8

Properties and Corrosion

Homogeneous bodies of materials—I was told—do not exist, homogeneous states of stress are not encountered.

OTTO MOHR

8.1 INTRODUCTION

Probably the most important property that is affected by corrosion is that of mechanical strength. Other properties are also affected by corrosion; however, they generally do not lead to failure, as is often the case with changes in strength. Strength loss is not the only mechanical effect of corrosion, because there are also many cases where the effects of corrosion lead to increased strength. Increases in strength due to corrosion are the result of healing of cracks and flaws in the surface layers of a specimen due, quite often, to the diffusion of impurities from the bulk to the surface. This change in chemistry

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at the surface may lead to the formation of a compressive layer on the surface because of differential thermal expansion between the surface layer and the bulk. Compressive surface layers may also form as a result of surface alteration layers having a larger specific volume than the bulk.

Environmentally enhanced strength loss may arise through the following phenomena:

1.Cracking of the surface alteration layers due to excessive mismatch in thermal expansion between the surface and the bulk

2.Melting of secondary phases at high temperature

3.Lowering of the viscosity of a glassy grain boundary phase at high temperature

4.Surface cracking caused by polymorphic transitions in the crystalline phases at the surface

5.Alteration that forms low strength phases

6.Formation of voids and pits, especially true for corrosion by oxidation

7.Crack growth

The term used to describe these phenomena is called stress corrosion or stress corrosion cracking (SCC), which occurs when a material is subjected to a corrosive environment while being under the influence of an external mechanical load. Stress corrosion cracking implies that the pair of parameters, applied stress and corrosive environment, must both be active. Removal of either the applied stress or the corrosive environment will prevent cracking.

Oxidation often leads to compositional and structural alteration, especially of surface layers and grain boundary phases, of a ceramic that subsequently leads to considerable changes in the physical properties. Such alterations can lead to changes in density, thermal expansion, and thermal and electrical conductivity. The influence that these changes exert upon mechanical properties can be deduced only through a thorough investigation of the mechanisms and kinetics of

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Properties and Corrosion

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corrosion. For example, the oxidation of silicon-based ceramics has been shown to be either active or passive depending upon the partial pressure of oxygen present during exposure (see Chapter 5, Section 5.2.2 for a discussion of the oxidation of SiC and Si3N4). When the pO2 is low, gaseous SiO is formed, leading to rapid material loss and generally to a loss in strength. When the pO2 is high, SiO2 is formed leading to strength increases depending upon the actual temperature and time of exposure, and whether or not the strength test is conducted at room or an elevated temperature. The investigator should be well aware that conducting mechanical property tests in air (which may also include moisture) at elevated temperatures constitutes exposure to a corrosive environment for many materials.

The failure of ceramics after long exposure to a constant applied load, well below the critical stress, is called static fatigue or delayed failure. If the load is applied under constant stress rate conditions, it is called dynamic fatigue. If the load is applied, removed, and then reapplied, the failure after long-time cycling is called cyclic fatigue. It is now well known that brittle fracture is quite often preceded by subcritical crack growth that leads to a time dependence of strength. It is the effect of the environment upon the subcritical crack growth that leads to the phenomenon termed stress corrosion cracking. Thus fatigue (or delayed failure) and stress corrosion cracking relate to the same phenomenon. In glassy materials, this delayed failure has been related to glass composition, temperature, and the environment (e.g., pH). Failure is a result of the chemical reaction that takes place preferentially at strained bonds at the crack tip with the rate being stress sensitive. Some crystalline materials exhibit a delayed failure similar to that in glasses.

The experimental relationship between crack velocity and the applied stress (i.e., stress intensity factor KI) is therefore of utmost importance. Attempts to fit various mathematical relationships to the experimental data have led to an

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assortment of equations of either the commonly used power law type or of some exponential form. The power law:

(8.1)

where A is a material constant (strong dependency upon environment, temperature, etc.), n is the stress corrosion susceptibility parameter (weak dependency upon environment), and KI is the applied stress intensity. KIC, which denotes the critical stress intensity factor, has been used most often because of its simplicity. It is the value of n (and also A) that determines a material’s susceptibility to subcritical crack growth. Final lifetime predictions are very sensitive to the value of n. The power law, however, does not always lead to the best representation. Jakus et al. [8.1] evaluated the prediction of static fatigue lifetimes from experimental dynamic fatigue data for four different materials and environments. These were hot-pressed silicon nitride at 1200°C in air, alumina in moist air, optical glass fiber in air, and soda-lime glass in water. They found that the exponential forms of the crack velocity equations allowed better predictions of lifetimes for the silicon nitride and optical glass fiber, but the power law form of the crack velocity equation allowed better predictions for alumina and soda-lime glass. Thus they concluded that one should collect data for several different loading conditions, and then select the crack velocity equation that best represents all the data for making lifetime predictions. Matthewson [8.2] has reported that one particular optical fiber material gave a best fit to the exponential form when tested in ambient air but gave a best fit to the power law when tested at 25°C in a pH=7 buffer solution. Matthewson suggested that one kinetics model unique to all environments probably does not exist, and that since the power law yields the most optimistic lifetimes, it is unsatisfactory for design purposes.

Crack velocity can be evaluated by direct and indirect methods. In the direct methods, crack velocity is determined as a function of the applied stress. These involve testing by techniques such as the double cantilever beam method, the double torsion method, and the edge or center cracked specimen method. Indirect methods,

Copyright © 2004 by Marcel Dekker, Inc.