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Cathodic protection by means of dispersed anodes
Corrosion Science, 1962, Vol. 2 pp. 275-280. Pergamon Press Ltd. Printed in Great Britain.
CATHODIC PROTECTION BY MEANS OF DISPERSED ANODES* T. K. R o s s Department of Chemical Engineering, Manchester College of Science and Technology, Manchester, England Abstract--Cathodic protection has limited application in chemical plant because of the difficulty of locating anodes at suitable positions. The problem may be overcome by dispersing sacrificial anodes on the cathodic portions of the surface, and the paper describes two applications of this principle. Preliminary results are presented in which the action of anodes in fixed and flowing systems are examined. The effectiveness of sacrificial anode action is discussed, and some measurements reported. R6sum~-L'application de la protection cathodique est limit6e dans l'industrie chimique gt cause de la difficult6 de placer les anodes ~t des endroits convenables. Le probl+me peut 6tre r6solu en dispersant les anodes sacrificielles sur les parties cathodiques de la surface et la communication d6crit deux applications de ce principe. L'auteur pr6sente des r6sultats pr61iminaires darts lesquels est examinee raction d'anodes dans des syst6mes fixes et des syst~mes fluides. L'efficacit6 de Faction de l'anode sacrificielle est discut6e et quelques mesures sont indiqu6es. Zusammenfassung--Die Anwendung von kathodischem Schutz in der chemischen Industrie ist durch die Schwierigkeit begrenzt, Anoden an geeigneter Stelle der Anlagen anzubringen. Diese Schwierigkeit kann fiberwunden werden, indem Opferanoden auf den kathodischen Bereichen der Oberfl/ichen der Anlagen verteilt angebracht werden. Die Arbeit besch.reibt zwei Anwendungsbeispiele dieses Prinzips. Vorl~ufige Ergebnisse der Wirkung der Anoden in rubenden und fliessenden Systemen werden mitgeteilt. Der Wirkungsgrad der Opferanoden wird erOrtert, und es wird fiber einige Messungen berichtet. CATHODIC p r o t e c t i o n p r o v i d e s a m e t h o d o f controlling c o r r o s i o n b y intervention in the electrochemical processes concerned and, in principle, consists o f p o l a r i z i n g the c o r r o d i n g surface b y means o f an external e.m.f., which m a k e s its p o t e n t i a l sufficiently m o r e negative to its s u r r o u n d i n g s so t h a t the a n o d e s on the surface cease to give a n y current. H o w e v e r , most o f the established a p p l i c a t i o n s o f this principle have been w o r k e d o u t for cases where the c a t h o d e has a regular a n d p r e d i c t a b l e g e o m e t r y . I n chemical engineering m a n y instances are e n c o u n t e r e d where the c a t h o d e s are n o t o n l y c o m p l e x a n d variable in form, b u t also where their activity is m u c h increased b y m o v e m e n t o f the corrosive e n v i r o n m e n t . One has only to consider the variety o f a p p a r a t u s in which the exchange o f energy a n d m a t t e r between solid a n d fluid phases is effected in chemical p l a n t s to a p p r e c i a t e the o r d e r o f the p r o b l e m . A n example o f this n a t u r e frequently e n c o u n t e r e d in chemical p l a n t p r o t e c t i o n involves the effect o f an active c a t h o d e , such as arises when c a r b o n o r g r a p h i t e equipm e n t is e m p l o y e d . The presence o f g r a p h i t e heat exchangers o r c a r b o n t o w e r fillings m a y i m p o s e severe c o r r o s i o n u p o n connected or c o n t a i n i n g m e t a l c o m p o n e n t s , a n d the i m p o r t a n c e o f this in a p a r t i c u l a r a p p l i c a t i o n has been d e m o n s t r a t e d . 1 I n this instance the c a t h o d i c r e a c t i o n was clearly c o n t r o l l e d b y the rate at which o x y g e n *Manuscript received 31 May 1962. The substance of this paper was communicated to the Corrosion S:ience Society on 17 Apr.il 1962. 275
276
T.K. Ross
dissolved in the solution on the carbon surface, which in turn was dependent upon the gas stream velocity. It is interesting to note that chemical engineering design seeks to improve the dissolution rate of selected components of the system, but the corrosive consequences are not always appreciated. The dissolution rate \ ~ J of oxygen in a film of liquid distributed over a carbon surface of area (A) conforms to an equation such as dw
- - K , . A . ( C ~ - - Ci)
dt
