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The corrosion product morphology found on sacrificial zinc anodes
Corrosion Science, 1977, Vol. 17. pp. 377 to 384. Pergamon Press. Printed in Great Britain
THE CORROSION PRODUCT MORPHOLOGY ON SACRIFICIAL ZINC ANODES*
FOUND
J. PERKINS," a n d R. A. BORNHOLDT+ + Materials Science Group, Department of Mechanical Engineering, Naval Postgraduate School, Monterey, CA 93940, U.S.A. Abstract--The morphology of anodic deposits on zinc was studied by scanning electron microscopy and potentiostatic techniques. The anodic film structure for zinc-steel couples in sea water is a porous three-dimensional network of discrete single crystal plates of ZnO. The individual ZnO crystals observed are 10--100 ~.m dia., with 106 plates/cm 2 of anode surface. The crystallites develop with hexagonal basal planes parallel to broad plate surfaces, as the result of an apparent dissolutionprecipitation mechanism, with limited electrochemical effects on the basic morphology. Details of the plate morphology and implications regarding defect structure and growth mechanisms are discussed. INTRODUCTION
ZINC HAS been used as a galvanic anode for the cathodic protection of ships' hulls for some time. One of the historical service difficulties in this application has been the development of apparent anode passivity, with a resulting loss of protection due to current blockage. 1,2 In the past, this occurrence has been correlated with the development of certain types of corrosion product film structures. ~-5 The present study has examined the effect of oceanographic and electrochemical variables on the growth of anodic corrosion films on zinc anode materials such as are typically applied to protect contemporary naval vessels. This study has placed emphasis on direct microscopic examinations of corrosion product morphology via scanning electron microscopy (SEM). As a metal corrodes it frequently forms a surface film. This corrosion product can take many forms depending on the detailed features of the metal and the corrosive environment. Once a film has formed, for example, as a result of oxidizing gaseous exposure or aqueous electrochemical corrosion, any one of the following may happen" (i) general dissolution of the film followed by base metal reaction; (ii) dissolution of the film at only a few discrete points with subsequent base metal reaction localized at these points; (iii) as (ii) but With corrosion gradually extending over the whole surface; (iv) as (ii) but followed by plugging of the resultant pores by corrosion products; (v) no film attack or, alternatively, further thickening of the film to the extent of stifling further attack. The last statement can be taken to include passivity, i.e. loss of chemical reactivity, a desirable condition for many metal applications. Zinc sacrificial anodes are intended to corrode. Unfortunately, under certain conditions, zinc will passivate due to the formation of a protective film.2,5 There is evidence that this film consists of a metal-excess zinc oxide, s,4 *Manuscript received 25 November 1974; in revised form 6 July 1976. "l'Associate Professor of Materials Science. :I:LCDR, USN. 377
378
J. PERKINSand R. A. BORNHOLDT
I n the present investigation, a n effort was made to o b t a i n further insight into the tendency of a sacrificial a n o d e zinc to form protective films. This investigation concentrated o n direct microscopic observations, studying e n v i r o n m e n t a l factors affecting film f o r m a t i o n a n d morphology. The electrolyte in all tests was n a t u r a l ( M o n t e r e y Bay) seawater. Both l a b o r a t o r y a n d ocean test exposures were conducted. The m o r p h o l o g y of zinc corrosion products in zinc-steel couples was examined by s c a n n i n g electron microscopy as a f u n c t i o n of actual ocean depth a n d in the l a b o r a t o r y as a f u n c t i o n of equivalent pressure a n d anodic potential. EXPERIMENTAL PROCEDURE Laboratory corrosion cells at atmospheric pressure
