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3.5.3 Physical Property Measurement

Gravimetry and Density

The evaluation of weight change during a reaction in many cases is sufficient to determine that corrosion has taken place. Weight change in itself, however, is not always detrimental. In the case of passive corrosion, a protective layer forms on the exposed surface. This would indicate that corrosion had taken place, but it is not necessarily detrimental since the material is now protected from further corrosion.

If at all possible, one should perform weight change experiments in a continuous manner on an automated thermal analyzer rather than performing an interrupted test where the sample is removed from the furnace after each heat treatment and weighed. In the interrupted test, one runs the risk of inaccurate weight measurements due to handling of the sample.

Density measurements are another form of gravimetry, but in this case, the volume change is also measured. Many times, volumetric changes will take place when a material has been held at an elevated temperature for an extended time. This implies that additional densification or expansion has taken place. Additional densification, although not necessarily a form of corrosion, can cause serious problems in structural stability. Expansion of a material generally implies that corrosion has taken place and that the reactions present involve expansion. Again, these may not be degrading chemically to the material but may cause structural instability.

One must exercise care in comparing density data obtained by different methods. Generally, the apparent density obtained from helium pycnometry is slightly higher than that obtained from water absorption*. For example, the data for a sample

* Helium is more penetrating than water and thus yields a smaller volume determination. This is dependent upon the pore size distribution.

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of fusion cast α/β alumina gave 3.47 g/mL by water absorption compared to 3.54 g/mL by helium pycnometry. Helium pycnometry lends itself to the determination of densities of corroded samples.

Porosity-Surface Area

The evaluation of the porosity of a corroded sample generally presents the investigator with a rather difficult task. Most often, the best method is a visual one. Determination of the variations in pore size distribution in different zones of the sample may be a significant aid to the analysis. With modern computerized image analysis systems, one has the capability of evaluating porosity and pore size distributions rather easily [3.16]. One must be aware of the fact that sample preparation techniques can greatly affect the results obtained by image analysis.

The determination of the porosity of an uncorroded specimen, however, is extremely important in determining the surface area exposed to corrosion. Two samples identical in every way except porosity will exhibit very different corrosion characteristics. The one with the higher porosity or exposed surface area will exhibit the greater corrosion. This is therefore not a true test of corrosion but is valuable in the evaluation of a particular as-manufactured material. Not only is the value of the total volume of porosity important, but the size distribution is also important.

The porosity test by water absorption is not sufficient since the total porosity available for water penetration is not equivalent to the total porosity available for gaseous penetration. Although water absorption is a convenient method to determine porosity, it yields no information about pore size, pore size distribution, or pore shape. Mercury intrusion, however, does yield information about pore size distribution in the diameter range between 500 and 0.003 µm. One must remember that the size distribution obtained from mercury intrusion is not a true size distribution but one calculated from an equivalent volume. By assuming the pores to be cylindrical, one can calculate an approximate surface area from the total

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volume intruded by the mercury. A sample that has been used for mercury intrusion should not be subsequently used for corrosion testing since some mercury remains within the sample after testing. For applications involving gaseous attack, a method that measures gas permeability better evaluates the passage of gas through a material. Permeability tests, however, are not as easy to perform as porosity tests. A major problem with the permeability test is sealing the edges of the sample against gas leakage.

Determination of the surface area directly by gas adsorption (BET*) or indirectly by mercury intrusion may not correlate well with the surface area available to a corrosive liquid since the wetting characteristics of the corrosive liquid are quite different from that of an adsorbed gas or mercury. Thus one should exercise caution when using data obtained by these techniques.

Mechanical Property Tests

Probably the most widely used mechanical property test is that of modulus of rupture (MOR). One generally thinks of corrosion as lowering the strength of a material; however, this is not always the case. Some corrosive reactions may, in fact, raise the strength of a material. This is especially true if the MOR test is done at room temperature. For example, a hightemperature reaction may form a liquid that more tightly bonds the material when cooled to room temperature. A method that is often used is first soaking the samples in a molten salt and then performing a MOR test. This evaluates both the hightemperature strength and the effects of corrosion upon strength. Long-term creep tests or deformation under load tests can yield information about the effects of alteration upon the ability to resist mechanical deformation. For a more detailed discussion of the effects of corrosion upon mechanical properties, see Chap. 8.

* BET is an acronym for the developers of the technique, Brunauer, Emmett, and Teller.

Copyright © 2004 by Marcel Dekker, Inc.