If at all possible, one should perform weight changeexperiments in a continuous manner on an automated thermalanalyzer rather than performing an interrupted test wherethe sample is remov
Trang 1136 Chapter 3 3.5.3 Physical Property Measurement
Gravimetry and Density
The evaluation of weight change during a reaction in manycases is sufficient to determine that corrosion has taken place.Weight change in itself, however, is not always detrimental Inthe case of passive corrosion, a protective layer forms on theexposed surface This would indicate that corrosion had takenplace, but it is not necessarily detrimental since the material isnow protected from further corrosion
If at all possible, one should perform weight changeexperiments in a continuous manner on an automated thermalanalyzer rather than performing an interrupted test wherethe sample is removed from the furnace after each heattreatment and weighed In the interrupted test, one runs therisk of inaccurate weight measurements due to handling ofthe 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 beenheld at an elevated temperature for an extended time Thisimplies that additional densification or expansion has takenplace Additional densification, although not necessarily a form
of corrosion, can cause serious problems in structural stability.Expansion of a material generally implies that corrosion hastaken place and that the reactions present involve expansion.Again, these may not be degrading chemically to the materialbut may cause structural instability
One must exercise care in comparing density data obtained
by different methods Generally, the apparent density obtainedfrom helium pycnometry is slightly higher than that obtainedfrom 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.
Trang 2Methods of Corrosion Analysis 137
of fusion cast α/β alumina gave 3.47 g/mL by water absorptioncompared to 3.54 g/mL by helium pycnometry Heliumpycnometry lends itself to the determination of densities ofcorroded samples
Porosity-Surface Area
The evaluation of the porosity of a corroded sample generallypresents 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 computerizedimage analysis systems, one has the capability of evaluatingporosity and pore size distributions rather easily [3.16] Onemust be aware of the fact that sample preparation techniquescan greatly affect the results obtained by image analysis.The determination of the porosity of an uncorrodedspecimen, however, is extremely important in determining thesurface area exposed to corrosion Two samples identical inevery way except porosity will exhibit very different corrosioncharacteristics The one with the higher porosity or exposedsurface area will exhibit the greater corrosion This is thereforenot 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 sizedistribution is also important
The porosity test by water absorption is not sufficient sincethe total porosity available for water penetration is notequivalent to the total porosity available for gaseouspenetration 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 mustremember that the size distribution obtained from mercuryintrusion 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 usedfor mercury intrusion should not be subsequently used forcorrosion testing since some mercury remains within the sampleafter testing For applications involving gaseous attack, amethod that measures gas permeability better evaluates thepassage of gas through a material Permeability tests, however,are not as easy to perform as porosity tests A major problemwith the permeability test is sealing the edges of the sampleagainst gas leakage
Determination of the surface area directly by gas adsorption(BET*) or indirectly by mercury intrusion may not correlatewell with the surface area available to a corrosive liquid sincethe wetting characteristics of the corrosive liquid are quitedifferent from that of an adsorbed gas or mercury Thus oneshould exercise caution when using data obtained by thesetechniques
Mechanical Property Tests
Probably the most widely used mechanical property test is that
of modulus of rupture (MOR) One generally thinks ofcorrosion 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 theMOR test is done at room temperature For example, a high-temperature reaction may form a liquid that more tightly bondsthe material when cooled to room temperature A method that
is often used is first soaking the samples in a molten salt andthen performing a MOR test This evaluates both the high-temperature strength and the effects of corrosion upon strength.Long-term creep tests or deformation under load tests can yieldinformation about the effects of alteration upon the ability toresist mechanical deformation For a more detailed discussion
of the effects of corrosion upon mechanical properties, seeChap 8
* BET is an acronym for the developers of the technique, Brunauer, Emmett, and Teller.
Trang 4Methods of Corrosion Analysis 139 3.6 DATA REDUCTION
The corrosion data that have been reported in the literaturehave been in many forms This makes comparison betweenvarious studies difficult unless one takes the time to convertall the results to a common basis Those working in the area
of leaching of nuclear waste glasses have probably made themost progress in standardizing the reporting of data; however,
a major effort is still needed to include the entire field ofcorrosion of ceramics The work and efforts of organizationslike ASTM can aid in providing standard test procedures andstandard data reporting methods These are briefly described
in Chap 4
3.7 ADDITIONAL RELATED READING
Riga A.T., Patterson G.H., Eds.; Oxidative Behavior of Materials by Thermal Analytical Techniques; ASTM STP 1326, ASTM: West
Conshohocken, Pa., 1997; 247 pp.
