Fundamentals

23

The surface of the boulder shown in Fig. 2.5 was not horizontal like the paver but sloped approximately 45°.

Attack by Molten Salts

The corrosion of ceramic components in gas turbine engines generally occurs through the action of condensed salts formed from impurities in the fuel and/or combustion air. Similar corrosion mechanisms occur in glass furnace regenerators and on glass furnace crowns. The condensation of molten salts occurs below their dew point and is thus dependent upon the temperature and pressure of operation, along with the concentration of the impurities in the fuel or air (compare discussion under Dew Point Corrosion on page 44). Fox et al. [2.21] listed the dew points for sodium sulfate deposition, a few of which are given in Table 2.1. Not only did a higher pressure raise the dew point for condensation, but it also increased the deposition rate, which generally led to more severe corrosion [2.22]. The effects of molten Na2SO4 upon the dissolution of silica and the importance of Na2O activity and the partial pressure of oxygen is discussed in more detail in Chap. 5, Carbides and Nitrides, page 193.

A model developed by Cook et al. [2.23] in their study of hot corrosion of ceramic (alumina) barrier coatings by sodium, sulfur, and vanadium molten salts gave the rate of solution of

TABLE 2.1 Dew Points (°C) for Na2SO4 Condensation

a Solid, since melting point is 884°C. Source: Ref. 2.21.

Copyright © 2004 by Marcel Dekker, Inc.

24

Chapter 2

a ceramic when a steady-state condition prevailed for the rate of salt removal equal to the rate of salt deposition. This provided a salt layer of constant thickness. The ceramic solution rate was then dependent upon the rate of salt deposition:

(2.7)

where:
Mc	= mass of ceramic dissolved
A	=	surface area
Ms	= mass of salt deposited
C	=	concentration of ceramic in layer

At low deposition rates when salts become saturated, the solubility in the salt becomes important. Use of this model requires the calculation of the gas phase and condensed solution equilibria using a computer program such as that developed by the NASA-Lewis Research Center [2.24]. In addition to the steady-state assumption for salt deposition and removal, other assumptions included the parabolic rate law, known equilibrium solubilities, and congruent dissolution.

Corrosion by molten salts has several beneficial applications. One very important application where dissolution of a ceramic is desired is in the removal of the ceramic cores from metal castings manufactured by the investment casting technique. The solvent used for core removal must be highly reactive to the ceramic at rather low temperatures while not damaging the metal. The ceramic must be stable toward molten metal attack at high temperatures and highly reactive toward solvent attack at low temperatures. In a study of the leaching rates of

Al 2O3, Y2O3, La2O3, ZrO2, ThO2, and MgO by molten Li3AlF6, Borom et al. [2.25] found that the corrosion appeared to involve

a solid reaction layer and a boundary layer in the liquid. Vigorous solvent circulation was required to overcome the diffusion-controlled process. Thus it appeared that congruent dissolution was required for optimum core removal since

Copyright © 2004 by Marcel Dekker, Inc.

Fundamentals

25

incongruent dissolution may form reaction layers that require forced convection for removal.

Electrochemical Corrosion

Very few studies have been reported over the last 30 years; however, much work was performed in the 1950s and 1960s on what has been called galvanic corrosion of refractories by glasses. Galvanic corrosion as defined by the physical chemist must occur between two materials in contact with one another and both must be in contact with the same electrolyte. Much of what has been reported should more appropriately be called electrochemical corrosion. One of the first reports of the existence of an electrical potential between refractory and glass was that of Le Clerc and Peyches [2.26] in 1953. The setup is schematically represented in Fig. 2.6. In such a case, the molten glass acts as the electrolyte and the platinum wire acts as a reference electrode (i.e., standard oxygen electrode). The use of platinum as a reference electrode requires that the

FIGURE 2.6 Electrochemical cell to determine potential difference between a ceramic and a molten glass.

Copyright © 2004 by Marcel Dekker, Inc.

26

Chapter 2

atmosphere above the melt contain a reasonable oxygen partial pressure since the reaction:

(2.8)

must be present at the site where the platinum comes in contact with the melt. The overall accuracy of such potential measurements is critically dependent upon obtaining excellent electrical contact among the various components of the galvanic cell. An additional problem that undoubtedly causes variation or drift in the measurements is the formation of a reaction interface layer between the refractory and the molten glass.

Godrin [2.27] has published a review of the literature on electrochemical corrosion of refractories by glasses. It has been shown that a potential difference does exist in such systems; however, no quantitative relationship between corrosion and potential has been reported. Since a potential difference exists in corroding systems, it has been tempting to assume that the potential is at least partly responsible for the corrosion; however, the application of a bias potential has been unsuccessful in eliminating corrosion. Although not totally reliable, Godrin concluded that refractories that had an electrical potential with respect to glass that was positive 0.4 to 0.7 V were fairly resistant to corrosion, that refractories with a potential greater than 1.0 V had rather poor resistance, and that refractories that had a negative potential with respect to glass should not be used.

Pons and Parent [2.18] have concluded that the oxygen ion activity was a very important parameter in corrosion and that its role was determined by the difference in oxygen potential between the molten glass and the refractory oxide. An additional interesting case was that of two different oxide materials (i.e., a multiphase polycrystalline material) in contact with the same glass that had oxygen potentials on either side of that of the glass. In such a case, it was assumed that oxygen migrated from the oxide of higher potential toward that of lower potential. If the conduction mechanism of the two oxides

Copyright © 2004 by Marcel Dekker, Inc.

Fundamentals

27

were different (ionic vs. electronic), the situation would become more complex. When the oxygen potentials of the oxides were greater than the glass, oxygen ions were assumed to be transported from the ionic conductive oxide to the electronic conductive one, which may ultimately result in pitting caused by the release of oxygen. If the oxygen potential of the oxides was lower than the glass, alkali ions of the glass would be transported to the electronic conductive oxide with oxygen release at the interface between the two oxides.

Although in theory the application of a bias potential to minimize or eliminate corrosion, which implies that the corrosion process is one that involves charge transfer, should produce noticeable results, a major practical problem has been that of making the electrical connection to the ceramic. The other problems relating to the success of a bias potential in eliminating corrosion are the other factors in corrosion— chemical reaction, diffusion, viscosity, solubility, etc. This topic is one of considerable importance and should receive a lot more attention than it has in recent years. A standard text that discusses electrode effects in liquid electrolytes should be consulted by the interested reader [2.28].

Wall et al. [2.29], in their studies of graphite fiber/BMI

(bismalimide) composites in contact with various metals immersed into an electrolyte, developed an electrical equivalent circuit of the electrochemical interface. The classic electrical double layer (see discussion on page 30, Attack by Aqueous Media) is established at the surface of the graphite fibers where the electrons at the surface are separated from the ionic charges in solution. This forms a capacitor, called the double-layer capacitance, CDL. This capacitance is dependent upon the electrode surface area, which allows one to monitor surface roughening, surface adsorbed species, and ingress of solution between fiber and matrix by changes in capacitance. Charge leakage rate across the interface can be represented by a resistance, RF, in parallel with CDL that is inversely proportional to the reaction rate. Changes in exposed surface area of the fibers or changes in surface chemistry will affect this rate. The

Copyright © 2004 by Marcel Dekker, Inc.