Effect of Inhibitors on the Corrosion of Zinc in Dry-Cell Electrolytes
Total Page：16
File Type：pdf, Size：1020Kb
U . S. Department of Commerce Research Paper RP1863 National Bureau of Standards Volume 40, February 1948 Part of the Journal of Research of the National Bureau of Standards Effect of Inhibitors on the Corrosion of Zinc in Dry-Cell Electrolytes By Clarence K. Morehouse t 1 Walter J. Harner t and George W . Vinal This paper deals with a study of substitutes for mercury and chromate films in curtailing t he corrosion of t he zi nc anode of Leclanche dry cells at high temperatures. Organic com pounds containing t he carbonyl group (s uch as furfural), heterocyclic nitrogen-containing compounds like quin aldine, a nd certain commercial products were found to be effective in retarding the cOlTosion of zinc in dry-cell electrolytes. Al t hough these types of mat.erials retarded t he corrosion of zinc, dry cells made with " them did not have the expected increase in shelf-life or electrical output. The inhibitors r' eit her reacted with t he paste wall to decrease its strength, or formed an insoluble film over the ent ire surface of t,he zine anode that co nt ributed to its internal resistance. No paral lelism was observed between t he electrical output of the cells and t he percentage of inhibitor used in their construction. On the other hand, t he paste wall of the dry ce ll, which is usuall y a starch-flou r gel, was found to have inhibiting properties. Starch was inert, but flour h9d inhibiting proper t i.es. The active inhibiting agent of t he flour is gluten. The constit uents of glu ten, namely, glutenin, and gliadin Bnd mesonin in different states of aggregation were isolated, and the 19.st two were found to be effective in retarding t he cor rosion of zinc. These materials can be incorporated in dry cells and inerease t heir capacity at moderate temperatures. I. Introduction able electrical energy, and (2) the corrosion reac tion (local action), which is a side reaction, and ? When the Leelanche type of dry cell is subjected from which no electric energy is available. These to high-temperature storage, one of the sources of two reactions may be formulated a follows: failure is the increased rate of corrosion of the zinc 1. The dry-cell reaction [2 , 3] in which no H 2 anode [lJ.2 The zinc anode also corrodes at normal gas is formed: temperatures (18 ° to 30° C), and it is common practice either to amalgamate the zinc or to use Zn+ 2 YInOz= ZnO.Mnz0 3' elU'omate films to curtail local action. At high 2. Corrosion reaction in which H2 gas is formed: temperature neither of these methods is so effec tive as to yield good shelf life. In this paper Reaction at anodic areas: Zn-'Zn+++2€, alternate methods are described, including the use R eaction at cathodic areas: of inhibitors and modifications of the paste wall to increase its inhibiting properties. The theory of corrosion inhibitors is still in the Over-all corrosion reaction: formative stage, and their choice is largely a matter of trial and crror. However, the principles postu laLed for the corrosion of steel may be considered. The corrosion reaction may also be considered In the dry cell the reactions are of two types: (1) more simply as the discharge of hydrogen ions that the cell discharge reaction, which gives the avail- penetrate to the cathodic areas of the zinc can. I Now with Oli n Industries, Western Cartridge Co., East Alton, III . The corrosion and the cell-discharge reactions are 2 Figures in brackets indicate the literature references at the end of this paper. not the same, and it should be possible to retard the Corrosion of Zinc in Dry-Cell Electrolytes lSI corrosion reaction without materially affecting the ionic volume, very mobile, I1nd highly penetrating. discharge reaction. In order to accomplish this, Dry cells have been made wi th electrolytes com the cathodic areas of the zinc must be made more posed of salts of anions other than chloride but anodic, or the anodic areas must be rendered inef with little success. Chlorides are necessary from fective until current is drawn from the cell. the standpoint of electrical output, because they Amalgamation of the zinc and chromate films preven t the anode from becoming passive when function in this sense. the cell is discharged. Therefore, studies were In the steel industry, organic substances are confined to concentrated aqueous solutions of added to the acid pickling bath to inhibit the cor ammonium chloride with or witbout additions of rosion of the metallic iron. The theory of these zinc chloride. The corrosion rate of the zinc inhibitors is that they form an ionizable salt with anode may be