High Temperature Oxidation And Corrosion Of Metals Pdf
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- Introduction to the high-temperature oxidation of metals
- High‐Temperature Oxidation of Metals
- ISO 21608:2012
- Heat resistant Alloys
The fundamentals of high temperature oxidation and corrosion of metals and alloys are discussed on thermodynamic, kinetic and morphological points of view. Special attention is paid to the compacity of corrosion scales, the nature of the diffusing defects and the location of the slow process es.
High-temperature corrosion is a mechanism of corrosion that takes place in gas turbines , diesel engines , furnaces or other machinery coming in contact with hot gas containing certain contaminants. Fuel sometimes contains vanadium compounds or sulfates which can form compounds during combustion having a low melting point. These liquid melted salts are strongly corrosive for stainless steel and other alloys normally inert against the corrosion and high temperatures. Other high-temperature corrosions include high-temperature oxidation ,  sulfidation and carbonization.
Introduction to the high-temperature oxidation of metals
An improved understanding of high-temperature alloy oxidation is key to the design of structural materials for next-generation energy conversion technologies. An often overlooked, yet fundamental aspect of this oxidation process concerns the fate of the metal vacancies created when metal atoms are ionized and enter the growing oxide layer. In this work, we provide direct experimental evidence showing that these metal vacancies can be inseparably linked to the oxidation process beginning at the very early stages.
The simultaneous and subsequent oxidation of these regions fills the vacated space and promotes adhesion between the growing oxide and the alloy substrate. These structural transformations represent an important deviation from conventional metal oxidation theory, and this improved understanding will aid in the development of new structural alloys with enhanced oxidation resistance. The oxidation behavior of structural alloys at high temperatures is central to the function of power plants, aircrafts, and many other high-temperature applications.
If the vacancies are not all so annihilated, then they may be transferred into the underlying alloy in a process known as vacancy injection. Although initially a controversial topic, 4 it is now well established that vacancy injection occurs during high-temperature oxidation in many alloy systems. This is because considerable differences exist for the two systems, most notably the boundary conditions for diffusion.
In general, attempts at quantifying the extent of vacancy condensation in systems that resemble an oxidizing structural alloy have shown that it accounts for only a small fraction of the vacancies theoretically generated during oxidation. The extent of vacancy condensation has important implications related to the growth stresses associated with oxidation.
For example, volume expansion that accompanies internal oxidation of an alloy matrix or of secondary phases such as metal carbides 18 is generally assumed to create stresses in the alloy.
However, no such stresses need arise if this oxidation serves to fill the space created by vacancy condensation. Thus, the volume change associated with oxidation relative to that made available by vacancy condensation is an important parameter in considering the stability of a growing oxide layer. Difficulties in investigating the behavior of injected metal vacancies is partly related to the short time- and length-scales at which vacancy formation and migration occur.
This is especially true of the early stages of oxidation, which can be difficult to capture experimentally, and requires instrumentation capable of resolving and visualizing structural and compositional changes approaching the atomic scale. In this contribution, we present insights into this behavior by assessing the surface of a structural alloy during the early stages of oxidation.
Alloy was selected for the study, as representative of a class of solution-strengthened Ni superalloys, which are used for many applications requiring exceptional mechanical strength and oxidation resistance at high temperatures.
Following exposure details provided in the methods section below , the material was analyzed by cross-sectional scanning transmission electron microscopy STEM using a high-angle annular dark-field HAADF detector.
From bottom to top, the image shows the alloy substrate, the oxide formed during exposure, and a layer of Pt deposited on the sample surface to prevent ion beam damage during preparation of the specimen. Thus, the alloy substrate appears lighter in contrast while the oxide appears darker. The origin of this contrast variation is revisited below. Although the majority of the oxidized surface appeared similar to that shown Fig.
This suggests these areas are voids that have formed during oxidation. Figure 2 also reveals that the oxide layer is Cr-rich and contains no Ni. STEM cross-sectional analysis of the oxidized surface.
A small amount of material is also observed throughout the dark contrast region adjacent to the voids indicated with dashed arrows in Fig. This Al-oxide network is dispersed throughout an otherwise voided zone, and together these features comprise the region of dark contrast observed in Fig. It should be noted that O observed in the alloy substrate Fig.
This also has the effect of diluting the O signal arising from the porous Al-oxide network, as the maps are presented as non-normalized raw counts see methods section below. Surprisingly, the same regions are also seen to contain Ni. We showed previously that Ni in the sample was present nearly exclusively in the metallic state. An additional feature of interest is also observed: a portion of the oxide layer is seen to protrude downward into the alloy substrate near the middle of Fig.
Although this could be related to a microstructural feature or groove present at the sample surface prior to oxidation, analysis of an unexposed sample 22 revealed a very smooth surface, suggesting that the oxide has in fact grown inward in this region.
