Corrosion of Hard Materials

Abstract

Corrosion may be defined as the physical and chemical alteration of a material due to its interaction with the environment of interest. It must be emphasized that corrosion resistance is not a material property but a system property and real environments have a high variability in both chemical and physical boundary conditions.Corrosive alteration leads to a change in the desired original properties and so corrosion usually, but not exclusively, has adverse effects and causes a decrease in strength, toughness, hardness, and increases wear. There are many reasons for the degradation of properties by corrosion. Decrease of the cross-section carrying the load, pitting, stress induced by phase changes, and grain separation are some of the common features.Some hard materials, namely Sic and Si3N4, are designed to serve at high temperatures, where corrosion resistance is an essential feature. This is the reason why a large proportion of the ceramic literature on corrosion, including recent textbooks [ 1-51, deals with these materials.In order to react with the environment, the material must not be in equilibrium with it. The hard materials covered in this chapter are non-oxides and hence this condition is fulfilled for all of them in almost every application environment, including ambient air. The applicability of hard materials thus rests on the kinetics of the processes.Before discussing individual materials we will introduce important terms and concepts in corrosion like attack modes, media classes, location of attack, kinetics, and their parameters. We feel that this is necessary for understanding of the discussion of the materials, because there is no generally accepted standard procedure that allows us to compare the corrosion behavior of materials as easily as other properties like mechanical strength or hardness. Corrosive MediaThe compositional variability of environments is so vast that only very few attempts have been made to classify corrosion behavior from the point-of-view of the discrete environment at hand. A noteworthy exception is the Dechema handbook [6],in which valuable information may be found if the corrosive agent is simple and known (e.g. acetates, chlorine, steam). However, its main thrust is on the corrosion of metals.

Corrosion of Ceramic Materials

“…Corrosion in halogenides and SO 2 /SO 3 is not treated (see Jacobson, 1993;Marra, Kreidler, Jacobson, & Fox, 1988;Nickel & Gogotsi, 2000;Nickel et al, 2010;Presser, Heon, & Gogotsi, 2011;Readey, 1998;Van der Biest, Barnes, Corish, & Norton, 1987). Presser and co-workers (Presser et al, 2011) provide an overview of the reaction of different carbides with Cl 2 .…”

“…The equation has the physical meaning that the diffusion through the barrier is rate-controlling and as a result, the thickness of the barrier increases with ffiffi t p . More complicated equations have been derived for short reaction times (formation of the stable oxide layer) (Nickel et al, 2010;Nickel & Gogotsi, 2000). The above equation is valid for steady-state systems, i.e.…”

“…Impurities in the gas phase can also change the diffusion coefficients in the oxide scale. Alkaline, sulphate and vanadium impurities in combustion gases have a significant negative effect on the oxidation stability of Si 3 N 4 and SiC (Jacobson, 1993;Klemm, 2010;Nickel et al, 2010;Nickel & Gogotsi, 2000;Petzow & Herrmann, 2002).…”

“…Corrosion behavior is therefore a property not of the material, but of the system including the material, the media and conditions such as temperature, pressure, medium type and volume or flow rate, and tribological as well as mechanical loads. Therefore, a simple statement concerning corrosion resistance of a material cannot be given because the material may be stable under one set of conditions, but unstable under another set of conditions (Jacobson, 1986;Klemm, 2010;Nickel & Gogotsi, 2000;Nickel, Quirmbach, & Pötschke, 2010;Petzow & Herrmann, 2002;Schmalzried & Schwetz, 2010).…”

Composite Stable to Corrosive Media in SiC-AL<sub>2</sub>O<sub>3</sub>-Si<sub>2O</sub>N<sub>2</sub> System

“…Table 13 gives th ucible test which makes clear that as a result of 10 and 40 hours testing refractory crucibles are stable to the above mentioned metals and no metal traces are noticed on crucibles surface [20][21][22][23][24][25][26][27][28].…”