CORROSION-EROSION

RESISTANCE

STEELS

ACID

IN SULFURIC

N. A. Adugina, M. and G. E. Lazarev

OF STAINLESS

MEDIA

M.

UDC 620.193:669. 15-194.661.256

Kristal',

In the N I I k h i m m a s h ( S c i e n t i f i c - R e s e a r c h Institute of Chemical Machinery) studies have been conducted on the c o r r o s i o n - e r o s i o n r e s i s t a n c e of the austenitic s t e e l s Kh18N10T, Kh17N13M3T, and 0Kh23N28M3D3T in sulfuric acid solution at concentrations up to 200 g / l i t e r at t e m p e r a t u r e s up to 100~ and in sulfuric acid contaIning oxidant additives, with quartz sand added as an a b r a s i v e , p a r t i c l e size 0.3-0.4 ram. The studies w e r e conducted in two units made f r o m 0Kh23N28M3D3T steel, with v a r i o u s intensities of m e c h a n i c a l action of the a g g r e s s i v e m e d i u m s t r e a m . In the f i r s t unit (UNI-3) ( s t r e a m velocity, 4.6 m / s e c ) the action of the working m e d i u m in a p p a r a t u s with a mixing device was imitated; in the second unit (UNI-1) ( s t r e a m velocity, 29 m / s e c ) , the intense action of the working m e d i u m which is o b s e r v e d in the operation of disk s p r a y d r i e r s , operating wheels of p u m p s , or other equipment was imitated.

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Fig. 1. Dependence of r a t e of c o r r o s i v e and c o r r o s i v e - e r o sive breakdown of austenitic s t e e l s , K, on sulfuric acid conc e n t r a t i o n , c. T e s t s conducted in a UNI-3 unit at 100~ 1) Kh18N10T steel; 2) 0Kh23N28M3D3T steel; - - - ) corrosive breakdown; ) c o r r o s i v e - e r o s i v e breakdown. Fig. 2. Dependence of s t a t i o n a r y potential, E (a) and of r a t e of c o r r o s i o n - e r o s i o n breakdown, IL (b) and of c o r r o s i v e breakdown (c) of austenitie s t e e l s on nitric acid concentration, c, in sulfuric acid (100 g / l i t e r ) . T e s t s conducted in a UNi-3 at 100~ 1) s t e e l Khl8N10T; 2) steel 0Kh23N28M3D3T. T r a n s l a t e d f r o m K h i m i c h e s k o e i Neftyanoe M a s h i n o s t r o e n i e , No. 6, pp. 17-18, June, 1972.

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.

527

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Fig. 3. Dependence of c o r r o s i o n - e r o s i o n breakdown ofaustenitic steels, &P, on concentration of sulfuric acid, c, at 80~ (a) and on t e m p e r a t u r e , t, when c = 100 g / l i t e r (b). Tests conducted in a UNI-1 unit: 1) KhlSN10T steel; 2) Kh17N13M3T steel; 3) 0Kh23N28M3D3T steel.

The UNI-3 unit is an apparatus with a mixing device, having a capacity of 3.0 l i t e r s (internal diam e t e r , 120 mm) ; the r a t e of rotation of the mixing device, which had a m i x e r of the turbine type, was 1470 rpm. During the test p e r i o d samples 20x 80 x 2.0 m m were f i r m l y fastened p a r a l l e l to the walls of the apparatus. The solid: liquid ratio was 2:3 (by weight). In the UNI-1 [1], the a g g r e s s i v e medium, which contained 0.5% by wt. of quartz sand, was fed by a s c r e w pump in two horizontal pipe sections on whose ends distributing nozzles were fastened. During rotation of the s c r e w pump and pipe sections, the s t r e a m of sludge was thrown against the samples under the action of centrifugal force, the samples being fastened at the level of the nozzles at an angle of 30 ~ to the direction of motion of the s t r e a m , which caused wear of the samples.

