EFFECT

O F pH O F W O R K I N G

STRENGTH

MEDIA

OF A MAGNESIUM

V , E~ B e l y a k o v , S. V , a n d V~ V . R o m a n o v

ALLOY

ON L O N G - T I M E MA2-1

Pushkina,

UDC 620.197.8

The reduction in the l o n g - t i m e strength of magnesium alloys in electrolyte solutions is in m o s t c a s e s a s s o c i a t e d with two phenomena [1]: s t r e s s - c o r r o s i o n c r a c k i n g [2] and c o r r o s i o n damage. The loss of strength due to c o r r o s i o n is a s s o c i a t e d e i t h e r with a reduction in the specimen c r o s s - s e c t i o n a r e a due to uniform dissolution of the m a t e r i a l or with the appearance of effective s t r e s s r a i s e r s in the case of nonuniform c o r r o s i o n . The aim of this investigation was quantitatively to determine the contribution of each of these f a c t o r s in reducing the l o n g - t i m e strength of one of the typical magnesium alloys in chloride solutions in relation to their pH. The experimental work was c a r r i e d out on a type MA2-1 magnesium alloy (4.45% AI; 1.12% Zn; 0.56% Mn; 0~ Fe; 0~ Si; 0.0011% Ni; 0~ Be; r e m a i n d e r Mg) in the f o r m of 1.5-mm-thick sheet. T e n sile s p e c i m e n s (70 m m long, 5 m m wide} w e r e cut in the rolling direction; t h e i r s u r f a c e was polished with e m e r y paper, the l a s t polishing operation being done with p a p e r No. 14, rubbing the s p e c i m e n in the longitudinal direction. The specimens w e r e then d e g r e a s e d with acetone, washed in distilled w a t e r , pickled in a solution containing 150 g / l i t e r CrO 3 and 20 g / l i t e r NaNO2, washed again in water, dried with a filter pap e r and placed in a d e s i c c a t o r (over calcium chloride} for 18-20 hr. The water-divide line was insulated with type B F - 2 adhesive to a depth of 5 m m . The s p e c i m e n s w e r e tested in uniaxial tension on a type V P - 8 machine. The t e s t base was 33 h, this being the time during which a complete l o s s of 10ng-time strength of s p e c i m e n s tested in the m o s t acid solution was o b s e r v e d . The tests w e r e c a r r i e d out in 0ol N (with r e s p e c t to chlorine ions} solutions c h a r a c t e r i z e d by the following pH values: 1.05; 2.00; 4.00; 7.00; 11.50; 14.00. Distilled w a t e r and c h e m i cally pure reagents were used in the p r e p a r a t i o n of the solutions; t h e i r pH was adjusted by HC1 o r NaOH additions. A type L P U - 0 1 i n s t r u m e n t with a g l a s s electrode was used to m e a s u r e pH~ To e n s u r e that pH r e m a i n e d constant, the tests were c a r r i e d out in running solutions; this was done by using special v e s s e l s (125 ml capacity) with taps. The e l e c t r o l y t e flow rate was about 3 m l / m i n so that the m a x i m u m variation in pH during an e x p e r i m e n t was 0.015-0.02. The rate of c o r r o s i o n of s p e c i m e n s in a s t r e s s - f r e e state was d e t e r m i n e d g r a v i m e t r i c a l l y ; values K a r e cited f o r a test period of 30 h~ The electrode potential of u n s t r e s s e d s p e c i m e n s was m e a s u r e d with the aid of a calomel e l e c t r o d e ; the r e s u l t s were r e c a l c u l a t e d in t e r m s of the hydrogen scale. The values eh d i s c u s s e d below r e l a t e to s t e a d y - s t a t e conditions, i.e., 30 min a f t e r the s t a r t of an experiment. The test t e m p e r a t u r e was 25" C~ In e v e r y case the r e s u l t s cited a r e a v e r a g e s of 4-7 replicate m e a s u r e m e n t s . As p r e v i o u s l y stated, the total loss of l o n g - t i m e strength of the alloy A ~ under conditions studied c o m p r i s e s the loss of strength due to s t r e s s - c o r r o s i o n cracking A(r sc and the loss due to c o r r o s i o n damage A ~ , i.e. [31, SC

A% = A~ + A~.

(1}

Such f a c t o r s as hydrogen e m b r i t t l e m e n t and pitting c o r r o s i o n , which s o m e t i m e s reduce the long-time strength of metals, do not in this case come into play. Baikov Institute of Metallurgy, A c a d e m y of Sciences of the USSR, Moscow. T r a n s l a t e d f r o m F i z i k o Khimieheskaya Mekhanika Materialov, Vol. 6, No. 5, pp. 7-10, S e p t e m b e r - O c t o b e r , 1970. Original a r t i c l e submitted June 10, 1969. 9 1973 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $15.00.

