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The control of high temperature corrosion of engineering carbons and graphites
Corrosion Science, Vol. 33, No. 4, pp. 527-543, 1992 Printed in Great Britain.
THE
0010-938X/92 $5.00 + 0.00 © 1992 Pergamon Press plc
CONTROL OF HIGH TEMPERATURE CORROSION OF ENGINEERING CARBONS AND GRAPHITES L u l s DEVINE DE CASTRO* a n d BRIAN M c E N A N E Y t School of Materials Science, University of Bath, Bath BA2 7AY, U.K.
Abstract--Mechanisms of corrosion of engineering carbons and graphites by reaction with gases at high temperature are reviewed and illustrated by the airburn reaction of anode carbons. Methods of controlling corrosion include external coatings and internal modifications of the carbon structure. Single-layer glaze coatings, derived from B203, are widely used for applications up to -900°C. Multilayer coatings consisting of a refractory SiC primary layer and a glass-forming secondary layer which seals cracks are used at higher temperatures. Up to -1200°C, B203-based glazes are used; if BzO3 is replaced or supplemented by SiO2-based glass-formers and/or refractory particulates, the maximum temperature is 1600--1800°C. So far, few coatings have proved successful above 1800°C, except for short term exposures. Internal modifications to the carbon substrate include glass-forming additions, e.g. B-compounds and refractory particulates, e.g. ZrC, ZrB 2 and SiC.
INTRODUCTION
CARBONS and graphites form a unique class of ceramic materials, because in non-oxidising environments they retain their strength and stiffness up to about 2400°C when significant creep occurs; 1this upper temperature limit is considerably in excess of those for other engineering ceramics, e.g. SiC and Si3N 4. Other attractive properties are low densities (resulting in high specific mechanical properties) high thermal conductivities and low thermal expansion coefficients (resulting in excellent thermal shock resistance). As a consequence, carbons and graphites have found many applications in high temperature engineering. The major markets, both in tonnage and value terms, are aluminium smelting,2 where carbon anodes and graphite cathodes are used, and graphite electrodes for electric arc steel-making. 3 Substantial quantities of carbon materials are used in other areas of extractive metallurgy as electrodes, refractories and crucible components. Specialised materials are used as moderator and structural components in gas-cooled nuclear reactors 4 and in the defence/aerospace industry for aircraft brakes, rocket engine components, nose tips and leading edges of re-entry vehicles. Carbon materials are also leading candidates for first wall duties in fusion energy devices. 5 A major disadvantage of carbon materials is their low oxidation resistance. Depending upon materials, environment and application, carbon begins to react with oxidising gases at appreciable rates from about 400°C. The overall corrosion reactions of carbons in oxygen-containing gases are C -~- 0 2 = C O 2
(1)
*Present address: Centro Technol6gico do Exdrcito, Av. das Am6ricas, 28705, Guaratariba, 23020 Rio de Janeiro, Brasil. tAuthor to whom correspondence should be addressed. Manuscript received 23 September 199l. 527
528
L.D. DECASTROand B. MCENANEY
and 2C
+ 0 2 =
2CO,
(2)
with (1) dominating at low temperatures and (2) at temperatures greater than about 700°C. Corrosion in gases containing steam or carbon dioxide follows the overall reactions C + HzO = CO + Hz (3) C + COz = 2CO.
(4)
Since no protective scale is formed, the high temperature properties of carbon materials cannot be exploited unless steps are taken to control corrosion. Several studies 6'7 have shown that 5-10% weight loss due to corrosion by oxidising gases reduces the strength of various carbon materials by - 5 0 % . This paper surveys both mechanisms of high temperature, gaseous corrosion of engineering carbons and graphites and methods for controlling corrosion. Radiolytic corrosion of nuclear graphites, 8 low temperature corrosion by liquids, and gasification of particulate carbons, e.g. coal chars, are not directly considered.
MANUFACTURE OF ENGINEERING CARBONS AND GRAPHITES Most engineering carbons and graphites are polygranular materials which are manufactured from sized filler particles and a binder (Fig. la). The major source of
(a)
I
(b)
I
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)
=oot)
~eat
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~inal heat eatment ) Carbon
product
[
~raphi6~~
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'
1 C(¢CoCpOsite )
Grodrap hire uct
FIG.1. Manufacturingsteps for: (a) polygranularcarbonsand graphites;(b) carboncarboncomposites.
)
FIG. 2. Microstructure of: (A) a polygranular graphite, showing (a-b) filler particle; (c) binder phase; and (d) porosity; (B) a carbon-carbon composite, showing (a,b) mutually perpendicular fibre bundles; (c) pores; and (d) cracks. 529
FIG. 11. The development of a sol gel-derived SiC coating on an electrode graphite,52 showing the effect of repeated application and firing: (a) fired once; (b) fired twice; (c) fired three times. EDX images for Si.
530
Corrosion of engineeringcarbonsand graphites
531
filler is petroleum coke which has been calcined to about 1350°C and the binder is usually a pitch derived from coal tar. When heated the binder/filler mixture is a thermoplastic body which can be formed by moulding or extrusion. The baking step which converts the pitch binder to carbon is critical, since there is a loss of volatiles and shrinkage during carbonisation of the binder, which must be accommodated by very slow heating rates. Impregnation with a resin or pitch is used to increase the density of the final product. Carbon products, e.g. anodes for aluminium smelting, 2 are produced after the baking step by heat treatment to about 1200°C. Heattreatment in the range 2700-3000°C (Fig. la), is required to produce graphite electrodes, e.g. for electric arc steel-making; Piper 9 gives further details of fabrication methods. The demands for high performance, high temperature materials from the defence/aerospace industries have prompted the development of an extensive range of carbon-carbon (C-C) composites, i.e. carbon fibre reinforcement in a carbon matrix, m In the fabrication of C-C composites (Fig. lb), textile engineering methods are used to produce preforms with a wide range of multi-directional fibre arrays. There are three classes of commercially-available carbon fibres: ex-polyacrylonitrile (PAN fibres), and ex-mesophase pitch (MP fibres) and ex-rayon fibres. The carbon matrix has been formed by carbonising thermosetting resins, pitches and by chemical vapour infiltration, CVI, methods. It is usually necessary to repeat the impregnation/ carbonisation cycle several times to aeheive an adequate matrix density, to The microstructure of a polygranular graphite (Fig. 2A), shows a coke filler particle, a-b, and binder carbon, c. The presence of networks of pores in carbon materials, e.g. d, allows ingress of corroding gases to the interior causing internal corrosion. The microstructure of a three-directional C-C composite (Fig. 2B), shows mutually perpendicular fibre bundles, a, b, and a network of cracks and pores, c, d; the origins of porosity in different types of carbon have been discussed elsewhere. 11 MICROSTRUCTURE OF ENGINEERING CARBONS AND GRAPHITES All carbons and graphites have more-or-less disordered microstructures which can be related to that of the graphite single crystal. For example, in electrode graphites the degree of graphitic structure is extensive, but products such as anode carbons, which have only been heated to about 1200°C, have much more disordered structures. A basic concept is the carbon layer plane, which in the graphite single crystal is an extended, planar, hexagonal array of carbon atoms. In disordered carbons layer planes may be limited in extent and contain many defects and heteroelements (e.g. H,O,N,S) bound to their edges; the nature of hetero-elements depends upon the precursors and final heat-treatment temperature. X-ray diffraction patterns of even the most disordered carbons ~2 show diffuse bands in the approximate positions of the (002) and (100) graphite lines, indicating a turbostratic structure, i.e. carbon layer planes stacked in parallel groups, but with no threedimensional graphitic ordering. The structure of carbons revealed by X-ray diffraction has been elaborated in recent years using electron microscopy. Oberlin et al. 13,14 propose the term 'Basic Structural Unit', BSU, to describe the smallest periodic feature that can be imaged by 002, 10 or 11 dark fields, The smallest BSU, of size less than 1 nm, are envisaged as small carbon layer planes, comprising less than 12 hexagonal rings, in stacks of two or three. In some carbons several BSU may lie approximately parallel, constituting a
532
L.D. DE CASTROand B. McENANEY
LMO Fxc. 3. Schematic representation of Basic Structural Units, BSU, and Local Molecular Ordering, LMO, in a non-graphitic carbon; adapted from Ref. 13.
region of Local Molecular Ordering, L M O (Fig. 3), which can range from the nanometric to the millimetric scale. L M O greater than about 1/~m in size are termed lamellae, which can be either flat or crumpled, depending upon the degree of disorder within the carbon layer planes. In the graphitisation process, represented schematically15 in Fig. 4, isolated BSU grow into distorted colt, mns which at higher temperatures undergo lateral condensation to produce crumpled lamellae. At the highest temperatures the defects within carbon layer planes are removed to produce extensive fiat lamellae with three-
FIG. 4. A schematicrepresentation of the graphitisation process; reproduced from Ref. 15 with permission.
