were observed, with two modifications (lamellar and interwoven) occurring within the aggregate morphological class. Parabolic kinetics for the growth of the product phases are observed for each couple. The magnitudes of the para-bolic rate constants for the couples which exhibit the layered arrangement are comparable with calculated values. A technique for controlling the product morphology is discussed, and a process for producing porous metal or oxide screens is introduced.

Internal oxidation of Ag-in alloys: Stress relief and the influence of imposed strain

The kinetics of internal oxidation of silver-indium alloys containing 3.5, 5.9, and 9.8 at.% In in air at temperatures 773 to 973 K were established by TGA with no load applied to the specimens. Silver nodules free of oxide particles were observed to form at the surface during internal oxidation. The volume of these silver nodules was comparable to the total volume increase caused by internal oxidation. The alloys were also creep tested during oxidation in air at creep rates varying from 10−7 to 5×10−5 s−1 at 773, 873, and 973 K. The parabolic rate constants kp for the internal oxidation of the solute were determined from the measured widths of the internal oxidation zones. A small increase in kp was observed with increased strain rate. The large volume change associated with internal oxide formation resulted in a stress gradient between the stress-free surface and the internal oxidation front which is under a high compressive stress. Stress relief occurred by transport of silver to the surface. A Nabarro-Herring creep type mechanism based on lattice diffusion of Ag cannot account for the high rate of silver transport to the surface. Pipe-diffusion controlled creep is proposed as the mechanism of stress accommodation by silver diffusion.

In situ observation of whiskers, pyramids and pits during the high-temperature oxidation of metals

A hot-stage, environmental scanning electron microscope has been used to observe the in situ development of oxide whiskers, pyramids, and pits in the oxidation of copper and nickel at elevated temperatures. The effects of oxidation temperature, metal deformation, and the presence of water vapor on these irregular oxidation features were studied. In each case, the feature results from the presence of a central screw dislocation which provides ledges for the extension of the oxide lattice, but the specific geometries are decided by factors such as surface diffusion along the dislocation core, the rate of the molecular dissociation step, and the balance of surface energy and dislocation line tension forces.