539

To determine Aft t u n s t r e s s e d specimens w e r e held in various solutions for periods required to p r o duce f r a c t u r e of s t r e s s e d specimens tested in these solutions. They were then removed f r o m the solution, washed in distilled water, dried with a f i l t e r paper, held in a d e s i c c a t o r , and subjected to standard tensile tests to determine t h e i r ~v1 (i.e., U.T.S). The loss of strength due to c o r r o s i o n damage was then calculated as i

!

A% = (Jv-- Cry,

(2)

where av is the U.T.S. of the alloy in the initial state and a v1 is its UoT.S. a f t e r c o r r o s i o n (in a s t r e s s - f r e e state) in a given solution for an appropriate time. A f t e r determining Aalv the value AGs c was found f r o m Eq. (1). As shown by test results r e p r o d u c e d in Fig. 1, the l a r g e s t loss of l o n g - t i m e strength of the alloy studied (Aa.r = 23.8 k g / m m 2) takes place in an acid solution with pH = 1.05 (curve 1). The loss of strength in solutions with pH in the interval 2.0-11.5 is s m a l l e r (9.1-8.9 k g / m m 2) and p r a c t i c a l l y constant. A solution with pH = 14 has p r a c t i c a l l y no effect on the l o n g - t i m e strength of alloy MA2-1. When curve I was constructed, the s c a t t e r of results obtained was 2.5% in solutions with pH = 1.054.0 and up to 23% in solutions with pH > 7. One's attention is drawn by the fact that in a wide range of values pH (1.05-11.5) the loss of longtime strength due to s t r e s s - c o r r o s i o n cracking r e m a i n s p r a c t i c a l l y constant at 4.0 k g / m m 2 (curve 3)~ On the other hand, the loss of strength due to c o r r o s i o n alone (curve 2) is strongly dependent on pH of the solution. The maximum value of A(rl~ (17.9 k g / m m 2) was r e c o r d e d for specimens tested in a solution with pH = 1.05. With pH i n c r e a s i n g to 14.0,A(rlT gradually d e c r e a s e d to 1.0 k g / m m 2. It should be noted that curves 1, 2, and 4 have the s a m e f o r m , i.e., the effect of pH on Aav, AOIT{and rate of c o r r o s i o n is the same in c h a r a c t e r . In other words, the c h a r a c t e r of the influence of pH on Aa T determines the f o r m of the curve of the total loss of long-time strength and the specific weight of all the f a c t o r s contributing to its reduction. At pH = 1.05 A(r1 constitutes 75.2% of A(r~, d e c r e a s i n g to 74.8% at pH = 2.0, 52.9% at pH = 4.0, and about 30% at pH = 7.0 and 11.5. At pH = 14.0 a small l o s s of long-time strength (AeT = 1o0 k g / m m 2) is equal to A a iT. Curve 4 indicates that the rate of c o r r o s i o n of alloy MA2-1 in solutions studied is f a s t e s t at pH = 1.05; with i n c r e a s i n g pH it gradually d e c r e a s e s to negligibly small values at pH = 14.0. It should be noted that the c o r r o s i o n in neutral solution takes the f o r m of the formation of fine (small diameter) deep pits; with i n c r e a s i n g pH the n u m b e r and depth of pits d e c r e a s e and t h e i r d i a m e t e r i n c r e a s e s . D e c r e a s i n g pH leads to an i n c r e a s e in the n u m b e r and size of pits; as a result, the surface of specimens tested in a solution with pH = 2.0 is uniformly c o v e r e d with c o r r o s i o n pits, general c o r r o s i o n taking place in a solution with pH = 1.05. As shown by curve 5, the electrode potential of alloy MA2-1 is shifted toward m o r e positive values with i n c r e a s i n g pH. The above-described effects of pH on the c o r r o s i o n and e l e c t r o c h e m i c a l c h a r a c t e r i s t i c s of this alloy a r e in a g r e e m e n t with data obtained by o t h e r w o r k e r s and a r e attributable to the dissolution of magnesium hydroxide in acid solutions and its low solubility in alkaline media [1]. If it is borne in mind that o u r test base was relatively short, it may be concluded f r o m the results obtained that alloy MA2-1 in 0.1 N (with r e s p e c t to C1 ions) solutions suffers a substantial loss of longtime strength (about 9 k g / m m 2) at pH = 1.05. This effect is a s s o c i a t e d with the influence of both weakening f a c t o r s ( s t r e s s - c o r r o s i o n cracking and c o r r o s i o n damage) which, in turn, reflect the relatively negative electrode potential of m a g n e s i u m and low protective power of films f o r m e d on its s u r f a c e in solutions conraining chlorine ions; this c r e a t e s conditions favorable for the formation of highly effective specific c o r r o sion couples (responsible f o r s t r e s s - c o r r o s i o n cracking [2]) and m i c r o e l e m e n t s (responsible f o r general corrosion). The t e s t results indicating that the absolute values of the loss of l o n g - t i m e strength due to s t r e s s c o r r o s i o n cracking a r e not dependent on pH,which has a m a r k e d influence on the loss of strength due to general c o r r o s i o n d a m a g e , a r e attributable to the fact that pH of the solutions used has a much s t r o n g e r influence on the kinetics and c h a r a c t e r of general c o r r o s i o n than on the kinetics of s t r e s s c o r r o s i o n cracking.