Corrosion of engineering carbons and graphites
533
dimensional graphitic ordering. Thus, there is a textural continuum between disordered carbons and materials which develop extended graphitic ordering. G A S - - C A R B O N REACTION M E C H A N I S M S
The oxidation of the graphite single crystal by gases is highly anisotropic, because the reactivity of carbon atoms at the edges of graphite basal planes, is orders of magnitude higher than that within basal planes. ~6 Point defects and emergent dislocations in the basal planes are also reactive sites, as abundantly illustrated using etch decoration techniques. 17 In the study of gas-carbon reactions this has led to the concept of active sites. At each active site oxygen-containing functional groups may form during gasification and reactant chemisorption, migration of intermediates, and product desorption may take place sequentially. Hetero-elements and catalytic mineral matter present in the carbon may form additional active sites. ~s In the absence of inorganic catalysts, a weli-graphitised material will have a lower active site concentration than a disordered material, leading in turn to a lower corrosion rate. 19-21 Intrinsic reactivities
Because of their importance in gasification of fossil fuels, the mechanisms of gascarbon reactions have been extensively studied. A comprehensive account is outside the scope of this review, but there are excellent treatments of the subject as it applies to coal char gasification. 22,23Quantitative treatments of gas-carbon reaction kinetics start with a measure of overall reactivity which is often expressed on a mass-specific basis, R [(g g-l s-l]: R = (dX/dt)/(1 - X ) ,
(5)
where X is the fractional conversion at time t. An illustration of the relationship between overall and intrinsic reactivities is provided by the C-CO2 reaction, for which there is substantial agreement as to the mechanism 24,2~ k I
CO 2 "~ Cf k~---_~CO + C(O)
(6)
C(O) -~ CO,
(7)
where Cf and C(O) represent an unoccupied and occupied active site respectively. Under most conditions reaction (6) is fast and reversible and reaction (7) is ratecontrolling. Assuming steady-state conditions gives a Langmuir-Hinshelwood expression for the intrinsic reaction rate Ri [sites (g C ) - l s a] R i = (Ct)kt(CO2)/[]
+ a(CO) + b(CO2)],
(s)
where (Ct) is the total active site concentration, (CO2) and (CO) are the concentrations of these gases at the surface of the carbon, a = k _ f l k 3 and b = k l / k 3. The overall reactivity R = mcRi, where m c is the mass of a carbon atom. There is much evidence that R is directly proportional to (Ct).26 A direct proportionality between R and (Ct), similar to equation (8), can be obtained for the C - H 2 0 reaction; the
534
L.D. DECASTROand B. MCENANEY Zone III
b
i o
~
Zone I ]zr
I
FIG. 5. A schematicArrhenius plot for reaction of ~ases with porous carbons. mechanism of the C-O2 reaction is much more complex, but direct proportionalities between R and (Ct) have been established for certain conditions in this case also. 23
Mass transport effects Engineering graphites contain significant porosity (Fig. 2), so that in practical situations the overall rate of reaction is influenced by in-pore diffusion and by boundary layer diffusion at higher temperatures. Equation (8) is derived assuming that the overall gasification rate, R, is determined solely by the reactivity of the carbon surface. Evidence for diffusional effects is provided by Arrhenius plots for gas--carbon reactions which are often curved with the apparent activation energy decreasing with increasing temperature, the decrease being attributed to the increasing influence of gaseous diffusion. An idealized representation of this trend is in Fig. 5. At low temperatures, Zone I, the concentration of reactant gas within the pores of the carbon is everywhere the same as the bulk gas concentration, Co, and R = mcR i. With increasing temperature, there is a decrease in the reactant gas concentration within the pores, Zone a, caused by both (i) the chemical reaction rate becoming greater than the rate of supply of gas through the pores in the char and (ii) the resistance offered by the outwardly-diffusing product gas. Within Zone a, the concentration of reactant gas at the centre of the carbon will decrease progressively as temperature rises, until it eventually falls to zero. This represents the transition to Zone II. Further increases in temperature produce a significant outward flux of product gas from the external surface of the carbon and this causes reduction in the reactant gas concentration in a boundary layer; this corresponds to Zone b. The concentration of reactant gas at the external surface of the carbon, Cs, falls progressively with increasing temperature until eventually Zone III is reached, where Cs approaches zero. CORROSION OF ANODE CARBONS The role of chemical and diffusional factors may be illustrated by the corrosion of anode carbons used in aluminium smelting; a sketch of the arrangement of a pre-
Corrosion of engineering carbons and graphites
535
Alumina cover
Frozen electrolyte Molten
Electrolyte Metal
Cathode block L o c a t i o n o f a p r e - b a k e d c a r b o n a n o d e i n a Hall-Heraultcell.
FIG. 6.
baked anode in a Hall-Heroult cell is in Fig. 6. The reduction of alumina to aluminium is accompanied by consumption of the carbon anode. Under the current density conditions usually employed 2 in industry the dominant reaction is 2A120 3 + 3C = 4A1 + 3CO 2.
(9)
There is additional, unwanted consumption of the anode due to a number of factors including the airburn and carboxy secondary reactions which contribute 8-15 wt% and 5-6 wt% respectively to total anode consumption. 2 The airburn reaction occurs on the upper surfaces of the anode by a combination of reactions (1) and (2), depending on the temperature. An Arrhenius plot of thermogravimetric data for airburn of anodes (Fig. 7), 27 shows pronounced curvature. The apparent activation energy in the temperature range 580--620°C is
7 ZoneIII rate
5'
C
~
'
~
ZoneII rate
4-
o
3' 0
_= 2
1
0.0011
!
!
|
0.0012
0.0013
0.0014
0.0015
[1/[']/[1/I(] FIG. 7.
Arrhenius plot for the airburn reaction of anode carbons; adapted from Ref. 27.
536
L.D. DECASTROand B. MCENANE¥
15 kJ mo1-1, indicative of boundary layer effects influencing the rate. Under conditions where boundary layer diffusion and in-pore diffusion are influencing the corrosion, the mass-specific rate, R, is given by R = Co[1/K' - 1/g"] -~
(a0)
where K' and K" are the mass transfer coefficients in pores and in the boundary layer respectively and Co is the concentration of oxygen in air. If it is assumed that CO2 is the sole product of the reaction and that stagnant conditions remain, then R in Zone III is R = ADCo/6 = K"Co,
(11)
where D is the free gas diffusivity of 02 in the boundary layer of width d and A is a stoichiometric coefficient. Using these equations it is possible to compute Zone II and Zone III rates. 27 Figure 7 shows that below 450°C the airburn reaction rate is influenced by in-pore diffusion; as the temperature increases there is a transition until at temperatures above 570°C boundary layer diffusion controls the corrosion rate. The carboxy reaction occurs as a result of corrosion by CO 2 produced at the anode face by reaction (4) at temperatures of -950°C. Under the influence of hydrostatic pressure, the CO 2 permeates the pore network in the carbon causing corrosion up to 10 cm behind the anode face. The rate of the carboxy reaction is therefore influenced by in-pore diffusion. 27 CONTROL OF CORROSION OF CARBONS AND GRAPHITES There is a vast literature on methods for controlling the corrosion of carbon and graphites at high temperatures, so that only a brief review can be attempted; more detailed surveys have been published. 28'29 In principle corrosion control is possible by modifying reactant gas composition. For example, equation (8) shows that the C-COz reaction can be inhibited by increasing the CO content of the gas. An important example of this approach, which is outside the scope of this review, is the control of radiolytic corrosion of nuclear graphites by CO2-based gases in British Magnox and A G R nuclear reactors by additions of CH4, CO and H20. 4'8 However, the great majority of methods seek to modify the structure of the component, either by use of external coatings and/or by internal modifications of the carbon material. E x t e r n a l coatings
Selection of a coating depends upon factors such as the corrosive gas composition and hydrodynamics and the time-temperature profile of the component. Thus a coating which may be satisfactory for a single short period of exposure at high temperatures, e.g. rocket re-entry conditions, may not be suitable for prolonged exposure at lower temperatures with multiple thermal cycling, e.g. jet engine components. The qualities required of a coating to protect a carbon substrate from oxidation at high temperatures include: 29 (i) refractoriness, i.e. low volatility and good high temperature physical/ mechanical properties; (ii) chemical compatibility with carbon and the corrosive atmosphere;
Corrosion of engineering carbons and graphites
Temperature/ 10 i. O I--
2500
2000
i
a
silica
(K)
1670 1430 i
537
1250
t
I
1110 i
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,
silicon nitride
-i 10
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silicon carbide O -
-
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I
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4
5
6
7
8
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(K-l)
Equilibrium vapour pressures of some coating materials.