Further, a region of darker contrast defined by the white dashed circle in Fig. The same region corresponds to low X-ray counts for all elements, suggesting it is lower in density.
Further, artifacts created during ion beam thinning of the specimen which could, for example, preferentially attack certain regions of the sample resulting in density variations cannot be ruled out. Because Ni in the alloy has participated only minimally in the oxidation process, 22 the distribution of Ni ions can be used to visualize what remains of the alloy substrate. Further, an inward-growing oxide that contains both Al and Cr extends to the bottom of the Cr-depleted region, similar to that observed by STEM analysis Fig.
It should be noted that trajectory aberrations arising from differences in evaporation field can lead to spatial overlap and therefore to artificial density variations in the APT reconstruction that are not characteristic of actual density variations in the material. Therefore, it is not possible to quantify density variations using APT. To gain insight into the low-density regions and to visualize their relationship with the surrounding oxidation processes in three dimensions, iso-composition surfaces sometimes referred to as iso-concentration surfaces were generated in a nm thick slice of the same region of APT reconstruction as shown in Fig.
In addition to the single low-density region observable in the ion maps Fig. An iso-density surface 26 of Ni for this same volume shown in Fig. Thus, regions consisting of residual alloy and Al-oxide clusters are observed in the vicinity of low-density regions in the substrate.
The location of the iso-composition surfaces in the alloy substrate immediately below the oxide layer and the co-existence of Ni-metal and Al-oxide in the same region, is in good agreement with STEM analysis Fig. The profile shows the detected total number of atoms and the number of Ni, O, Al, and Cr atoms as a function of distance in the volume defined by the 3-nm diameter cylinder a 1-nm bin width was used.
Error bars represent the square root of the number of detected atoms. To further investigate the relationship between the low-density and adjacent oxidized regions, a one-dimensional atom count profile was obtained through the inward-growing oxide region as shown in Fig. Ions present in the volume defined by the 3-nm diameter cylinder shown in Fig. Thus, the profile presents the number of detected atoms of each element as a function of position.
As described above, this analysis does not provide a quantitative assessment of the density, however qualitative variations are revealed. For clarity, Ni ions are shown, whereas the other relatively high-concentration alloying elements that participated minimally in the oxidation process Co and Mo are omitted. These elements followed the Ni trace at values close to what is expected for the bulk alloy shown in Fig.
S3 of the supplementary information and thus the Ni trace is representative of what remains of the alloy substrate following oxidation. As with the iso-composition surfaces Fig. On both sides of the oxidized region, the total number of atoms decreases considerably. This is in part owing to the lack of oxide in these regions. There is no compensating increase in the number of other atoms in these regions. This confirms the existence of regions of decreased density within the partially oxidized surface of the residual alloy substrate.
As mentioned above, possible reconstruction artifacts must be considered when interpreting the APT results. Specifically, the lower evaporation field of the oxidized regions could lead to artificial density decreases in adjacent metal regions. Despite this, several features and their consistency with STEM observations point to the existence of real density variations in the material.
In addition, as the degree of overlap depends on the difference in evaporation field, one would expect the composition profile Fig. On the contrary, the primary alloying elements Ni, Co, and Mo are observed in the low-density regions at ratios close to what is expected for the bulk alloy Fig. Although the complex nature of the sample does complicate this observation, it is still further evidence that spatial overlaps have not contributed significantly in these regions.
In short, although the precise density variation and spatial distribution of such regions cannot be determined, these observations cannot be explained by artifacts alone and do suggest real density decreases in the material.
This process is illustrated schematically in Fig. Many of these vacancies are not annihilated, but instead coalesce to form vacancy clusters, leading to low-density regions near the surface of the alloy substrate.
Such molecular diffusion might occur along grain boundaries, 33 or through transient nanopores, which form preferentially at the grain boundaries. The oxide formed by this inward growth fills some of the space made available by the departing metal atoms, and this results in regions of low-density that contain both Ni-metal and Al-oxide on the near-atomic scale Figs 3 and 5. The above processes likely occur in parallel—as vacancies coalesce, Al is oxidized at the free surfaces formed in the alloy, and new Al-oxide in turn provides interfacial sinks for continued coalescence of vacancies.
This results in the accumulation of vacancies and thus decrease in density near the oxidized regions, as observed in Fig.
As vacancies continue to coalesce and Ni and other elements not participating in oxidation diffuse down into the alloy substrate, voids that are large enough to observe in the TEM cross-section are eventually formed adjacent to the Al-oxide.
This results ultimately in an Al-oxide network inside an otherwise voided region—precisely what is observed in Fig.
