Alloys parabolic corrosion

Figure 11. Parabolic corrosion of experimental aluminum alloy A288 (Al 1% Ni,0.5% Fe,0.1% Ti) in stagnant water at 260°C (9)...

Measuring the time and temperature dependencies in independent experiments gives the values for x, a, and k needed to perform a double extrapolation (time and temperature) to obtain end-of-life corrosion penetration. Limitations in available datasets can lead to considerable uncertainties associated with such extrapolations. While parabolic kinetics is often assumed, this assumption must be carefully assessed. A significant amount of data suggests that Alloy 800H corrosion in superheated steam follows linear kinetics (e.g.. Pearl et al. [65]). Brush [66] developed a model for the corrosion kinetics (Eq. (4.3)) that consisted of a constant term and a hnear term ... 

Mrowec et examined the resistance to high-temperature corrosion of Fe alloys with Cr contents between 0.35 and 74 at% Cr in 101 kPa S vapour. They found that the corrosion was parabolic, irrespective of the temperature or alloy composition, and noted that sulphidation takes place at a rate five orders of magnitude greater than oxidation at equivalent temperatures. At less than 2% Cr, the alloys formed Fe, j.,S growing by outward diffusion of Fe ions, with traces of FeCr2S4 near the metal core. 

Titanium is susceptible to pitting and crevice corrosion in aqueous chloride environments. The area of susceptibility for several alloys is shown in Figure 7 as a function of temperature and pH. The susceptibility depends on pH. The susceptibility temperature increases parabolically from 65°C as pH is increased from zero. After the incorporation of noble-metal additions such as in ASTM Grades 7 or 12, crevice corrosion attack is not observed above pH 2 until ca 270°C. Noble alloying elements shift the equilibrium potential into the passive region where a protective film is formed and maintained. 

Sulfidation tests of the different alloys were carried out over the temperature range 750-950°C. Plots of the square of mass gain vs. time in different H2-H2S atmospheres at 900°C are shown in Fig. la,b,c.The kinetics of Ni36Al and Ni45Al follow the parabolic rate law with rather high corrosion rates. In gas mixtures containing less than 0.5 vol.% H2S no sulfidation but alumina growth was observed. 

In petroleum refineries, process streams containing hydrogen also frequently contain hydrogen sulfide. This causes sulfidic corrosion. You know from experience that increasing the chromium content of a steel increases its resistance to corrosion by high-sulfur crudes. However, do not jump to the conclusion that chromium alloying always improves resistance to sulfidic corrosion. It does so if the operation is dirty, as it usually is in crude streams, or if the corrodents are elemental sulfur or sulfur compounds that do not decompose to release hydrogen sulfide. This increased resistance to sulfur corrosion depends on formation of a protective scale. With such scales, the corrosion rate is parabolic — it decreases with exposure time. 

The corrosion product plays an important role in the corrosion resistance of zinc, particularly in aqueous and certain atmospheric environments (cf. low-alloy or weathering steels). In such cases, the corrosion kinetics may be parabolic in nature, the rate decreasing with time (cf. aluminum), an aspect that has been extensively investigated. Taylor and Tolley [184] have examined the characteristics of sprayed zinc coatings and concluded that the decrease in permeability was... 

In Fig. 11, a laboratory apparatus to test coupons of metallic alloys and ceramics is presented. The coupons are coated periodically with thin deposits of Na2S04 or a Na2S04"NaV03 mixture, and the gas composition is controlled to maintain the stability of the deposit [32], The specimens are removed at selected time intervals to examine the corrosion. Typical results obtained for AI2O3 [32,33] are presented in Figs. 12 and 13, showing a porous zone that developed in accordance with a parabolic rate. 

If an alloy is to have acceptable resistance against high-temperature corrosion, it must react with the environment to form a continuous and adherent slow-growing scale which has sufficient mechanical properties to withstand the effects of both growth and thermal stresses. As discussed in Chapter 3 of this volume, ideal protective scale growth obeys diffusion-controlled, parabolic kinetics, i.e. ... 

Extensive testing of alloys has shown that many alloys establish parabolic time dependence after a minimum time of 1000 hours in air at temperatures above 900°C. If the surface corrosion product (scale) is removed or cracked so that the underlying metal is exposed to the gas, the rate of oxidation can be much faster. The influence of partial pressure on oxidation above 900°C is specific to each alloy, as illustrated for some common alloys in Fig. 15.17. Most alloys do not show a strong influence of the concentration upon the total penetration. Alloys such as Alloy HR-120, and Alloy 214 even exhibit slower oxidation rates as the concentration increases. These alloys are rich... 

Let us now consider a maximum acceptable scale thickness of 1 pm after 10 kh operation. This thickness is expected to avoid spallation of the oxide scale and to prevent corrosion of the metaUic conduction paths between the grains. As shown in Fig. 5, the alloy support should have an oxidation rate constant lower than 10 g cm s . The same plot can also be used to conversely predict the expected lifetime of the support by knowing the parabohc rate constant of the alloy. One should notice that these calculations are based on an ideal parabolic oxidation behavior of the alloy. Some deviations may be expected due to Cr evaporation in air from the scale, which is enhanced in the presence of steam. This linear evaporation term is not introduced in Eq. (1). 

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