水蒸气对高温氧化的影响
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À1 DG 1 ¼ 246; 440 À 54:8 T J mol
(10.1) (10.2)
Consequently, the value of pO2 in gases containing free molecular oxygen is essentially unchanged by reaction (10.1) if water vapour is added. In pure steam or inert gas–water vapour mixtures, the equilibrium value of pO2 is determined by the extent of H2O dissociation. In the case of pure steam, the dissociation of one mole yields x moles of H2 and x=2 moles of O2, with x to be calculated from the equilibrium expression. Using the method of Section 2.1, we find K2 1 ¼ x3 PT 2ð 1 À xÞ 2 ð 1 þ x=2Þ (10.3)
where PT is the total pressure. Because K1 is small, x ( 1 and (10.3) is well approximated by 2 1=3 2K 1 (10.4) x¼ PT pO2 ¼ xPT (10.5)
Equilibrium oxygen partial pressures calculated in this way are seen in Figure 10.2 to be high enough to form Fe2O3 on iron at low temperatures. As expected, pO2 increases with temperature and total pressure. At high temperatures, hematite formation in steam is not possible, unless the pressure is large. If impurity amounts of O2(g) are present in the steam, then pO2 is set by the impurity level rather than H2O dissociation. However, it seems likely that any such impurities would be quickly scavenged by reaction with oxidizing metal, and the water dissociation equilibrium thereby restored. A final factor to be recognized in considering the reacting gas phase is hydrogen generation. In the case of pure steam, the oxidation process M þ H2 O ¼ MO þ H2 (10.6) produces hydrogen. Depending on the mass transfer rates between the scale surface and steam phases, the local situation could be similar to that reached in synthesis gas or a laboratory H2/H2O mix. In the presence of free molecular oxygen, however, any hydrogen generated in this way is presumably oxidized rapidly to water vapour. Water vapour can, in principle, interact with an oxidizing metal in a number of ways. It can participate in surface reactions, thereby modifying the scale–gas
455
456
Chapter 10 Effects of Water Vapour on Oxidation
The nature of the problem is illustrated in Figure 10.1, where the results of oxidizing a ferritic 9% Cr steel at 6501C in N2/O2 and N2/O2/H2O are compared. The dry gas produced an extremely thin (B50 nm) scale with a chromium-rich layer adjacent to the alloy, and protective behaviour was achieved. This was not the case in wet gas, where a porous, multiphase scale grew rapidly. The mechanisms whereby water vapour changes the phase constitution, microstructure and growth rate of the oxidation product are of both fundamental interest and practical importance. Water vapour is present in many gases of industrial importance. Atmospheric air contains water vapour at levels which vary with temperature and relative humidity. A temperature range of 18–281C corresponds to saturation values (i.e. at 100% relative humidity) of pH2 O ¼ 0:02 À 0:04 atm. As is seen later, these levels are sufficient to affect the oxidation rates of many alloys. Thus the results of laboratory experiments using uncontrolled atmospheric air are subject to these affects. Conversely, oxidation rates to be expected in, for example, air pre-heaters cannot be predicted from laboratory data obtained using dry air.
10.1. Introduction
457Water vapouris invariably a constituent of combustion gases, and can therefore affect corrosion in engines, direct fired furnaces and recuperators. It is also present in synthesis gas and coal gas, along with hydrogen. Similar mixtures are generated in fuel cell anode gas streams. Finally, pure steam is the working fluid in many power generating systems as well as being handled as a process stream in a diversity of chemical plants. The water molecule is very stable with respect to its dissociation H2 O ¼ H2 þ 1 2O2 as reflected by the free energy change
Contents
10.1. INTRODUCTION
In 1988, Kofstad [1] wrote, ‘‘It is well known that most technical steels oxidise faster in water vapour or in air or combustion gases containing water vapour than in dry oxygen. The reasons for this are poorly understood’’. At a subsequent Workshop on High Temperature Corrosion [2], it was concluded that understanding remained incomplete. Since then, considerable experimental effort has led to a better definition of the problem, and an improved level of understanding. However, there is still much to learn. As noted by Saunders and McCartney in 2006 [3], ‘‘It is well known that the oxidation rate of steels in steam is about an order of magnitude greater than in air or oxygen, but the mechanism responsible for this increased rate is still unclear’’.
mounting
Ni-coating
oxide scale
steel
(a)
mounting
Ni-coating
hematite magnetite voids Fe3O4 + (Fe,Cr, Mn) 3O4
(b)
steel
Figure 10.1 Cross-section of P91 steel after 100 h exposure at 6501C to (a) N2-1% O2 and (b) N2-1% O2-2% H2O. Reprinted from [4] with permission from Elsevier.