where K~ is the transfer coefficient, and Cg and Ci are the bulk and carbon/liquid interface oxygen concentrations respectively. If the oxygen is consumed in a cathodic reaction under diffusion control, Ci approaches zero, and as the rate of the cathodic reaction is related to d,' by Faraday's Law it is possible to calculate values of the dt
transfer coefficient from experimental results. Comparison of such results with the known value of Kg for oxygen absorption confirms that this particular cathodic reaction is diffusionally controlled. It is quite possible for granulated or preformed carbon shapes to have a surface area of 100-500 ft2/ft 3, so that a cathodic area of 200-1000 ft2/ft3 of supporting metal surface is possible, with resulting heavy corrosion of the latter. In this instance attempts to reduce corrosion of associated metals by substitution or surface coating were made, but none was entirely satisfactory. Cathodic protection was of course considered, but the practical difficulty of installing the anode was considerable and its effect upon flow patterns within the bed would have been serious. If portions of a suitably anodic metal are prepared of the same external shape and size as the carbon particles, they may be mixed among the cathodic bed without interfeting with flow patterns or necessitating any special electrical arrangements. Each dispersed anode may be considered to be surrounded by a cathodic surface to which it must be capable of supplying sufficient current to satisfy its diffusional processes without drainage from the protected surfaces. Thus, if N anodes are equally distributed in a cylindrical packed bed of height H and diameter D, the solid shapes in the bed having a surface/volume factor (a), the surface of cathode attributed to each anode 7r D 2 H a
approximates to - - - when wall effects are neglected. Also, if the anodes are of 4N mean effective radius r, their individual surface area is 4rcr ~. Now the total current (I) required on a total cathode area (A) may be calculated from the transfer coefficient, for I =
dw F _ GAGr d--i x J d
_ G,w
H aCF 4J
(1)
where J is the electrochemical equivalent of the diffusing substance and F is Faraday's Number. The current density on the individual anodes (ia) if they satisfy the needs of their particular cathodic zones becomes
Cathodic protection by means of dispersed anodes
27"~
l-O0
n--..
0.75
o"
n-° .9
+ko
0.50
~
0.25
o (D
0
0'25
0!50
0.~5
,00
Dispersed onode rotio, NyN
FiG. 1. Comparison of corrosion rate (R') obtained in presence of N' anodes, with the unprotected corrosion rate (R) when the calculated minimum number of anodes is N. + 10 mesh activated charcoal, and 10 mesh granulated zinc. [] ¼in. carbon Rashig rings and ¼in. × ~ in. magnesium cylinders. c, ~ in. × & in. carbon discs, and same size aluminium discs. A 10 mesh activated charcoal, and ~ in / { in. magnesium cylinders. ia -- ~ D 2 H a -
-
16rcr~N
KgCgF X
-
-
J
and, if there is a practical limit to (ia) at which passivation occurs, say (ij ) , the minimum value of N may be calculated from U -- D2H aKgCgF 16i,,'r2J
(2)
The numerator of this expression contains all the terms associated with the chemical engineering design, and the denominator comprises those factors connected with the choice of anode, although of course the effective radius (r) (reflecting the anode shape) may be set by hydrodynamic considerations. Fig. 1 illustrates some results obtained with variously shaped anodes of zinc, magnesium and aluminium dispersed in four kinds of carbon bed. The reduction of corrosion is plotted against the ratio of anode numbers used to that predicted from the above equation. The technique previously reported 1 was used to determine the practical value of iJ for each anode material when embedded in a carbon bed. This is necessary, because the dispersed anodes are themselves subject to the same mass transfer processes as the surrounding cathodes. Thus when an anode has a tendency to become passive under oxidizing conditions it is more inclined to do so when exposed to a dynamic system, and the attained value of (i~') is also a function of velocity. The minimum value of N was then calculated from (2) and chosen proportions of this number of anodes (N') admixed with carbon shapes. The resulting corrosion (R') of the metal surfaces was expressed as a fraction of the unprotected rate (R) and plotted against the fraction of calculated anodes used (N'/N).
T. K. R o ~ >
J
o.
-0" 5 0
-0"60
o o
-0"70
c
-°'8°o
~'5
5to
;J5
,oc
Distance along tube diameters FIG. 2. Potential of a 1 in. bore mild steel pipe at various distances downstream from the point of addition of dispersed magnesium anodes. Solution of 0'001 N. NaC1 flowing at 2 ft/s., with 0-12 >: 0-05 in. magnesium cylinders added at the rate of 200/1.
