Introduction To High Temperature Oxidation And Corrosion PdfBy Maia U. In and pdf 20.01.2021 at 08:56 7 min read
File Name: introduction to high temperature oxidation and corrosion .zip
- High Temperature Oxidation and Corrosion of Metals, Volume 1
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- Dry & Wet Corrosion
Professor Wranglen's book is based on his experience in giving an introductory corrosion science and engineering course to engineering students for over 10 years. This book is relatively short and claims only to be an introduction to the subject. This, however, does not do justice to the book.
High Temperature Oxidation and Corrosion of Metals, Volume 1
Insufficient resistance against high-temperature oxidation restricts the number of possible applications. A clear dependency of the scale growth kinetics on W content and oxidation temperature is demonstrated by thermogravimetry and subsequent analysis of cross-sections.
By means of electron backscattered diffraction EBSD , the evolution of microstructures in the outer oxide layers were examined depending on the oxidation temperature. Sequential exposure of samples in 16 O 2 - and 18 O 2 -containing atmospheres proved counter-current material transport. The combination of focused ion beam FIB and secondary ion mass spectroscopy SIMS visualised the formation of new oxide phases mainly on the outer and inner interface of the oxide scale.
An elaborate review of available transport paths for oxygen is given during the discussion of results. Conventional Fe-, Ni- or Co-base alloys proved their suitability for various technical applications, especially during operation in harsh conditions at elevated temperatures. Due to their good mechanical performance, Ni-base Superalloys were unchallenged for decades as alloys that are used for turbine blades in sections of gas turbines where they withstand aggressive service conditions.
Nevertheless, there is still considerable research effort to unravel mechanistic details of the complex oxide scale growth on alumina-forming high-temperature alloys. The in-service capability of Ni-base Superalloy is limited by their melting point and is therefore close to its maximum.
At constant Al levels, Pettit 20 demonstrated significantly higher tendency for the transition from internal oxidation to the formation of a continuous Al 2 O 3 layer with increasing temperatures. For the description of the scaling mechanisms in the Co-Al-W model system, which lead to the development of barrier layers, especially at lower temperatures, detailed knowledge of transport processes during oxidation are essential.
Two-stage tracer exchange experiments are established for drawing conclusions on elementary transport mechanisms during oxidation at high temperatures. Visualisation of oxygen diffusion paths on mechanically polished cross-sections after tracer exchange experiments was demonstrated by Ooi et al. Due to the combination of the two data sets, tracer intensities can directly be attributed to specific morphological features in the complex oxide scales at a quality level that is hardly obtained by comparable studies.
The acronyms used in this study indicate the nominal W content of the respective single-crystal alloy. Oxide scales after thermogravimetry and two-stage tracer exchange experiments are analysed in detail. During the discussion, all acquired results on relevant elementary processes are supplemented by classical model predictions from literature. The compositions with higher W contents exhibit significantly slower oxidation rates over the whole duration of experiments. For higher W contents, the initial period shortens.
For lower temperatures, transitions in the reaction kinetics are less pronounced. Furthermore, the samples demonstrate lower increase in weight during the experiment. All elucidated oxide scales on ternary Co-Al-W model alloys reveal three distinct layers.
On top of the initial alloy surface, an outer oxide layer d 1 formed. This layer can be expected to consist almost completely of CoO. Below d 1 , an inner oxide layer d 2 including a conglomerate of various oxide phases is clearly separated from the internal precipitation region d 3.
The narrow zone including the interface between the deepest formed oxide species and alloy is generally referred to as IOF. In comparison with the inner oxide layer, d 3 consists of dark Al 2 O 3 precipitates embedded in the unoxidised matrix. The micrograph taken from the cross-section of 10Wsx exhibits considerable amount of discontinuous alumina segments in the internal precipitation regions.
Due to the slow lateral growth of these discontinuous alumina segments, they only provide a localised and hence insufficient barrier to oxygen transport. Nevertheless, the individual thickness of distinguished scale regions is strongly dependent on the W level in the ternary system. The general trend of higher W contents leading to lower oxidation rates is also reflected in the dimensions of the individual layers.