(iii) low permeability to oxygen and carbon; (iv) adherence at the carbon/coating interface. These requirements amount to a formidable specification, which becomes more stringent as the operating temperature of the component increases. Successful formulations can be classified into single layer coatings, used at lower temperatures, and multilayer coatings for higher temperatures. Single layer coatings are mainly glazes which are fluid in the service temperature range; glazes can be self-healing, provided they wet the carbon surface. The most successful glazes are based upon B203, 3°-32 which melts around 500°C, i.e. close to the temperature at which carbons begin to oxidise. BzO 3 has good wetting properties for carbon, although a disadvantage is that it is readily hydrolysed in moist atmospheres. B20 3 remains fluid up to -1200°C when vaporisation becomes significant (Fig. 8), particularly in moist atmospheres. P-containing glazes derived from a wide variety of inorganic and organophosphorus compounds have also been shown to be effective in controlling corrosion of carbons and graphites up to -900°C. 33,34 There is an extensive patent literature on the formation of oxidation resistant coatings by thermal and air spraying on arc furnace electrode graphites to control side-wall corrosion which can account for up to - 4 0 % of the electrode wastage. 35 Typically, an aluminium bond coat followed by a powdered, glass-forming coating are applied to the electrode and the coating is melted on to the graphite surface using an electric arc. Recent examples of electrode coatings include Ni-Si and AI-Si powders in a nitrocellulose lacquer36 and polyphosphate formulations.37 In modern electric arc practice water-cooling has led to a reduction in the demand for coated electrodes. Control of the airburn reaction of anodes in aluminium is made by the simple expedient of covering the upper surfaces of the anode with alumina powder or crushed cryolite-based electrolyte bath, 38 see Fig. 6. The permeability of oxygen in B203-based glazes sets an upper limit to their application in the range 800-900°C. 39For higher temperatures coatings must include
538
L.D. DECASTROand B. MCENANEY TABLE 1.
COEFFICIENTS OF THERMAL EXPANSION FOR CARBON MATERIALS AND CERAMICS
Material Carbon--carbon composite Coarse-grainedgraphite Medium-grainedgraphite Fine-grainedgraphite Vitreous silica Siliconnitride Siliconcarbide Hafniumcarbide Zirconiumcarbide
CTE/(10-6 °C- 1) at 1000°C -0.05 3.1 4.3 7.0 0.04 2.8 6.1 6.6 6.7
refractory compounds with low permeabilities to oxygen and carbon. A major problem results from the low coefficients of thermal expansion, CTE, of carbon materials relative to many candidate coating refractories. 29 If the CTE of the coating exceeds that of the carbon substrate, then microcracking occurs on cooling from the application temperature or upon thermal cycling. From calculations of the oxidation rate for a C--C composite with a model cracked coating, Luthra 39 has shown that 10 nm wide cracks can provide paths for rapid diffusion of oxygen. CTE values for carbons range from -0.05 x 10 -6 K -1 for some C-C composites to 7 × 10-6 K -1 for fine-grained graphites (Table 1). The low values of CTE are partly because the extensive crack networks in engineering carbons and graphites (Fig. 2), can accommodate thermal expansion. 4° The most widely-used component of multilayer coatings on carbons is SiC. The attractions of SiC are its refractoriness, its low CTE ( - 6 x 10 -6 K -1) and low oxygen permeability. It also has a low rate of oxidation and the SiOz film formed is protective. A practical advantage is the number of established methods for forming SiC coatings (see below). The upper temperature limit (-1800°C) for long term use of SiC is set by dissociation of the SiO2 protective layer (Fig. 8), and its carbothermic reduction. Figure 9 shows that significant pressures of CO and SiO vapours can be generated at 1800°C, which can cause disruption of protective coatings. Application methods for multilayer coatings on carbons and graphites include: pack processes, conversion coatings and chemical vapour deposition. An early patent 41 for a two-layer coating describes an inner SiC layer (formed by a pack process using SiC and Si) and an outer B203-based glaze; part of the function of the glaze is to fill cracks in the SiC layer which are formed as a result of CTE mismatch (Table 1). The concept of a (cracked) primary SiC layer in conjunction with a crack sealing, glass-forming layer has formed the basis for many coating designs. In many cases, as the service temperature of the coating increases, B203-based glazes are supplemented with or replaced by SiO2-based glass-formers , sometimes incorporating refractory particulates. An example of a successful pack process is the coating system used to protect the C-C composite nose tips and leading edges of NASA
Corrosion of engineering carbons and graphites Temperature/ 2500
1
2000 I
0,D
== m m
(K)
1667 i
I
~ .
539
1429 ,
1250
I
SiO + C = SiO + CO
-1 -2
¢1.
-3
-4 o "5"
-6
2SiO + S i C = 3 S i O + C O 2
i
I
I
5
6
7
8
10^4/T Fl6. 9.
Equilibrium gas pressures for reduction reactions of Si02.
Space Shuttles. 42 The component is heated to 1750-1850°C in a pack containing A1203, SiC and Si, with minor amounts of B and MgO, to form a 5-30 mm primary SiC-based coating. A secondary SiO2-based layer is formed by repeated impregnation with tetraethyl-o-silicate (TEOS) or liquid alkali silicates and other refractory compounds, e.g. SiC, ZrO 2 or Si3N4.43-45 To form conversion coatings carbon articles are heated in a reaction vessel at 1800°C above a mixture of SiC and SiO2. As Fig. 9 shows, at this temperature a SiO-containing atmosphere is created by 2SiOz + SiC = 3SiO + 2C0
(12)
and then SiC is produced by the reaction SiO + 2C = SiC + CO.
(13)
This method produces a SiC concentration gradient in the coating, resulting in a gradation in CTE. A SiC conversion layer applied to carbon rocket nozzles exhibited much less tendency to crack and spall under oxidation conditions than SiC coatings produced by other means, 46 presumably as a result of the gradation in CTE. For these reasons, SiC-based conversion coatings are popular choices as primary layers. Many types of multilayer coatings have been developed using chemical vapour deposition, CVD, including (i) a CVD SiC coating overlaid with various glassformers; 47 (ii) a SiC conversion coating followed by a CVD SiC layer;48 (iii) a SiC pack layer followed by a CVD SiC layer ;49 (iv) a SiC conversion coating overlaid by a
540
L. D. DE CASTROand B. MCENANEY at
b
C
Fro. 10. Some corrosion control systems for a C-C composite: (a) a cracked SiC primary layer and a B20 3 glaze secondary layer incorporating refractory particles; (b) a SiC conversion primary layer and a CVD SiC secondary layer; (c) an internal 13203-based glaze incorporating refractory particles and a cracked SiC external coating.
pack SiC layer and CVD Si3N4.50Various designs of multilayer coatings are sketched in Fig. 10. Recently, SiC coatings have been produced on various carbon substrates using a sol-gel method. 51'5z A gel was produced by acid-catalysed hydrolysis of TEOS to which a phenolic resin was added as a carbon source. Repeated brush application to the carbon surface followed by firing to 1450°C converted the coating to SiC. Figure 11 shows the development of an external coating on an electrode graphite, accompanied by substantial penetration of the internal pore structure. Figure 12 shows the effect of the coating on the oxidation rate of the graphite in air at 920°C.
Internal modifications A possible disadvantage of external coatings is that cracking can lead to rapid and catastrophic failure of the substrate. For this reason, internal modifications of the carbon have been made to provide an alternative or supplementary method of corrosion control. In one method refractory compounds are incorporated into the graphite during manufacture to act as oxidation inhibitors or to form glassy layers on
100 95
9o
~'~
85.
80
Fro. 12.
0
, 5
~ 10
\
t ' ~ r 15
20
2rs
30
TIME (rain) Effect of a sol gel-derived SiC coating on the oxidation resistance of an electrode graphite in air at 920°C: (A) coated; (B) uncoated.
Corrosion of engineeringcarbons and graphites
541
the surface of the carbon under service conditions. Early examples are the JTA grade graphites, developed by Union Carbide Inc. for re-entry applications, 53 which incorporate SiC, Zr, ZrB2 and ZrC. Corrosion control in the JTA materials results from the development of a B 2 0 3 - S i O 2 glaze coating incorporating particles of the Zr-containing refractory compounds on exposure to air at elevated temperatures; JTA graphites perform satisfactorily for several hours on exposure to air at 1700°C. Similar formulations were studied in Russian work, 54 where it was shown that additions of V and Nb compounds improved oxidation resistance in the range 12001800°C. Other carbon-refractory composites developed for corrosion control include C-B4 C55 and C-Zr-ZrC, C-Ti-TiC. 56 A similar range of refractory compounds have been added to C-C composites during manufacture.57 For example, McKee 5s added ZrB2 and B, B4C , SiC and ZrB2 particles to the resin precursor of the matrix in a C-C composite; a B203-based coating was formed which was resistant to repeated thermal cycling at temperatures up to 1200°C. Matrix-modified C-C composites have also been developed by French workers using CVI of SiC, TiC and BN. 59~2 The most successful of these hybrid matrix composites was C-C/SiC which showed both improved mechanical properties and good oxidation resistance in air up to 1500°C. The fibres in C-C composites may also be inhibited by pretreatment. Boron and phophorus compounds in submonolayer quantities confer significant oxidation resistance to carbon materials due to active site poisoning 63'64 (barrier glazes are formed at higher concentrations). These principles have been applied to the inhibition of oxidation of carbon fibres. 65'66 Recently, internal borate-based glass formers encapsulated by a cracked external CVD coating of SiC have been developed (Fig. 10C). 29 T h e s e systems perform satisfactorily in burner rig tests for 20 h up to 1540°C. Corrosion control above 1800°C The problems of devising coatings which can perform satisfactorily at temperatures greater than 1800°C are formidable. Iridium has attracted attention, because its vapour pressure, oxygen permeability and oxidation rate are low. 67 Ir is also compatible with carbon to -2300°C, but a disadvantage is its high CTE. Wright 6s studied the fabrication of Ir and Ir alloy coatings on graphite by plasma arc spraying followed by gas pressure bonding. At 2000°C swelling of the coating occurred due to generation of CO by carbothermic reduction of iridium oxide (formed by reaction of Ir with residual oxygen) at the coating/substrate interface. Chown 69 showed that coatings incorporating refractory carbides and borides, e.g. ZrC and ZrB 2 were satisfactory for short times up to -2200°C, but non-oxide ceramics become increasingly susceptible to oxidation as the temperature increases. On the other hand, refractory oxides, e.g. ZrO2, HfO2 have high oxygen permeabilities and are susceptible to carbothermic reduction. Strife and Sheehan 29 suggest that development of complex multilayer coatings is required to overcome these problems.
