Oxide scales thickness

After the most severe oxidation exposure of the current study (w), the oxidation layers were rare and only about 100 nm thick for A and 500 nm for B. For material B an estimate for the parabolic oxidation rate constant less than 0.0005 nf/h was calculated from the oxidation scale thickness of 500 nm (Fig.lc). The value is low when compared to parabolic rate constants for SiC in pure atmospheric oxygen at 900°C. However, direct comparison is misleading since in hot gas filters SiC is covered by the binder. This means that water needs time for diffusion through the binder before the oxidation occurs. In addition an initial oxidation layer of unknown thickness may be present. 

One example is the hot corrosion of a preoxidized nickel specimen by a thin Na2S04 melt film in a 0.1 wt. % SO2-O2 gas mixture at 1200 K [29]. By variation of the oxide scale thickness and the purity of the material, different regimes of corrosion were investigated passive state, pseudopassive state, and active state. The passive state of 99.9975% of pure nickel, preoxidized in pure O2 for 2 h at 1200 K is controlled by diffusion of 8207 in the salt melt. The corresponding Nyquist plot of impedance data shows linear behavior in the low-frequency range withaslope of45° (Fig. 16).The semicircle at higher frequencies was attributed to the resistance of the NiO layer itself The active state was established on less pure nickel... 

M. Ahrens, R. Vassen, D. Stover, Stress distributions in plasma sprayed thermal barrier coatings as a function of interface roughness and oxide scale thickness. Surface and Coatings Technology 161... 

In graphical form, the parabolic oxidation is represented by a horizontal parabola as shown in Fig. 2-7 a. The smaller kp is, the lower is the course of this parabola and the more protective is the situation for oxidation. For the experimental determination of kp, i.e., for the oxidation rate constant, the data measured as oxide scale thickness or mass gain as a function of time are plotted in a parabolic way. That is, the square of the scale thickness or the mass gain measured is plotted on the ordinate while time is plotted in a linear form on the abscissa. Fig. 2-7 b. In such a plot the oxidation kinetics... 

Figure 2-49. Schematic diagram of theoretical mass gain by oxidation and mass loss by evaporation of volatile metal chlorides. Furthermore, the measured kinetics of metal recession, oxide scale thickness growth, and overall mass change AmM are shown for oxidizing/chloridizing atmospheres (Grabke, 1991).

Here x is the oxide scale thickness, is the linear rate constant and t is the time. Scale formation which is parabolic with time obeys the following rate law and indicates diffusion through the oxide scale is rate controlling ... 

A tristationary behaviour appears in a certain temperature range. It is also revealed by the variation of the oxidized scale thickness. Independent of the stability study, the S-shaped curve gives the possibility of a hard transition between the two quasi-stationary states during the growth of the scale (see Figure 4). 

The model was applied for the oxidation of a y-Ni-27Cr-9Al alloy at 1353 K and a p02 of 20 kPa to calculate the amounts of a-Al203, Cr203 and NiO formed as well as the course of the A1 concentration in the alloy at the O/M interface as a function of oxidation time. The total oxide scale thickness dox, as obtained from the XRD experiment (see Fig. 33.4), was used as input in the model calculations. 

Total oxide scale thickness as a function of oxidation time for the oxidation of a y-Ni-27Cr-9AI alloy at p02 = 20 kPa and a temperature of 1353 K and 1443 K. For each oxidation time, the value for d x was obtained from the XRD experiments as the sum of the amounts of a-Al203, Cr203, NiO, NiCr204 and NiAl204 formed (see Section 33.3.1 for details). 

The values (as determined with XRD) for the amounts of a-Al203, Cr20s and NiO (Fig. 33.5) developed, as well as for the total oxide scale thickness (Fig. 33.4), increase very rapidly during the early stages of oxidation in accordance with the large oxide scale thickness observed after 0.2 h of oxidation (Fig. 33.8). This rapid increase in the oxide scale thickness is associated with a severe A1 and Cr depletion in the alloy near the 0/M interface [4]. As shown in Fig. 33.7, immediately after the onset of oxidation, the calculated A1 concentration in the alloy at the 0/M interface drops rapidly towards a value close to zero (in our opinion due to a very short period of exclusive a-Al203 formation at the onset of oxidation, stage I in Fig. 33.7, see Ref. [4] for details). Such a low value for the A1 concentration in the alloy at the 0/M interface was also observed experimentally after 10 min of oxidation [4]. 

Figure A.13 [1] shows the effect of steam temperatures on steam oxide scale thickness. With increasing steam temperatures, materials with an improved steam oxidation resistance have to be used for superheater and reheater tubes. Spalled steam oxide scales have the potential to plug steam flows and erode turbine components. Using high chromium content or fine grained stainless steel tubes is...

Fig. 3 Evolution of Cr concentration (in weight %) in the bulk and in the oxide scales (thickness 1 and 1.5 /tm) as a function of particle size. Selected alloy has 22 wt % Cr...

Similar calculations can be done for an oxide scale thickness of 1.5 pm. In this case, larger grains of around 80 pm size should be selected to reach a minimum of 17 wt % Cr in the bulk. It should be noted that chromia scale is the limiting conductive pathway between the functional anode and the power leads. Therefore, increasing the thickness of the oxide scale would contribute to increase cell resistance, and ultimately, to spallation of the layer. 

Where L is the oxide scale thickness (in cm), kp is the parabohc growth rate constant referring to as the oxygen mass uptake (in g cm s ), t is time (in s), p is the density of the scale (in g cm ), and 0 is the weight fraction of oxygen in the scale. A plot of the scale thickness as a function of the oxidation time for various parabolic growth rate constants is shown in Fig. 5. 

Fig. 6 Measured oxidation rate constants for selected uncoated and coated FSS [9, 10]. The grey square referred to as acceptable values set for the oxide scale thickness and parabolic oxidation rate constants after 10 kh operation. Reproduced here with kind permission from The Electrochemical Society 2009...

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