DEMONSTRATION REAGENT FOR CORROSION OF ALUMINUM WILSON L. ORR University of Southern California, School of Aeronautics, Santa Maria, California
Tm ferroxyl reagent as described in Tars JOURNAL by Meldrum' has been used very successfully in demonstrating the mechanism of corrosion in iron and steel, but for aeronautical engineering students it was desirable to extend these demonstrations to aluminum and aluminum alloys if possible. A similar reagent was developed using "Aluminon" (Aurin Tricarboxylic acid) and thymol blue as the indicators. Aluminon develops F ~ ~ Y 2. I B as ~ 1w i , - ~ . ~~t ~ a red color in the presence of aluminum ions and thus indicates the anodic areas in the corrosion of aluminum. The thymol blue changes from a pale yellow to a blue electrons may be conducted to any point on the metal over the pH interval of 8.0 to 9.0 and develops a blue where they can be removed in some chemical reaction. color in the area of the cathodic reaction. In the uresence of oxvcen and moisture the usual " The electrolytic theory of corrosion has been amply reaction is described elsewhere. Meldrum briefly outlined it for 03 2HzO 4- 4e40H- (Cathodic reaction) the corrosion of iron, and it is essentially the same for other metals. Metals in general have a certain ten- The cations formed may be removed by passing into dency to lose electrons and become ions. The greater solution or by migrating along -the metal surfaces until this tendency the higher the oxidation potential and the their charge is effectively neutralized by some anions. higher the metal is placed in the electromotive series. The areas where these reactions take place in the corHowever, a given metal may exhibit differencesin oxi- rosion of aluminum can be observed by the use of the dation potential between various areas in the same reagent described below. piece of metal due to strains or other conditions. This PREPARATION OF REAGENTS reaction, in aluminum, for example, One gram of sodium chloride and 1.3 g. of agar are Al -+ Al+3 + 3e- (Anodic reaction) gently boiled in 100 ml. of water for about an hour to will take place in the area where the oxidation potential peptise the agar and then filtered through a cloth. is the highest but cannot proceed unless the electrons Then 13 ml. of aluminon reagent and 15 ml. of thymol and ions formed can be removed to prevent polarization blue indicator solution are added and the pH is adjusted or accumulation of charges in the various areas. The to 8, or just short of the formation of a blue color, with dilute sodium hydroxide. The reagent is then ready ' MELDRUM,W. B., J . C ~ e n rEnuc., . 25, 254 (1048). for use. The aluminon reagent is made by dissolving 0.1 g. of aluminon in 100 ml. of water. The reagent is used in the same manner as the ferroxyl reagent. Enough of the reagent is poured into a Petri dish or other suitable container to form a thin layer and allowed to cool. A uiece of aluminum wire o; other alumi?um object is then placed on this foundation layer and more reagent is added to cover it completely. At this point the reagent should not be too hot as.distortiou of the color zones may appear. LEGEND The gelling may be hastened by setting the dish in a RED - A N O D I C A R E A pan of cold water. The colors start to develop in about Al - ~ l ' * i 3 e one hour, but sufficient development usually .requires
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JOURNAL OF CHEMICAL EDUCATION
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several hours. It is usually best to prepare the samples one day for observation the next day. The samples may be kept for several days if stored in a humidifier so the gel does not dry out. The metal should be thoroughly cleaned before placing it in the reagent. If the article is smooth, clean the surface with steel wool and then in dilute hydrochloric acid followed by a water rinse to remove the acid. If the surface is uneven it is better to use only the acid. TYPICAL DEMONSTRATIONS
If a piece of aluminum wire about an inch long is placed in the gel, after about twelve hours the colors will develop as shown in Figure 1. The anodic reaction takes place a t the ends of the wire where strains were introduced in cutting the wire, and the blue color showing the cathodic zone is more diffuse, covering most of the area between the ends. The effect of strain can also be shown by sharply bending a piece of wire (Figure 2) or filing notches in it (Figure 3). However, bending a piece of aluminum wire does not always set up sufficient strain to accelerate corrosion in the bend unless the bend is quite sharp. Much better results are obtained if instead of pure aluminum an alloy is used which will "work-harden."
Figure 5.
a4 s t Aluminum Alloy C h d Sheet
If a copper wire is wound around a piece of aluminum wire the colors develop much faster, showing how contact with a less active metal accelerates corrosion (Figure 4). A small piece of Alclad, notched through the clad surface in one place, enables the student to see how the sacrificial pure aluminum coating over the aluminum alloy protects the main stock (Figure 5). This reagent used with the ferroxyl reagent allows the alert instructor to devise many impressive demonstrations that are especially appropriate. It should always be kept in mind, however, that corrosion depends very largely on environment, and the conditions of exposure in the reagent are not identical to conditions under which the metals are often used. Some reputable source should be consulted for the effects of various environment conditions on the corrosion of any individual metal. One example will illustrate this point. Zinc is anodic to aluminum in most neutral and acid solutions: hence in such solutions contact withzinc results in protection of the aluminum article. In alkaline solutions, the potential reverses so that in these media, contact with einc can cause accelerated attack of aluminum.*
' UHLIG,H. H., "Corrosion Handbook,'' John Wiley & Sons, Inc., New York, 1948, p. 48.