CONCLUDING REMARKS As with other areas of corrosion, systems to control high temperature oxidation of carbons and graphites must be designed for particular service conditions; there is no single coating which is satisfactory for all duties. A wide range of glazes, many based upon B203, are used in traditional high temperature applications of engineering carbons and graphites up to -900°C. Higher temperatures require multilayer
542
L.D. DE CASTROand B. MCENANEY
coatings, s o m e t i m e s c o m b i n e d with i n t e r n a l modification of the c a r b o n material. U p to - 1 2 0 0 ° C m u l t i l a y e r coatings c o n t a i n i n g cracked SiC refractory layers of various types a n d B 2 0 3 - b a s e d crack-sealing glazes are used; at higher t e m p e r a t u r e s volatilisation of B203 occurs particularly in moist a t m o s p h e r e s . If B203 is replaced or s u p p l e m e n t e d by SiO2-based glass formers, the m a x i m u m t e m p e r a t u r e can be e x t e n d e d to 1600-1800°C, w h e n significant volatilisation of SiO2 occurs. If SiO2 is in direct c o n t a c t with the c a r b o n substrate, t h e n c a r b o t h e r m i c r e d u c t i o n can occur from - 1 5 0 0 ° C (Fig. 9), a n d the gaseous p r o d u c t s can cause d i s r u p t i o n of the coating. V a r i o u s designs of m u l t i l a y e r coatings for applications at t e m p e r a t u r e s a b o v e 1800°C are b e i n g actively d e v e l o p e d , b u t so far few have p r o v e d successful, except for shortt e r m exposure. REFERENCES 1. W. V. GREEN,J. WEERTMANand E. G. ZUKAS,Mater. Sci. Engng. 6, 199 (1970). 2. K. GRJOTHEIMand B. J. WELCH,Aluminium Smelter Technology, 2nd edn. Aluminium Verlag, Dusseldorf (1988). 3. D. J. SWINDEN, The Arc Furnace, Electroproduction Teaching Monograph 1. The Electricity Council, London (1986). 4. J. V. BEST,W. J. STEPHENand A. J. WICKHAM,Prog. Nucl. Energy 16, 127 (1985). 5. A. MIYAHARAand T. TANABE,J. NucL Mater. 155-167 (Pt A), 49 (1988). 6. I. M. PICKUP,B. McENANEYand R. G. COOKE,Carbon 24, 535 (1986). 7. J. ZHAO,R. C. BRADTand P. L. WALKERJR, Carbon 23, 9 (1985). 8. B.T. KELLY,Prog. Nucl. Energy 16, 73 (1985). 9. E. L. PIPER,in Encyclopaedia of Chemical Technology (eds M. GRAYSONand D. ECKROTH),Vol. 4, pp. 576-588. Wiley, New York (1978). 10. J. D. BUCKLEY,Ceram. Bull. 67, 364 (1988). 11. B. MCENANEYand T. J. MAYS,in Introduction to Carbon Science (ed. H. MARSH),pp. 153-196. Bunerworths, London (1989). 12. R. E. FRANKLIN,Proc. R. Soc. A209, 196 (1951). 13. A. OBERLIN,in Chemistry and Physics of Carbon (ed. P. A. THROWER),Vol. 22, pp. 1-143. Marcel Dekker, New York (1989). 14. A. OBERLINand G. TERRIERE,J. Micros. 18,247 (1973). 15. H. MARSH,Carbon 29,703 (1991). 16. S. E. STEINand R. L. BROWN,Carbon 23, 105 (1985). 17. R. T. YANG,in Chemistry and Physics of Carbon (ed. P. A. THROWER),Vol. 19, pp. 163-210. Marcel Dekker, New York (1984). 18. D. W. MCKEE,in Chemistry and Physics of Carbon (ed. P. A. THROWER),Vol. 16, pp. 1-112. Marcel Dekker, New York (1981). 19. N. R. LAINE,F. J. VASTOLAand P. L. WALKERJR, J. phys. Chem. 67, 2030 (1963). 20. P. CAUSTONand B. MCENANEY,Fuel 64, 1447 (1985). 21. P. EHRBURGER,F. LouYs and J. LAHAYE,Carbon 27, 389 (1989). 22. R. H. ESSENHIGH,in Chemistry of Coal Utilisation (ed. M. A. ELLIOT)2rid Supplementary Vol., p. 1172. Wiley, New York (1981). 23. N. M. LAURENDAU,Prog. Energy Combust. Sci. 4, 221 (1978). 24. S. ERGUN,J. phys. Chem. 60,480 (1956). 25. M. MENTSERand S. ERGUN,U.S. Bureau of Mines Bull. 664 (1973). 26. B. MCENANEY,in Fundamental Issues in Control of Carbon Gasification Reactivity (eds P. EHRBURGER and J. LAHAYE),pp. 175-204. Kluwer, Dordrecht (1991). 27. N. BIRD,B. MCENANEYand B. A. SADLER,in Light Metals 1990 (ed. C. M. BICKERT),pp. 467-472. TMS, Warrendale, PA (1990). 28. D. W. MCKEE,in Chemistry and Physics of Carbon (ed. P. A. THROWER),Vol. 23, pp. 173-232. Marcel Dekker, New York (1991). 29. J. R. STRIFEand J. E. SHEEHAN,Ceram. Bull. 67, 369 (1988). 30. P. EHRaURGER,P. BARRANEand J. LAHAYE,Carbon 24,495 (1986). 31. G. NERIand A. GEMMI,U.K. Patent, 2 023 557 (1980).
Corrosion of engineering carbons and graphites
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32. E. M. GOL'DSHTEIN,M. S. ORENBAKHand M. S. GORPINENKO,Inorg. Mater. (U.S.A.) 16, 1353 (1980). 33. D. W. MCKEE, C. L. SP1ROand E. J. LAMBY,Carbon 22, 285 (1984). 34. A. E. KINDLER, in Ext. Abstr. 17th Am. Carbon Conf., pp. 407-408. American Carbon Society, Lexington, KY (1985). 35. W. E. SCHWABE,J. Met. 24.65 (1972). 36. J. L. SMIALEKand G. C. RYBICKI,U.S. Patent, 4 535 035 (1985). 37. P. VAST, G. PALAVIT,L. MONTAGNE,J. L. BOULIEZand J. CORDIER,in Carbon '88 (eds B. MCENANEY and T. J. MAYS),pp. 582-585. IOP Publishers, Bristol (1988). 38. A. M. FITCHETT, D. J. MORGANand B. J. WELCH, in Light Metals 1988 (ed. L. G. BOXALL),pp. 291294. TMS Warrendale, PA (1988). 39. K. L. LUTHRA, Carbon 26, 217 (1988). 40. A. L. SUTTONand V. C. HOWARD,J. Nucl. Mater. 7, 58 (1962). 41. H. V. JOHNSON,U.S. Patent, 1 948 382 (1934). 42. D. M. SHUFORD,U.S. Patent, 4 471 023 (1984). 43. D. M. CURRY,D. M. SCOTTand C. N. WEBSTER,in Proc. 24th SAMPE Symp., Book 2, pp. 1524-1539. SAMPE, San Francisco (1979). 44. J. W. PATTEN, R. W. MOSS and B. A. FORCHT,U.S. Patent, 4 500 602 (1985). 45. R. A. HOLZL, U.S. Patent, 4 515 860 (1985). 46. K. A. KLEEPINC;,Joint Propulsion Conference, AIAA-86-1499 (1986). 47. W. HUETTNER, R. WEISSand J. SCHEIFFARTH,in Carbon '88 (eds B. MCENANEVand T. J. MAYs), pp. 651~53. IOP Publishers, Bristol (1988). 48. L. DANIS, S. CRUZENand W. SCHIMMEL,ASME Meeting (March 1981), Paper No. 81-GT-35 (1981). 49. R. D. VELTRIand F. S. GALASSO,U.S. Patent, 4 476 178 (1984). 50. R. D. VELTRIand F. S. GALASSO,U.S. Patent, 4 472 476 (1984). 51. L. D. DE CASTROand B. MCENANEV,in Proc. lnt. Symp. Carbon, Vol. 1, pp. 366-369. The Carbon Society of Japan, Tsukuba (1990). 52. L. D. DE CASTROand B. MCENANEY,in Ext. Abstr. 20th Am. Carbon Conf., pp. 420--421. American Carbon Society, Santa Barbara (1991). 53. J. D. BUCKLEr,NASA Technical Note TN D-4231, Langley Research Center (April 1967). 54. D. A. KRIVOSHEIN,M. A. MAURAKH,V. S. DERGUNOVA,Yu. N. PETROVand Yu. N. LEBEDEV,SoY. Powd. Metall; Met. Ceram. 17,460 (1978); transl, from Proshkov. Metallurg. 6, 60 (1978). 