THE

RESISTANCE

TO THE

ACTION

OF ACID-RESISTANT OF ALTERNATING

]~. l~. M a z o , L. L . K. U s h a k o v a ,

COATINGS

MEDIA

G. K h o d s k i i , a n d L . A. K u r a k e v i c h

UDC 620.193.423:666.29

The published data on the r e s i s t a n c e of enamel coatings to alternating media is limited and deals mainly with the chemical r e s i s t a n c e of a coating specially developed for a p a r t i c u l a r purpose [1, 2] or the effect of alternating media on some c h e m i c a l l y r e s i s t a n t experimental coatings developed in the Institute of General and Organic C h e m i s t r y of the B e l o r u s s i a n SSR [3]. The purpose of the p r e s e n t p a p e r was to make a c o m p a r a t i v e study of s e v e r a l industrial and e x p e r i mental a c i d - r e s i s t a n t coatings in o r d e r to find their dependence on the effects of alternate acid and alkali solutions (20.24% HC1 and 2N NaOH). Industrial a c i d - r e s i s t a n t coatings (LK-1, A-32 [4]); the experimental coatings (Nos. 1 and 2) developed in our Institute; and coating A developed in the S c i e n t i f i c - R e s e a r c h I n s t i tute of Enamelling and Chemical Machinery, were used in these studies. The s p e c i m e n s were plates m e a s u r i n g 114 • 114 era, p r i m e d on both sides and coated with the e x p e r i m e n t a l enamel. The methods of determining the chemical r e s i s t a n c e of the coating have p r e v i o u s l y been d e s c r i b e d in [5]. The a g g r e s s i v e r e a g e n t s were changed s u c c e s s i v e l y after 3 h. After each 3-h cycle the specimens w e r e washed, dried, and weighed, and then subjected to the action of the next f r e s h l y p r e p a r e d solution. The total length of time for the e x p e r i m e n t s was 36 h (12 cycles). The experiments were c a r r i e d out at a t e m p e r a t u r e of 96~ and the amount of r e a g e n t was 10 m l / c m 2 of s u r f a c e . After the t r e a t m e n t the weight l o s s e s were d e t e r m i n e d and the e x t r a c t analyzed for SiOz, CaO, TiO2, and ZrO2, using the n o r m a l methods. The amount of oxide t r a n s f e r r e d into the solution was calculated in m o l e / c m 2 of t r e a t e d s u r f a c e and was denoted by nSiO 2, nCaO, etc., as was done in the p a p e r published by Dubrovo and Shmidt [5]. F i g u r e 1 shows a graphical r e p r e s e n t a t i o n of the kinetic c u r v e s of the t r a n s f e r of silica f r o m the coating to the solution during t r e a t m e n t by alternating media. They a r e linear. It is c l e a r f r o m Fig. 1 that during the t r e a t m e n t a slow hag-down p r o c e s s o c c u r s which, however, does not change the linear form of the c u r v e s . The significant slowing down can, a c c o r d i n g to Grebenshchikov's t h e o r y , be explained by the formation of a p r o t e c t i v e silica film. As a r e s u l t of the alkali t r e a t m e n t the protective film is dissolved and c o r r o s i o n is a c c e l e r a t e d . This is e x p r e s s e d on the graph by the fact that the " a c i d - a l k a l i ~ points a r e further a p a r t than the " a l k a l i - a c i d " points. The r e s i s t a n c e of the coating to the action of the alternating media i n c r e a s e s both with an increase in the density and improved p r o p e r t i e s of the film and with an i n c r e a s e in the r e s i s t a n c e of the internal s t r u c t u r e of the coating to the effect of the reagents. This is illustrated in Fig. 1 by the s m a l l e r slope of the curve and the shortening of the distances between the experimental points. The A-32 coating is c h a r a c t e r i z e d by the g r e a t e s t distance between the " a c i d - a l k a l i " points and the No. 2 coating by the s h o r t e s t distance. The experimental data obtained for the kinetics of the weight l o s s e s and the t r a n s f e r of the c o m p o nents into the solution w e r e analyzed using the following e m p i r i c a l equation: x = kt,