The absolute current requirements of a cathode upon which oxygen absorption is taking place may of course be predicted from the rate of that absorption, and hence the total cathodic current can be calculated from mass transfer data. Thus the theoretical protective current required in a particular condition can be predicted and compared to that actually used as calculated from the loss in weight of the anodes. The dispersed sacrificial anode may also be extended to the field of corrosion control in fluid systems, as, for instance, in applying cathodic protection to the inside of a long straight pipe carrying a corrosive fluid. This problem becomes one of locating anodes at sufficiently frequent intervals, for a single sacrificial anode located at one end will only protect a zone some 3-4 tube diameters in length. But, if small sacrificial anodes are introduced in such a way as to be suspended in and transported by the liquid they may give adequate protection over any length (Fig. 2). It is interesting to note that the only portion of tube not adequately protected under these conditions was the first 3 or 4 dia where the glass nozzle used to introduce anodes was sited; the use of a nozzle of protective metal overcame this. This particular result was obtained with a fixed loading of anodes, and the degree of protection or potential shift is naturally related to the number of anodes in a given volume of liquid. Thus the loading required to obtain any desired protected potential can be selected, and many investigations of this type have been carried out for different anode materials. What can be done in one tube can also be done in several parallel tubes, so that tube bundles can also be protected. An integral requirement of this form of dispersed anode protection is that the anodes are freely suspended in and transported by the environment and so their form and shape is important. As a general principle one requires an anode size and shape such that the settling forces exerted upon it are at least balanced by drag forces exerted by the corroding fluid. Under these conditions free motion of the solid particles
Cathodic protection by means of dispersed anodes
279
results, and their repeated contacts with the vessel wall develop the necessary protective current. Also the size of an anode dictates its total life in a circulating system. There are clearly two factors of importance in the interaction between the dispersed anode and the protected surface, namely the frequency of such contacts and their effectiveness. I f the frequency of contacts for a single particle isf/sec., the period of duration of such a contact 0 sec., and the contact resistance Re, then the current transfered per anode (I) is given by
I -- (Ep - - E,,)fO Rc when Ep and E, are the mean potentials of the protected surface and the anodic particle. Also this current transfer is related to the rate of anodic dissolution by the expression dW
F
z=-Z- x ~
dW. where - - is the rate of weight loss of N particles of electrochemical equivalent J, dt
and F is Faraday's Number. dW
F
Thus, d---t- × fO SO,
"
8~
(Ep -- E,)fO
_
NJ
Rc
dW
F
X
--
at
NJ(E,, -- Eo)
350
~o"
300
.
c E 25O
:u O~
l 2OO
~50
IO0
5O laJ
/
.! w
l l"
°o &
o!s IOglo Reynolds
~o number,
!~
z'-o z'-5
3o
dpV/@
Fro. 3. The calculated value of the effectivenessfactor obtained with various magnesium anodes dispersed in 0.001 N NaCI flowing in a 2 in. bore mild steel pipe. dp = mean anode diameter V = flow velocity "t = kinematic viscosity of solution.
280
T.K.
Ross
dW
Thus from data o n ~ f for (N) particles, and observations of (Ep) and (Ea), the "effectiveness factor" ( ; 0 ) of the anodes may be calculated. It seems to be impossible at present to separate the terms f and 0 except by substituting inert particles of the same dispersive properties as anodes in a constant number (N). From anode weight loss measurements then, the effectiveness factor between anode and wall may be deduced. Fig. 3 indicates how this factor, which directly controls the effectiveness of dispersed anodes, varies with dynamic conditions. This curve is very similar in shape to that relating the fluidization characteristics of the anodes to the flow rate, so that in general it may be said that the anodes are at their most effective when properly fluidized by the environment. At present the behaviour of dispersed protection is being pursued along various lines. The effect of various anode-metal systems are being explored and also the particulate nature of the anode contact is being examined. By substituting part of the bed with inert glass particles of the same fluidization properties, the effect of electrical substitution without the loss of dynamic properties may be studied. This line of work suggests the fact that most ceramic shapes could be covered with the appropriate amount of anodic material, e.g. by coating them or by compressing mixtures of metallic and inert powders, with the result that the anodic behaviour could be controlled to the desired level. REFERENCE 1. T. K. Ross and A. H. MORSHEDIAU,Trans. Inst. Chert1. Engrs. 38, 43 (1960).