55. K. MIYASAKI,T. HAGIOand K. KOBAYASHI,J. Mater. Sci. 16,752 (1981). 56. V. I. KOSTIKOV,A. G. RAKOCH,G. A. KRAVETSKII,B. L. AIRAPETOV,G. D. POSOS'EVA,P. N. DRUGOV and I. G. SUBBOTINA,Inorg. Mat. (U.S.A.) 17,352 (1981); trans, from lsv. Akad. Nauk. S.S.S.R., Neorgan. Mat. 17, 520 (1981). 57. L. C. EHRENREICH,U.S. Patent, 4 119 189 (1978). 58. D. W. MCKEE, Carbon 26, 659 (1988). 59. R. NASLAIN,P. HAGENMULLER,F. CHRISTIN,L. HERAUDand J. J. CHOURY,in Advances in Composite Materials. ICCM-3 (ed. A. R. BUNSELL),Vol. 2, pp. 1084-1097. Pergamon, Paris (1980). 60. R. NASLAIN,J. Y. ROSSIGNOL, P. HAGENMULLER,F. CHRISTIN, L. HERAULDand J. J. CHOURY,Rev. Chim. Min. (France) 18,544 (1981). 6l. J. Y. ROSSIGNOL,R. NASLAIN,P. HAGENMULLERand L. HERAULD,in Proc. 4th Int. Conf. Composite Materials, Vol. 2, pp. 1227-1237. North Holland, Amsterdam (1982). 62. H. HANNACHE,J. M. QUENISSETand R. NASLAIN,J. Mater. Sci. 19,202 (1984). 63. P. MAGNE,H. AMARIGLIOand X. DUVAL, Bull. Soc. Chim. France 6, 2005 (1971). 64. D. W. MCKEE, Carbon 24, 737 (1986). 65. L. E. JONES and P. A. THROWER,J. Chim. Phys. 84, 1431 (1987). 66. K. SAITOand Y. Ko~o, U.S. Patent, 4 197 279 (1980). 67. J. M. CRISCIONE,R. A. MERCURI,E. P. SCHRAM,A. W. SMITHand H. F. VOLK,AFML-TDR-64-173, Part 2 (1964). 68. T. R. WRIGHT,T. R. BRAECKELand D. E. KIZER,Battelle Mere. Inst., Technical Report, AFML TR68-6, AD831448 (1968). 69. J. CHOWN,R. F. DEACON,N. SINGERand A. E. S. WHITE,in Special Ceramics (ed. P. POPPER),p. 81. Academic Press, London (1963).
THE
0010-938X/92 $5.00 + 0.00 © 1992 Pergamon Press plc
CONTROL OF HIGH TEMPERATURE CORROSION OF ENGINEERING CARBONS AND GRAPHITES L u l s DEVINE DE CASTRO* a n d BRIAN M c E N A N E Y t School of Materials Science, University of Bath, Bath BA2 7AY, U.K.
Abstract--Mechanisms of corrosion of engineering carbons and graphites by reaction with gases at high temperature are reviewed and illustrated by the airburn reaction of anode carbons. Methods of controlling corrosion include external coatings and internal modifications of the carbon structure. Single-layer glaze coatings, derived from B203, are widely used for applications up to -900°C. Multilayer coatings consisting of a refractory SiC primary layer and a glass-forming secondary layer which seals cracks are used at higher temperatures. Up to -1200°C, B203-based glazes are used; if BzO3 is replaced or supplemented by SiO2-based glass-formers and/or refractory particulates, the maximum temperature is 1600--1800°C. So far, few coatings have proved successful above 1800°C, except for short term exposures. Internal modifications to the carbon substrate include glass-forming additions, e.g. B-compounds and refractory particulates, e.g. ZrC, ZrB 2 and SiC.
INTRODUCTION
CARBONS and graphites form a unique class of ceramic materials, because in non-oxidising environments they retain their strength and stiffness up to about 2400°C when significant creep occurs; 1this upper temperature limit is considerably in excess of those for other engineering ceramics, e.g. SiC and Si3N 4. Other attractive properties are low densities (resulting in high specific mechanical properties) high thermal conductivities and low thermal expansion coefficients (resulting in excellent thermal shock resistance). As a consequence, carbons and graphites have found many applications in high temperature engineering. The major markets, both in tonnage and value terms, are aluminium smelting,2 where carbon anodes and graphite cathodes are used, and graphite electrodes for electric arc steel-making. 3 Substantial quantities of carbon materials are used in other areas of extractive metallurgy as electrodes, refractories and crucible components. Specialised materials are used as moderator and structural components in gas-cooled nuclear reactors 4 and in the defence/aerospace industry for aircraft brakes, rocket engine components, nose tips and leading edges of re-entry vehicles. Carbon materials are also leading candidates for first wall duties in fusion energy devices. 5 A major disadvantage of carbon materials is their low oxidation resistance. Depending upon materials, environment and application, carbon begins to react with oxidising gases at appreciable rates from about 400°C. The overall corrosion reactions of carbons in oxygen-containing gases are C -~- 0 2 = C O 2
(1)
*Present address: Centro Technol6gico do Exdrcito, Av. das Am6ricas, 28705, Guaratariba, 23020 Rio de Janeiro, Brasil. tAuthor to whom correspondence should be addressed. Manuscript received 23 September 199l. 527
528
L.D. DECASTROand B. MCENANEY
and 2C
+ 0 2 =
2CO,
(2)
with (1) dominating at low temperatures and (2) at temperatures greater than about 700°C. Corrosion in gases containing steam or carbon dioxide follows the overall reactions C + HzO = CO + Hz (3) C + COz = 2CO.
(4)
Since no protective scale is formed, the high temperature properties of carbon materials cannot be exploited unless steps are taken to control corrosion. Several studies 6'7 have shown that 5-10% weight loss due to corrosion by oxidising gases reduces the strength of various carbon materials by - 5 0 % . This paper surveys both mechanisms of high temperature, gaseous corrosion of engineering carbons and graphites and methods for controlling corrosion. Radiolytic corrosion of nuclear graphites, 8 low temperature corrosion by liquids, and gasification of particulate carbons, e.g. coal chars, are not directly considered.
MANUFACTURE OF ENGINEERING CARBONS AND GRAPHITES Most engineering carbons and graphites are polygranular materials which are manufactured from sized filler particles and a binder (Fig. la). The major source of
(a)
I
(b)
I
l C
I ( oo)
)
=oot)
~eat
I
~inal heat eatment ) Carbon
product
[
~raphi6~~
0mpregnate
'
1 C(¢CoCpOsite )
Grodrap hire uct
FIG.1. Manufacturingsteps for: (a) polygranularcarbonsand graphites;(b) carboncarboncomposites.
)
FIG. 2. Microstructure of: (A) a polygranular graphite, showing (a-b) filler particle; (c) binder phase; and (d) porosity; (B) a carbon-carbon composite, showing (a,b) mutually perpendicular fibre bundles; (c) pores; and (d) cracks. 529
FIG. 11. The development of a sol gel-derived SiC coating on an electrode graphite,52 showing the effect of repeated application and firing: (a) fired once; (b) fired twice; (c) fired three times. EDX images for Si.
