On the formation of porosity and microchannels in growing scales

It is well known that oxide scales develop porosity and microchannels that permit inward transport of molecular species from the ambient gas even under conditions when there is no evidence of cracking of the scales. It is proposed that such porosity and microchannels develop as a result of grain growth and of plastic deformation (grain-boundary sliding, diffusion creep, etc.) under compressive stresses in the scales. The presence of small amounts of impurities enriched at grain boundaries in the scales may greatly affect deformation and mechanical and transport properties in scales.     

Chromium transport through Cr2O3 scales. II. Changes in scale morphology during high vacuum treatment of oxidized chromium specimens

Chromium specimens oxidized at 1200 and 1300° O to give Cr₂O₃ scales with varying thicknesses have been high vacuum annealed for extended periods at temperature. During the high vacuum anneal chromium is transported through the scale and evaporates from the scale surface. Initially the rate of chromium evaporation decreases with time as a result of recrystallization and densification of the scale. On extended high vacuum treatment the rate of chromium evaporation again increases and major changes in scale morphology takes place. The outer scale surfaces develop hollows in the oxide grains while the grains protrude from the scale at the inner surfaces. The morphological changes are interpreted in terms of differences in diffusion rates along grain boundaries and through the lattice and resultant variations in surface energy along the surfaces.     

Carburization of Fe-Ni-Cr steels in CH4-H2 mixtures at 850 – 1000°C

Three heat resistant FeNiCr steels have been carburized in mixtures of CH₄ + H₂ in the temperature interval 850 – 1000°C. The carbon activity in the gas phase was larger than one. Under these conditions, the carburization process involved carbon deposition on the outer surface of the alloy, dissolution and diffusion of carbon into the alloy as well as carbide formation ((Cr, Fe)₇C₃ + (Cr, Fe)₂₃C₆). The rate determining step was diffusion of carbon into the alloy. Two models, which both were based on Fick's 2. law, have been applied on the results. Wagner's model for internal oxidation, which give an analytical solution to Fick's 2. law, did not fit the obtained results. With the model based on a finite difference technique, which gives a numerical solution to Fick's 2. law, it was possible to obtain a good fit. However, the kinetical and thermodynamical data used in the calculation deviate from those found in literature.     

On the oxidation of iron in CO2 + CO mixtures. III: Coupled linear parabolic kinetics

High-purity iron has been oxidized at 1000–1200° C in CO₂ and in CO₂ + CO with different compositions and at different total gas pressures (0.1–1 atm.). The experimental work has comprised thermogravimetric reaction rate measurements and characterization of the wüstite scales by metallography and x-ray diffraction. The overall results have been analyzed in terms of a classical model for coupled linear/parabolic kinetics, where it is assumed that the surface of growing wüstite scales has exactly the same defect structure and defect concentrations as that of bulk wüstite equilibrated in the same gaseous atmospheres. Important discrepancies are found between the predicted and the experimentally observed reaction behavior. Thus, both the linear and parabolic rate constants are found to be dependent on the partial pressure of CO₂ and the total gas pressure of the CO₂ + CO gas mixtures, and furthermore, the reaction in CO₂ + CO is slower than in O₂ and in H₂O + H₂ with the same oxygen activity. In order to explain the experimental results, it is suggested that CO and CO₂ molecules interact with the wüstite surface and thereby affect the defect structure and defect concentrations in a thin surface layer, and that this, in turn, affects both the linear and parabolic reaction rates.     

On the distribution of Ni3S2 in corrosion layers of NiO−Ni3S2

Qualitative studies have been made of the interfacial tension between Ni_3±xS₂ and different oxides (NiO, CoO, and feldspar). The results suggest reasons why Ni_3±xS₂ is distributed as an interconnected network along NiO grains in rapidly growing scales formed during reaction of nickel with SO₂ and SO₂+O₂ at temperatures from about 550 to 850°C. In the system NiO–Ni_3±xS₂ interfacial energies may stabilize a network of sulfide along edges of NiO grains. Ni_3±xS₂ does not wet the silicate material (feldspar), and this is consistent with previous interpretations that silicates formed by segregation of silicon to NiO interfaces disrupt the sulfide network in scales and thereby provide improved corrosion resistance.     

On the High-Temperature Oxidation of Cu-Rich Cu-Ni Alloys

Cu-2 wt.%Ni and Cu-5wt.%Ni were oxidized at 800to 1050°C and oxygen pressures from from 5 ×10^-4 to 1 atm. The oxidation, as measured bythermogravimetry, was approximately parabolic. The oxidescales could be divided in two main regions: An outerregion consisting of copper oxides and an inner porousregion, which consists of Cu₂O with dispersedNiO particles. NiO particles exists as internal-oxide particles. The interface between the two layersreflects the original surface, which shows that theouter part grows by outward Cu diffusion via vacancies.The inner part grows by outward diffusion of copper and inward transport of gaseous oxygen by thedissociative-transport mechanism. The amount ofporosity, the relative thickness of the inner layercompared to the total thickness of the scale, and theoxidation rate as a function of Ni content was dependenton the reaction conditions.     

Chromium transport through Cr2O3 scales I. On lattice diffusion of chromium

Cr specimens preoxidized at 1100–1300°C to give Cr₂O₃ scales with varying thicknesses and microstructures have been treated at temperature in high vacuum. During the high vacuum treatment the specimens lose weight due to outward Cr transport through the Cr₂O₃ scales. The initial rate of weight loss gradually diminishes, but eventually the weight loss reaches a linear rate. Concurrently the Cr₂O₃ scale exhibits grain growth and densifies. It is concluded that the mode of outward chromium transport gradually changes during the high vacuum treatment: from lattice and grain-boundary diffusion and possibly vapor transport along microcracks during the initial stage to lattice diffusion only for the densified scales. It is concluded that chromium diffuses by an interstitial type mechanism. Self-diffusion coefficients of Cr in Cr₂O₃ at the Cr-Cr₂O₃ phase boundary have been calculated from the linear rates of chromium transport for different defect structure situations.     

On Oxygen Diffusion in tetragonal zirconia

Oxygen diffusion in dense scales of tetragonal zirconia at 1100–1300°C has been studied by the so-called interruption kinetic method (or the Rosenberg method). Assuming that oxygen vacancies are the predominant defects in zirconia (ZrO₂), studies by this method provide values of the oxygen self diffusion coefficient in this oxide at its lower limit of stability, D, and the value in tetragonal zirconia can be expressed as D =2.2 · 10^?3 exp (?140(kJ/mole)/RT). The studies furthermore show that the maximum non-stoichiometry in tetragonal zirconia, X in ZrO_2?x, is small and has a value about x* ～ 0.03 at 1000–1300°C.     

Metal dusting phenomenon during carburization of FeNiCr-alloys at 850–1000°C

Three austenitic FeNiCr steels, G 4848, G 4852 and 36 XM, have been corroded in CH₄ + H₂ mixtures at 850, 900 and 1000°C. Under these conditions a new metal dusting phenomenon has been observed where closed cavities near the exposed surfaces of the alloys are filled with a powdery mixture of graphite, iron metal, chromium carbide and also in cases SiO₂. It is tentatively concluded that the powder is formed through the decomposition of (Cr, Fe)₇C₃.