NEW MATERIALS AND CORROSION CONTROL
CORROSION ITS
RESISTANCE
ALLOYS I.N.
IN S O D I U M
Gladkii
OF
ALUMINUM
CHLORIDE
and Yu.P.
AND
MEDIA UDC 620.193.4:669.71
Chmyrev
Many y e a r s of p r a c t i c e in the U s o l ' s k Salt Extraction Plant have shown that the construction m a t e r i a l (KS aluminum alloy, containing 95-96% A1, 2% Mg, 1.4% Mn, 0.7% St, and 0.2% Sb) f r o m which the imported salt drying drums were made had v e r y high c o r r o s i o n r e s i s t a n c e in the conditions of salt extraction [1]. This led to a study of utiltzability of aluminum and its alloys manufactured by the Soviet industry in contact with table salt and its solutions. The investigations have been c a r r i e d out by the All-Union Scientific Res e a r c h Institute of the Salt Industry. According to data of the All-Union Scientific R e s e a r c h and Planning Institute of the Aluminum, Magnesium, and Electrode Industries the best c o r r o s i o n r e s i s t a n c e in sodium chloride media, could be expected f r o m AMg5 and AMg2 a l u m i n u m - - m a g n e s i n m alloys. Some l i t e r a t u r e data [2, 3] indicate also that at given conditions (determined pH values, etc.) these alloys should have a high c o r r o s i o n r e s i s t a n c e in c h l o r i d e containing media. The c o r r o s i o n tests were c a r r i e d out on A2M aluminum sheets, and AD1 and AM2 aluminum alloys. The chemical composition of these m a t e r i a l s in p e r c e n t is given in Table 1. The m a t e r i a l s were tested under l a b o r a t o r y conditions and in the conditions of the s a l t - e x t r a c t i n g industry. The l a b o r a t o r y c o r r o s i o n tests were c a r r i e d out by p a r t i a l and complete i m m e r s i o n in solutions of chemically purified and m o t h e r liquor b r i n e s of the Sla~yan Salt Plant, in an artificial b r i n e p r e p a r e d f r o m table salt f r o m the above plant, and in brine f r o m the Bol'sho[ Kalkaman Lake (Table 2). The specimens were tested at r o o m t e m p e r a t u r e (20-23~ and at the boiling t e m p e r a t u r e of the s o l u tions in flasks equipped with reflux c o n d e n s e r s . T h r e e specimens of each m a t e r i a l were p l a c e d in each flask on glass rods. The tests at r o o m t e m p e r a t u r e were c a r r i e d out with and without s t i r r i n g . The c o r r o s i o n r e s i s t a n c e of the m a t e r i a l s was a s s e s s e d by the metal loss in g / m 2 and the c o r r o s i o n rate in m m / y r . The c h a r a c t e r of the c o r r o s i o n was a s s e s s e d visually and with a binocular m i c r o s c o p e . The results of the l a b o r a t o r y c o r r o s i o n tests c a r r i e d out on aluminum and its alloys are given in Table 3. The data show that the lowest c o r r o s i o n rate for A2M aluminum and its AD1 and AMg2 alloys were found in the natural brine f r o m Bol'shoi Kalkaman Lake, which had a pH of 7.2. The s u r f a c e of the s p e c i mens of these metals remained p r a c t i c a l l y unchanged after six months of testing in the lake b r i n e ; only a few shallow c o r r o s i o n spots were detected on the surface of A2M aluminum and AMg2 a l u m i n u m - - m a g n e s ium alloy s p e c i m e n s . All these m a t e r i a l s can be considered as c o r r o s i o n r e s i s t a n t in the lake brIne.
TABLE 1 Alloy A2M AD1 AMg2
Cu
Mn
Mg
Si 0.50 0.60
0.02 0.05 0.42
1.23
A1 Rest
Impurities not more than Fe
Si
0.50 0.3 0.1
0.35 0.14
~
/
others 0.10 0.13
Note: Accordingto data of the All-Union Scientific Research Institute of the Aluminum, Magnesium, and Electrode Industries (Leningrad).
T r a n s l a t e d f r o m Khimlcheskoe i Neftyanoe Mashinostroenie, No, 8, pp. 18-19, August, 1969.
619
Lake ...............
7.6
1.2276 1.2014 1.2263
13.0 8.8
1,1986
Purified, from 81avyan plant. Mother liquor from 81avyan plant ...............
