INFLUENCE OF CORROSIVE MEDIA ON CERAMIC INSULATORS E. Ya. Medvedovskii and F. Ya. Kharitonov

UDC 666.593.2:620.193.4

In developing ceramics intended for service in corrosive media it is essential to consider the design of the apparatus, the working conditions, the composition of the media and their properties (temperature, pressure, flow rate, etc.); the service conditions of the items being designed (mechanical and electrical loadings, thermal-shock resistance, the duration of the applied loads, and the action of the corrosive media, etc.); the processes for which the items were designed and the apparatus, their working parameters and the processes taking place during the reaction of the ceramic with the media; the composition and properties of the ceramic material used, the potential of the production technology, and also the compatability with other materials used in the equipment. Ceramic components contain various production and operational stress concentrators, and so the ceramics that operate in corrosive media need to provide for the stressed state and the influence of the media on mechanical breakdown. When the ceramic material reacts with the corrosive media in a dynamic schedule, when the media wash away the reaction materials at a certain velocity (it can be assumed that in this reaction the concentration of substances in the media hardly alters with time), the rate of corrosion will depend only on the concentration of substance in the design material. Then, in the simplest case, the rate of reaction can be represented by the ratio: ,v=K(A), where K is a constant for the reaction rate. Denoting by A 0 the initial concentrationof substance A (ceramic) and by X - the reduction in concentration of the original substance during time t, stoichiometrically equal to the concentration of the reaction product, we obtain a constant for the reaction rate in an integral form: l

K=t|n

A/(Ao--X)

Inserting in the previous equation the known value K = K0exp(-E/RT) we obtain In A/Ao--X)=Kotexp(--E/RT). Considering that the strength of any material depends on the dimensions of the largest defect, and diminishes with increase in the concentration of faults in the material, as a first approximation we can write In S = l n So--K2texp(--E/RT), (1) where S is the

instantaneous strength, So is the initial strength

In(S/So)=--K,InA/(Ao--X); K2 i s a c o n s t a n t c o v e r i n g t h e c o n s t a n t t h e d e f e c t K1 (K2 = KoK1).

K0 a n d a c o n s t a n t

characterizing

the dimension of

A n a l y s i s o f Eq. ( 1 ) shows t h a t i f t h e p a r a m e t e r s o f t h e medium a r e c o n s t a n t , then the strength of the material as a result of aging obeys an exponential law, and the durability is inversely proportional to the concentration of corrosive components in the medium. From the analysis of Eq. (i) in applying it to a ceramic design it is seen that the first member characterizes the relationship between the strength and the time. Since the strength of the ceramic during storage in normal air-humidity conditions, as a rule, does not alter, the sign of the first number in Eq. (i) is always positive. The second member in this equation characterizes the influence of the corrosion medium on the strength of the ceramic, which in most cases causes a marked reduction in strength. Thus, when the ceramic is used in such a medium its actual strength depends on these two factors. Elektrofarfor Production Association. 15-16, October, 1992. 470

Translated from Steklo i Keramika, No. 10, pp.

0361-7610/92/0910-0470512.50 9 1993 Plenum Publishing Corporation

To prolong the life of insulators and other items made from ceramics, or those lined with ceramics, it is usual to increase the thickness of the wall, i.e., to give a certain "tolerance" to the corrosion. However, in the case of corrosion in chemically active media (water vapors, active acid vapors, alkalis) when the corroding processes occur not only on the surface but also within the volume, an increase in the wall thickness may speed up such processes, as a result of which the durability of the design is not increased [i]. In evaluating the design element working in corrosive media, stresses are selected, bearing in mind a definite strength reserve [2]:

