Electrolytic Control
C
ORROSION is one of the most important chemical reactions taking place in this age of metals. The annual corrosion bill is estimated to be well over one billion dollars. This billion dollar chemical reaction is passed over rather lightly in college chemistry, and it is not until the chemist enters the metal industry that he realizes its importance and implications. Most corrosion processes are electrolytic in nature, resulting from the contact of two dissimilar metals in the presence of an electrolyte, or from any of the other conditions that give rise to an electric cell. Two articles, one by Burnse and the other by B r ~ w ngive , ~ the general picture of the electrolytic nature of corrosion. These electrolytic corrosion reactions obey the ordinary laws of electrochemical reactions. Furthermore, i t is reasonable to expect that if we could oppose the current produced by such a reaction by one equal to i t the reaction should be stopped. Several general principles should be kept in mind: (1) The anodic member of a metal couple is usually the one that corrodes. ( 2 ) The electrochemical series is not a true indication of the metal which is anodic in natural corrosive media. For example, in a l-molar sodium chloride solution zinc is anodic to aluminum, as is true in most natural surroundings. (3) In the control of corrosion by application of external currents. side reactions sometimes take lace. Thus, aluminum, zinc, and lead even when cathodic, are attacked by the accumulation of alkali around the cathode if the current density is too high. In the course of his work the author has performed a few experiments which seemed suitable for demonstrations of the electrolytic nature and control of corrosion processes. These are not the only ones that might have been selected, as other metals and other conditions than those to be described would serve the same purpose. EXPERIMENTAL
The corrosive action of sodium chloride solution (0.05 per cent) upon iron and aluminum strips was selected for this demonstration. These metals are particularly suitable because their action with salt solutions is rapid and the corrosion products are voluminous, so that results are clearly visible. Since aluminum is amphoteric, it behaves somewhat differently than iron, although if time is limited the demonstrations involving iron are better. Metal strips 4 in. X 1 in. X in., drilled and fitted for electrical connections, were used. The inert electrode was a graphite rod one quarter in. in diameter. Electrolytic control of corrosion is best shown by applying voltage to the metal strip immersed in the salt solution, using the graphite rod as the other electrode. Direct current of proper voltage is necessary Present address, Goodyear Research Laboratories, Akron, Ohio. BURNS,R. A,, "Mechanism of corrosion processes," Am. Soc. Testing Materials Bull., 126, 17-20 (1944). a BXOWN, R. H., ''Gakanic corrosion," ibid.,pp. 21-6.
RONALD B. SPACHT' The National Bronze and Aluminum Foundry CoCleveland, Ohio
and if possible the temperature should be kept between^ 140" and 150°F. if good results are to be obtained~ within 24 hours. A Slomin Electroanalyzer is very well. suited for this demonstration. The polarity and the voltage of the metal strip may^ be changed to give different results. Four setups involving iron .are shown in the following diagram. Shaded portions indicate the approximate quantities of Fe(OH)a observed after 24 hours.
NACL SOLUTION
I
The following table shows the conditions used and the approximate results obtained. Code l~mbn
Anadc
Cofhodr
Applied volfaac
1 2 3 4
Fe C
C Fe C Fe Control
1 2.2
5
C
2
6
Al C
Al C
A1 Control
1
7 8
1
None 2
Nome
Obsewafion
Large deposit of Fc(OH)r Moderate Fe(0H)r Slight deposit Fe(0Hjz Less Fc(OHh than in number I but more than number 2 or L Moderate AI(0H)s Large deposit AI(0H)r Slight deposit AI(0H)s More AIlOHh than number 7.
DISCUSSION
Examination of the first four results shows that the iron is protected only when made the cathode and is completely protected only when a certain minimum voltage is exceeded. Making iron the anode accelerates its corrosion, as shown in number 1. Aluminum behaves differently than iron. In general, i t corrodes less rapidly. Aluminum will corrode when made the cathode if the current density is too high as shown in number 5 in the table. This is explained by its reaction with the accumulated alkali around the cathode a t these high current densities; however, a t one volt practically all corrosion is stopped when aluminum is the cathode. ACKNOWLEDGMENT
The author wishes to express his appreciation to the National Bronze and Aluminum Foundry Company for permission to publish this paper.
