This review deals with the procedures used in the anodic oxidation of titanium and its alloys, the nature and properties of the oxide films, their uses, and the trends in research and development.

Anodic oxidation of titanium and its alloys

The focus of this review is the state of knowledge of the oxidation behaviour in steam of alloys with potential for use as pressure parts in steam boilers. The rate of oxide growth on steam-touched surfaces and the characteristics of that oxide are of increasing interest as the quest for improvements in cycle efficiency leads to progressively higher operating temperatures and pressures. The consequences of increased rate of growth of these oxides are of concern because of implications for tube overheating and oxide exfoliation. Mitigation of such problems requires a mechanistic understanding of the influences of alloy composition and microstructure, and especially of the evolution with time of specific scale structures. Similarly, the relative effects of factors such as time, temperature and operating parameters must be understood. The oxidation behaviour of the class of ferritic steels that forms the bulk of the heat transfer surface in steam boilers is of particular importance since allo...

A review of the oxidation behaviour of structural alloys in steam

Crystallization in anodic oxide films

Several Fe-Cr-Ni austenitic stainless steels (Cr wt.%: 13–25, Ni wt.%: 15) were oxidized in steam for 1000 hr at 500–900°C. The oxide scales were examined and categorized with respect to the chromium concentration and the grain size of the base metal. Experiments showed three conditions for the critical bulk Cr concentration and the oxidation temperature at which the oxidation behavior changed drastically. Metallographic examination showed that two of these three conditions resulted from the internal-external transition of Cr2O3 either on the metal surface or along the grain boundaries of the base metal. Attempts were made to interpret these conditions from the available oxidation theories. Atkinson's treatment was employed with some modification to incorporate the grain-boundary diffusion of Cr in the base metal. The calculation basically explained the internal-external transition for the oxidation of these steels.

Internal-external transition for the oxidation of Fe-Cr-Ni austenitic stainless steels in steam

The influence of superficially applied CeO2 coatings on the isothermal-oxidation behavior, preceded by nonisothermal heating, as well as cyclic-oxidation behavior of three grades of austenitic stainless steel (AISI-316, −321, and −304), in dry air is reported. The superficial coating had a thickness of 2.1 μm. The linear heating rate employed was 6 K min−1 up to a maximum temperature of 1423 K, and the isothermal holding temperature was 1273 K. The results clearly depict that CeO2 coatings not only reduced the rates of scale growth for all three varieties of steel but also imparted improved scale adhesion to the respective alloy substrates, as evident from the fact that the coated steels could withstand a number of thermal cycles without scale rupture. In the bare condition, 321-grade steel exhibited the best performance. However, in the presence of the coating, the improved performances of 316 and 321 grades were almost identical, whereas the 304 variety showed improvement only in the first cycle of exposure. The kinetics results have been substantiated by postoxidation analyses of the alloy/scale combinations by SEM, EDS, EPMA, and XRD techniques to reveal the role of rare-earthoxide coatings on the observed behavior.

Improvement in the oxidation behavior of austenitic stainless steels by superficially applied, cerium oxide coatings

The corrosion behavior of the 18‐8 stainless steels and some nickel alloys in superheated steam was found to be dependent on the surface preparation. Those treatments which left a severely cold worked surface resulted in reduced corrosion attack in subsequent exposure to steam as compared with annealed metal surfaces. The corrosion rate of electropolished annealed type 304 stainless steel in oxygenated (30 ppm) steam rose rapidly as the temperature increased from 540° to 650°C. At 600° and 650° the measured rate decreased with increasing time during 80 day static tests. The corrosion appeared more linear with time in dynamic tests at 650°C (∼7 mg/dm2‐day). In these tests there appeared to be no effect of velocity (30–91 m/sec) or oxygen and hydrogen content on the corrosion rate. Flaking and loss of the outer corrosion coating was severe in the dynamic tests. The corrosion rate of type 406 was too small to be precisely measured in all of the tests, static and dynamic, in which it was simultaneously exposed with type 304. A coating equivalent to about metal penetration was formed on the initial exposure of 406. No flaking of the outer coating was evident in 80 day tests. The corrosion attack of the 400 series steels in static oxygenated steam went down with increasing chromium content, particularly above about 13 w/o Cr. Both type 430 (16 w/o Cr) and 446 (25 w/o Cr) exhibited greater short term corrosion resistance than type 304 to 650 °C, 42 kg/cm2 steam. The nickel based alloys, particularly Inconel 625, were also more corrosion resistant than type 304 in short static and dynamic tests in 650 °C, 42 kg/cm2, superheated steam.

W. E. Ruther

Papers

Corrosion of Steels and Nickel Alloys in Superheated Steam

The Anodizing of Zirconium and Other Transition Metals in Nitric Acid