540

( 5

jO001 7g-20

600

I/~ - , 2

+/.JO

J.O0

- d

~

-

9

.+

2

~

~

4

10

~2 pH

Fig. 1. Effect of pH on the total l o s s of static l o n g - t i m e s t r e n g t h A~r alloy MA2-1 (curve 1), the l o s s of s t r e n g t h due to c o r r o s i o n d a m a g e AcT1 (curve 2), the l o s s of s t r e n g t h due to s t r e s s c o r r o s i o n cracking A(rSe (curve 3), the r a t e of c o r r o s i o n K (curve 4) and the e l e c t r o d e potential eh (curve 5) in 0.1 N (with r e s p e c t to C1 ions) solutions.

L e t us examine this p r o b l e m m o r e c l o s e l y . Lowering pH below 7.0 leads to a continuous i n c r e a s e in the intensity of dissolution of p r o t e c t i v e s u r f a c e f i l m s ; this r e s u l t s in an i n c r e a s e in the n u m b e r of anodic regions on the m e t a l s u r f a c e and f a c i l i t a t e s both the anodic and cathodic r e a c t i o n s . This explains the i n c r e a s e in the u n i f o r m i t y of c o r r o s i o n and in the weight l o s s e s and the shift of the e l e c t r o d e potential t o w a r d the negative values o b s e r v e d in acid solutions. T h e s e f a c t o r s influence to a l e s s e r extent the e f f e c t i v e n e s s of the specific c o r r o s i o n couples affecting the kinetics of s t r e s s - c o r r o s i o n c r a c k i n g , b e c a u s e t h e i r o p e r a t i o n is controlled mainly by the anodic r e a c t i o n s and b e c a u s e the anodes of the s p e c i f i c couples a r e f a r r e m o v e d f r o m the m e t a l s u r f a c e and o p e r a t e u n d e r conditions of c r e v i c e c o r r o s i o n and s t r e s s concentration [2]. On the o t h e r hand, i n c r e a s i n g pH i n c r e a s e s the adhesion of the p r o t e c t i v e s u r f a c e f i l m s , reduces the n u m b e r of anodic s u r f a c e m i c r o e l e m e n t s , and inhibits the anodic r e a c t i o n s ; this at f i r s t r e d u c e s the r a t e of c o r r o s i o n and l a t e r , at higher pH l e v e l s , c o m p l e t e l y inhibits the c o r r o s i o n . As shown by the r e s u l t s of this investigation, however, the f o r m a t i o n and consolidation of p r o t e c t i v e s u r f a c e f i l m s in alkaline solutions has no effect on the l o s s of l o n g - t i m e s t r e n g t h of alloy MA2-1 due to s t r e s s - c o r r o s i o n c r a c k i n g which, as in the c a s e of acid solutions, is a s s o c i a t e d with different d e g r e e s of influence of this f a c t o r on the o p e r a t i o n of s u r f a c e m i c r o e l e m e n t s and specific couples. It is only at pH = 14.0 that surface f i l m s on m e t a l acquire sufficient p r o t e c t i v e power to s u p p r e s s the operation of specific couples. The r e s u l t s obtained lead to the following conclusions. The m a g n e s i u m alloy MA2-1 t e s t e d f o r 33 h in 0.1 N (with r e s p e c t to chlorine ions) solutions s u f f e r s a substantial l o s s of its l o n g - t i m e strength. The pH of such solutions has a m a r k e d effect on the m e c h a n i s m and d e g r e e of the l o s s of s t r e n g t h m a i n l y by a f f e c t ing the s p e c i f i c weight of the l o s s of s t r e n g t h due to c o r r o s i o n d a m a g e . The l o s s of l o n g - t i m e strength due to s t r e s s - c o r r o s i o n r e m a i n s constant (at about 4 k g / m m 2) in a wide r a n g e of pH (1.05-11.5). The l o s s of l o n g - t i m e s t r e n g t h at pH = 14 is r e l a t i v e l y s m a l l (1.0 k g / m m 2) and a s s o c i a t e d only with the c o r r o s i o n d a m a g e . LITERATURE 1.

2. 3.

CITED

V. V. Romanov, C o r r o s i o n of Magnesium [in Russian], Izd. AN SSSR (1961). V. V. Romanov, Metallurgy, Metallography, and P h y s i c . c h e m i c a l Methods of Investigation" T r a n s actions of Baikov Metallurgical Institute AS USSR [in Russian], No. 13, Izd. Nauka (19627. V. V. Romanov, FKhMM, No. 2 (19687.

541