Automotive Catalytic Converters
4.4 Modifications to the Alloy Composition
duced from 1102 hours to 849 hours as the aluminium content is increased from 8 to 13wt%
[30].
These studies have shown that increasing the aluminium content within the foil may be of benefit to the oxidation lifetime of the alloy, but that there is an upper limit of 8wt%. Above this the improvement is marginal and at even higher levels may be detrimental. As increasing the aluminium content is detrimental to the alloys mechanical properties, particularly ductility, research should be undertaken on methods to enrich the aluminium content in prefabricated components. Chemical vapour deposition processes show promise [31,32], but from the above analysis the aluminium content of the foil must be limited to below 8wt%.
4.4.2 The Addition of Multiple Active Elements
From Table 1 it can be seen that many, and sometime multiple, active elements have been con- sidered to improve the oxidation behaviour and scale adhesion of alumina forming ferritic steels. Generally the level of addition is less than 0.3wt% metallic addition with the more fa- voured additions including yttrium, lanthanum, cerium, zirconium, hafnium and possibly tita- nium at low levels [33].
The property improvement which can be achieved by the addition of reactive elements (RE’s) depends on the type of reactive element used, its amount, distribution and form within the alloy. Numerous papers exists in the literature in this area as evident from the many review papers in reference [8]. Most studies confirm that the dominant mechanisms are tramp element scavenging, such as removing sulphur impurities [34,35] or modifying the transport properties within the oxide scale, due to some modification of the oxide grain boundaries [36]. Both of these mechanisms have been observed, and prevail, for the alumina forming ferritic steels. The questions are therefore, which active element? How much? And in what form?
As evident from available commercial alloys (Table 1), research and development favours Y, Ce, La, Zr and Hf and discussion will be limited in the main to these elements. The role of titanium is an interesting one, as should it be considerable as a minor alloying addition, a tramp element (impurity) or an active element? Recent work has shown that small additions of tita- nium may be beneficial and it may indeed be acting as an active element [33].
In FeCrAl-RE foils reactive elements are commonly added as metallic additions, with addi- tions as low as 0.05wt% providing optimum oxidation behaviour [30]. In fact, the lowest mass gain has been measured for yttrium additions of 0.02wt% [29], however, this alloy showed evidence of spalling after 1000 h at 1100 °C and therefore the slightly higher value is consid- ered optimum. If the yttrium content is increased further to 0.07 or 0.08wt% then internal oxi- dation is observed at 1100 °C, which is though to be detrimental to the behaviour of foil mate- rials [37].
Hafnium, like yttrium has a beneficial effect on both scale adherence and oxide growth rate [29]. Similarly zirconium may be considered beneficial, when added singly, but in combination with yttrium may be detrimental to thin foil material by increasing the depth of internal oxida- tion [29,38]. Thus to a Fe-20Cr-5Al alloy containing 0.04wt% Y, the addition of 0.04wt% Zr may be beneficial, however, increasing either the yttrium content or zirconium content above this level can cause extensive internal oxidation, penetrating up to 120mm from each free sur- face [29]. However, a 0.04wt% addition of hafnium in addition to yttrium is beneficial at tem- peratures in the range 1100–1350 °C and is the basis of the commercial Aluchrom YHf alloy.
Equally 0.10wt% Zr, without yttrium is also beneficial, as demonstrated by the excellent high temperature oxidation behaviour of Kanthal APM. This complexed interplay suggests that
there are optimum levels of yttrium, hafnium and zirconium and that these levels may vary with the aluminium content of the alloy.
The choice of the optimum combination of active elements within thin foil FeCrAl-RE mate- rials is a clear area for future research as the appropriate selection should reduce oxidation rates and improve alloy adherence without adversely affecting the foil mechanical properties through internal oxidation. Care has to be taken to limit “overdoping” because of the increased oxidation rates and internal oxidation damage that it produces. The role of titanium, a benefi- cial element or detrimental impurity, is still an open debate. Recent compelling papers by Quadakkers and coworkers [33,39,40] suggest that for small additions it behaves like hafnium and zirconium, but that in combination with other active elements it is easy to overdope the alloy [37,38].
4.4.3 The Interaction of Reactive Elements with Alloy Impurities
Minor additions of titanium, zirconium and hafnium have been shown to be beneficial in lim- iting internal oxidation attack. [29, 37–40]. This is associated with their ability to tie up carbon and nitrogen impurities as tiny carbo-nitrides [39,40].
Other tramp elements, such as phosphorus, vanadium calcium etc and minor additions such as silicon must also be considered.
Calcium and vanadium are thought to be benign, not showing a clear effect at concentrations of 5 and 80ppm respectively. Phosphorus equally is benign at 1100 °C and 1200 °C, however at 1300 °C it can drastically decrease the lifetime to chemical failure, associated with a sub- stantial increase in oxide growth rate [40,41]. This detrimental behaviour is associated with phosphorus segregation at oxide grain boundaries and the metal oxide interface [41].