CHAPT ER
10
Effects of Water Vapour on Oxidation
10.1. Introduction 10.2. Volatile Metal Hydroxide Formation 10.2.1 Chromia volatilization 10.2.2 Chromia volatilization in steam 10.2.3 Effects of chromia volatilization 10.2.4 Silica volatilization 10.2.5 Other oxides 10.3. Scale–Gas Interfacial Processes 10.4. Scale Transport Properties 10.4.1 Gas transport 10.4.2 Molecular transport 10.4.3 Molecular transport in chromia scales 10.4.4 Ionic transport 10.4.5 Relative importance of different water vapour effects on chromia scaling 10.5. Water Vapour Effects on Alumina Growth 10.6. Void Development in Growing Scales 10.7. Understanding and Controlling Water Vapour Effects References 455 458 459 462 463 466 468 468 472 472 475 480 484 487 488 489 490 492
(10.1) (10.2)
Consequently, the value of pO2 in gases containing free molecular oxygen is essentially unchanged by reaction (10.1) if water vapour is added. In pure steam or inert gas–water vapour mixtures, the equilibrium value of pO2 is determined by the extent of H2O dissociation. In the case of pure steam, the dissociation of one mole yields x moles of H2 and x=2 moles of O2, with x to be calculated from the equilibrium expression. Using the method of Section 2.1, we find K2 1 ¼ x3 PT 2ð 1 À xÞ 2 ð 1 þ x=2Þ (10.3)
where PT is the total pressure. Because K1 is small, x ( 1 and (10.3) is well approximated by 2 1=3 2K 1 (10.4) x¼ PT pO2 ¼ xPT (10.5)
Equilibrium oxygen partial pressures calculated in this way are seen in Figure 10.2 to be high enough to form Fe2O3 on iron at low temperatures. As expected, pO2 increases with temperature and total pressure. At high temperatures, hematite formation in steam is not possible, unless the pressure is large. If impurity amounts of O2(g) are present in the steam, then pO2 is set by the impurity level rather than H2O dissociation. However, it seems likely that any such impurities would be quickly scavenged by reaction with oxidizing metal, and the water dissociation equilibrium thereby restored. A final factor to be recognized in considering the reacting gas phase is hydrogen generation. In the case of pure steam, the oxidation process M þ H2 O ¼ MO þ H2 (10.6) produces hydrogen. Depending on the mass transfer rates between the scale surface and steam phases, the local situation could be similar to that reached in synthesis gas or a laboratory H2/H2O mix. In the presence of free molecular oxygen, however, any hydrogen generated in this way is presumably oxidized rapidly to water vapour. Water vapour can, in principle, interact with an oxidizing metal in a number of ways. It can participate in surface reactions, thereby modifying the scale–gas
455
456
Chapter 10 Effects of Water Vapour on Oxidation
The nature of the problem is illustrated in Figure 10.1, where the results of oxidizing a ferritic 9% Cr steel at 6501C in N2/O2 and N2/O2/H2O are compared. The dry gas produced an extremely thin (B50 nm) scale with a chromium-rich layer adjacent to the alloy, and protective behaviour was achieved. This was not the case in wet gas, where a porous, multiphase scale grew rapidly. The mechanisms whereby water vapour changes the phase constitution, microstructure and growth rate of the oxidation product are of both fundamental interest and practical importance. Water vapour is present in many gases of industrial importance. Atmospheric air contains water vapour at levels which vary with temperature and relative humidity. A temperature range of 18–281C corresponds to saturation values (i.e. at 100% relative humidity) of pH2 O ¼ 0:02 À 0:04 atm. As is seen later, these levels are sufficient to affect the oxidation rates of many alloys. Thus the results of laboratory experiments using uncontrolled atmospheric air are subject to these affects. Conversely, oxidation rates to be expected in, for example, air pre-heaters cannot be predicted from laboratory data obtained using dry air.
10.1. Introduction
457Water vapouris invariably a constituent of combustion gases, and can therefore affect corrosion in engines, direct fired furnaces and recuperators. It is also present in synthesis gas and coal gas, along with hydrogen. Similar mixtures are generated in fuel cell anode gas streams. Finally, pure steam is the working fluid in many power generating systems as well as being handled as a process stream in a diversity of chemical plants. The water molecule is very stable with respect to its dissociation H2 O ¼ H2 þ 1 2O2 as reflected by the free energy change
Contents
10.1. INTRODUCTION
In 1988, Kofstad [1] wrote, ‘‘It is well known that most technical steels oxidise faster in water vapour or in air or combustion gases containing water vapour than in dry oxygen. The reasons for this are poorly understood’’. At a subsequent Workshop on High Temperature Corrosion [2], it was concluded that understanding remained incomplete. Since then, considerable experimental effort has led to a better definition of the problem, and an improved level of understanding. However, there is still much to learn. As noted by Saunders and McCartney in 2006 [3], ‘‘It is well known that the oxidation rate of steels in steam is about an order of magnitude greater than in air or oxygen, but the mechanism responsible for this increased rate is still unclear’’.
mounting
Ni-coating
oxide scale
steel
(a)
mounting
Ni-coating
hematite magnetite voids Fe3O4 + (Fe,Cr, Mn) 3O4
(b)
steel
Figure 10.1 Cross-section of P91 steel after 100 h exposure at 6501C to (a) N2-1% O2 and (b) N2-1% O2-2% H2O. Reprinted from [4] with permission from Elsevier.
CHAPT ER
10
Effects of Water Vapour on Oxidation
10.1. Introduction 10.2. Volatile Metal Hydroxide Formation 10.2.1 Chromia volatilization 10.2.2 Chromia volatilization in steam 10.2.3 Effects of chromia volatilization 10.2.4 Silica volatilization 10.2.5 Other oxides 10.3. Scale–Gas Interfacial Processes 10.4. Scale Transport Properties 10.4.1 Gas transport 10.4.2 Molecular transport 10.4.3 Molecular transport in chromia scales 10.4.4 Ionic transport 10.4.5 Relative importance of different water vapour effects on chromia scaling 10.5. Water Vapour Effects on Alumina Growth 10.6. Void Development in Growing Scales 10.7. Understanding and Controlling Water Vapour Effects References 455 458 459 462 463 466 468 468 472 472 475 480 484 487 488 489 490 492