(I) (2)

x--A=kt,

where x is the weight l o s s , g / c m 2 or the amount of the eomponents which a r e t r a n s f e r r e d into the s o l u tion, m o l e / c m 2 ; k is the rate constant; t is the duration of the t r e a t m e n t ; and A is a constant indicating Institute of General and Inorganic C h e m i s t r y , Aeademy of Sciences of the B e l o r u s s i a n SSR. T r a n s lated from Steklo i K e r a m i k a , No. 12, pp. 13-14, D e c e m b e r , 1974. 9 1975 Plenum Publishing Corporation, 227 West 17th Street, New York, iV. Y. 10011. No part QFthis publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilmb~g, recording or otherwise, without written permission of the publisher. A copy of this article is available from the publisher for $15.00.

857

TABLE

TABLE 2

i

~- ~

H ~ [m~

k, mole I

o ~

%

k.lo',

Enamel

/cm t' h

~b~..~ a SiO= n CaO 3 6 9 12 15 18 21 24 27 30 33 36 Average

2,67 12,1 18,9 29,3 36,2 47,0 53,8 65,1 72,0 84,4 91,2 04,6

3,22.10 - 6 3,18.10 - 6 2,87.10 - 6 2,77.10 - 6 2,88.10 - 6 3,00.I0 - 6 2,89.10 - 6 3,00.10 .-6 2,92.10 - 6 3,04.10 - 6 2,97.10 - 6 3,10.I0 - 6 2,986.10 - 6

9*Experimental data. Calculated data . . . . . .

1,95~ 27,2. 10,91 9,9 19,87 5;1 28,8~ 1,6 37,74 4,3 46,74 0,55 55,7 3,5 64,6~ 0,7 73,63 2,3 83,5~ 0,97 92,52 1,4 00,49 3,9

I

LK-I A-32

199,1 2,706[ 0,257 81,9 4,08 0,563 A I 48 0 1,43010,314 ExperimentaiNo. 1 98,0 2,986 0,379 Ex~erimentalNo. 2 29,1 0,770 0,480

that, during the initial period of the effect, the p r o c e s s is o c c u r r i n g in a c c o r d a n c e with another law. The value of A is d e t e r m i n e d by a graphical extrapolation of the s t r a i g h t line to i n t e r s e c t the ordinate. It is well known from [7] that these equations can also d e s c r i b e the alkali effect of the p r o cess.

An example of the calculation of the rate constant ~2) d e s c r i b i n g the t r a n s f e r of the silica f r o m the e x p e r i m e n t a l coating into the solution ( x - A = kt; A = - 7 ) is given in Table 1. It is c l e a r f r o m this table that the calculated and experimental data are in s a t i s f a c t o r y a g r e e m e n t . F r o m the second cycle of t r e a t m e n t (6 h) onwards the deviations do not exce0d 10%. Sometimes (in the case of other coatings) this figure can be as high as 15%. Slightly g r e a t e r deviations a r e noted for the f i r s t cycle. The applicability of this equation f o r d e s c r i b i n g the c u r v e s for the effect of the a l t e r n a t i n g media can be used as an indication that the limiting stage in this case is the kinetic p r o c e s s in spite of the slowing down which is observed.