Laboratory (as well as all other) sea water corrosion tests were conducted using zinc-steel couples. The zinc composition is given in Table 1 and conformed to MIL-A-18001He; a low carbon steel was used. The zinc samples were 1.25 × 1.25 × 0.6 cm, with the broad face drilled with a 0.5 cm hole and mounted flush to the steel surface. The area ratio of the couple was approx. 50 : 1 (cathode to anode), one-side area ratio (not counting edges). This closely simulates the area ratio used by the U.S. Navy s for cathodic protection on unpainted ship hulls. TABLE I . CoMPOSITION OF THE ZINC ANODE MATERIAL
Wt.~, Pb Fe Cd Cu AI Si Zn
0.002 0.002 0.003 0.0003 0.001 0.001 Balance
Prior to coupling, the zinc samples were ground on abrasive paper to a 600 grit finish, measured, and weighed. Nylon nuts and bolts were used to join the couples. The mild steel was descaled and free of rust. Laboratory corrosion cell couples were exposed at atmospheric pressure for periods up to four weeks, with duplicate couples for each time examined. The zinc samples were subsequently lightly rinsed in distilled water, dried, weighed, and mounted on SEM stubs for examination of corrosion film structure and morphology by X-ray diffraction (XRD) and SEM, respectively. Film structures throughout the study were obtained using a Norelco diffractometer and CuKa radiation. Laboratory pressure cell tests
Zinc-steel cells were investigated in the laboratory as a function of equivalent ocean depth using a stainless steel pressure chamber (6 in. dia. x 12 in. long). A glass liner insulated the couples from the walls. The chamber was filled with sea water to within ½ in. from the top and the couples were suspended from the cover plate. Desired equivalent depth pressures up to 200 ft were obtained by pressurizing with argon gas, chosen because of its low sea water solubility. The zinc samples were ground, measured, and weighed prior to testing. Exposures at equivalent depths of 50, 100, 150 and 200 ft for times up to 14 d were made. After the desired exposure they were rinsed, dried, weighed (to obtain the sample weight gain), and mounted on SEM stubs; or alternately rinsed, dried, scrubbed, and weighed (to obtain the metal weight change). Ocean exposure arrays
Actual ocean exposure tests were also conducted, consisting of exposure of couples at various depths and for various times with the primary aim of developing film structures and corrosion morphologies wliich could be compared with laboratory control samples. Couple preparation was identical to that of laboratory test samples. Monterey Bay was chosen as the test site as it afforded
Corrosion product morphology found on sacrificial zinc anodes
379
proximity and provided desired water depths. In addition, oceanographic data was readily availablq (Table 2). The ocean exposure location was 3 km west of Moss Landing, California Power Station (U.S. Coastal and Geological Survey Chart No. 5402). TABLE 2. OCEANOGRAPHICDATA FOR MONTEREY BAY SEA WATER (DATA PROVIDED BY U.S. NAVAL POSTGRADUATE SCHOOL OCEANOGRAPHY DEPARTME~rr, 1970 SURVEY AT 60 m DEPTH)
Salinity Oxygen Temperature
32.8-33 parts per thousand 1.33 ml/I. 8°C
Microacopic examination of corroMonproduct morphology The method of examining corrosion films by metallographic sectioning requires extreme care during the mounting and polishing operations in order to preserve the surface structure, and in the case of delicate or fragile structures, it is particularly difficult to avoid damage during these operations. For this reason, an observation method requiring no sample handling would be ideal. Direct observation by optical microscopy is found to be unrewarding for rough surfaces due to lack of depth of focus. Scanning electron microscopy provides several orders of magnitude improvement in depth of focus while also having considerably higher magnifications. This technique allows direct study of the morphology of corrosion products which may then be correlated with models of the corrosion processes in order to interpret observed corrosion behavior. In this study, a Cambridge $4-10 scanning electron microscope (SEM) was used to study the corrosion product morphology. Photomicrographs of the surfaces of samples exposed under the various corrosive conditions were obtained. Surface qharging by the electron beam was observed to varying degrees, but was generally not a serious problem. All samples were able to be examined without deposition of a conductive coating at an accelerating potential of 20 KV.