Alterations in the growth rates of the outer oxide layer can be one reason for these pronounced changes. For the growth of NiO, quantitative studies demonstrated that increasing grain size causes considerable changes in the overall rate constants. Due to the large interaction volume of the electron beam with the sample, the lateral resolution is restricted. Electron micrographs not shown that were observed in parallel with the recording of EBSD data indicated that regions without any signal or clear allocation to any grain orientation can be correlated to pores within the outer oxide layers see also Fig.
Above this layer, significant porosity impedes the assignment of measurement spots to grain orientations. The size of grains that developed closer to the outer scale interface decreases.
At the outer scale interface, the nucleation and growth of new, small grains could only be observed to a minor extent. In Fig. For all three compositions, the displayed IPF maps exhibit regions of coarse grains that dominate d 1. For the sample with the highest W level, growth of small grains on the outer region of d 1 is still evident. Displayed are representative sections of the outer oxide layer on a 7Wsx, b 9Wsx and c 10Wsx.
Each sample exhibits comparably large pores between the columnar grains see also Fig. Following the general trend of slower reaction kinetics with higher W contents, the thickness of the epitaxial region of small grains above the original alloy surface decreases for 9Wsx and 10Wsx. Besides this fine-grained sections, no preferential orientation was observed in any CoO layer. As demonstrated in Fig. The oxide scales can be distinguished in the above described three individual layers.
As expected, the overall thickness of the scales decreases. The determined IPF maps of the investigated oxide scales are displayed in Fig. Instead, the observed grain sizes are widely comparable in the two elucidated oxide scales. Neither a considerable epitaxial inner zone nor any preferred orientations are evident after exposure at lower temperatures. Nevertheless, all grains are slightly more extended into the growth direction, which indicates minor differences between the activation energies of nucleation and lattice expansion during grain growth.
The previous results provide mainly information on the expansion of the outer oxide layer. Growth of d 1 can only be sustained by considerable diffusion of cations to the outer scale interface. To gain more detailed insights into the transport of material during the early stages of scale formation on ternary Co-base model alloys, two-stage oxidation experiments were conducted on this alloy series.
By using the distinguishable stable oxygen isotopes 16 O and 18 O, the well-acclaimed technique delivers direct evidence for the transport paths of oxygen and provides supplementary information on the diffusion of cations during short-term exposure in the considered temperature window. In contrast to the thermometric experiments in synthetic air, the atmosphere during the tracer exchange experiments was pure oxygen.
Error bars represent the SD of five measurements. Each value represents the mean of five measurements. Standard deviations are given as error bars. As the general tendency towards slower oxidation kinetics with increasing W contents remains unaffected from this scatter, results from both experimental approaches can be combined to one reliable study on mass transport mechanisms during scale growth on Co-Al-W systems.
To facilitate an easier allocation of tracer distribution to specific morphological features of the investigated oxide scales, the total positive secondary ion SI images of the analysed sample regions were recorded immediately prior to acquisition of SIMS maps.
The same lateral resolution was used for both data sets. Displayed are cross-sections of multilayered oxide scales on a 7Wsx, b 9Wsx and c 10Wsx. See the text for the explanation of white circles, arrows and the marked regions. Due to the changed contrast conditions in SI micrographs, d 3 appears less clear in the displayed cross-sections.
The distribution and size of pores in the outer oxide layers is widely comparable to the one obtained during thermogravimetric exposure. Continuous outward diffusion of metal cations results in a distinctly demarcated region of labelled oxygen on the outer oxide interface. Furthermore, enrichment of 18 O isotopes is particularly evident around pores and cracks in d 1 of 9Wsx and 10Wsx.
Closer inspection of the outer oxide layer on 10Wsx also reveals grain boundaries that are decorated with 18 O white arrows in Fig. The internal precipitation region d 3 can be divided into two parts. Several authors suggested the usage of isotopic fractions if 18 to eliminate unwanted influences that are caused by localised charging, instrumental factors or sample topography. The relative tracer enrichment during the second oxidation stage can be determined by normalising the calculated if 18 values using the actual 18 O 2 isotopic fractions in the gas that was used during the first stage g 1 and the second stage g 2 of the experiment: Nevertheless, also the calculation of if 18 values can provide misleading results for measurement points with comparably low intensities for both distinguished oxygen isotopes.
Thus, the application of Eq. Undoubtedly, the second measurement has higher relevance to the desired conclusions on transport mechanisms in the grown oxide scales.
