describes an electrical conductance method by Mars G. Fontam INTERESTING corrosion test A m e t h o d that is beginning to find wider use in laboratory and plant studies is based on the change in conductance of a metal specimen as it decreases in thickness because of corro-
Figure 1. Pictorial d i a g r a m of electrical circuits A.
B. C.
Potentiometer Storage battery Standard resistor 1 .OO ohm
D.
Ammeter
E. Variable resistor F. Specimen G. Thermostat
sion. This column reviews an excellent report issued in August 1955 by the Air Force. This report is Technical Note WADC T N 55-178, (‘An Elec-
Figure 3.
December 1955
trical Conductance Test Method for Measuring Corrosion,’’ by D. Roller, Materials Laboratory, Wright Air Development Center, Air Research and Development Command, United States Air Force, Wright-Patterson Air Force Base, Ohio. The electrical conductance method possesses several advantages over other methods for determining resistance of metals and alloys to corrosion. The main advantages are (1) the corrosion rate can be determined without removal of the specimen from the system (2) possible errors due to cleaning, handling, and weighing are eliminated (3) large amount of data obtained per man-hour, and (4) the rate of corrosion can be continually followed and also automatically recorded. Advantages 1 and 2 are particularly desirable where specimens are not readily accessible such as in a high pressure, high temperature system where a shutdown is needed for specimen removal. This method is also particularly adaptable for measuring low rates of corrosion where cleaning or removal of corrosion products may introduce relatively large
M a n u a l equipment
errors. In most cases, dirt, scale, or other corrosion products do not appreciably affect the conductance of the metal specimen. The main limitation of this method for general corrosion testing is that corrosion must be comparatively uniform and free from appreciable pitting. However, this same limitation applies to methods involving determination of actual weight losses of the specimens. The method briefly consists of passing a small known current through the specimen, measuring the voltage drop across the specimen, and calculating the conductance using Ohm’s Lam. Conductance depends on the crosssectional area of the specimen and decreases with this area (or thickness). Direct comparison of conductance values with actual weight losses showed a straight line relationship. Results of this method check well with results of older or conventional test methods. Figure 1 shows a diagram of the electrical circuits involved. The setups can be manual or automatic. Current
Figure 4.
Automatic recording equipment
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Corrosion is supplied by a 6-volt, three-cell acid storage battery. Heating effects due to current passage are negligible because of the short time required to take a reading. For elevated temperature tests, the original conductance of the metal or alloy specimen can be obtained by placing the specimen in a noncorrosive solution which is a t the desired temperature.
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Plant tests
The writer recently observed an automatic electrical conductance test method in operation in a high production plant running a t high temperatures and pressures. Excellent correlation between this test and actual service was obtained where materials were being evaluated. I n addition, this method is used to determine whether or not changes in the corrosivity of the plant streams occurred and also the effects of process variations on corrosion of the equipment. This cannot be discussed in detail here, but the writer encouraged publication by the plant of the techniques used and results obtained. Acknowledgment
The writer hereby gratefully acknowledges permission by the Materials Laboratory to review this report and also for supplying the photographs used herein.
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studies and tests in vapors, such as high humidity.
Figure 2. Magnesium ribbon specimens in two types of holders Uncorroded permanent leads
Corroded removable leads
Theoretically, specimens of most any size and shape could be used, but for best sensitivity and ease of calculation thin (20 mils or less) strips or ribbon samples are used. Specimens used in this investigation were 15 X '/s X 0.005 to 0.015 inch. Figure 2 shows typical specimens used. Cast specimens are, of course, not readily obtainable in these dimensions. Itissometimes desirable to use statistical analysis of the large amount of data that can be obtained by the electrical conductance test method. This also applies to corrosion data obtained by other methods. Figure 3 shows the apparatus for manual operation, and Figure 4 shows automatic recording equipment. Through the use of timing devices, the automatic setup can be made to periodically read a number of specimens. Additional work by the Air Force with this method includes inhibitor
Corrosion Testing Types of tests, reasons for conducting tests, materials, and specimens Surface preparation, measuring, weighing, and exposure techniques Duration of tests Plant exposure, effect of temperature, laboratorycontrolled temperature baths, and plant tests for heating surfaces Aeration and expression for corrosion rates Nitric acid testing of stainless steels Procedures for cleaning specimens after exposure Nomograph for converting corrosion rates and densities and costs of materials Procedures for galvanic or twometal tests Stress corrosion Concentration cell corrosion tests
Date Discussed
April 1954
June 1954 July 1954
August 1954 September 1954 October 1954
December 1954
February 1955 April 1955 June 1955 November 1955
Correspondence concerning this column will be forwarded if addressed .to the author, % Editor, INDUSTRIAL AND ENGISEERINQ CHEMISTRY,
1155-16th
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
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Vol. 47, No. 12