FromSyntheticBol'shoi ............. Kalkaman
11.4
~
2
Brine
TABLE
~
1
0.016
0.014 0.028
0.032
ions
188,430
9"442 1201"197
O.Oll I179.707 9.011 193.040
-
111"730
[31.277 0.941
2,487
Composition, g/liter
I 0.912
[ 1.066 0.303
0.698
1157"209
[296.278 318.259
310.658
I
[
-
.i
.'.
.~
1142"138
-
I 14.702
salts
I 0.065
0.055 0.112
0.131
1.470
#
1.454 O.226
0.825
z
G
TABLE 3 Corrosion medium
Alloy
Temperature, ~
Test conditions
Test duration, h
Corrosion rate g/em2.h
mm/yr
A2M. . . . . . . AD1 . . . . . . . AMg2 . . . . . .
Purified brine from Slavyan salt plant
20-23
Full immersion
750
0.0349 0.0487 0.0055
0.1131 0.1581 0.0180
A2M . . . . . . AD1 . . . . . . . AMg2 . . . . . .
The same
20-23
Partial immersion
747
0,0174 0.0226
0.0559 0.0732 0.0180
A2M . . . . . . AD1 . . . . . . . AMg2 . . . . . .
A2M ADI AMg2 A2M ADI
Full immersion
755
0.0558 0.0166
0.0491 0.1810 0.0541
0.0155
Full immersion with stirringof the solution
750
0.0102 0.0043 0.0094
0,0330 0.0139 0.0304
20-23
Full immersion
750
0.2701 0.3130 0.2397
0.8767
......
Mother liquor from 81avyan plant
0.7776
......
The same
20-23
Partial immersion
750
0.1644 0.1946 0.2044
0.5333 0.6358 0.6631
20-23
Full immersion
750
0.4902 1.1291 0.1496
1.5904 3.6633 0.4853
A2M . . . . . . AMg2
105
I I
20-23
A2M . . . . . . AD1. . . . . . . AMg2. . . . . . ADI
0,0399
.......
!
....... ...... ......
with stirring of the solution
.......
AMg2 . . . . .
1.0263
A2M . . . . . . AD1 . . . . . . AMg2 . . . . .
Synthetic brine
20-23
Full immersion
750
0.0109 0.0115 0.0135
0.0354 0.0375 0.0439
A2M . . . . . . AD1 . . . . . . AMg2. ....
The same
20-23
Partial immer-
750
0.0145 0.0122 0.0163
0.0471
0.0068 0.0077
0.0221
0.0074
0.0240
A2M
......
ADI
......
AMg2
.....
sion Natural brine fron Bol'shoi Kalkaman Lake
20-23
Full immersion
1344
mark
0.0396 0.0529
0.0251
Note: The data represent mean values of the results obtained on three specimens.
T h e s e a l l o y s showed the l o w e s t c o r r o s i o n r e s i s t a n c e in the m o t h e r l i q u o r f r o m the S l a v y a n Salt P l a n t , the pH of w h i c h was about 13. All s p e c i m e n s of a l u m i n u m and its a l l o y s t e s t e d (with p a r t i a l and full i m m e r s i o n ) in the m o t h e r l i q u o r s h o w e d p i t s w h i c h l a t e r l e d to holes t h r o u g h the s p e c i m e n . T h e c o r r o s i o n a t t a c k was p a r t i c u l a r l y s e v e r e along the w a t e r l i n e (in the c a s e of p a r t i a l i m m e r s i o n ) . T h e p i t t i n g c h a r a c t e r of the c o r r o s i o n of t h e s e m a t e r i a l s was a l s o o b s e r v e d in the s y n t h e t i c b r i n e and p a r t i e u l a x l y in the p u r i f i e d b r i n e of the S l a v y a n Salt P l a n t , having a pH of 8.8 and 11.4, r e s p e c t i v e l y . M a n y o b s e r v a t i o n s have s h o w n that the c o r r o s i o n r a t e as well as the c h a r a c t e r of c o r r o s i o n of a l u m i n u m and its alloys in s a t u r a t e d b r i n e s was m a i n l y affected b y the pH of the m e d i u m . B r i n e s having a p H n e a r to 7 s h o w e d the l o w e s t c o r r o s i o n effect on a l u m i n u m and its a l l o y s . All m a t e r i a l s t e s t e d in s a t u r a t e d b r i n e s showed a p p r o x i m a t e l y the s a m e c o r r o s i o n r e s i s t a n c e . In p u r i f i e d b r i n e f r o m the S l a v y a n Salt P l a n t only the AMg2 a l u m i n u m - - m a g n e s i u m a l l o y c a n b e r e g a r d e d as " r e s i s t a n t " (with c o m p l e t e i m m e r s i o n ) ; A 2 M a l u m i n u m and A D l m u s t b e r e g a r d e d as having a ' , r e g i i c e d r e s i s t a n c e " ( m a r k 6). H o w e v e r , a l r e a d y v i s u a l i n s p e c t i o n showed that the c o r r o s i o n of a l u m i n u m and its a l l o y s in p u r i f i e d and m o t h e r l i q u o r b r i n e s f r o m the S l a v y a n Salt P l a n t and i n the s y n t h e t i c s a t u r a t e d b r i n e had the s a m e l o c a l i z e d c h a r a c t e r , i . e . , the c h a r a c t e r of p i t t i n g c o r r o s i o n . F u r t h e r a c t i o n of c o r r o s i o n p r o c e s s e s c o n v e r t e d i n d i v i d u a l s p o t s into l a r g e open p i t s .