530
Corrosion of engineeringcarbonsand graphites
531
filler is petroleum coke which has been calcined to about 1350°C and the binder is usually a pitch derived from coal tar. When heated the binder/filler mixture is a thermoplastic body which can be formed by moulding or extrusion. The baking step which converts the pitch binder to carbon is critical, since there is a loss of volatiles and shrinkage during carbonisation of the binder, which must be accommodated by very slow heating rates. Impregnation with a resin or pitch is used to increase the density of the final product. Carbon products, e.g. anodes for aluminium smelting, 2 are produced after the baking step by heat treatment to about 1200°C. Heattreatment in the range 2700-3000°C (Fig. la), is required to produce graphite electrodes, e.g. for electric arc steel-making; Piper 9 gives further details of fabrication methods. The demands for high performance, high temperature materials from the defence/aerospace industries have prompted the development of an extensive range of carbon-carbon (C-C) composites, i.e. carbon fibre reinforcement in a carbon matrix, m In the fabrication of C-C composites (Fig. lb), textile engineering methods are used to produce preforms with a wide range of multi-directional fibre arrays. There are three classes of commercially-available carbon fibres: ex-polyacrylonitrile (PAN fibres), and ex-mesophase pitch (MP fibres) and ex-rayon fibres. The carbon matrix has been formed by carbonising thermosetting resins, pitches and by chemical vapour infiltration, CVI, methods. It is usually necessary to repeat the impregnation/ carbonisation cycle several times to aeheive an adequate matrix density, to The microstructure of a polygranular graphite (Fig. 2A), shows a coke filler particle, a-b, and binder carbon, c. The presence of networks of pores in carbon materials, e.g. d, allows ingress of corroding gases to the interior causing internal corrosion. The microstructure of a three-directional C-C composite (Fig. 2B), shows mutually perpendicular fibre bundles, a, b, and a network of cracks and pores, c, d; the origins of porosity in different types of carbon have been discussed elsewhere. 11 MICROSTRUCTURE OF ENGINEERING CARBONS AND GRAPHITES All carbons and graphites have more-or-less disordered microstructures which can be related to that of the graphite single crystal. For example, in electrode graphites the degree of graphitic structure is extensive, but products such as anode carbons, which have only been heated to about 1200°C, have much more disordered structures. A basic concept is the carbon layer plane, which in the graphite single crystal is an extended, planar, hexagonal array of carbon atoms. In disordered carbons layer planes may be limited in extent and contain many defects and heteroelements (e.g. H,O,N,S) bound to their edges; the nature of hetero-elements depends upon the precursors and final heat-treatment temperature. X-ray diffraction patterns of even the most disordered carbons ~2 show diffuse bands in the approximate positions of the (002) and (100) graphite lines, indicating a turbostratic structure, i.e. carbon layer planes stacked in parallel groups, but with no threedimensional graphitic ordering. The structure of carbons revealed by X-ray diffraction has been elaborated in recent years using electron microscopy. Oberlin et al. 13,14 propose the term 'Basic Structural Unit', BSU, to describe the smallest periodic feature that can be imaged by 002, 10 or 11 dark fields, The smallest BSU, of size less than 1 nm, are envisaged as small carbon layer planes, comprising less than 12 hexagonal rings, in stacks of two or three. In some carbons several BSU may lie approximately parallel, constituting a
532
L.D. DE CASTROand B. McENANEY
LMO Fxc. 3. Schematic representation of Basic Structural Units, BSU, and Local Molecular Ordering, LMO, in a non-graphitic carbon; adapted from Ref. 13.
region of Local Molecular Ordering, L M O (Fig. 3), which can range from the nanometric to the millimetric scale. L M O greater than about 1/~m in size are termed lamellae, which can be either flat or crumpled, depending upon the degree of disorder within the carbon layer planes. In the graphitisation process, represented schematically15 in Fig. 4, isolated BSU grow into distorted colt, mns which at higher temperatures undergo lateral condensation to produce crumpled lamellae. At the highest temperatures the defects within carbon layer planes are removed to produce extensive fiat lamellae with three-
FIG. 4. A schematicrepresentation of the graphitisation process; reproduced from Ref. 15 with permission.
Corrosion of engineering carbons and graphites
533
dimensional graphitic ordering. Thus, there is a textural continuum between disordered carbons and materials which develop extended graphitic ordering. G A S - - C A R B O N REACTION M E C H A N I S M S
The oxidation of the graphite single crystal by gases is highly anisotropic, because the reactivity of carbon atoms at the edges of graphite basal planes, is orders of magnitude higher than that within basal planes. ~6 Point defects and emergent dislocations in the basal planes are also reactive sites, as abundantly illustrated using etch decoration techniques. 17 In the study of gas-carbon reactions this has led to the concept of active sites. At each active site oxygen-containing functional groups may form during gasification and reactant chemisorption, migration of intermediates, and product desorption may take place sequentially. Hetero-elements and catalytic mineral matter present in the carbon may form additional active sites. ~s In the absence of inorganic catalysts, a weli-graphitised material will have a lower active site concentration than a disordered material, leading in turn to a lower corrosion rate. 19-21 Intrinsic reactivities
Because of their importance in gasification of fossil fuels, the mechanisms of gascarbon reactions have been extensively studied. A comprehensive account is outside the scope of this review, but there are excellent treatments of the subject as it applies to coal char gasification. 22,23Quantitative treatments of gas-carbon reaction kinetics start with a measure of overall reactivity which is often expressed on a mass-specific basis, R [(g g-l s-l]: R = (dX/dt)/(1 - X ) ,
(5)
where X is the fractional conversion at time t. An illustration of the relationship between overall and intrinsic reactivities is provided by the C-CO2 reaction, for which there is substantial agreement as to the mechanism 24,2~ k I
CO 2 "~ Cf k~---_~CO + C(O)
(6)
C(O) -~ CO,
(7)
where Cf and C(O) represent an unoccupied and occupied active site respectively. Under most conditions reaction (6) is fast and reversible and reaction (7) is ratecontrolling. Assuming steady-state conditions gives a Langmuir-Hinshelwood expression for the intrinsic reaction rate Ri [sites (g C ) - l s a] R i = (Ct)kt(CO2)/[]
+ a(CO) + b(CO2)],
(s)
where (Ct) is the total active site concentration, (CO2) and (CO) are the concentrations of these gases at the surface of the carbon, a = k _ f l k 3 and b = k l / k 3. The overall reactivity R = mcRi, where m c is the mass of a carbon atom. There is much evidence that R is directly proportional to (Ct).26 A direct proportionality between R and (Ct), similar to equation (8), can be obtained for the C - H 2 0 reaction; the
534
L.D. DECASTROand B. MCENANEY Zone III
b
i o
~
Zone I ]zr
I
FIG. 5. A schematicArrhenius plot for reaction of ~ases with porous carbons. mechanism of the C-O2 reaction is much more complex, but direct proportionalities between R and (Ct) have been established for certain conditions in this case also. 23
Mass transport effects Engineering graphites contain significant porosity (Fig. 2), so that in practical situations the overall rate of reaction is influenced by in-pore diffusion and by boundary layer diffusion at higher temperatures. Equation (8) is derived assuming that the overall gasification rate, R, is determined solely by the reactivity of the carbon surface. Evidence for diffusional effects is provided by Arrhenius plots for gas--carbon reactions which are often curved with the apparent activation energy decreasing with increasing temperature, the decrease being attributed to the increasing influence of gaseous diffusion. An idealized representation of this trend is in Fig. 5. At low temperatures, Zone I, the concentration of reactant gas within the pores of the carbon is everywhere the same as the bulk gas concentration, Co, and R = mcR i. With increasing temperature, there is a decrease in the reactant gas concentration within the pores, Zone a, caused by both (i) the chemical reaction rate becoming greater than the rate of supply of gas through the pores in the char and (ii) the resistance offered by the outwardly-diffusing product gas. Within Zone a, the concentration of reactant gas at the centre of the carbon will decrease progressively as temperature rises, until it eventually falls to zero. This represents the transition to Zone II. Further increases in temperature produce a significant outward flux of product gas from the external surface of the carbon and this causes reduction in the reactant gas concentration in a boundary layer; this corresponds to Zone b. The concentration of reactant gas at the external surface of the carbon, Cs, falls progressively with increasing temperature until eventually Zone III is reached, where Cs approaches zero. CORROSION OF ANODE CARBONS The role of chemical and diffusional factors may be illustrated by the corrosion of anode carbons used in aluminium smelting; a sketch of the arrangement of a pre-
Corrosion of engineering carbons and graphites
535
Alumina cover
Frozen electrolyte Molten
Electrolyte Metal
Cathode block L o c a t i o n o f a p r e - b a k e d c a r b o n a n o d e i n a Hall-Heraultcell.
FIG. 6.
baked anode in a Hall-Heroult cell is in Fig. 6. The reduction of alumina to aluminium is accompanied by consumption of the carbon anode. Under the current density conditions usually employed 2 in industry the dominant reaction is 2A120 3 + 3C = 4A1 + 3CO 2.
(9)
There is additional, unwanted consumption of the anode due to a number of factors including the airburn and carboxy secondary reactions which contribute 8-15 wt% and 5-6 wt% respectively to total anode consumption. 2 The airburn reaction occurs on the upper surfaces of the anode by a combination of reactions (1) and (2), depending on the temperature. An Arrhenius plot of thermogravimetric data for airburn of anodes (Fig. 7), 27 shows pronounced curvature. The apparent activation energy in the temperature range 580--620°C is
7 ZoneIII rate
5'
C
~
'
~
ZoneII rate
4-
o
3' 0
_= 2
1
0.0011
!
!
|
0.0012
0.0013
0.0014
0.0015
[1/[']/[1/I(] FIG. 7.
Arrhenius plot for the airburn reaction of anode carbons; adapted from Ref. 27.
