The significance of velocity exponents in identifying erosion-corrosion mechanisms

The modes of erosion-corrosion are diverse and may vary from being ''erosion-dominated'', where erosion of metal is the dominant process, to ''corrosion-dominated'', where erosion of oxide scale is the dominant process. The intermediate situation in which erosion of transient oxide is the predominant process is termed ''erosion-corrosion-dominated'' and describes the regime in which continual formation and removal of oxide occurs down to the scale/metal interface, This paper considers some of the recent erosion-corrosion data and evaluates the velocity exponents. The critical factors which affect velocity exponents in these environments are identified, and some general principles and provisos are outlined when attempting to use such a technique to identify the mechanism of erosion-corrosion on the material surface. It is shown that the velocity exponents derived for ''erosion-dominated'' conditions are similar to those evaluated for ''ductile'' erosion processes. However, for ''corrosion-dominated'' conditions the exponents are significantly lower than those derived for ''brittle'' erosion processes at room temperature. For ''erosion-corrosion-dominated'' conditions the situation is more complex with velocity exponents being strongly dependent on temperature, alloy composition and relationship between velocity and particle flux. It is concluded that velocity exponents may be used only in very specific cases to identify erosion-corrosion mechanisms as the relationship between erosion-corrosion rate and velocity is complex and is a function of a wide range of parameters

Inhibition of metal dusting of alloy 800H by laser surface melting

Metal dusting is a catastrophic carburisation phenomenon that occurs at temperatures of 450-850°C in atmospheres of high carbon activity. The resistance of alloys to corrosion, including metal dusting, relies on the formation of a dense, adherent oxide layer that separates the alloy from the corrosive environment. For such an oxide layer to be protective, it must achieve full surface coverage, be crack-free and be established before significant material degradation has occurred. Formation of a protective oxide scale can be enhanced by increasing the population of rapid diffusion paths for the protective elements (e.g. Cr and Al) to reach the alloy surface. In this work, laser surface melting has been used to improve the metal dusting resistance of Alloy 800H by creating a rapid solidification microstructure and, thereby, increasing the density of rapid diffusion paths. Oxidation during laser processing has been found to be detrimental to metal dusting resistance. However, it has been demonstrated that the resulting oxide can be removed without compromising metal dusting resistance. Results of exposure to a metal dusting atmosphere (20% H2 80% CO at 650°C) are presented. Samples have been examined in plan and cross-section using optical and scanning electron microscopy. Selected samples were also examined by electron probe microanalysis and X-ray diffraction

Inhibition of metal dusting using thermal spray coatings and laser treatment

Alloy 600 and Alloy 800H are susceptible to metal dusting. Both alloys were thermally sprayed with two different corrosion resistant coatings: Ni50Cr and Ni31Cr11Al0.6Y. Laser remelting was used to enhance further the effectiveness of these coatings to resist metal dusting by eliminating interconnected porosity and improving coating adhesion. Uncoated, coated and laser-treated coated samples of Alloy 600 and Alloy 800H were exposed to a mixed gas atmosphere (20% H2, 80% CO at 650°C). Samples were examined in plan and cross-section using optical and scanning electron microscopy, electron probe microanalysis and X-ray diffraction. The extent of carbon deposition was tracked by mass difference measurements at intervals during exposure. The thermally sprayed coatings enhanced metal dusting resistance by acting as physical barriers to carbon ingress. The NiCrAlY coating performed well on both substrates. The NiCr coating itself underwent metal dusting and spalled from Alloy 800H due partly to CTE mis-match stresses. Laser treatment of both coatings successfully eliminated interconnected porosity and hence enhanced metal dusting resistance

Accounting for nature: assessing habitats in the UK countryside.

Countryside Survey 2000 (CS2000) and the Northern Ireland Countryside Survey 2000 (NICS2000) have been designed to provide detailed information about the habitats and landscape features that are important elements of our countryside. They can tell us about the ‘stock’ of these resources, that is how much of them we have and where they are to be found, and they can give us an insight into their condition based on the variety and abundance of the plant species associated with them. Using information from previous surveys, we can also gain an understanding of how the stock and condition of these habitats and landscape features are changing over time. We can build up a sort of balance sheet or an account of natural assets in the UK countryside. In this report we look in particular at the period between the last two surveys, 1990 and 1998

Mechanisms of scale formation and subsequent annealing on some nickel-, copper-, and cobalt base alloys

2 volsAvailable from British Library Document Supply Centre- DSC:D76666 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

Computer simulation of the effect of pre-oxidation in erosion-corrosion environments

The combined effects of erosion and corrosion can lead to a number of material wastage scenarios in industrial environments. In some cases, for example at high temperatures in fluidized bed environments, the wear may be corrosion dominated where chipping of an oxide scale is the predominant process, while in more aggressive environments such as gas turbine conditions, formation of a protective scale may never occur. Thus, pre-oxidation to form a protective scale or coating of a substrate may be effective in reducing material loss in corrosion-dominated environments since particle energies may be low enough to prevent significant deformation of the underlying surfaces. A computer model has been developed to simulate the erosion of a preformed scale or a coating on a metal substrate. This model uses computer graphics to demonstrate the sequential stages of erosion-corrosion. By incorporating established erosion and corrosion algorithms to calculate the wastage rate, the surface topography and extent of wear can be shown after exposure to erosion-corrosion conditions of varying severity. Thus, one can extrapolate from this technique whether erosion of oxide or metal is likely to be dominant wastage mechanism. This model was developed to describe the erosion-corrosion of a preformed oxide scale on a metal and comparisons are made between the erosion results for pre-oxidized surfaces and the graphics after computer simulation of erosion-corrosion. However, since the factors which determine adhesion of a preformed scale may be similar in many respects to those of a coating, the model may equally describe the erosion-corrosion behaviour of a coated metal. Hence, the paper shows the stages of the model development and compares the experimental results with those of the model

Identification of transitions through erosion-corrosion regimes using erosion-corrosion maps

Paper identifying transitions through erosion-corrosion regimes using erosion-corrosion maps