621
TABLE 4
The c o r r o s i o n p r o c e s s e s were significantly intensified by the stirring of the solutions. In all solutions with pH valves exceeding 8, an intense hydrogen evolution was observed on Alloy g/m2.h mm/yr specimens of aluminum and its alloys. In solutions of pH 7.07.6 (for example, in the brine f r o m Bol'shoi Kalkaman Lake) the A2M. . . . . . . . 0.013 0.042 c o r r o s i o n p r o c e s s e s were significantly retarded. In this case AD1. . . . . . . 0.006 0.021 hydrogen evolution was not observed. The surface of the speAM~ . . . . . . 0.012 0.042 cimens remained p r a c t i c a l l y unchanged after six months of t e s t ing. The c o r r o s i o n rate determined in these tests places A2M and its AD1 and AMg2 alloys into the group of " r e s i s t a n t " m a t e r i a l s , according to their c o r r o s i o n r e s i s t ance in lake brine. Corrosion rate
C o r r o s i o n tests on aluminum and i t s alloys in the conditions of the drying p r o c e s s for table salt in the Slavyan Plant was of g r e a t p r a c t i c a l interest. Specimens of A2M aluminum and of AD1 and AMg2 aluminum alloys were fixed in special f r a m e s which were then placed into the drying drum. C o r r o s i o n tests were c a r r i e d out on nine specimens (three specimens of each metal) in wet salt c r y s t a l s at 280 ~ for 2520 h. The water content of the salt varied, Results of the tests axe shown in Table 4. After 3 months of testing under plant cofiditions a visual inspection of the surface of the specimens showed no significant changes. The tests showed that A2M aluminum and AD1 and AMg2 p o s s e s s a v e r y high c o r r o s i o n r e s i s t a n c e against hot wet salt (mark 4). A2M aluminum and AD1 and AMg2 alloys can be placed by their c o r r o s i o n rate in hot salt into the third group of r e s i s t a n t m a t e r i a l s . They can be applied as constructional and lining m a t e r i a l s f o r drying equipment (drying drums, KS apparatus, etc.) in the p r o duction of table salt. The carbon and stainless steels used nowadays in the salt industry f o r lining of drying equipment (for example Kh18N10T) are subject to strong c o r r o s i o n attack (carbon steels are subject to strong total c o r r o s i o n , stainless steels to the formation of c o r r o s i o n c r a c k s [1]). LITERATURE
1. 2.
3. 4.
I~ N. Gladkii, C o r r o s i o n and Its Prevention in the Salt Industry [in Russian], TsINTIpishcheprom, Moscow (1966). The Metal W o r k e r ' s Handbook, Vol. 5, Book 1, V. S. Vladislav (editor) [in Russian], Mashgiz, Moscow (1960). G. L. Cherepakhova, A. V. Shreider, and L Ya, Klinov, Khim. i Neft. Mashinostr., No. 6 (1966). D. S. Nadezhdin, I. N. Gladkii, Yu. P. Chmyrev, and A. I. Naumenko, Trudy UkrNIIsol', No. 6, 14
(1962).
622
CITED
