Resistance Measurement as a Tool for Corrosion Studies N. P. Singh, S. C. Gupta, and 6. R. Sood Physics Department, Punjabi University, Patiala, India
The importance of corrosion and introduction of simple experiments dealing with the phenomenon of corrosion have been highlighted i n recent publications ( 1 4 . One method used i n corrosion experiments ( 2 , 3 )i s the weightloss method. From the measured weight loss i n a given time for a specimen of known area, the corrosion rate in mg cm-2 h-' or some other related parameter can be calculated. We describe another method of studying corrosion of metals i n a given solution by monitoring the resistance of the specimen (thin wire or thin strip) a s a function of time. This method h a s two advantages over t h e weight-loss method. Corrosion can be studied continuously a s a function of time, and from the slope of the (resistance)-m vs. time plot, corrosion rate and any variation i n i t can be observed. Resistance measurement can, i n principle, be made with better accuracy (one part i n t e n thousand). Thus, corrosion rate and any variation i n it can be studied with more precision. Experimental In our experiment by monitoring the resistance of a 44gauge (diameter 1.1 x 104 m) copper wire of length L = 0.20 m, dipped in a solution (HCl, HNO3, and Hz0 i n the ratio of 1:3:8 by volume) the corrosion rate a s a function of time was studied. The solution was chosen to obtain about 15 widespread readings a t 5-min interval in 2 h. (For other corrodants the span of time shall be determined by the corrosion rate.) The experimental setup is shown i n Figure 1. The experiment was carried out a t room temperature (20 "C). Resistance measurements were carried out using the standard four-lead method to eliminate the lead-resistance error. In this method two separate leads are used for current (whichis kept constant), and two leads are used for
measuring potential difference across the specimen. This ensures that the measured potential difference is across the specimen alone, so the lead resistance does not come into the picture. The ratio of the measured potential difference and the constant current is the resistance of the specimen. In this method no zero adjustments are needed. If sufficiently thick connecting wires are used, even a standard multimeter can be used for resistance measurements. Discussion Resistance of a wire of diameter D, length L, and resistivity p, is given by
As corrosion sets in, the diameter of the wire is reduced giving rise to an increase i n resistance. A plot of (R)-'nvs. time should provide information about the corrosion rate. As the diameter of the wire is reduced, the volume and the mass of the wire i s reduced. Weight loss for initial and final diameter can be obtained, if needed, from the equation Am=mj-mf=
-
npd&
~3
4
(3)
B R A 5 5 RODS
COPPER W ~ R E
---------_
P
BEAKER
where pa is the density of copper. Because the surface area is changing during corrosion, weight loss per unit area per unit time can be written a s
Figure 1. Experimental setup. Volume 72 Number 5 May 1995
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gives the rate a t which the diameter decreases. This value for our specimen is 3.8 x m min-I. The corresponding value of corrosion rate i n mg ern-' h-' i s 10. Acknowledgment I n Figure 2 (R)-In i s plotted as a function of time. A straight-line graph points towards uniform corrosion of the wire, leading to a uniform decrease in diameter. The slope of the line is proportional to the corrosion rate. For any two values of resistance, weight loss may be calculated from eq 4. Slope of the straight-line graph in Figure 2 is 5.8 x n-ln min-'. Multiplying this slope value by
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Journal of Chemical Education
We would like to thank the referee for useful comments. Literature Cited 1. Onuehukwu,A.I. J. Cham. Educ 1986.63.269-270, 2. Onuehukwu,A.I. J Cham. Educ 1888.65.934. 3. Riga C. 0.;Sylvia L.: DaCoata, S. L. F.A.;Agostinho,S. M. L. J Chem. Edue. ,988, 66,441442, 4. References cited in 1 3 .