Clearly, these interactions between the reactive elements and minor alloy impurities is an important area of future study. Results strongly indicate that optimum oxidation resistance can be achieved through a combination of multiple active elements. The choice of the right levels, however, can only be made when their interaction with alloy impurities is fully quantified.
Thus in future alumina forming ferritic steels, not only the major alloy additions need con- trolling,but also the combination of active elements and the type, amounts and distributions of tramp elements and other manufacturing alloy impurities.
5 Concluding Remarks
In this paper, pertinent materials issues in the development of future, metal foil, automotive catalytic converters have been addressed by reference to the effect they have on component lifetime.
The predicted life of alumina forming ferritic alloys, under oxidising conditions, was mod- elled based on an aluminium reservoir approach. The reserve of aluminium depends on the original alloy aluminium content (Co), the aluminium level at which breakaway oxidation oc- curs (CB) and the local volume/surface area ratio (V/A). Thus the aluminium reservoir is given by
( )
0.89 . .
(1 )
m o B
ox B
C C V
C A
r -
r - .
Consumption of aluminium results from the oxidation/corrosion processes, through,
· Early scale formation and the growth of a transient oxide. At low temperatures, this may be a transition alumina, for example g-alumina, while at high temperatures a-alumina rapidly forms. For FeCrAl-RE foils 950 °C has been shown to be a critical temperature, below this temperature g-alumina is stable for appreciable times (in excess of 60 h at 900 °C). Since these transition aluminas grow at a faster rate than a-alumina, this can have a significant ef- fect on catalyst life.
· Formation of a stable, protective oxide. Ideally this is a-alumina, whether d, q, g, or a- alumina form depends on the temperature, environment (i.e. water vapour content) and al- loy composition.
Spalling is not normally expected to occur for weak FeCrAl-RE foil materials and is usually only observed when the section thickness increases. If spallation occurs the alumina scale fails by mechanically induced chemical failure (MICF) and this can substantially reduce component life, not least because under MICF breakaway triggers when the aluminium level falls below some critical level, circa 1.7wt%Al. For foil materials, stress relaxation processes usually en- sure that internal stresses are not sufficient to cause failure and the foil will ultimately fail by intrinsic chemical failure (InCF) where CB approaches zero. However, geometric factors, such as sharp corners, holes, welds and changes in shape can provide stress raisers and may lead to local spallation.
· Ultimately, consumption of scale forming elements due to oxidation, leads to non protective conditions.
Less stable oxides are formed, coupled with possible internal oxidation and nitridation. This stage is critically dependent on the alloy composition, the type of active elements present and the composition of the gas phase. This internal oxidation rapidly depletes any remaining scale forming elements.
· Finally, Breakaway oxidation ensues.
Figure 9: Stochastic life prediction model for the chemical failure of FeCrAl-RE, alumina forming alloys [4]
1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06
0.001 0.010 0.100 1.000 10.000
Volume/Surface Area [mm]
kp*tB [ (mm)2 ]
Aluchrom Yhf - 1100C Aluchrom Yhf - 1200C Aluchrom Yhf - 1300C Aluchrom Yhf - 1300C Kanthal AF - 1300C Kanthal AF - 1300C Kanthal AF - 1200C Kanthal APM - 1300C Kanthal APM - 1300C Kanthal APM - 1200C PM2000 - 1300C PM2000 - 1300C PM2000 - 1200C PM2000 - 1200C (Improve) Aluchrom Yhf 1200C (Iso) Kanthal APM 1200C (Iso) PM2000 1200C (Iso) C0=5.5 wt.%
CB=1.7 wt.% (MICF) CB=0.0 wt.% (InCF) InCF
> = 0.0
>=1.0 5%
0.1%
>=0.1
5%
0.1%
>=0.01 5%
0.1%
MICF
Thus, chemical failure reflects a balance between the available aluminium reservoir and its rate of consumption due to a complex interplay of oxidation parameters. Key among these is the oxide rate constant, which may follow parabolic or sub-parabolic kinetics, and for thicker sectioned components, or highly constrained geometries, the critical oxide thickness to spall. It has been proposed that the (k × tB) product is a temperature independent parameter, that defines the lifetimes of an alumina forming ferritic steels, whether a foil or sheet materials [4]. Fig- ure 9 presents a plot of (kp× tB) against V/A ratio for a range of alumina forming ferritic steels over the temperature range 1050 °C–1400 °C, confirm the hypothesis that the (kp × tB) product may be used to provide a temperature independent estimate of component life.
In Figure 9 alloy lives between a few tens of hours and 20,000 h are plotted, superimposed on the plot are statistical corrosion models, that define the risk of failure [4], two levels of risk are plotted: a 5% chance of failure and a 0.1% chance of failure assuming oxidation follows parabolic kinetics. All foil samples were observed to fail by intrinsic chemical failure (InCF), when unconstrained. A life model, based on intrinsic chemical failure (> = 0.0; CB = 0.0wt%) and parabolic kinetics, provides a conservative estimate of foil component life for a range of FeCrAl-RE materials over the temperature range 1100 °C–1400 °C.
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