CATHODIC PROTECTION BY MEANS OF DISPERSED ANODES* T. K. R o s s Department of Chemical Engineering, Manchester College of Science and Technology, Manchester, England Abstract--Cathodic protection has limited application in chemical plant because of the difficulty of locating anodes at suitable positions. The problem may be overcome by dispersing sacrificial anodes on the cathodic portions of the surface, and the paper describes two applications of this principle. Preliminary results are presented in which the action of anodes in fixed and flowing systems are examined. The effectiveness of sacrificial anode action is discussed, and some measurements reported. R6sum~-L'application de la protection cathodique est limit6e dans l'industrie chimique gt cause de la difficult6 de placer les anodes ~t des endroits convenables. Le probl+me peut 6tre r6solu en dispersant les anodes sacrificielles sur les parties cathodiques de la surface et la communication d6crit deux applications de ce principe. L'auteur pr6sente des r6sultats pr61iminaires darts lesquels est examinee raction d'anodes dans des syst6mes fixes et des syst~mes fluides. L'efficacit6 de Faction de l'anode sacrificielle est discut6e et quelques mesures sont indiqu6es. Zusammenfassung--Die Anwendung von kathodischem Schutz in der chemischen Industrie ist durch die Schwierigkeit begrenzt, Anoden an geeigneter Stelle der Anlagen anzubringen. Diese Schwierigkeit kann fiberwunden werden, indem Opferanoden auf den kathodischen Bereichen der Oberfl/ichen der Anlagen verteilt angebracht werden. Die Arbeit besch.reibt zwei Anwendungsbeispiele dieses Prinzips. Vorl~ufige Ergebnisse der Wirkung der Anoden in rubenden und fliessenden Systemen werden mitgeteilt. Der Wirkungsgrad der Opferanoden wird erOrtert, und es wird fiber einige Messungen berichtet. CATHODIC p r o t e c t i o n p r o v i d e s a m e t h o d o f controlling c o r r o s i o n b y intervention in the electrochemical processes concerned and, in principle, consists o f p o l a r i z i n g the c o r r o d i n g surface b y means o f an external e.m.f., which m a k e s its p o t e n t i a l sufficiently m o r e negative to its s u r r o u n d i n g s so t h a t the a n o d e s on the surface cease to give a n y current. H o w e v e r , most o f the established a p p l i c a t i o n s o f this principle have been w o r k e d o u t for cases where the c a t h o d e has a regular a n d p r e d i c t a b l e g e o m e t r y . I n chemical engineering m a n y instances are e n c o u n t e r e d where the c a t h o d e s are n o t o n l y c o m p l e x a n d variable in form, b u t also where their activity is m u c h increased b y m o v e m e n t o f the corrosive e n v i r o n m e n t . One has only to consider the variety o f a p p a r a t u s in which the exchange o f energy a n d m a t t e r between solid a n d fluid phases is effected in chemical p l a n t s to a p p r e c i a t e the o r d e r o f the p r o b l e m . A n example o f this n a t u r e frequently e n c o u n t e r e d in chemical p l a n t p r o t e c t i o n involves the effect o f an active c a t h o d e , such as arises when c a r b o n o r g r a p h i t e equipm e n t is e m p l o y e d . The presence o f g r a p h i t e heat exchangers o r c a r b o n t o w e r fillings m a y i m p o s e severe c o r r o s i o n u p o n connected or c o n t a i n i n g m e t a l c o m p o n e n t s , a n d the i m p o r t a n c e o f this in a p a r t i c u l a r a p p l i c a t i o n has been d e m o n s t r a t e d . 1 I n this instance the c a t h o d i c r e a c t i o n was clearly c o n t r o l l e d b y the rate at which o x y g e n *Manuscript received 31 May 1962. The substance of this paper was communicated to the Corrosion S:ience Society on 17 Apr.il 1962. 275
276
T.K. Ross
dissolved in the solution on the carbon surface, which in turn was dependent upon the gas stream velocity. It is interesting to note that chemical engineering design seeks to improve the dissolution rate of selected components of the system, but the corrosive consequences are not always appreciated. The dissolution rate \ ~ J of oxygen in a film of liquid distributed over a carbon surface of area (A) conforms to an equation such as dw
- - K , . A . ( C ~ - - Ci)
dt
where K~ is the transfer coefficient, and Cg and Ci are the bulk and carbon/liquid interface oxygen concentrations respectively. If the oxygen is consumed in a cathodic reaction under diffusion control, Ci approaches zero, and as the rate of the cathodic reaction is related to d,' by Faraday's Law it is possible to calculate values of the dt