The calculated rate constants were used to evaluate the chemical r e s i s t a n c e of the coatings. The use of the constants is most convenient since it makes it possible to reduce the amount of experimental w o r k and to l i m i t it to 12 c y c l e s , taking into account the s m a l l changes in the values after the first cycles. The constants c h a r a c t e r i z e the slope of the c u r v e s thus making it possible to give a more c o r r e c t evaluation of the c h e m i c a l r e s i s t a n c e in those c a s e s where intersection of the c u r v e s is o b s e r v e d in subsequent c y c l e s or a f t e r extrapolation with r e s p e c t to time beyond the limits of the experiment. The rate constants d e s c r i b i n g the t r a n s f e r of S iO 2 into the solution were used as the maLn indicators of the c h e m i c a l r e s i s t a n c e . The p r o c e s s can be evaluated more objectively f r o m these constants than from the weight-change constants. It is well known that the weight l o s s e s depend on s u r f a c e phenomena (e.g., adsorption) which m a y lower their values and affect the value of the constants, The p r e s e n c e in the' solution of s i l i c a whose amount is d e t e r m i n e d a n a l y t i c a l l y can be used as a c l e a r sign of the destruction of the coating. The constants of Eqs. (1) and (2) d e s c r i b i n g the change in the weight l o s s e s and the t r a n s f e r of the components into the solution a r e shown in Table 2. It follows f r o m Table 2 that the biggest constant for the t r a n s f e r of S i O 2 into the solution belongs to the A-32 coating and the s m a l l e s t c h a r a c t e r i z e s the No. 2 e x p e r i m e n t a l coating, which is in good a g r e e m e n t with the data illustrated in Fig. 1. The rate constants d e s c r i b i n g the p r o c e s s of S i O 2 extraction f r o m the r e m a i n i n g coatings are also found to be in good a g r e e ment with Fig. l . Thus, in the group of industrial coatings used in the study the A coating has the highest r e s i s t a n c e to the action of the alternating media but it is not as good as the experimental coating No. 2. It should be pointed out that f r o m the rate constants d e s c r i b i n g the change in weight loss the A-32 coating can be c o n s i d e r e d as more r e s i s t a n t than the LK-I. coating. This, however, is contradicted by the data f r o m a visual examination and m i c r o s c o p i c study of the s p e c i m e n after t r e a t m e n t in a g g r e s s i v e media. Marked c o r r o s i o n of the s u r f a c e after three t r e a t m e n t cycles was observed visually for the A-32 s p e c i m e n s . After four cycles the c o r r o s i o n (roughness) extended over the whole specimen. For the L K - I coating even a f t e r six cycles only t r a c e s of c o r r o s i o n in the f o r m of a slight roughness are observed. A m i c r o s c o p i c study of s i l v e r e d s p e c i m e n s of the coatings a f t e r t r e a t m e n t was made using an MIM-7 m e t a l l o g r a p h i c m i c r o s c o p e with a magnification x 350. It was established that the A-32 coating c o r r o d e s s ignificantly m o r e than the L K - 1 . The m a r k e d c o r r o s i o n begins even in the f i r s t cycle of t r e a t m e n t and is

858

/i o

/

r~

/

lO0 /

/

x"

/

i 5.

i

I ,/ 1

o

o

E < J

Fig. 1. The kinetic c u r v e s for the t r a n s f e r of silica f r o m a coating into solution during the action of a l t e r nating media: 1) LK-1 coating; 2) A32; 3) A; 4) e x p e r i m e n t a l coating No. 2; 5) e x p e r i m e n t a l coating No. 1 (unshaded s y m b o l s denote t r e a t m e n t with 20.24% HC1; shaded s y m b o l s , t r e a t m e n t with a 2N solution of NaOH).

6 9 12 15 M Z# 24 2750JJ36

Duration of treatment y, h characterized by the appearance of a large number of dark patches on the site of a bubble. In the LK-I coatings, traces of corrosion are marked after the sixth cycle and appear in the change in shape of some bubbles. This study of the surface of specimens confirms the correctness of the evaluation of the chemical resistaneeofcoatings from the rate constants describing the transfer of SiO2 into the solution.

LITERATURE io

2. 3. 4.

5. 6.

7.

V. V. I~. No. V. I~.

]~. Mishel'andN. ]~. Mishel'and N. 1~. Mazo, L. G. 2 (1972). V. Vargin and L. 1~. Mazo, L. G.

CITED

P. Romina, Zh. PriM. Khim., 39, No. 4 (1966). P. Romina, Zh. PriM. Khim., 4-~, No. 5 (1968). Khodskii, L. K. Ushakova, et al.-~-Izv. Akad. Nauk Z. Zasukhina, Steklo i Keramika, No. 9 (1969). Khodskii, L. K. Ushakova, et al., Izv. Al~ad. BSSR,

BSSR,

Ser. Khim.

Ser. Khim.

Nauk,

Nauk,

No. 5

(1971). S. K. Dubrovoand Yu. A. Shmidt, Izv. Akad. Nauk SSSR, Otd. Khim. Nauk, No. 3 (1955). l~. I~. Mazo, L. G. Khodskii, L. K. Ushakova, and F. T. Zhos, Izv. Akad. Nauk BSSR, Ser. Khim. Nauk, No. 1 (1972).

859