The thresholds for oxygen ion intensities were individually determined for each experiment. Foss et al. The underlying principle was already presented earlier 29 and is slightly adapted in this study. Due to the irregular outer oxide interface, usage of the average if 18 across the entire width of the displayed oxide scale overestimates the expansion of d 1. The calculation of if 18 depth profiles from sections where the outer oxide scale is nearly parallel to the initial sample surface can be demonstrated to be considerably more meaningful.
The profiles were calculated Eq. As already described above, counter-current transport mechanisms are confirmed for ternary Co-base model alloys irrespective of the W content. However, more specific indications on the prevailing nature of oxygen transport through the internal oxidation zone can be extracted from the displayed figure. The only valid explanation for this rise in labelled oxygen is the oxidation of phases in the former zone of internal precipitates embedded in the matrix.
In other words, there is an additional oxidation front following the progress of the IOF into the sample. For 9Wsx, two significantly different sections of the outer oxide scale were considered for the calculation of if 18 depth profiles. The first region of the scale, Fig. After this, the computed pseudo depth profile exhibits features of a classical concentration profile developed during diffusion in a semi-infinite media. Towards the internal precipitation region, a moderate increase can be observed.
The second region of the multilayered oxide on 9Wsx, Figs 7b and 8c , was deliberately chosen to represent the interior of one large grain of d 1. Contrary to the first profile, no indications of considerable oxygen transport via diffusion can be found in d 1. The sample with the highest W content demonstrates particularly low 18 O intensity in the transition zone from the inner oxide layer to the internal precipitation region Fig. During the investigation of samples that were oxidised in synthetic air with field emission scanning electron microscopy, no evident porosity was observed in d 3.
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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. High temperature oxidation and other corrosion types are commonly modelled using the Deal-Grove model to account for diffusion and reaction processes. Two types of sulfate -induced hot corrosion are generally distinguished: Type I takes place above the melting point of sodium sulfate and Type II occurs below the melting point of sodium sulfate but in the presence of small amounts of SO 3. In Type I the protective oxide scale is dissolved by the molten salt.
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. The examples of such type are iron rusts at room temperature and deteriorate faster than nickel and chromium that are attacked slowly with time.
Dry & Wet Corrosion
Oh, M. High temperature oxidation of Fe Mn Si alloys results in the formation of a transformed surface layer that contains a lower concentration of manganese and a higher concentration of silicon than the underlying metal. Because the corrosion resistance of iron-silicon alloys is very sensitive to the concentration of silicon in the metal, the transformed layer is often more corrosion-resistant than the original alloy.
The significant role of alloying elements with respect to the exposed medium is studied in detail. The surface morphology has expressed the in situ nature of the alloy and its affinity toward the environment. The EDS and XRD analysis has evidently proved the presence of protective oxides formation on prolonged exposure at elevated temperature. The predominant oxide formed during the exposure at high temperature has a major contribution toward the protection of the samples. The nickel—iron-based superalloy is less prone to oxidation and hot corrosion when compared to the existing alloy in gas turbine engine simulating marine environment.
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. The influence of gases other than oxygen and alloying additions are described. In a second part, the effects of high energy beams are presented and the location of their influence is discussed. Major or minor modifications are described, leading to enhancement or inhibition of the corrosion rate. The use of high energy beams as corrosion protection tools is envisaged.
Not a MyNAP member yet? Register for a free account to start saving and receiving special member only perks. As described in chapter 2 , the primary purposes of high-temperature structural coatings are to enable high temperature components to operate at even higher temperatures, to improve component durability, and to allow use of a broader variety of fuels in land-based and marine-based engines. Although high-temperature coatings protect the substrate, the demarcation between coating and substrate either metal or nonmetal is becoming increasingly blurred. The demanding requirements of high-temperature service in both isothermal and cyclic modes have recast the way researchers think about coated structures. These structures can be considered part of a continuum; at the limit the coating will be a progressive modification of the substrate and therefore must be concurrently designed with the substrate.
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Не знаю, о ком вы говорите, - поправил его Беккер, подзывая проходившую мимо официантку. Он купил две бутылки пива и протянул одну Двухцветному. Панк изумленно взглянул на бутылку, потом отпил изрядный глоток и тупо уставился на Беккера.