transfer coefficient from experimental results. Comparison of such results with the known value of Kg for oxygen absorption confirms that this particular cathodic reaction is diffusionally controlled. It is quite possible for granulated or preformed carbon shapes to have a surface area of 100-500 ft2/ft 3, so that a cathodic area of 200-1000 ft2/ft3 of supporting metal surface is possible, with resulting heavy corrosion of the latter. In this instance attempts to reduce corrosion of associated metals by substitution or surface coating were made, but none was entirely satisfactory. Cathodic protection was of course considered, but the practical difficulty of installing the anode was considerable and its effect upon flow patterns within the bed would have been serious. If portions of a suitably anodic metal are prepared of the same external shape and size as the carbon particles, they may be mixed among the cathodic bed without interfeting with flow patterns or necessitating any special electrical arrangements. Each dispersed anode may be considered to be surrounded by a cathodic surface to which it must be capable of supplying sufficient current to satisfy its diffusional processes without drainage from the protected surfaces. Thus, if N anodes are equally distributed in a cylindrical packed bed of height H and diameter D, the solid shapes in the bed having a surface/volume factor (a), the surface of cathode attributed to each anode 7r D 2 H a
approximates to - - - when wall effects are neglected. Also, if the anodes are of 4N mean effective radius r, their individual surface area is 4rcr ~. Now the total current (I) required on a total cathode area (A) may be calculated from the transfer coefficient, for I =
dw F _ GAGr d--i x J d
_ G,w
H aCF 4J
(1)
where J is the electrochemical equivalent of the diffusing substance and F is Faraday's Number. The current density on the individual anodes (ia) if they satisfy the needs of their particular cathodic zones becomes
Cathodic protection by means of dispersed anodes
27"~
l-O0
n--..
0.75
o"
n-° .9
+ko
0.50
~
0.25
o (D
0
0'25
0!50
0.~5
,00
Dispersed onode rotio, NyN
FiG. 1. Comparison of corrosion rate (R') obtained in presence of N' anodes, with the unprotected corrosion rate (R) when the calculated minimum number of anodes is N. + 10 mesh activated charcoal, and 10 mesh granulated zinc. [] ¼in. carbon Rashig rings and ¼in. × ~ in. magnesium cylinders. c, ~ in. × & in. carbon discs, and same size aluminium discs. A 10 mesh activated charcoal, and ~ in / { in. magnesium cylinders. ia -- ~ D 2 H a -
-
16rcr~N
KgCgF X
-
-
J
and, if there is a practical limit to (ia) at which passivation occurs, say (ij ) , the minimum value of N may be calculated from U -- D2H aKgCgF 16i,,'r2J
(2)
The numerator of this expression contains all the terms associated with the chemical engineering design, and the denominator comprises those factors connected with the choice of anode, although of course the effective radius (r) (reflecting the anode shape) may be set by hydrodynamic considerations. Fig. 1 illustrates some results obtained with variously shaped anodes of zinc, magnesium and aluminium dispersed in four kinds of carbon bed. The reduction of corrosion is plotted against the ratio of anode numbers used to that predicted from the above equation. The technique previously reported 1 was used to determine the practical value of iJ for each anode material when embedded in a carbon bed. This is necessary, because the dispersed anodes are themselves subject to the same mass transfer processes as the surrounding cathodes. Thus when an anode has a tendency to become passive under oxidizing conditions it is more inclined to do so when exposed to a dynamic system, and the attained value of (i~') is also a function of velocity. The minimum value of N was then calculated from (2) and chosen proportions of this number of anodes (N') admixed with carbon shapes. The resulting corrosion (R') of the metal surfaces was expressed as a fraction of the unprotected rate (R) and plotted against the fraction of calculated anodes used (N'/N).
T. K. R o ~ >
J
o.
-0" 5 0
-0"60
o o
-0"70
c
-°'8°o
~'5
5to
;J5
,oc
Distance along tube diameters FIG. 2. Potential of a 1 in. bore mild steel pipe at various distances downstream from the point of addition of dispersed magnesium anodes. Solution of 0'001 N. NaC1 flowing at 2 ft/s., with 0-12 >: 0-05 in. magnesium cylinders added at the rate of 200/1.