Potentiostatic measurements Anodic polarization studies were performed using a Model 200A TRW research potentiostat Prior to testing, the samples were ground to a 600 grit finish and drilled and tapped to fit the sample holder. The ASTM G5-71/7 test procedure was followed, with the exceptions of the sea water electrolyte, and one minute time intervals between readings. The primary purpose of the potentiostatic investigations was to accelerate and control corrosion of samples so that direct correlation could be made with the SEM and XRD studies. EXPERIMENTAL RESULTS AND DISCUSSION
Morphology of anodic films T h e c o r r o s i o n p r o d u c t f o r m e d on polycrystalline zinc d u r i n g a n o d i c electrocrystallization in sea w a t e r when c o u p l e d with steel is f o u n d to be zinc oxide (ZnO), as shown by X - r a y diffraction p o w d e r p a t t e r n s o f the in place a n o d i c film. T h e hexagonal Z n O films typically a d o p t a m o r p h o l o g y consisting o f n u m e r o u s small plates (see Fig. 1). These single crystal plates are 10-100 ~m in size (average dia.) with a particle density o f a p p r o x . 106 plates/cm ~ o f base m e t a l surface. This structure, which is p o r o u s o n a fine scale, is desirable with respect to sacrificial a n o d e a p p l i c a tion, as it allows continuous access of the electrolyte to the base anode metal, and
therefore allows electrons to be continuously provided to the cathode. No evidence was found of a compact sealing layer of any kind adjacent to the anode surface, in natural couples at r.t. and atmospheric pressure; this is seen clearly in Fig. 2. The appearance of the substrate and the nature of the crystal morphology indicates that the film is the result of a dissolution-precipitation type mechanism. Maintenance of a porous film of this sort should lead to desirable linear corrosion
380
J. PERKINSand R. A. BORNHOLDT
kinetics for sacrificial anodes. However, under certain external conditions of potential, pressure, solution chemistry, and time, these structures may be envisaged as sealing off the surface, leading to effective passivation. This is demonstrated in Fig. 3 for a couple identical to that of Figs. 1 and 2 exposed at 16 m depth for 200 h, resulting in a dense compact anodic film and loss of cathodic protection. This structure probably represents the manifestation of a polarity reversal in the zinc-steel couple;8,9 comparison of the film structure with that of a sample subjected to a complete potentiostatic anodic sweep followed by a cathodic sweep (Fig. 4) lends credence to this conclusion. It is apparent that for a wide range of anodic potentials, the crystallization of ZnO on zinc involves anodic dissolution and oriented three-dimensional nucleation of discrete crystallites of quite small size. The crystallite plates tend to be oriented such that growth occurs primarily normal to the base metal surface, as typified in Figs. 1 and 2. X-ray diffraction patterns of the in-place films verify this crystallographic texture, exhibiting, in particular, a pronounced decrease in the hexagonal ZnO basal plane peak intensity. Sideplates are observed to nucleate from the broad faces of plates, evidently having specific crystallographic orientations with respect to the ZnO plate surface on which they originate (see Fig. 5). Even at plate intersection points (see Figures for example), no plate thickening whatsoever is demonstrated, indicating the pronounced lack of growth tendency perpendicular to broad plate faces. It is also evident that the substrate polycrystalline zinc alloy exerts virtually no influence on the orientation of the initial or subsequent ZnO crystallites. It may be reasonably deduced from the vivid development of hexagonally-shaped plates, some of which are remarkably regular (Fig. 6), that the crystals grow with basal (0001) ZnO planes parallel to the exposed broad faces of the plates. Growth proceeds almost exclusively by lateral extension, corresponding to growth perpendicular to other, more rapid-growing crystallographic planes. These are clearly either the (I 120) or (1070) ZnO prism planes. Electrochemical considerations Potentiostatic polarization experiments indicate no tendency for the zinc anode material to passivate in stagnant sea water. A typical anode sweep is shown in Fig. 7. Examination of the anodic corrosion product from several points on the anodic polarization curve indicates that the average plate diameter increases with coulombic input, as expected (1.25, 2.5 and 5.0 ~tm for 3, 30 and 300 C, respectively), and that the nucleation frequency also increases for greater anodic polarization (see Fig. 8).
Electrochemical considerations relating to film nucleation and growth processes are complex under the conditions examined. Generally speaking, crystal morphology in dissolution-precipitation crystallization will be dependent on the average applied surface potential, and, of course, in local variations in this potential. As the ZnO erystallites deposit on the surface, the average surface activity changes, as does the base metal current density. The specific changes in these important conditions will depend upon the growth form and surface nucleation density of the crystallites, and vice versa. Thus, when an experiment is conducted at constant average current density, the base metal overpotential can be expected to change markedly as the new
FIc. I. Scanning electron micrograph of an array of ZnO crystal plates formed on zinc exposed in a zinc-steel couple at room temperature and atmospheric pressure for 100 h. FIG. 2. The appearance of the zinc substrate surface at an early stage of crystal development indicates a dissolution-precipitation mechanism. FIG. 3. Compact film formed in sea exposure, at 16 m depth for 216 h. FIG. 4. Film morphology developed by anodic-cathodic potentiostatic sweep in sea water.
