Page 715 - Corrosion Engineering Principles and Practice

668 C h a p t e r 1 5 H i g h - Te m p e r a t u r e C o r r o s i o n 669

By using Fig. 15.3 in an appropriate fashion, one could determine
the partial pressure of oxygen that would cause any of the listed metals
to corrode, as well as the CO/CO ratios or the H /H O ratios to produce
2
2
2
a similar oxidation. If all these gases were actually mixed together, the
situation would of course become more complicated and beyond the
scope of the present introduction to the subject. One could further add
to such complexity by introducing other gases such as sulfur dioxide,
hydrogen sulfide, chlorine gas, or other contaminants which, in addition
to oxides, tend to form sulfides, chlorides, and so forth.

15.2.2 Vapor Species Diagrams
Vapor species that form in any given high-temperature corrosion
situation often have a strong influence on the rate of attack of the
metallic substrate, the rate generally being accelerated when volatile
corrosion products are formed. Gulbransen and Jansson have shown
that metal and volatile oxide species are important in the kinetics of
high-temperature oxidation of carbon, silicon, molybdenum, and
chromium [5]. Six types of oxidation/corrosion rate-controlling
mechanisms have been identified:
1. At low temperature, diffusion of oxygen and metal species
through a compact oxide film
2. At moderate and high temperatures, a combination of oxide
film formation and oxide volatility
3. At moderate and high temperatures, the formation of volatile
metal and oxide species at the metal-oxide interface and
transport through the oxide lattice and mechanically formed
cracks in the oxide layer
4. At moderate and high temperatures, the direct formation of
volatile oxide gases
5. At high temperature, the gaseous diffusion of oxygen through
a barrier layer of volatilized oxides
6. At high temperature, spalling of metal and oxide particles
The diagrams most suited for presentation of vapor pressure data
in oxide systems are log(p M x O ) versus log(p ) at constant temperature
y
O 2
and Arrhenius diagrams of log(p M x O ) versus 1/T at constant oxygen
y
pressure. The following example will illustrate the construction of the
first type of these diagrams by considering the Cr-O system at 1200°C
or 1473 K. Only one condensed oxide, Cr O , is formed under these
2
3
conditions of high-temperature oxidation [4]. The thermochemical
data for producing the vapor equilibria diagram shown in Fig. 15.4 are
given in Table 15.2. The thermochemical data for Cr O (s), Cr(s), and
3
2
the four possible gaseous species Cr(g), CrO(g), CrO (g), and CrO (g)
2
3
were obtained from a commercial database [6].