Water Effects on Corrosion Behavior of Structural Ceramics

Abstract

Environmental degradation poses a serious limitation on the utility of engineering materials such as metals, plastics, glasses, and structural ceramics. However, at least water would seem to be a harmless environment for ceramics, which are considered to be the most corrosion-resistant of all materials. When we drink water from a glass or a ceramic cup, we never think about an aqueous alteration of these materials. We cannot imagine that our ceramic mug may be dissolved in water or fail due to stress corrosion during breakfast. Even hot water which we use to make coffee or tea does not seem to be a hazardous environment for our china and earthenware. Why then are we going to discuss the effect of water on structural ceramics? The answer is simple: Under conditions of normal use of structural ceramics, which include high temperatures, high pressures, and mechanical stresses (Figure 1), even the most stable and corrosion-resistant materials may interact with water and water may become a hostile environment. It can be adsorbed on the surface of ceramics and act as a solvent for oxide ceramics or as an oxidant for non-oxide ceramics (Figure 2). However, water can cause a degradation of glasses and oxide ceramics even at room temperature if mechanical stresses are simultaneously applied (so-called stress corrosion). Water or humid air can decrease significantly the mechanical properties of certain zirconia ceramics at only slightly elevated temperatures (~200°C).