Results of the test conducted showed that the wear of all the steels investigated in water containing the abrasive was identical. In tests in the UNI-3 unit, depending on the intensity of mechanical action, it was 0.25 r a m / y e a r ; but in the UNI-1 unit it was about 105 r a m / y e a r (0.45 g/h). The c o r r o s i o n r e s i s t a n c e of the steels in the concentration range of sulfuric acid and the t e m p e r a t u r e range which were studied was diverse. The rate of c o r r o s i o n - e r o s i o n degradation of the steels depended on the extent to which they were alloyed. The experimental data show that in sulfuric acid solutions which contain solid p a r t i c l e s , Khl8N10T steel has the least r e s i s t a n c e to c o r r o s i o n - e r o s i o n action of the medium, and 0Kh23N28M3D3T steel has the most. However, the extent of the effect of the c o r r o s i o n f a c t o r in the rate of overall c o r r o s i o n - e r o s i o n degradation of the steels depends on the magnitude of mechanical action. At a r e l a t i v e l y weak mechanical action of the medium, when the r a t e s of c o r r o s i o n and e r o s i o n are c o m m e n s u r a t e , or when a predominance of r a t e of c o r r o s i v e degradation over e r o s i v e degradation is observed, the c o r r o s i o n f a c t o r has g r e a t importance. In this case, a small reduction of c o r r o s i o n r e s i s t a n c e of 0Kh23N28M3D3T, within the range of one point (from 0.4 to 0.55 r a m / y e a r ) in the ten-point scale of c o r r o s i o n r e s i s t a n c e (GOST 1319-68) upon changing the sulfuric acid concentration f r o m 30 to 200 g / l i t e r , which is p e r m i s s i b l e in the absence of erosion, r e d u c e s the r e s i s t a n c e of the steel against c o r r o s i o n - e r o s i o n wear two-fold (Fig. 1). The l a r g e r r i s e in rate of c o r r o s i o n - e r o s i o n degradation of 0Kh23N28M2D3T steel as c o m p a r e d with the increase in its c o r r o s i o n rate is probably caused by d e s t r u c t i o n of the p a s s i v e state of the steel surface, with its c o m plete activation, and p o s s i b l y also by a c e r t a i n facilitation of o c c u r r e n c e of e r o s i o n p r o c e s s e s due to weakening of the surface l a y e r s of the steel under the effect of the c o r r o s i o n factor. Such a phenomenon has a l r e a d y been observed in Khl8N10T steel. It should be noted that a d e c r e a s e in the c o r r o s i v e activity of the medium in the c o r r o s i o n - e r o s i o n action of a g g r e s s i v e media (especially under the operating conditions of mixing devices where the m e c h a n i ca] action is small, about 4.6 m / s e c ) r e d u c e s the wear of the steels. Thus, when nitric acid is added to a solution of sulfuric acid, steels Kh17N13M3T and 0Kh23N28M3D3T can be t r a n s f o r m e d f r o m the active state to the p a s s i v e one (Fig. 2). F o r example, in a sulfuric acid solution the s t a t i o n a r y potential of steels Kh17N13M3T and 0Kh23N28M3D3T abruptly shifts in the positive direction, f r o m - 0 . 2 3 to 0.17 V, r e s p e c tively. Thereupon the rate of c o r r o s i o n of Khl7N13M3T steel is reduced f r o m 1.2 to 0.025 r a m / y e a r ; and that of steel 0Kh23N28M3D3T, f r o m 0.4 to 0.01 r a m / y e a r ; correspondingly, the wear of these steels is r e d uced, respectively, f r o m 2.80 to 0.45 r a m / y e a r and f r o m 0.70 to 0.25 r a m / y e a r . The high c o r r o s i o n - e r o s i o n r e s i s t a n c e of the steels in solutions containing sulfuric acid and nitric acid is apparently caused by p r e s e r vation of a passive state of the metallic surface, in spite of its constant renewal at a rate of 0.25 m m / y e a r . The c o r r o s i o n f a c t o r also exerts a significant effect on the wear of steels during intensive mechanical action in sulfuric acid solutions (under conditions were the rate of e r o s i v e breakdown in water is s e v e r a l o r d e r s of magnitude g r e a t e r than the rate of corrosion). Thus, in sulfuric acid at a concentration of 100 g / l i t e r and a t e m p e r a t u r e of 80~ the c o r r o s i o n - e r o s i o n breakdown of Khl8N10T steel is 1.5 t i m e s as g r e a t as that of KhlTN13M3T steel, and 5 times as g r e a t as that of 0Kh23N28M3D3T steel (Fig. 3).

528

The c o r r o s i v e f a c t o r also e x e r t s a l a r g e effect on the i n c r e a s e in c o r r o s i v e - e r o s i o n breakdown of KhlSN10T and Kh17N13M3T steels when the concentration and t e m p e r a t u r e of the sulfuric acid are i n c r e a sed. Under s i m i l a r conditions, the c o r r o s i o n - e r o s i o n wear of 0Kh23N28M3D3T steel, which is stable in sulfuric acid of the concentrations studied (the c o r r o s i o n rate of this steel at 80~ does not exceed 0.20 m m / y e a r ) does not differ f r o m the wear in water in the absence of an a g g r e s s i v e medium. In this case the extent of c o r r o s i v e breakdown is slight and only mechanical breakdown is observed on the working s u r face of the metal, upon constant renewal of the working s u r f a c e at a r a t e which c o n s i d e r a b l y exceeds the rate of c o r r o s i o n . A c o r r o s i v e action of the medium is not displayed even upon breakdown of the p a s s i v e state of the metal. Thus, the studies p e r f o r m e d have shown that the c o r r o s i o n - e r o s i o n r e s i s t a n c e of stainless steels in surface acid media, r e g a r d l e s s of the intensity of mechanical action, is determined by the extent of alloying. To ensure a high c o r r o s i o n - e r o s i o n r e s i s t a n c e of metals, equal to t h e i r e r o s i o n r e s i s t a n c e in water, it is n e c e s s a r y to use m a t e r i a l s which have a high c o r r o s i o n r e s i s t a n c e . F o r example, for conditions under which tests of the steels mentioned were conducted, their c o r r o s i o n rate should not exceed 0.20 r a m / y e a r (with intensive mechanical action) or 0.01 m m / y e a r (under r e l a t i v e l y weak mechanical action). LITERATURE

CITED

!

1.

E.L.

Aronov et al., Zavodskaya Laboratoriya, 33, No. 6 (1967).

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