FIG. 5. Sideplates are observed at various crystallographic orientations to the main ZnO plate on which they originate; the plate intersections are free of thickening; exposure at room temperature and pressure for 100 h. FIG. 6. Many of the ZnO plates have extremely regular hexagonal profiles; exposure at atmospheric pressure for 100 h. FXG. 8. Increased nucleation frequency evident at greater anodic polarization exposure at 0.9 V, 100 mA/cm'-' for 5 rain.
Corrosion product morphologyfound on sacrificialzinc anodes
383
1.0
o
0.5
tO
17
=>
.o_ -I-
/
R I7
-0.5
~o fl_ -I.0
~
...__.v . / v - v ' v ~ v ' ~
v
Io -z
FIG. 7.
v
, I0-' C u r r e n t density,
II0 mo/cm 2
I2 10
Typical anodic polarization curve for zinc in sea water obtained using one minute time intervals between steps.
crystals form on the surface. In view of potentiostatically controlled experiments conducted in this study, changes in surface potential can be expected to affect nucleation frequency significantly. In practice, as anodic electro-crystallization proceeds, there is, of course, a change in actual exposed base metal surface area, giving an increased current density (assuming a significantly higher resistance for the deposited crystals). This is accompanied by a change in the average surface activity of the electrode, corresponding to the difference in activity between the original surface and the surfaces of the electro-crystallized particles; this will be reflected by a change in the average exchange current density, I o, of the electrode. In the present study, it is observed that in the range of conditions experienced (with the exception of certain actual sea exposures, where severely different environmental conditions were experienced), no pronounced changes in crystal morphology (other than size and nucleation frequency) are caused by electrochemical variations (potential, current density) for anodic electro-crystallization of ZnO on zinc in sea water. However, in general, ZnO crystals grown on zinc electrodes in this study are observed to be oriented with primary growth directions distributed near the surface normal direction, i.e. along field lines; few plates are observed to align anywhere near parallel to the base metal surface. This observation is considered to reflect an influence of electric field orientation and resulting concentration profiles on nucleation events, with much less influence on growth mechanisms. A final comment regarding electrochemical considerations is that a number of previous indirect studies of anodic film formation on zinc and other materials have perhaps overemphasized the importance of film resistance. As is obvious from the present direct observations of film morphology, such measurements will have limited significance if there exists a high degree of microscopic porosity over wide ranges of electrochemical conditions.
384
J. PERKINSand R. A. BORNHOLDT S U M M A R Y AND C O N C L U S I O N S
The pronounced growth deficiencies in the c-axis direction and pyramidal normal directions of the hexagonal ZnO structure for anodically electro-crystallized ZnO crystals is most fortunate with regard to avoiding development of a coherent sealing anodic film on sacrificial anode materials. Unlike many materials subject to corrosion conditions, the development of dense coherent films, in which growth (and current passage) is limited by solid state diffusional processes, is not desirable in the case of corrosion of sacrificial anodes. This study has led to the following specific conclusions regarding anodic ZnO crystallization. 1. The anodic film on unbiased zinc-steel couples in sea water at room temperature is zinc oxide (ZnO). 2. The morphology of anodically electro-crystallized ZnO is a porous film consisting of many discrete single crystal plates. 3. The individual ZnO plates in anodically electro-crystallized films are on the order of 10-100 ~m diameter, with a diameter-to-thickness ratio of approx. 50 : 1. These are present with a density of c a . 106 plates per cmL 4. Individual anodic ZnO plates have hexagonal basal planes parallel to the broad plate surfaces. The plates have a hexagonal outline with blunt edges, indicating growth predominantly perpendicular to prism planes. 5. No surface ledges or spirals are observed on anodic ZnO crystals, indicating that defect growth mechanisms are not operative. Growth occurs largely by plate broadening, with limited thickening. Growth is apparently by a dissolution-precipitation mechanism. Acknowledgements--This work was supported by the Office of Naval Research (ONR), Washington, D.C.
REFERENCES 1. R. B. TEELand D. B. ANDERSON,Corrosion 12, 53 (1956). 2. B, H. TYrELLand H. S. PREISER,ASNEJ., p. 701, November (1956). 3. K. HtmER, 3". eleetrochem. $oe. 100, 376 (1953). 4. H. FRY and M. WHrrAK~R,7. eleetroehem. Soe. 106, 606 (1959). 5. R. F. AsH'rONand M. T. HEPWORTH,Corrosion 24, 50 (1968). 6. U.S. Military Specification MIL-A-18001H, Anodes, Corrosion Preventive, Zinc; Slab, Disc, and Rod Shaped, 28 June (1968). 7. ASTM Standard G5-71, Standard Reference Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements. 8. R. B. HOXENGand C. F. PRUTTON,Corrosion 5, 330 (1949). 9. P. T. GILBERT,3". eleetroehem. Soe. 99, 16 (1952).
