Effect of Addition Agents upon the Corrosion Rate of Aluminum by Alkalies Effect of Various Substances F. H. RHODESAND F. W. BERKER,Cornel1 University, Ithaca, N. Y. N THE pickling of steel with acid it is well known that the action of the acid upon the metal may be minimized b y adding t o the acid certain inhibitors that retard the corrosion of the metal without at the same time seriously interfering with the solution of the scale. I n the present investigation the effects of various inorganic salts and organic compounds upon the rate of corrosion of aluminum by alkalies has been determined, with a view to the possibility of finding some substance that would minimize or prevent the corrosion of the metal by alkaline solutions. Some work on the effects of addition agents on the rate of corrosion of aluminum by alkalies has already been published, Rohrig (5)found that the addition of sodium silicate to a solution of sodium carbonate decreases the rate of action of the solution on aluminum, although the silicate increases the rate of attack of the metal by sodium hydroxide in moderately
Into the reaction tube, M , was placed sufficient of the soluton to be tested to cover the test strip of aluminum. Gaseous hydrogen was passed through the solution for about 3 minutes in order to saturate it with hydrogen and thus eliminate any error due t o the solubility of the gas in the solution. The reaction tube wvas then immersed in a bath of water maintained at a constant temperature of 30" C. and allowed t o stand until the solution had attained the temperature of the bath. A cleaned and weighed strip of aluminum, 5 cm. long, 1 cm. wide, and 0.0635 cm. thick was introduced, and the reaction vessel was connected immediately with a Hempel buret in which the volume of evolved hydrogen was measured. Readings were taken every minute for 20 minutes. At the end of this period, the aluminum was removed, rinsed, and again weighed. The loss in weight of the metal served as a check upon the amount of hydrogen evolved as a measure of the total extent of the corrosion. The aluminum used was commercial sheet aluminum, 99.3 per cent pure.
EFFECTS OF ADDITIONAGENTSON CORROSION ALKALIESALONE. I n the first series of experiments the rate of corrosion of aluminum in sodium hydroxide solutions of various concentrations was measured. The p H of each solution was determined, using a standard hydrogen electrode balanced against a saturated calomel electrode. The results are shown in Figure 2, in which the values for p H and for rates of corrosion are plotted against the concentrations, and in Figure 3, in which is shown the variation in the rate of corrosion with the change in pH. The rates of action of molar and 0.5 molar solutions of potassium hydroxide were also measured :
m
ALKALI
CONCENTRATION RATIO
Sodium hydroxide Potassium hydroxide Sodium hydroxide Potassium hydroxide
F
FIGURE 1. APPARATUS FOR DETERMINATION OF CORROSION RATE E, F. M,
Control stopcocks Reaction bulb
S. Capillary spiral 2'. Inlet tube for hydrogen X. Hydrogen inlet control
Y . Inlet for alkaline solution
Molar 1 1 0.5 0.5
Cc. Hn/min. 2.50 2.13 1.60 1.42
Although potassium hydroxide is more nearly completely dissociated than is sodium hydroxide in solutions of these concentrat,ions, there is a distinct and consistent difference in the rate a t which they attack aluminum. SALTS. I n the next series of experiments the effects of salts were investigated. I n each experiment the salt, in the concentration indicated, was dissolved in a 0.5 molar solution of sodium hydroxide. SALT
None NaCl NaCl
CONCN.
RATE
Molar Cc. Hl/min. 1.6 0.5 1.96 1.0 1.93 1.5 1.89
...
SALT KCl NazSOo
KnSO,
COXCN.
Molar 0.5 0.5 0.5 0.5
RATE
Cc. Hdmin 1.66 1.98 1.83 3.98
concentrated solutions. Seligman and Williams (6) also NaCl NrtsPO4 investigated the effects of sodium silicate as a n addition The addition of sodium chloride increases the activity of agent in the cleaning of aluminum by alkalies. The use of silicate as a retarder in this process was patented by Lea ( 3 ) . the hydroxyl ions from the sodium hydroxide and markedly Rohrig (5) found that certain colloidal substances, such as accelerates the corrosion. The acceleration is most marked glue, agar agar, and gelatin, exhibited inhibiting action. in solutions of 0.5 molar concentration and decreases as the Other substances for which similar action has been claimed concentration rises above this value. -4 similar effect is include ammonium compounds, dichromates, sodium molyb- observed with solutions of sodium chloride in molar alkali. date, permanganates, and salts of the noble met.als (1, 2, 4). Sodium sulfate shows a n effect similar to that of sodium chloIn the present work the rate of action of the alkaline solu- ride but even more pronounced. The addition of sodium phostions on the aluminum was determined by measuring the phate increases the rate of corrosion very greatly, the increase rate of evolution of the hydrogen formed by the reaction. being due in part at least t o the hydrolysis of the salt and the The construction of the apparatus used is shown in Figure 1. resulting increase in concentration of hydroxyl ions. 1336
I N D U S T R I A L h N D E N G IS E E R I N G C 11 E M I S T R Y
December, 1933 CONCN. OF NaCl
SaClH
IN
MOLAR
,Volar 0 1 2
RaTE Ce. H d m i n . 2.5 2.8 2.8
OXIDIZINGAGENTS. Since the use of certain oxidizing agents as inhibitors in the cleaning of aluminum has been suggested, the effects of potassium chromate and of potassium permanganate upon the rate of attack of the metal by sodium
it seemed probable that organic compounds of high molecular weight and containing an acidic g r o u p f o r example, phenolic compounds-might be effective in decreasing the rate of action of the alkali on the metal. Accordingly, several simple and complex phenols were tested: CONCN. I N
MOLAR SOLN.