Gp ~cer/K., where o is the permitted stress, MPa; Oce r is the rupture strength of the ceramic, MPa; and K is th~ coefficient of strength reserve. The authors of [9] make recommendations on the choice of the reserve-strength values in relation to the type of medium (neutral, corrosive, explosive, flammable, and toxic) and working schedules for the equipment (temperature of working medium, excess pressure, vibration, contact with coarse abrasive particles). Depending on the conditions of service the value of the coefficient comes within the range 3-12. The selection of ceramic and the design of the insulator should be based on the fact that, as a result of the imperfect nature of the technology, the heterogeneous structure, the presence of thermal stresses, etc., the specific strength of the material is much lower than that determined on standard specimens. This is confirmed by a comparison of the strengths of standard specimens and those cut from articles (large insulators), and the Weibull modulus values for support-rod and transmission porcelain insulators, equal to 5-7. In evaluating transmission electric insulators (e.g., voltage 6-35 V class) the guaran ~ teed minimum destructive load, taking account of the properties of the material, is assumed to be 20-40% higher than the calculated minimum strength of the insulator, and the permitted working load in relation to the moisture content is selected as 2.5-3.5 times less than the minimum strength of the insulator. Thus, the ratio of the minimum guaranteed breakdown load to the maximum calculated working load is about 3-5 [3]. Bearing in mind the effect of the corrosive medium this ratio is still high. In making the strength calculations use is made of the working pressure, i.e., the total maximum excess pressure in the equipment during normal processing, and for a hydraulic pres-' sure of a column of liquid situated in the apparatus, if it exceeds 5% of the working pressure. When the apparatus is used without excess pressure, for the calculated value we use the hydrostatic pressure of a column of liquid but not less than 0.03 MPa. The pressure during hydraulic tests of ceramic equipment is taken to be 1.5 times higher than the working pressure in the apparatus [2]. In calculating the maximum permissible rate of corrosion, as applied to transmission (hollow cylindrical) insulators and the wall thickness, we should take account of the strength of the material, which changes with time because of the action of the corrosive medium (it acts on the internal surface of the insulator). It is assumed here that the corrosion of the ceramic is developed over the internal surface uniformly, and its rate is linear with time; a reduction in the wall thickness during corrosion is not accompanied by a deterioration in the mechanical properties of the undamaged part of the article, and in particular cracking does not occur. We also assume that during corrosion there is a reduction in the thickness of the insulator wall. For a transmission insulator with external D o and internal d o diameters 6 o = (D0-d0)/2) the rate of uniform corrosion will equal v ~ = (60--6)I T=

(wall thickness

(d--do)/2~

(2)

where 6 and d are respectively the thickness of the corroded wall and the internal diameter after time T; 6 = (D0-d)/2 or d = D0-26, T is the period of service of the article. For such insulators situated under the pressure of a corrosive medium P

~__KPd -

-

2~ K 9

where 6K is the stress due to the corrosion of the insulator (tensile stress).

471

Replacing d by the value D o = 26 and completing the simple conversion, to calculate the thickness of the insulator wall 6

"KPDo

=2(~+KP)

it is possible

(3)

.

Inserting the value 6 into Eq. (2) we obtain

KPDo /T; 2 (e,+KP)

%=(6o

(4)

KP Do ~='2 (6o-- v~T) --KP. The rate of corrosion of ceramic insulators can be determined by periodically checking the state of their internal surfaces during prolonged service or by making specimens as "predictors". The expression for the compressive stress of a transmission

insulator takes the form

Kmg KqHg 6cCmp= (a/4) (D~--d ~) : ~ (z/4)(b~'--d2) = K~gH,

(6)

where m is the mass of the insulator, g is the acceleration of free fall, q is the mass of an insulator i m high, H is the height of the insulator; and 7 is the apparent density of the material. Hence:

-

d=-~/D~--4KqHE/(~comp)

By s e l e c t i n g c e r t a i n v a l u e s l i m i t i n g v a l u e s o f c , 6, d, v K.

f o r a number o f p a r a m e t e r s

( 7) in (3)-(7)

we can c a l c u l a t e

the

U s i n g ( 5 ) and ( 6 ) i t i s p o s s i b l e t o d e t e r m i n e t h e f o r c e s a c t i n g on t h e t r a n s m i s s i o n i n s u l a t o r u n d e r t e n s i o n ( u n d e r t h e a c t i o n o f c o r r o s i o n ) F and c o m p r e s s i o n Fcomp F

~

KPd ~,,

KPd2~H

o~='-~--~a-=--.----.~--i

KPd2~H

= ~

Feomp ='TKyg H ( D~--d 2)

,

.