altered somewhat, but not signif the acid, whereby positive inhibitor ions are formed icantly, by changing the concentration of the chlo that are attracted by electrical forces to the ride or by using zinc containing various percent cathodic areas. They are not discharged but ages of other metals. In this work, commercial replace the iron ions in a H elmholtz double layer, zinc containing 0.25 to 0.60 percent of cadmium, and when a state of equilibrium is attained they 0.20 to 0.60 percent of lead, and less than 0.03 form a protective layer over the metallic surface. percent of iron was used. The effectiveness of this layer depends on its ability to prevent hydrogen ions from penetrating II. Experimental Procedure to the cathodic areas where they are discharged. The rate of corrosion of the zinc was deter The size, nature, position on the metallic surface, mined by a method suggested and used by D. N . ' and the closeness of packing of the inhibitor ions Craig, in which the amount of hydrogen that is will determine the effectiveness of this protective evolved under controlled conditions is measured. layer. The apparatus consisted of an inverted test tube \ These types;:>f inhibi tors are classified as cath closed by a rubber stopper through which passed odic inhibitors. Attention was given to this type, a capillary tube (fig. 1). Within the test tube the because in the dry cell it is desirable that the cor sample of zinc and electrolyte were placed, the rosion of the zinc be inhibited by stopping the electrolyte completely filling the test tube and corresponding cathodic reaction and not the capillary tube. In the inverted position, the gas anodic one, which is necessary for the production liberated at the zinc rises in the tube and expels of electrical energy by the cell. An anodic inhib an equal volume of electrolyte through the capil- ( itor, unless used in adjusted amounts, would not lary tube. The tube was weighed from day to I be desirable in that it would not only stop the day, and the volume 01 gas evolved was calculated cell-discharge reaction but might lead to localized from the change in weight and the known density corrosion and cause perforation of the zinc can. of the solutions. Measuremerits were made at Chromates act as anodic inhibitors, and experience 54° C, a temperature reached in storage in the I ' with chromate films indicates that they eiLher tropics during the past war. In some experi become ineffective when current is drawn from ments ther ate of corrosion was determined by a ~ the cell, or that the chromates function differently loss-in-weight method, considerecl later. in the presence of the paste wall (starch-flour gels) . III. Results In many corrosion problems it is possible to re Data given in tables 1 and 2 show the effects of duce and sometimes eliminate the corrosion by amalgamation and of chromate films on the rate of altering the existing environments. With dry corrosion of zinc in ammonium chloride main- l cells, however, the environments are necessary tained at 54°C. AA-size zinc cans (17/32 in. in from the standpoint of electrical output and little diameter and 1% in. in height) were used. Neither changes can be made in them. The electrolyte in amalgamation of the zinc nor use of chromate the usual type of dry cell consists of aqueous solu films was as effective as desired at 54° C. Any t.ions of ammonium and zinc chlorides. There is excess of mercury decreases the mechanical an abundance of chloride ions, which are small in strength of the cans. 152 Journal of Research L TABLE 1. Effect of amalgamation on the corrosion of zinc electrodes (~ t 54° C "....-wire hook [AA-size zinc caos totally immersed io 20% NH.OI so lu tions] Hydrogen evolved a t 54° C for- ligCI, per AA-sizc zi nc can / 1 day 2 days '/ el ectrolyte / / j / u ml ml solution 0.00 ______ ___ ____ ________________ _ 8. I 15.6 /~/// .01. ____________________________ _ 4.8 II. I .02 ____ ___ _____ _________________ _ ////// 4.6 14. 4 .04 _______________ _______ __ __ ___ _ 3. I 9.1 ///// .08 ____________ _____________ ___ __ 1.9 8.7 .10 _______ ____ __________________ _ 1.9 7.2 / //// / .15 ______________________ _____ __ _ 1. 8 6.3 .20 ______ ______ __ __________ _____ _ 1.7 7. 2 The inhibiting properties of a large number of materials were studied. They may be classed as commercial or industrial by-products, organic compounds, inorganic compounds, and colloidal ru bber materials. The organic compounds may be sub stopper divided into noncyclic and heterocyclic nitrogen-, oxygen-, or sulfur-containing compounds_ I so cycli c compounds would not be expected to be gloss tubing effective inhibitors because of their symmetry.