John Stringer

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

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 reactive element effect in high-temperature corrosion☆

Abstract When small quantities (typically 1 wt.% or less) of a number of reactive elements are added to high-temperature alloys containing chromium or aluminum, a number of beneficial effects result. This is called the “reactive element effect” (REE). Some or all aspects of the REE can be developed when the reactive element is added as an alloy addition, when it is present as a dispersion of oxide particles in the alloy, when it is ion implanted into the surface, or when a coating of the reactive element oxide is applied to the surface of the alloy. In this paper, the experimental evidence is briefly summarized, and different models that have been proposed to account for one or more aspects of the REE are assessed in terms of this evidence.

The effect of reactive element additions on the selective oxidation, growth and adhesion of chromia scales

Abstract The addition of minor amounts of reactive elements to Cr 2 O 3 or Al 2 O 3 forming alloys has been known to produce a number of beneficial effects in improving their oxidation resistance. The mechanism by which these elements influence oxidation is, however unclear. In this paper, the effect of reactive element addition, whether as surface or alloying additives, on the development, growth and adhesion of Cr 2 O 3 scales is discussed. It is pointed out that the development of a continuous external Cr 2 O 3 layer and the elimination of base metal oxidation are not coupled processes; reactive element additions can eliminate the latter, but not always affect the former. The incorporation of a reactive element in oxide scales is essential for modifying scale growth. However, a slower oxidation rate or a change in the overall oxide growth direction are not critical factors in improving scale adhesion. In light of these and other experimental results, current theories concerning the reactive element effect are critically evaluated.

Oxide scale adhesion and impurity segregation at the scale/metal interface

The chemistry at scale/metal interfaces was studied using scanning Auger microscopy after removal of the scale in ultra-high vacuum using an in situ scratching technique. Al2O3 and Cr2O3 scales formed between 900°C and 1100°C on Fe-18 wt.% Cr-5 wt.% Al and on Ni-25 wt.% Cr alloys, respectively, were investigated. The adhesion of these scales was determined qualitatively by way of micro-indentation and scratching on the surface oxide. All of the alumina scales fractured to the same degree to expose the metal surface, regardless of the oxidation temperature. The chromia-forming alloy on the other hand, developed more adherent scales at lower oxidation temperatures. About 20 at.% sulfur was found at the metal surface in all cases, and its presence was not only detected on interfacial voids, but also on areas where the scale was in contact with the alloy at temperature. Results from this study clearly demonstrated that sulfur as an alloying impurity does segregate to the scale/alloy interface. However, for alumina scales and chromia scales, the effect of this segregation on oxide adhesion is noticeably different.

The influence of ion-implanted yttrium on the selective oxidation of chromium in Co-25 wt.% Cr

Specimens of Co-25 wt.% Cr, Co-25 wt.% Cr-1 wt.% Y, and yttriumimplanted Co-25 wt.% Cr alloy were oxidized at 1000°C in 1 atm O2. The implantation dosage ranged between 1016 to 1018 ions/cm2. The unimplanted binary alloy oxidized to a duplex Co-rich scale, but the Y-containing ternary alloy formed a continuous Cr2O3 layer. When the implantation dosages were lower than a nominal 1018 ions/cm2, the alloy failed to develop a similar continuous Cr2O3 layer as that observed with the Y-containing alloy. A temporarily stable external Cr2O3 scale was formed on the most heavily implanted specimen (1×1018 Y+/cm2). This Cr2O3 scale consisted of very fine-grained oxide, which is permeable to the outward transport of Cr and Co. Internal oxidation pretreatment of the ion-implanted specimens converting the Y metal to its oxide prior to the oxidation experiment, can enhance the development of an external Cr2O3 scale, but this scale is also unstable. Results suggest that the selective oxidation of chromium in an ordinarily non-Cr2O3 -forming alloy can be due to the reactive element oxides acting as preferential nucleation sites on the alloy surfaces, but the subsequent growth of these scales may require a continuous supply of reactive elements in the alloy.

