Neil Birks

Some principal mechanisms in the simultaneous erosion and corrosion attack of metals at high temperatures

Abstract Combined erosion-corrosion conditions can cause especially severe degradation of materials because the erosive component often adversely affects the protective properties of the corrosion product. This paper describes the various ways that erosive and corrosive processes may in teract with one another. Depending upon the magnitudes of these components a number of regimes are described for metals and alloys. The pure erosion and pure corrosion regimes prevail when the corrosion component or the erosive component respectively is negligible. In the erosion-enhanced corrosion regime the erosion component causes the corrosion product to be less protective because of spalling, by decreasing its thickness, or by causing the transport processes through this product to be changed. The oxidation-affected erosion regime involves processes whereby oxidation products affect the erosion properties of the metals.

Forming continuous alumina scales to protect superalloys

The development of continuous Al2O3 scales on alloys via selective oxidation is an extremely effective approach to obtain high-temperature oxidation resistance. Depending on the alloy system, it may be necessary to optimize the selective oxidation process. For all alloy systems, improvements in oxidation resistance can be obtained by optimizing the type and concentrations of reactive elements and by controlling the concentration of sulfur.

Penetration of sulfur through preformed protective oxide scales

Thermodynamic assessment of sulfur penetration through otherwise protective scales such as Cr2O3, Al2O3 has been carried out for Co-Cr- and Co-Cr-Al-base alloys. Limiting conditions for sulfide formation following gas molecular transport and solution-diffusion transport have been established and the results partially confirmed by experiments carried out on Co-10Cr, Co-25Cr, and Co-10Cr-5Al alloys in sulfurous atmospheres. The results show that molecular transport of sulfurous gas species through the growing oxide scale definitely occurs. It was not possible to confirm or disprove the solutiondiffusion mechanism.

Some interactions in the erosion-oxidation of alloys

The modes of interaction of erosion and high-temperature oxidation, occurring simultaneously on an alloy surface, have been studied using Ni-30Cr, MA754, Ni-20Al, and Co-22Cr-11Al-0.2Y alloys to examine the influence of chromia and alumina scale formation on the erosion of nickel and cobalt base alloys. The results have shown that, in the presence of a rapidly flowing oxidizing gas stream, the evaporation of volatile metal oxides becomes important at lower temperatures where normally it can be ignored. Otherwise, the erosion and oxidation of alloys parallels the behavior of pure metals but also introduces additional factors derived from lengthening of the period of transient oxidation and modification of concentration profiles in the alloy adjacent to the alloy/scale interface. Higher erosion intensities extend the transient oxidation behavior which adversely affects the formation of protective scales. As with pure metals, the presence of erosion and oxidation together always produced increased rates of degradation.

Interaction between erosion and high-temperature corrosion of metals: The erosion-affected oxidation regime

The combined erosion-oxidation of nickel, cobalt, and the oxides of these metals have been studied at 780°C in air to examine two regimes of interactionnamely, a regime of erosion-enhanced oxidation during which an oxide scale of constant thickness covers metal specimens, and a regime of oxidation-affected erosion that is characterized by a composite surface layer of metal, oxide, and erodent. In the case of cobalt, these two regimes have been documented and the transition from one regime to another described. For the range of conditions examined, only the oxidation-affected erosion regime was observed for nickel due to its lower oxidation rate compared to cobalt.

Effect of angle of incidence on the combined erosion-oxidation attack of nickel and cobalt

The combined erosion-oxidation of cobalt and nickel at 600 and 780°C over a range of particle-impact angles has been studied by using weight change vs. time measurements and extensive morphological and microstructural characterization of exposed specimens. The oxidation process has been found to affect the angular dependence of the degradation rates. The effect is especially significant at low-impact angles where ripples and flakes are developed upon the surfaces of specimens.

High-temperature corrosion resistance

The approaches which are currently used to develop high-temperature corrosion resistance in alloys are briefly described by considering oxidation, mixed gas, and hot corrosion degradation processes. These approaches are compared to those used to develop high temperature corrosion resistance in ceramics, and future trends that may be expected to be followed to obtain high temperature corrosion resistance are also reviewed.

Spalling types and mechanisms in the erosion corrosion of metals at high temperature

Abstract Spalling of scales has been observed during the isothermal erosion-oxidation attack of several metals. Some typical results are presented, showing that such spalling can arise from more than one mechanism.

Environmental effects during application of materials at temperatures above 1200 °C

Abstract The materials which may have properties for use at temperatures above 1200 °C are examined and shown to be some refractory metal alloys, a few intermetallics, a number of ceramics, and composites. It is shown that the most important form of environmentally induced degradation of these materials at temperatures above 1200 °C is oxidation. The oxidation of each of these types of material is then examined. It is shown that, in order for materials to possess adequate oxidation resistance at temperatures above 1200 °C, external scales of Al 2 O 3 or SiO 2 must be formed as reaction products.

The development of surface features on nickel during erosion-corrosion

Abstract The bombardment of metal surfaces under conditions of erosion, or corrosion-affected erosion where the corrosion component is small compared with erosion, leads to substantial physical rearrangement of that surface. It has been found that impingement normal to the surface induces a random pattern of hills and valleys, whereas impingement at an angle to the surface produces a pattern of ripples, aligned across the direction of the erosive stream. Recently, it has been found that, after long times at high temperature, a hill and valley pattern produced by normal erosive impact transforms into a ripple pattern by alignment of the hills and valleys in each metal grain. In this, the individual grains show patterns of ripples that lie at different orientations in the different grains, suggesting that this is determined by the slip system of the crystal. Currently, this phenomenon is being studied using polycrystalline nickel and a nickel bicrystal in order to determine the mechanism by which the ripple pattern evolves and which determines the orientation of the ripples.