The absolute current requirements of a cathode upon which oxygen absorption is taking place may of course be predicted from the rate of that absorption, and hence the total cathodic current can be calculated from mass transfer data. Thus the theoretical protective current required in a particular condition can be predicted and compared to that actually used as calculated from the loss in weight of the anodes. The dispersed sacrificial anode may also be extended to the field of corrosion control in fluid systems, as, for instance, in applying cathodic protection to the inside of a long straight pipe carrying a corrosive fluid. This problem becomes one of locating anodes at sufficiently frequent intervals, for a single sacrificial anode located at one end will only protect a zone some 3-4 tube diameters in length. But, if small sacrificial anodes are introduced in such a way as to be suspended in and transported by the liquid they may give adequate protection over any length (Fig. 2). It is interesting to note that the only portion of tube not adequately protected under these conditions was the first 3 or 4 dia where the glass nozzle used to introduce anodes was sited; the use of a nozzle of protective metal overcame this. This particular result was obtained with a fixed loading of anodes, and the degree of protection or potential shift is naturally related to the number of anodes in a given volume of liquid. Thus the loading required to obtain any desired protected potential can be selected, and many investigations of this type have been carried out for different anode materials. What can be done in one tube can also be done in several parallel tubes, so that tube bundles can also be protected. An integral requirement of this form of dispersed anode protection is that the anodes are freely suspended in and transported by the environment and so their form and shape is important. As a general principle one requires an anode size and shape such that the settling forces exerted upon it are at least balanced by drag forces exerted by the corroding fluid. Under these conditions free motion of the solid particles
Cathodic protection by means of dispersed anodes
279
results, and their repeated contacts with the vessel wall develop the necessary protective current. Also the size of an anode dictates its total life in a circulating system. There are clearly two factors of importance in the interaction between the dispersed anode and the protected surface, namely the frequency of such contacts and their effectiveness. I f the frequency of contacts for a single particle isf/sec., the period of duration of such a contact 0 sec., and the contact resistance Re, then the current transfered per anode (I) is given by
I -- (Ep - - E,,)fO Rc when Ep and E, are the mean potentials of the protected surface and the anodic particle. Also this current transfer is related to the rate of anodic dissolution by the expression dW
F
z=-Z- x ~
dW. where - - is the rate of weight loss of N particles of electrochemical equivalent J, dt
and F is Faraday's Number. dW
F
Thus, d---t- × fO SO,
"
8~
(Ep -- E,)fO
_
NJ
Rc
dW
F
X
--
at
NJ(E,, -- Eo)
350
~o"
300
.
c E 25O
:u O~
l 2OO
~50
IO0
5O laJ
/
.! w
l l"
°o &
o!s IOglo Reynolds
~o number,
!~
z'-o z'-5
3o
dpV/@
Fro. 3. The calculated value of the effectivenessfactor obtained with various magnesium anodes dispersed in 0.001 N NaCI flowing in a 2 in. bore mild steel pipe. dp = mean anode diameter V = flow velocity "t = kinematic viscosity of solution.
280
T.K.
Ross
dW
Thus from data o n ~ f for (N) particles, and observations of (Ep) and (Ea), the "effectiveness factor" ( ; 0 ) of the anodes may be calculated. It seems to be impossible at present to separate the terms f and 0 except by substituting inert particles of the same dispersive properties as anodes in a constant number (N). From anode weight loss measurements then, the effectiveness factor between anode and wall may be deduced. Fig. 3 indicates how this factor, which directly controls the effectiveness of dispersed anodes, varies with dynamic conditions. This curve is very similar in shape to that relating the fluidization characteristics of the anodes to the flow rate, so that in general it may be said that the anodes are at their most effective when properly fluidized by the environment. At present the behaviour of dispersed protection is being pursued along various lines. The effect of various anode-metal systems are being explored and also the particulate nature of the anode contact is being examined. By substituting part of the bed with inert glass particles of the same fluidization properties, the effect of electrical substitution without the loss of dynamic properties may be studied. This line of work suggests the fact that most ceramic shapes could be covered with the appropriate amount of anodic material, e.g. by coating them or by compressing mixtures of metallic and inert powders, with the result that the anodic behaviour could be controlled to the desired level. REFERENCE 1. T. K. Ross and A. H. MORSHEDIAU,Trans. Inst. Chert1. Engrs. 38, 43 (1960).