Improvements in oxidation resistance by dispersed oxide addition: Al2O3-forming alloys

The improvement in oxidation resistance produced by small additions of active elements to Al2O3-forming CoCrAl alloys is primarily dependent on the formation of oxide pegs which grow into the alloy around the internal oxide particles of the active element; void formation at the alloy-scale interface is also suppressed. The distribution of these pegs is critical and this paper demonstrates that an internal oxidation pretreatment can be used to convert the active element to its oxide in a controlled manner, thereby optimizing the peg distribution. Al2O3-forming CoCrAl containing 1% Hf or Ce is internally oxidized in a sealed quartz capsule containing a 50/50 powder mixture of CoAl-Al2O3; it was not possible to oxidize internally Y-containing alloys. The isothermal and cyclic oxidation resistance of these alloys is superior to that of alloys not given a prior treatment. Detailed metallographic examination indicates that the prior internal oxidation treatment produces a finer, more uniform distribution of oxide pegs penetrating into the alloy which is more efficient in combatting scale spallation. Furthermore, the lower residual hafnium content in the alloy minimizes large HfO2 precipitates and the formation of gross Al2O3 intrusions, which can initiate scale failure. Thus, by internally oxidizing the alloy first, the advantages of a high alloy Hf content (1%) in producing sufficient oxide pegs, but of the right size, coupled with minimal thickening of the surface scale, can be achieved.

The effect of surface-applied reactive metal oxides on the high temperature oxidation of alloys☆

Abstract The influence of surface-applied nitrate-converted CaO, CeO2, Y2O3, Y2O3, La2O3 HfO2 and ZrO2 on the oxidation behavior of Co-15wt.%Cr, Co-25wt.%Cr and Ni-25wt.%Cr alloys at 1000 and 1100 °C in O2 at 1 atm has been studied. None of the surface-applied oxides promoted Cr2O3 formation on the Co-15wt.%Cr alloy. Some (but not all) promoted Cr2O3 formation on Co-25wt.%Cr at 1000 °C and this appears to be because in these cases the surface-applied oxide acts as a physical barrier to O2. At 1100 °C, there was no apparent effect. For Ni-25wt.%Cr alloy, some of the reactive oxides had a profound effect on the oxidation behavior. The scale adhesion was greatly improved; the oxide growth direction was reversed and the growth rate of the oxide diminished. The results are compared and discussed with the effects found when the reactive element oxides are present as dispersions within the alloys.

Observations on the role of oxide scales in high-temperature erosion-corrosion of alloys

The results are presented of exposure to a controlled high-temperature erosive gas stream of a series of alloys, which were selected to represent the range of microstructures and mechanical properties available in commercial high-temperature alloys. Analysis of the kinetic and morphological data suggested that the high-temperature oxidation behavior of a given alloy plays a very important role in determining its erosion-corrosion behavior under the conditions studied. In terms of relative behavior, alloys which are weak but ductile at temperature, and which form tenacious oxide scales, exhibited the highest resistance to high-temperature erosion-corrosion. Simple models were developed to describe the expected interaction between high-temperature oxidation and erosion.

Coatings in the electricity supply industry: past, present, and opportunities for the future

The cost of electricity, the reliability of its supply, the impact on the environment, and the global sustainability, are all forcing the supply industry to make greater demands on the materials of construction of the generation, transmission, and distribution components. Not only is the performance being enhanced, but the requirements for durability, and the ability to identify potential failures, to estimate remaining life using non-intrusive methods, and to repair or replace quickly and effectively, are also subject to greater demands. Coatings play an important role in all of these aspects. In this paper, a very general summary of some of the more important issues is presented.

Role of coatings in energy-producing systems: An overview☆

Abstract Components in energy-producing systems required to operate at high temperatures may suffer a variety of degradation processes as a consequence of environment. These include oxidation, mixed oxidant attack, molten-salt-induced attack and erosion. One method of combating this attack is to use a protective coating, but there are a number of criteria that determine whether this is an economically desirable solution, as opposed to using an intrinsically more resistant alloy, redesigning the energy system or replacing the component more frequently. In this paper, aspects relating to the use of coatings are briefly reviewed.