Gibson, A.S.; LaFemina, J.P Structure of Mineral Surfaces In Physics and Chemistry of Mineral Surfaces; Brady, P.V., Ed.; CRC Press,
NY, 1996, 1–62.
Zipperian, D.C Microstructural Analysis Using Image Analysis In
Ceramic Transaction, Advances in Ceramic-Matrix Composites;
Bansal, N.P., Ed.; Am Ceram Soc., Westerville, OH, 1993; Vol.
38, 631–651.
Mason, C.W.; Handbook of Chemical Microscopy, 4th Ed.; John Wiley
Sons, Inc.: New York, 1983; Vol 1.
Cherry, R.J., Ed.; New Techniques of Optical Microscopy and Microspectroscopy, Topics in Molecular and Structural Biology;
Neidle, S., Fuller, W., Series Eds.; CRC Press: Boca Raton, FL, 1991.
Chinn, R.E., Ed.; Ceramography: Preparation and Analysis of Ceramic Microstructures ASM International & The Amer Ceram Soc.
2002, 214 pp.
Trang 5140 Chapter 3 3.8 EXERCISES, QUESTIONS, AND
PROBLEMS
1 List all the possible techniques that one may use to analyze
a corroded sample and the type of information obtained
2 Describe the differences between laboratory tests andfield trials
3 List the various parameters of a laboratory test thatcan be scaled from the actual environment and listthose that cannot How will this affect the overallinterpretation of the results of a lab test?
4 Discuss the errors that may arise when performing anaccelerated laboratory test In addition, whatcharacteristics of a small lab sample lead to errorscompared to the full-size installation?
5 Calculate the increased interface surface exposed bypolishing a sample at a 45° taper, if the originalperpendicular cross section had a 1-µm thick interface
6 What parameters are important in the grinding andpolishing of a sample and how do they affect the finalresult?
7 Discuss the information that one may obtain by examining
a corroded sample with the unaided human eye
8 Discuss the importance of the surface area of thecorroded sample to the volume of the corroding liquid
9 How does an interrupted weight change test vs.temperature interfere with the results? How can thisproblem be overcome?
REFERENCES
3.1 Weisser, M.; Bange, K Sophisticated methods available to
analyze glass corrosion Glass Res 2000, 9 (2), 16–17, 21.
3.2 Wachtman, J.B Characterization of Materials;
Butterworth-Heinemann: Boston, 1993.
3.3 Brady, P.V.; House, W.A Surface-controlled dissolution and
growth of minerals In Physics and Chemistry of Mineral
Trang 6Methods of Corrosion Analysis 141
Surfaces; Brady, P.V., Ed.; CRC Press: New York, 1996; 225–
305 Chp 4
3.4 Chanat, S Preparation techniques for analysis of fiber
reinforced ceramic matrix composites In Ceramic Transaction, Advances in Ceramic-Matrix Composites; Bansal, N.P., Ed.;
Am Ceram Soc.: Westerville, OH, 1993; Vol 38, 603–615 3.5 Damgaard, M.J.; Geels, K High capacity materialographic specimen preparation Struers J.Materialogr 2001; 7–11 Structure 38.
3.6 Macchesney, J.B.; Rosenberg, P.E The methods of phase equilibria determination and their associated problems In
Phase Diagrams: Materials Science and Technology; Alper, A.M., Ed.; Refractory Materials; Margrave, J.L., Ed.; Academic
Press: New York, 1970; Vol 6–1, 113–165 Chp 3 3.7 Eriksson, G Thermodynamic studies of high temperature equilibria XII SOLGASMIX, A computer program for calculation of equilibrium compositions in multiphase
7, 519–525.
3.11 Dickson, M.J The significance of texture parameters in phase
analysis by X-ray diffraction J Appl Cryst 1969, 2, 176–180.
3.12 Brime, C The accuracy of X-ray diffraction methods for
determining mineral mixtures Mineral Mag 1985, 49 (9),
531–538.
3.13 Schmidt, C.; Rickers, K In-situ determination of mineral solubilities in fluids using a hydrothermal diamond-anvil cell and SR-XRF: Solubility of AgCl in water Am Mineral 2003,
88 (2–3), 288–292.
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3.14 Lodding, A Characterization of corroded ceramics by SIMS.
In Corrosion of Glass, Ceramics and Ceramic Superconductors;
Clark, D.E., Zoitos, B.K., Eds.; Noyes Publications: Park Ridge,
NJ, 1992; 103–121 Chp 4.