This confirms that these features are related to the oxidation process, and provides further evidence that our results have not arisen simply from artifacts inherent to TEM and APT analysis. Finally, this suggests the mechanism proposed here persists for much longer exposure times, and is thus highly relevant to the long-term stability of the alloy. By the processes described above, voids or other low-density regions can become incorporated into the growing oxide layer.
Simultaneous internal and external oxidation of Cr is not expected for these conditions 38 and thus these regions cannot be explained simply by an internal oxidation process.
These perhaps originated as regions of low-density, which were subsequently filled by the inward diffusion of oxygen. Because regions of vacancy accumulation become at least partially filled with oxide, it is difficult to determine by the ex situ analyses shown here precisely what portion of the vacancies have participated. However, the extent of the subsurface features described above suggests that a considerable fraction of the metal vacancies generated during oxidation have indeed contributed to these processes.
These results provide direct experimental evidence in support of the Available Space Model, 39 which has been hypothesized to describe the growth of two-layer oxide structures commonly observed on Fe-Cr 40 , 41 , 42 , 43 and other 44 alloys. In the proposed model, the outer oxide layer is formed by the outward diffusion of metal cations. To summarize, we have shown that a considerable fraction of the atomic vacancies generated during high-temperature oxidation can be transferred into the underlying alloy beginning at the very early stages.
Contrary to the commonly held belief that such vacancies are primarily annihilated, we show that significant vacancy coalescence can occur that results in a variation in density below the oxide layer ranging from that of the bulk alloy to fully voided regions. These voids and other low-density regions are subsequently oxidized or otherwise incorporated into the growing oxide layer and thus the metal vacancies are shown to have a central role in the oxidation pathway of the alloy.
The detailed experimental conditions were reported previously. The alloy composition provided by the manufacturer is shown in Table 1. The samples were polished to a colloidal silica finish. However, the local roughness determined by STEM imaging of an unexposed sample was considerably smaller, 22 confirming that the sample features discussed in this paper were not simply an effect of surface finishing. The sample was exposed to No effect of carbon was observed for exposure to these conditions 22 and thus the experiment may be regarded as a short duration exposure in a mildly oxidizing environment.
This imaging mode contains minimal interference effects and is highly sensitive to density variations, where lower density regions appear darker in contrast. EDS maps are presented as raw counts i.
High‐Temperature Oxidation of Metals
In order to respond to the environmental issues and because of the introduction of new ways of manufacturing processing of electronic and other components, more and more facilities, equipment, and parts are being used at high temperatures. Consequently, demand for heat-resistant alloys is growing due to the following needs: improvement in product reliability; consistency in equipment for new processes; optimization of repair and maintenance processes; reduction of equipment maintenance frequency span; reduction of total investment costs by taking into consideration running and maintenance costs, etc. Based on accumulated during over 40 years technological experience in melting, processing, quality control and heat-resistance in various industries, Hitachi Metals ,Ltd. Okegawa Works manufactures various types of heat-resistant alloy products and also offers related consulting services. The following list, for reference, is a comprehensive introduction to the various types of alloys. Please note that the values presented are average and are not guaranteed.
Simultaneously, the effect of grain size of these metals and grain boundary displacement during oxidation process are described very clearly. The combined effect of crystal structure and grain size on the formation of oxide scale is studied in depth understanding with support from the literature search. High Temperature Corrosion. Generally, most of the metals used in common application technologies undergo deterioration on exposure to weather condition with time. The rate of corrosion varies widely from slower to faster degree depending on the type of material.
Skip to search form Skip to main content You are currently offline. Some features of the site may not work correctly. DOI: Birks and G. Meier and F. Birks , G. Meier , F.
High Temperature Oxidation and Corrosion of Metals, Second Edition, provides a high level understanding of the fundamental mechanisms of high temperature alloy oxidation. It uses this understanding to develop methods of predicting oxidation rates and the way they change with temperature, gas chemistry, and alloy composition. The book focuses on the design and selection of alloy compositions which provide optimal resistance to attack by corrosive gases, providing a rigorous treatment of the thermodynamics and kinetics underlying high temperature alloy corrosion. In addition, it emphasizes quantitative calculations for predicting reaction rates and the effects of temperature, oxidant activities, and alloy compositions.
This book is concerned with providing a fundamental basis for understanding the alloy-gas oxidation and corrosion reactions observed in practice and in the laboratory. Starting with a review of the enabling thermodynamic and kinetic theory, it analyzes reacting systems of increasing complexity. It considers in turn corrosion of a pure metal by a single oxidant and by multi-oxidant gases, followed by corrosion of alloys producing a single oxide then multiple reaction products.
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Heat resistant Alloys
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PDF | High Temperature Oxidation and Corrosion of Metals, Second Edition, provides a high level understanding of the fundamental.
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