536
L.D. DECASTROand B. MCENANE¥
15 kJ mo1-1, indicative of boundary layer effects influencing the rate. Under conditions where boundary layer diffusion and in-pore diffusion are influencing the corrosion, the mass-specific rate, R, is given by R = Co[1/K' - 1/g"] -~
(a0)
where K' and K" are the mass transfer coefficients in pores and in the boundary layer respectively and Co is the concentration of oxygen in air. If it is assumed that CO2 is the sole product of the reaction and that stagnant conditions remain, then R in Zone III is R = ADCo/6 = K"Co,
(11)
where D is the free gas diffusivity of 02 in the boundary layer of width d and A is a stoichiometric coefficient. Using these equations it is possible to compute Zone II and Zone III rates. 27 Figure 7 shows that below 450°C the airburn reaction rate is influenced by in-pore diffusion; as the temperature increases there is a transition until at temperatures above 570°C boundary layer diffusion controls the corrosion rate. The carboxy reaction occurs as a result of corrosion by CO 2 produced at the anode face by reaction (4) at temperatures of -950°C. Under the influence of hydrostatic pressure, the CO 2 permeates the pore network in the carbon causing corrosion up to 10 cm behind the anode face. The rate of the carboxy reaction is therefore influenced by in-pore diffusion. 27 CONTROL OF CORROSION OF CARBONS AND GRAPHITES There is a vast literature on methods for controlling the corrosion of carbon and graphites at high temperatures, so that only a brief review can be attempted; more detailed surveys have been published. 28'29 In principle corrosion control is possible by modifying reactant gas composition. For example, equation (8) shows that the C-COz reaction can be inhibited by increasing the CO content of the gas. An important example of this approach, which is outside the scope of this review, is the control of radiolytic corrosion of nuclear graphites by CO2-based gases in British Magnox and A G R nuclear reactors by additions of CH4, CO and H20. 4'8 However, the great majority of methods seek to modify the structure of the component, either by use of external coatings and/or by internal modifications of the carbon material. E x t e r n a l coatings
Selection of a coating depends upon factors such as the corrosive gas composition and hydrodynamics and the time-temperature profile of the component. Thus a coating which may be satisfactory for a single short period of exposure at high temperatures, e.g. rocket re-entry conditions, may not be suitable for prolonged exposure at lower temperatures with multiple thermal cycling, e.g. jet engine components. The qualities required of a coating to protect a carbon substrate from oxidation at high temperatures include: 29 (i) refractoriness, i.e. low volatility and good high temperature physical/ mechanical properties; (ii) chemical compatibility with carbon and the corrosive atmosphere;
Corrosion of engineering carbons and graphites
Temperature/ 10 i. O I--
2500
2000
i
a
silica
(K)
1670 1430 i
537
1250
t
I
1110 i
I
,
silicon nitride
-i 10
a.
0
O g
silicon carbide O -
-
-10 3
I
I
I
I
I
I
4
5
6
7
8
9
IO000/T
Fro. 8.
0
(K-l)
Equilibrium vapour pressures of some coating materials.
(iii) low permeability to oxygen and carbon; (iv) adherence at the carbon/coating interface. These requirements amount to a formidable specification, which becomes more stringent as the operating temperature of the component increases. Successful formulations can be classified into single layer coatings, used at lower temperatures, and multilayer coatings for higher temperatures. Single layer coatings are mainly glazes which are fluid in the service temperature range; glazes can be self-healing, provided they wet the carbon surface. The most successful glazes are based upon B203, 3°-32 which melts around 500°C, i.e. close to the temperature at which carbons begin to oxidise. BzO 3 has good wetting properties for carbon, although a disadvantage is that it is readily hydrolysed in moist atmospheres. B20 3 remains fluid up to -1200°C when vaporisation becomes significant (Fig. 8), particularly in moist atmospheres. P-containing glazes derived from a wide variety of inorganic and organophosphorus compounds have also been shown to be effective in controlling corrosion of carbons and graphites up to -900°C. 33,34 There is an extensive patent literature on the formation of oxidation resistant coatings by thermal and air spraying on arc furnace electrode graphites to control side-wall corrosion which can account for up to - 4 0 % of the electrode wastage. 35 Typically, an aluminium bond coat followed by a powdered, glass-forming coating are applied to the electrode and the coating is melted on to the graphite surface using an electric arc. Recent examples of electrode coatings include Ni-Si and AI-Si powders in a nitrocellulose lacquer36 and polyphosphate formulations.37 In modern electric arc practice water-cooling has led to a reduction in the demand for coated electrodes. Control of the airburn reaction of anodes in aluminium is made by the simple expedient of covering the upper surfaces of the anode with alumina powder or crushed cryolite-based electrolyte bath, 38 see Fig. 6. The permeability of oxygen in B203-based glazes sets an upper limit to their application in the range 800-900°C. 39For higher temperatures coatings must include
538
L.D. DECASTROand B. MCENANEY TABLE 1.
COEFFICIENTS OF THERMAL EXPANSION FOR CARBON MATERIALS AND CERAMICS
Material Carbon--carbon composite Coarse-grainedgraphite Medium-grainedgraphite Fine-grainedgraphite Vitreous silica Siliconnitride Siliconcarbide Hafniumcarbide Zirconiumcarbide
CTE/(10-6 °C- 1) at 1000°C -0.05 3.1 4.3 7.0 0.04 2.8 6.1 6.6 6.7
refractory compounds with low permeabilities to oxygen and carbon. A major problem results from the low coefficients of thermal expansion, CTE, of carbon materials relative to many candidate coating refractories. 29 If the CTE of the coating exceeds that of the carbon substrate, then microcracking occurs on cooling from the application temperature or upon thermal cycling. From calculations of the oxidation rate for a C--C composite with a model cracked coating, Luthra 39 has shown that 10 nm wide cracks can provide paths for rapid diffusion of oxygen. CTE values for carbons range from -0.05 x 10 -6 K -1 for some C-C composites to 7 × 10-6 K -1 for fine-grained graphites (Table 1). The low values of CTE are partly because the extensive crack networks in engineering carbons and graphites (Fig. 2), can accommodate thermal expansion. 4° The most widely-used component of multilayer coatings on carbons is SiC. The attractions of SiC are its refractoriness, its low CTE ( - 6 x 10 -6 K -1) and low oxygen permeability. It also has a low rate of oxidation and the SiOz film formed is protective. A practical advantage is the number of established methods for forming SiC coatings (see below). The upper temperature limit (-1800°C) for long term use of SiC is set by dissociation of the SiO2 protective layer (Fig. 8), and its carbothermic reduction. Figure 9 shows that significant pressures of CO and SiO vapours can be generated at 1800°C, which can cause disruption of protective coatings. Application methods for multilayer coatings on carbons and graphites include: pack processes, conversion coatings and chemical vapour deposition. An early patent 41 for a two-layer coating describes an inner SiC layer (formed by a pack process using SiC and Si) and an outer B203-based glaze; part of the function of the glaze is to fill cracks in the SiC layer which are formed as a result of CTE mismatch (Table 1). The concept of a (cracked) primary SiC layer in conjunction with a crack sealing, glass-forming layer has formed the basis for many coating designs. In many cases, as the service temperature of the coating increases, B203-based glazes are supplemented with or replaced by SiO2-based glass-formers , sometimes incorporating refractory particulates. An example of a successful pack process is the coating system used to protect the C-C composite nose tips and leading edges of NASA
Corrosion of engineering carbons and graphites Temperature/ 2500
1
2000 I
0,D
== m m
(K)
1667 i
I
~ .
539
1429 ,
1250
I
SiO + C = SiO + CO
-1 -2
¢1.
-3
-4 o "5"
-6
2SiO + S i C = 3 S i O + C O 2
i
I
I
5
6
7
8
10^4/T Fl6. 9.
Equilibrium gas pressures for reduction reactions of Si02.
Space Shuttles. 42 The component is heated to 1750-1850°C in a pack containing A1203, SiC and Si, with minor amounts of B and MgO, to form a 5-30 mm primary SiC-based coating. A secondary SiO2-based layer is formed by repeated impregnation with tetraethyl-o-silicate (TEOS) or liquid alkali silicates and other refractory compounds, e.g. SiC, ZrO 2 or Si3N4.43-45 To form conversion coatings carbon articles are heated in a reaction vessel at 1800°C above a mixture of SiC and SiO2. As Fig. 9 shows, at this temperature a SiO-containing atmosphere is created by 2SiOz + SiC = 3SiO + 2C0
(12)
and then SiC is produced by the reaction SiO + 2C = SiC + CO.
(13)
This method produces a SiC concentration gradient in the coating, resulting in a gradation in CTE. A SiC conversion layer applied to carbon rocket nozzles exhibited much less tendency to crack and spall under oxidation conditions than SiC coatings produced by other means, 46 presumably as a result of the gradation in CTE. For these reasons, SiC-based conversion coatings are popular choices as primary layers. Many types of multilayer coatings have been developed using chemical vapour deposition, CVD, including (i) a CVD SiC coating overlaid with various glassformers; 47 (ii) a SiC conversion coating followed by a CVD SiC layer;48 (iii) a SiC pack layer followed by a CVD SiC layer ;49 (iv) a SiC conversion coating overlaid by a
540
L. D. DE CASTROand B. MCENANEY at
b
C
Fro. 10. Some corrosion control systems for a C-C composite: (a) a cracked SiC primary layer and a B20 3 glaze secondary layer incorporating refractory particles; (b) a SiC conversion primary layer and a CVD SiC secondary layer; (c) an internal 13203-based glaze incorporating refractory particles and a cracked SiC external coating.
pack SiC layer and CVD Si3N4.50Various designs of multilayer coatings are sketched in Fig. 10. Recently, SiC coatings have been produced on various carbon substrates using a sol-gel method. 51'5z A gel was produced by acid-catalysed hydrolysis of TEOS to which a phenolic resin was added as a carbon source. Repeated brush application to the carbon surface followed by firing to 1450°C converted the coating to SiC. Figure 11 shows the development of an external coating on an electrode graphite, accompanied by substantial penetration of the internal pore structure. Figure 12 shows the effect of the coating on the oxidation rate of the graphite in air at 920°C.