During the simultaneous action of compressive force and corrosion (for a large insulator height, and marked corrosion in a corrosive medium) the resultant force will equal E F = ~ m p ,

and t h e r e s u l t a n t

stress

will

be ....

o

In this case the value 6 is variable in relation to the selected height of the insulator h (at the upper point, when h = 0, the insulator will be acted upon only by the forces caused by corrosion; at the lower point when h = H, oK will be acting, and this is assumed to be a constant, and Ocomp equal to 7gH). Using this expression and putting it into Eq. (4) we can determine the permitted service life of the article. For a large insulator height and active corrosion (when the actions of the compressive force and the corrosion are considerable) the influence of the force is of a more complex nature. For a short service period, there is uniform corrosion of the insulator over the height, and subsequently its contribution has a compressive force; in the lower sections of the insulator corrosion will be more intense than in the upper. The results can be used for developing and investigating the service of ceramic transmission insulators designed for service in electric-gas, high-voltage equipment, chemical 472

plant and other ceramic articles of similar design, and which work in contact with corrosive media. LITERATURE CITED lm

2. 3.

P. P. Budnikov and F. Ya. Kharitonov, Ceramic Materials for Corrosive Media [in Russian], Stroiizdat, Moscow (1971). V. F. Babich and K. P. Belous, Chemical Equipment from Ceramics [in Russian], Mashinostroenie, Moscow (1987). N. S. Kostyukov, N. V. Minakov, V. A. Knyazev, et al., Ceramic Insulators [in Russian], Energoatomizdat (1984).

ACOUSTIC-EMISSION INVESTIGATIONS OF STRUCTURAL CHANGES IN TUNGSTEN-CARBIDE METAL-CERAMICS DURING THERMAL AND RADIATION TREATMENT AND HYDROGENATION UDC 666.762:534.2:538..97

V. L. Ul'yanov, I. P.- Chernov, A. A. Botaki, and B. V. Chakhlov

Metal-ceramic alloys based on tungsten monocarbide (WC metal-ceramics) are promising design materials that can be used in various areas of technology. Previously we investigated the elastic properties of these materials at low temperatures (100-295 K) using acoustic methods [i]. The study of the mechanical properties of acoustic methods is connected with the need to to the structural changes in these materials. sonic elastic waves in a solid body, under the serve processes of multiplication and movement the waves (internal friction [2]), and plastic

ceramics under various external effects using study the attenuation of elastic waves due In particular, during the spread of ultraaction of the acoustic wave forces, we obof dislocations, leading to attenuationof deformation of the body.

The aim of the present work was to make an experimental investigation of the structural changes and the microplastic deformation (in the region of stress that is less than the yield point) in WC metal-ceramics during thermal (i00-ii00 K) radiation (gamma-quanta upto absorption doses of
8o~

( 2hd~2s2._):_~ R(U/Uo--1)fKmB ;

6.m~--6KmK .,;

se~UR co= ~ K l o

R '

~o

where h is the distance between the electrodes arranged on the lateral faces of the piezoquartz transformer with an attenuation decrement 6k; dz2, $22 and fK are its piezoelectric modulus, the constant of elastic pliability, and the resonance frequency, determined on the frequency meter; mB, m K and m0 are the masses of the vibrator, piezoquartz, and specimen; Tomsk Polytechnical University. October, 1992.

Translated from Steklo i Keramika, No. i0, pp. 17-19,

0361-7610/92/0910-0473512.50 9 1993 Plenum Publishing Corporation

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