3.15 Gibson, A.S.; LaFemina, J.P Structure of mineral surfaces.
In Physics and Chemistry of Mineral Surfaces; Brady, P.V.,
Ed.; CRC Press: New York, 1996; 1–62.
3.16 Exner, H.E., Hougardy, H.P., Eds.; Quantitative Image Analysis of Microstructures; DGM Informationsgesellschaft
mbH: Germany, 1988, 235 pp.
Trang 8Corrosion Test Procedures
When you can measure what you are speaking about andexpress it in numbers you know something about it; butwhen you cannot measure it, when you cannot express it
in numbers, your knowledge is of a meager andunsatisfactory kind
LORD KELVIN
4.1 INTRODUCTION
The American Society for Testing and Materials (ASTM) wasformed in 1898 through the efforts of Andrew Carnegie and thechief chemist of the Pennsylvania Railroad, Charles Dudley,who were both convinced that a solution was necessary to theunexplainable differences of testing results that arose betweentheir laboratories These early efforts were focused upon
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improving the understanding between seller and buyer of thequality of their products Although ASTM and otherorganizations have made considerable progress in eliminatingthe unexplainable differences in testing results betweenlaboratories, new materials and new applications continue topresent new and exciting challenges to the corrosion engineer.These challenges, however, are ones that must be overcome ifthere is to be honest competition in the world market of materials.Many of us have fallen into the habit of performing a testonly once and believing the results This is probably one of themost important things not to do when evaluating a particularmaterial for use under a certain set of conditions The results of
a test will generally vary to a certain degree and can varyconsiderably It is up to the testing engineer to know or determinethe test method variation All ASTM standards now contain astatement of precision and bias to aid the test engineer indetermining how his test fits into the overall imprecision of theprocedure developed by the standards committee In thedevelopment of an ASTM standard, a ruggedness test (ASTMStandard E-1169) is performed to determine the major sources
of variation This test should be performed for any laboratorytest that one might conduct to minimize the major sources oferror The idea of the ruggedness test is to determine the majorsources of variation of a procedure and then minimize thosevariations to within acceptable limits
Many standard tests have been developed through ASTM toevaluate the corrosion resistance of various ceramic materials.These various tests have been listed in Tables 4.1 and 4.2 andcan be found in the Annual Book of ASTM Standards, volumes2.05, 4.01, 4.02, 4.05, 12.01, 14.04, 15.01, and 15.02 A briefsummary of each of these is given below Standards that are inthe process of being developed have not been listed in Tables 4.1and 4.2 These draft standards can be found on the ASTM website.* ASTM designates some procedures as standard test methodsand others as standard practices The distinction between these
* The ASTM web site can be found at www.astm.org
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two is best given by their definitions ASTM defines test method
as a definitive procedure for the identification, measurement,and evaluation of one or more qualities, characteristics, orproperties of a material, product, system, or service that produces
a test result, and practice as a definitive procedure for performingone or more specific operations or functions that does not produce
a test result [4.1] Standard practices provide the user withaccepted procedures for the performance of a particular task.Test methods provide the user with an accepted procedure fordetermination of fundamental properties (i.e., density, viscosity,etc.) These standards must be updated or reapproved by theend of the 8th year after the last approval If not reapproved,the standard is then withdrawn
The Materials Characterization Center* (MCC) is anotherorganization that has developed standard test procedures [4.2].Several of these tests have been used extensively by thoseinvestigating the leaching of nuclear waste glasses Test MCC-
1 involves a procedure for testing the durability of monolithicglass samples in deionized or simulated groundwater at 40°C,70°C, and 90°C for 28 days One disadvantage of this test isthat no standard glass is used, thus eliminating corrections forbias It does, however, require the reporting of mass lossnormalized to the fraction of the element leached in the glasssample allowing one to make comparisons between glasses.Test MCC-3, in contrast, evaluates an agitated crushed glasssample to maximize leaching rates Test temperatures areextended to 110°C, 150°C, and 190°C Again, a standard glass
is not used Both of these tests have now been developed intoASTM standard test methods, C1220 and C-1285, respectively.With the global economy of today, the engineer must befamiliar with standards from countries other than the UnitedStates In addition to the individual countries that maintainstandards, there are also the International Organization for
* The MCC was created in 1980 by the U.S Department of Energy and is operated for the DOE by the Pacific Northwest Laboratories of the Battelle Memorial Institute in Richland, WA.
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TABLE 4.1 Continued
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TABLE 4.1 Continued
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TABLE 4.3 Standards Organizations and Their Acronyms