3
Methods of Corrosion Analysis
One must learn by doing the thing; for though you think you know it, you have no certainty until you try.
SOPHOCLES
3.1 INTRODUCTION
According to Weisser and Bange [3.1] it was Lavoisier who in 1770 first recorded the aqueous corrosion of a silicate glass predominantly by use of an analytical balance. The analysis of corrosion has been changing over the years with the greatest changes probably taking place within the last 25 years. These changes have been due mostly to the availability of sophisticated computerized analytical tools. It has taken many years for investigators to become familiar with the results obtained and how to interpret them. In some cases, special
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sample preparation techniques had to be perfected. Although one could conceivably employ all the various characterization methods described below, in most cases, only a few are needed to obtain sufficient information to solve a particular problem. The determination of the overall mechanism of corrosion requires a thorough detailed investigation using several characterization methods. Many times, though, the investigator has a limited amount of time and/or funds to obtain data and thus must rely on a few well-chosen tools. It should be obvious that considerable thought should be given to the selection of samples, test conditions, characterization methods, and interpretation of the results, especially if the data are to be used for prediction of lifetimes in actual service conditions. The reader is referred to the book by Wachtman [3.2] for a review of the principles involved in the various characterization techniques.
3.2 LABORATORY TEST VS. FIELD TRIALS
There are two general ways to approach a corrosion problem: either to conduct some laboratory tests to obtain information as to how a particular material will behave under certain conditions, or to perform a postmortem examination of field trial samples. It is best to perform the laboratory test first to aid in making the proper selection of materials for a particular environment and then perform the field trial. Laboratory tests, however, do not always yield the most accurate information since they rely on the investigator for proper setup; however, they are easier to control. The investigator must have a thorough understanding of the environment where the ceramic is to be used and must select the portions of the environment that may cause corrosion. For example, it is not sufficient to know that a furnace for firing ceramicware is heated by fuel oil to a temperature of 1200°C. One must also know what grade fuel oil is used and the various impurities contained in the oil and at what levels. In addition, parameters such as partial pressure of oxygen, moisture
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content, etc. may be important. Once all these various parameters are known, the investigator can set up an appropriate laboratory test.
One must also understand all the various things that cannot be scaled down to a laboratory test, such as viscosity of liquids, time, temperature, etc. Care must be exercised when attempting to perform an accelerated laboratory test, which is usually accomplished by raising the temperature or increasing the concentration of the corrosive medium or both. Since the mechanism of corrosion in the accelerated test may not be the same (generally, it is not the same) as that under actual service conditions, erroneous conclusions and inaccurate predictions may be obtained. The mechanisms must be the same for accurate application of laboratory test results to actual service conditions. Sample size is one parameter that is easily scaled; however, this can also cause problems. For example, when testing the corrosion of a ceramic by a liquid, the ratio of liquid volume present to the surface area of the exposed ceramic is very important. The investigator must remember that corrosion is controlled predominantly by thermodynamics and kinetics. Assuming that the proper laboratory tests have been conducted, the probability that any problems will arise is minimal.
The only way to analyze corrosion accurately is to conduct a field trial. This entails placing selected materials in actual service conditions, generally for an abbreviated time, and then collecting samples for analysis along with all the operational data of the particular environment. The size and amount of material or samples placed into actual service conditions for a field trial can be as little as one small laboratory test bar, or, for example, as large as a complete wall in a large industrial furnace. The larger the installation for the field trial, the more confidence one must have in the selection of materials. The larger installations are generally preceded by several laboratory tests and possibly a small-scale field trial. Abbreviated times may be as long as several years or as short as several days.
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Data such as temperature and time are the obvious ones to collect, but there exists a large amount of other data that should be examined. Many times, however, some of the more important data do not exist for one reason or another. For example, maybe the oxygen partial pressure was not determined during the duration of the service life of the ceramic. In some cases, it may be impossible to collect certain pieces of data during the operation of the particular piece of equipment. At these times, a knowledge of phase equilibria, thermodynamics, and kinetics can help fill in the gaps or at least give an indication as to what was present.