Internal modifications A possible disadvantage of external coatings is that cracking can lead to rapid and catastrophic failure of the substrate. For this reason, internal modifications of the carbon have been made to provide an alternative or supplementary method of corrosion control. In one method refractory compounds are incorporated into the graphite during manufacture to act as oxidation inhibitors or to form glassy layers on
100 95
9o
~'~
85.
80
Fro. 12.
0
, 5
~ 10
\
t ' ~ r 15
20
2rs
30
TIME (rain) Effect of a sol gel-derived SiC coating on the oxidation resistance of an electrode graphite in air at 920°C: (A) coated; (B) uncoated.
Corrosion of engineeringcarbons and graphites
541
the surface of the carbon under service conditions. Early examples are the JTA grade graphites, developed by Union Carbide Inc. for re-entry applications, 53 which incorporate SiC, Zr, ZrB2 and ZrC. Corrosion control in the JTA materials results from the development of a B 2 0 3 - S i O 2 glaze coating incorporating particles of the Zr-containing refractory compounds on exposure to air at elevated temperatures; JTA graphites perform satisfactorily for several hours on exposure to air at 1700°C. Similar formulations were studied in Russian work, 54 where it was shown that additions of V and Nb compounds improved oxidation resistance in the range 12001800°C. Other carbon-refractory composites developed for corrosion control include C-B4 C55 and C-Zr-ZrC, C-Ti-TiC. 56 A similar range of refractory compounds have been added to C-C composites during manufacture.57 For example, McKee 5s added ZrB2 and B, B4C , SiC and ZrB2 particles to the resin precursor of the matrix in a C-C composite; a B203-based coating was formed which was resistant to repeated thermal cycling at temperatures up to 1200°C. Matrix-modified C-C composites have also been developed by French workers using CVI of SiC, TiC and BN. 59~2 The most successful of these hybrid matrix composites was C-C/SiC which showed both improved mechanical properties and good oxidation resistance in air up to 1500°C. The fibres in C-C composites may also be inhibited by pretreatment. Boron and phophorus compounds in submonolayer quantities confer significant oxidation resistance to carbon materials due to active site poisoning 63'64 (barrier glazes are formed at higher concentrations). These principles have been applied to the inhibition of oxidation of carbon fibres. 65'66 Recently, internal borate-based glass formers encapsulated by a cracked external CVD coating of SiC have been developed (Fig. 10C). 29 T h e s e systems perform satisfactorily in burner rig tests for 20 h up to 1540°C. Corrosion control above 1800°C The problems of devising coatings which can perform satisfactorily at temperatures greater than 1800°C are formidable. Iridium has attracted attention, because its vapour pressure, oxygen permeability and oxidation rate are low. 67 Ir is also compatible with carbon to -2300°C, but a disadvantage is its high CTE. Wright 6s studied the fabrication of Ir and Ir alloy coatings on graphite by plasma arc spraying followed by gas pressure bonding. At 2000°C swelling of the coating occurred due to generation of CO by carbothermic reduction of iridium oxide (formed by reaction of Ir with residual oxygen) at the coating/substrate interface. Chown 69 showed that coatings incorporating refractory carbides and borides, e.g. ZrC and ZrB 2 were satisfactory for short times up to -2200°C, but non-oxide ceramics become increasingly susceptible to oxidation as the temperature increases. On the other hand, refractory oxides, e.g. ZrO2, HfO2 have high oxygen permeabilities and are susceptible to carbothermic reduction. Strife and Sheehan 29 suggest that development of complex multilayer coatings is required to overcome these problems.
CONCLUDING REMARKS As with other areas of corrosion, systems to control high temperature oxidation of carbons and graphites must be designed for particular service conditions; there is no single coating which is satisfactory for all duties. A wide range of glazes, many based upon B203, are used in traditional high temperature applications of engineering carbons and graphites up to -900°C. Higher temperatures require multilayer
542
L.D. DE CASTROand B. MCENANEY
coatings, s o m e t i m e s c o m b i n e d with i n t e r n a l modification of the c a r b o n material. U p to - 1 2 0 0 ° C m u l t i l a y e r coatings c o n t a i n i n g cracked SiC refractory layers of various types a n d B 2 0 3 - b a s e d crack-sealing glazes are used; at higher t e m p e r a t u r e s volatilisation of B203 occurs particularly in moist a t m o s p h e r e s . If B203 is replaced or s u p p l e m e n t e d by SiO2-based glass formers, the m a x i m u m t e m p e r a t u r e can be e x t e n d e d to 1600-1800°C, w h e n significant volatilisation of SiO2 occurs. If SiO2 is in direct c o n t a c t with the c a r b o n substrate, t h e n c a r b o t h e r m i c r e d u c t i o n can occur from - 1 5 0 0 ° C (Fig. 9), a n d the gaseous p r o d u c t s can cause d i s r u p t i o n of the coating. V a r i o u s designs of m u l t i l a y e r coatings for applications at t e m p e r a t u r e s a b o v e 1800°C are b e i n g actively d e v e l o p e d , b u t so far few have p r o v e d successful, except for shortt e r m exposure. REFERENCES 1. W. V. GREEN,J. WEERTMANand E. G. ZUKAS,Mater. Sci. Engng. 6, 199 (1970). 2. K. GRJOTHEIMand B. J. WELCH,Aluminium Smelter Technology, 2nd edn. Aluminium Verlag, Dusseldorf (1988). 3. D. J. SWINDEN, The Arc Furnace, Electroproduction Teaching Monograph 1. The Electricity Council, London (1986). 4. J. V. BEST,W. J. STEPHENand A. J. WICKHAM,Prog. Nucl. Energy 16, 127 (1985). 5. A. MIYAHARAand T. TANABE,J. NucL Mater. 155-167 (Pt A), 49 (1988). 6. I. M. PICKUP,B. McENANEYand R. G. COOKE,Carbon 24, 535 (1986). 7. J. ZHAO,R. C. BRADTand P. L. WALKERJR, Carbon 23, 9 (1985). 8. B.T. KELLY,Prog. Nucl. Energy 16, 73 (1985). 9. E. L. PIPER,in Encyclopaedia of Chemical Technology (eds M. GRAYSONand D. ECKROTH),Vol. 4, pp. 576-588. Wiley, New York (1978). 10. J. D. BUCKLEY,Ceram. Bull. 67, 364 (1988). 11. B. MCENANEYand T. J. MAYS,in Introduction to Carbon Science (ed. H. MARSH),pp. 153-196. Bunerworths, London (1989). 12. R. E. FRANKLIN,Proc. R. Soc. A209, 196 (1951). 13. A. OBERLIN,in Chemistry and Physics of Carbon (ed. P. A. THROWER),Vol. 22, pp. 1-143. Marcel Dekker, New York (1989). 14. A. OBERLINand G. TERRIERE,J. Micros. 18,247 (1973). 15. H. MARSH,Carbon 29,703 (1991). 16. S. E. STEINand R. L. BROWN,Carbon 23, 105 (1985). 17. R. T. YANG,in Chemistry and Physics of Carbon (ed. P. A. THROWER),Vol. 19, pp. 163-210. Marcel Dekker, New York (1984). 18. D. W. MCKEE,in Chemistry and Physics of Carbon (ed. P. A. THROWER),Vol. 16, pp. 1-112. Marcel Dekker, New York (1981). 19. N. R. LAINE,F. J. VASTOLAand P. L. WALKERJR, J. phys. Chem. 67, 2030 (1963). 20. P. CAUSTONand B. MCENANEY,Fuel 64, 1447 (1985). 21. P. EHRBURGER,F. LouYs and J. LAHAYE,Carbon 27, 389 (1989). 22. R. H. ESSENHIGH,in Chemistry of Coal Utilisation (ed. M. A. ELLIOT)2rid Supplementary Vol., p. 1172. Wiley, New York (1981). 23. N. M. LAURENDAU,Prog. Energy Combust. Sci. 4, 221 (1978). 24. S. ERGUN,J. phys. Chem. 60,480 (1956). 25. M. MENTSERand S. ERGUN,U.S. Bureau of Mines Bull. 664 (1973). 26. B. MCENANEY,in Fundamental Issues in Control of Carbon Gasification Reactivity (eds P. EHRBURGER and J. LAHAYE),pp. 175-204. Kluwer, Dordrecht (1991). 27. N. BIRD,B. MCENANEYand B. A. SADLER,in Light Metals 1990 (ed. C. M. BICKERT),pp. 467-472. TMS, Warrendale, PA (1990). 28. D. W. MCKEE,in Chemistry and Physics of Carbon (ed. P. A. THROWER),Vol. 23, pp. 173-232. Marcel Dekker, New York (1991). 29. J. R. STRIFEand J. E. SHEEHAN,Ceram. Bull. 67, 369 (1988). 30. P. EHRaURGER,P. BARRANEand J. LAHAYE,Carbon 24,495 (1986). 31. G. NERIand A. GEMMI,U.K. Patent, 2 023 557 (1980).
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