PHENOL
XaOH Molar
OF
...
None Phenol Phenol Phenol o-Cresol p-Cresol 8- Naph tho1 p-Aminophenol p-Arninophenol Resorcinol
FIGURE 2. EFFECTOF CONCENTR~. TION OF SODIUJl HYDROXIDE UPON P H OF SOLUTIOV AUD RATEOF HYDROG E S El OLUTION 1 2
Rate of evolution of hydrogen
CONCN. OF SODIUM
METASILICATE IN
PH
MOLAR NaOH Molar h-one 0.1 0.5 1.5
R4TE Cc. Hz/min. 2.5 2.7 2.7 2.2
13.57 13.32 13.2 13.10
With those solutions that contained rather high concentrations of silicate, the action of the alkali on the metal was not uniform but was much localized, and the metal was not smoothly etched but was badly pitted. ALCOHOLS.Aliphatic alcohols appeared to decrease the rate of action of molar sodium hydroxide solutions on aluminum, although the effect was not very marked. RATB C'c. H,/min. 2.5 1.95 2.15
None Methyl Ethyl
$LCOHOL
N-propyl N-butyl Glycerol
2.0 2.4 2.32
FIGURE 3. EFFECTS OF R a T E OF CORROsION O F
TAR ACIDS ON ALUMINUMBY SODIUM HYDROXIDE
'
13 D /24
1. 2. 3. 4.
//e 0
cc
/
2
Resorcinol Phloroglucinol Phloroglucinol Phloroglucinol Dihydroxynaphthalene p-Chlorophenol 2-Chloro-5-hvdroxvtoluerle N-hydroxypGenyldycine o ,o-Dihydroxybiphenyl
13.58 13.00 12.90 11.61 11.90
...
1 2 : 63 l2:62 11.56 9.86 12.51
... ...
9.55
...
ii:4o ...
2.5 2.0 1.7 .37 .35 .38 1.3 1 3 0.1 1.6 .6 0.0 1.4 .9
.
.o
.8 2.1 1.6
.o .o
I n no case did any of the phenolic compounds show a specific inhibiting action. It is true that the addition of phenols to a solution of sodium hydroxide tends to decrease the rate of action of the alkali on aluminum, but in no instance is the decrease greater than would be expected from the effect of the phenolic compound in lowering the pH of the solution. I n fact, in most instances a solution of a phenol in sodium hydroxide attacks aluminum more rapidly than does a solution of sodium hydroxide alone of the same pH, so that the phenols increase rather than decrease the rate of corrosion. GELATIN. Finally, one series of experiments was made to determine the effect of gelatin on the rate of attack of aluminum by a molar solution of sodium hydroxide. The following results were obtained: COICN. OF GELATIN I N MOLARS o m a . OF
NaOH
PH
RATE Cc./min
13.58 13.39
2.5 1.0 0.4 0.3
70 0.0 2 4 6
13:39
R.4TE
Cc. Hz/min.
With the exception of the butyl alcohol, each of the alcohols was present in molar concentration. Because of the comparatively slight solubility of butyl alcohol in a normal solution of sodium hydroxide, a molar solution of this substance could not be obtained. I n testing the action of this compound, the solution used was one saturated with the alcohol a t 30" C. /J 6
0.2 0.5 1.0 1.0 1.0 0.5 0.5 1.0 0.25 0.5 1.0 0.2 0.3 0.5 0.5 0.5 0.5 0.5 0.5
RATE Cc. Hg/min
PH
PH
hydroxide were measured. I n no case was any inhibiting effect observed. Sodium metasilicate was found to have a slight inhibiting action when present in comparatively large amounts:
ALCOHOL
1337
Solution of sodium hydroxide alone N sodium hydroxide containing phenol N sodium hydroxide containing resorcinol N sodium hydroxide containing phloroglu.
Gelatin has a very marked specific inhibiting effect and this effect is not due to the lowering of the pH of the solution. CONCLUSION From the present work it appears that the common inorganic salts of the alkali metals do not decrease the rate of action of alkalies on aluminum but actually increase the rate of corrosion. Sodium metasilicate in rather high concentrations does lower the rate of attack of the metal but tends to cause pitting. The simpler aliphatic alcohols show only very slight inhibiting effect. Phenolic compounds exhibit no specific inhibiting action, although they do behave to some extent as acids and thus lower the pH of the alkaline solution. Of the substances tested, the only ones that did show a specific inhibiting effect were glucose and gelatin. LITERATURE CITED
cinol
N,/M,r).
GLUCOSE.Glucose in molar concentration was found to inhibit the action of a molar solution of sodium hydroxide almost completely. The rate of evolution of hydrogen was reduced to the very low value of 0.4 cc. per minute. PHENOLS.I n the pickling of steel by acid, certain basic organic compounds of high molecular weight have been found to be efficient inhibitors. Reasoning from analogy,
(1) Aubert and Dixmier, Rev. mBt., 26, 309 (1929). (2) Creutsfeldt, Korrosion u. Metallschutr, 4, 102 (1928). (3) Lea, British Patent 127,772 (Sept. 9, 1918). (4) Mix and Genest, Britkh Patent 13,012 (May 28, 1910). (5) Rohrig, Chem.-Ztg., 47, 528 (1923). (6) Seligman and Williams, J. I n s t . Mefals, 28, 297 (1922). RECEIVEDJ u n e 5 , 1933.
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