Some of the important principles that determine the establishment, growth and long-term maintenance of protective Cr2O3, Al2O3 and SiO2 scales on hightemperature iron-, nickel- and cobalt-base alloys are reviewed and discussed. Emphasis is placed on the effects of alloying elements and other additions, such as third elements and reactive elements or oxide dispersions, on each of these processes. Particular attention is paid to transport processes in the scales and the importance of short-circuit paths. Some of the important parameters that influence the long-term mechanical stability of such scales are considered and evaluated.

The influence of alloying elements on the development and maintenance of protective scales

The oxidation behaviour of metals and alloys at high temperatures in atmospheres containing water vapour: A review

https://eprints.ncl.ac.uk/file_store/production/56373/74E3E1A7-8D83-4218-8DA6-EDC480E11A4F.pdf

An overview of the potential of quantitative coating adhesion measurement by scratch testing

Failure mode maps in the thin film scratch adhesion test

Currentunderstanding of the complex interrelationships among growth kinetics, microstructure, and adhesion of protective Cr2O3 and Al2O3 scales is critically reviewed. Similarities and differences in the behavior of these two systems are highlighted. The morphology of the alloy-scale interface appears to be a critical factor. Recent ideas are advanced to interpret the effect of oxygen-active elements on the development of a tortuous interface conducive to improved scale tenacity.

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Microstructure, adhesion and growth kinetics of protective scales on metals and alloys

In this review the role of stress in the high temperature oxidation of metals is examined. In the introductory sections the Wagner theory of oxide growth and its modification to account for short circuit diffusion are outlined. The direct influence of oxide growth stresses on defect diffusion is then examined and shown to be a significant factor when constrained volume changes accompany the growth of the oxide layer. Methods for evaluating growth stresses are critically reviewed and the possible origins of such stresses considered in detail. The response of the oxide layer to thermally and mechanically induced stresses is considered in the last part of the review. Under tensile conditions, through-thickness cracks can readily be produced but may also heal with fresh oxide if imposed strain rates are relatively low. Spallation of the oxide is not usually found under tensile conditions but occurs frequently under compressive stresses by either a ‘buckling’ or ‘wedginq’ process. Detailed descriptions...

Stress effects in high temperature oxidation of metals

Conditions for the initiation of oxide-scale cracking and spallation

Smart overlay coatings are a functionally gradient coating system designed to provide high temperature corrosion protection over a wide range of operating conditions. The SMARTCOAT design consists of a MCrAlY base, enriched first in chromium, then aluminium to provide a chemically graded structure. At elevated temperatures, above 900°C (1650°F), the coating oxidises to form a protective alumina scale. However, at lower temperatures this alumina scale does not reform rapidly enough to confer protection under type II hot corrosion conditions. The coating is therefore designed with an intermediate chromium-rich interlayer, which permits the rapid formation of chromia healing areas of type II corrosion damage. Laboratory and burner rig tests have been carried out on a series of developmental smart overlay coatings. These have shown that the development of an intermediate chromium-rich phase provides protection under low temperature hot corrosion conditions, while the aluminium-rich surface layer provides resistance to high temperature oxidation and type I hot corrosion. Thus, the single application of SMARTCOAT permits operation over a broad range of industrial and marine turbine conditions.

/pdf/smart-overlay-coatings-concept-and-practice-1e0kfq9rin.pdf

Smart overlay coatings: concept and practice

Oxidation tests in a CO2-based atmosphere at 1123 K have been conducted on a series of 20Cr-25Ni stainless steels containing a dispersion of TiN particles and with silicon contents ranging from 0.05 to 2.35 wt.%. In all cases, a chromia surface film whose thickness increased parabolically with time was formed. The parabolic rate constant was a minimum at an intermediate alloy silicon content when an interlayer of silica was formed between the outer oxide and the metal. The significance of this duplex structure on the processes determining the oxidation rate and the extent of oxide spalling is discussed. An additional feature of the attack was the formation of subsurface voidage whose depth again increased parabolically with time. The appropriate rate constant also showed a minimum at an intermediate silicon level and the relationships between this form of attack and the development of the surface oxides are highlighted.

Influence of silicon additions on the oxidation resistance of a stainless steel

It is proposed that bond coats in thermal-barrier coating (TBC) systems, particularly those deposited by plasma spraying, can contain regions which are diffusionally isolated from the bulk of the coating This can arise through the internal formation of alumina layers as a consequence of the ingress of molecular oxygen into the relatively porous structure Such isolated regions, termed diffusion cells, will experience enhanced depletion of aluminum as a result of the continued thickening of the alumina layer at their surface This process has been demonstrated for a CoNiCrAlY bond coat after oxidation in air at 1100°C A consequence of this enhanced depletion is that chemical failure will occur sooner in diffusion cells and voluminous breakaway oxides will form above them at the interface of the bond coat and the ceramic top coat The associated spatial variation in oxidation and displacement rates across the surface of the bond coat are expected to aid delamination of the outer ceramic layer

Hugh Evans

Papers

Stress effects in high temperature oxidation of metals

Conditions for the initiation of oxide-scale cracking and spallation

Smart overlay coatings: concept and practice

Influence of silicon additions on the oxidation resistance of a stainless steel

Diffusion Cells and Chemical Failure of MCrAlY Bond Coats in Thermal-Barrier Coating Systems