3.3SAMPLE SELECTION AND PREPARATION
It should be obvious that powders will present a greater surface area to corrosion and thus will corrode more rapidly than a solid sample. One may think this to be a good way to obtain a rapid test, but saturation of the corroding solution may cause corrosion to cease, or even cause a reverse reaction (i.e., crystal growth), giving misleading results. This points to the extreme importance of the surface area to volume ratio (SA/V) of the ceramic to the corroding solution. Another factor related to this is that during corrosion, the surface may change, altering the SA/V ratio effect. Surface areas during dissolution have been reported to increase presumably due to opening of etch pits, microfissures, etc. [3.3].
Selecting samples for analysis provides another challenge to the investigator. Foremost in the selection process is selecting an area for analysis that is representative of the overall corrosion process. If this cannot be done, then many samples must be analyzed. Much of the modern analytical equipment necessitates the analysis of very small samples, thus one must be very sensitive to the selection of representative samples or at least evaluate multiple samples.
Much care must be given to preparing samples that contain an adherent reaction product surface layer. It is best to select a
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sample that is many times larger than required by the final technique and then mounting this in some metallurgical mountant (e.g., epoxy). After the larger sample has been encased, then smaller samples can be safely cut from the larger piece.
Solid samples, when prepared for laboratory tests, should be cleaned in a noncorrosive solution to remove any loose particles adhering to the surface and any extraneous contamination. Brady and House [3.3] have reported that initially, an accelerated, nonlinear dissolution may occur from high-energy sites caused by grinding and incomplete removal of ultrafine particles. Best results are obtained if the cleaning is done in an ultrasonic cleaner. These cleaning solutions can be obtained from any of the metallographic supply companies. If the sample is mounted into one of the epoxy-type metallographic mountants, one must be aware that some cleaning solutions will react with the mountant. It is best to use supplies from one manufacturer to avoid these problems.
If as-manufactured samples are used for corrosion tests, one should remove a thin surface layer by grinding and cleaning before performing the corrosion tests. In this way, remnants from such things as powder beds or encapsulation media used in the production of the material can be eliminated and therefore not interfere with the corrosion process.
Quite often, the as-manufactured surface of a ceramic will have a different microstructure or even chemistry than the bulk. This often manifests itself as a thin surface layer (as much as several millimeters thick) that contains smaller grain sizes (more grain boundaries) and possibly a lower porosity. If the corrosion test corrodes only this thin surface layer, again, misleading results will be obtained. One way to solve this problem is to remove the surface layer by grinding. Grinding, however, must be done with some thought to the final surface roughness since, again, this will affect the SA/V ratio. Diamond-impregnated metal grinding disks should be used rather than silicon carbide paper disks or silicon carbide loose grit. Loose grit and the grinding media from paper disks have a tendency to become
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lodged within the pores and cracks of the sample being prepared. The final grinding media grit size should be no greater than 15 µm. It is best to clean samples after each grinding step in an ultrasonic cleaner with the appropriate cleaning solution.
Surface roughness of solid samples is an item that is often overlooked. Not only does a rough surface increase the area exposed to corrosion, but it may also lead to problems with some analytical techniques. For example, when the surface roughness is on the order of the reaction layer thickness caused by corrosion, errors will be present in the depth profiles obtained by secondary ion mass spectroscopy (SIMS). In those cases when surface analyses are planned, one should prepare solid samples to at least a 5-µm finish.
Grinding and polishing of samples that contain a reaction product surface layer should be done so that the reaction layer is not damaged, or the interface obscured. If part of the sample is metal, then polishing should be done in the direction ceramic toward metal to eliminate smearing the metal over the ceramic. If very thin reaction layers are present, one can prepare taper sections to increase the area that is examined. Sample preparation of composites presents some additional problems since materials of very different characteristics will be presented at the surface being polished. Chanat [3.4] of Buehler Ltd. has offered some good advice for mounting, sectioning, and polishing. The most effective method involves the use of diamond abrasives with low nap cloths for FRCMC*. High nap cloths can induce excessive relief at boundaries of different materials.
Many tips on how to prepare samples can be obtained by reading the various technical journals published by the manufacturers of consumable grinding and polishing supplies. One particular article that offers some new ideas was that of Damgaard and Geels [3.5]. They emphasized the importance of polishing disk diameter and velocity, indicating that both are directly proportional to material removal rates. Although
* FRCMC=fiber-reinforced ceramic matrix composite.
Copyright © 2004 by Marcel Dekker, Inc.