THE CORROSION PRODUCT MORPHOLOGY ON SACRIFICIAL ZINC ANODES*
FOUND
J. PERKINS," a n d R. A. BORNHOLDT+ + Materials Science Group, Department of Mechanical Engineering, Naval Postgraduate School, Monterey, CA 93940, U.S.A. Abstract--The morphology of anodic deposits on zinc was studied by scanning electron microscopy and potentiostatic techniques. The anodic film structure for zinc-steel couples in sea water is a porous three-dimensional network of discrete single crystal plates of ZnO. The individual ZnO crystals observed are 10--100 ~.m dia., with 106 plates/cm 2 of anode surface. The crystallites develop with hexagonal basal planes parallel to broad plate surfaces, as the result of an apparent dissolutionprecipitation mechanism, with limited electrochemical effects on the basic morphology. Details of the plate morphology and implications regarding defect structure and growth mechanisms are discussed. INTRODUCTION
ZINC HAS been used as a galvanic anode for the cathodic protection of ships' hulls for some time. One of the historical service difficulties in this application has been the development of apparent anode passivity, with a resulting loss of protection due to current blockage. 1,2 In the past, this occurrence has been correlated with the development of certain types of corrosion product film structures. ~-5 The present study has examined the effect of oceanographic and electrochemical variables on the growth of anodic corrosion films on zinc anode materials such as are typically applied to protect contemporary naval vessels. This study has placed emphasis on direct microscopic examinations of corrosion product morphology via scanning electron microscopy (SEM). As a metal corrodes it frequently forms a surface film. This corrosion product can take many forms depending on the detailed features of the metal and the corrosive environment. Once a film has formed, for example, as a result of oxidizing gaseous exposure or aqueous electrochemical corrosion, any one of the following may happen" (i) general dissolution of the film followed by base metal reaction; (ii) dissolution of the film at only a few discrete points with subsequent base metal reaction localized at these points; (iii) as (ii) but With corrosion gradually extending over the whole surface; (iv) as (ii) but followed by plugging of the resultant pores by corrosion products; (v) no film attack or, alternatively, further thickening of the film to the extent of stifling further attack. The last statement can be taken to include passivity, i.e. loss of chemical reactivity, a desirable condition for many metal applications. Zinc sacrificial anodes are intended to corrode. Unfortunately, under certain conditions, zinc will passivate due to the formation of a protective film.2,5 There is evidence that this film consists of a metal-excess zinc oxide, s,4 *Manuscript received 25 November 1974; in revised form 6 July 1976. "l'Associate Professor of Materials Science. :I:LCDR, USN. 377
378
J. PERKINSand R. A. BORNHOLDT
I n the present investigation, a n effort was made to o b t a i n further insight into the tendency of a sacrificial a n o d e zinc to form protective films. This investigation concentrated o n direct microscopic observations, studying e n v i r o n m e n t a l factors affecting film f o r m a t i o n a n d morphology. The electrolyte in all tests was n a t u r a l ( M o n t e r e y Bay) seawater. Both l a b o r a t o r y a n d ocean test exposures were conducted. The m o r p h o l o g y of zinc corrosion products in zinc-steel couples was examined by s c a n n i n g electron microscopy as a f u n c t i o n of actual ocean depth a n d in the l a b o r a t o r y as a f u n c t i o n of equivalent pressure a n d anodic potential. EXPERIMENTAL PROCEDURE Laboratory corrosion cells at atmospheric pressure
Laboratory (as well as all other) sea water corrosion tests were conducted using zinc-steel couples. The zinc composition is given in Table 1 and conformed to MIL-A-18001He; a low carbon steel was used. The zinc samples were 1.25 × 1.25 × 0.6 cm, with the broad face drilled with a 0.5 cm hole and mounted flush to the steel surface. The area ratio of the couple was approx. 50 : 1 (cathode to anode), one-side area ratio (not counting edges). This closely simulates the area ratio used by the U.S. Navy s for cathodic protection on unpainted ship hulls. TABLE I . CoMPOSITION OF THE ZINC ANODE MATERIAL
Wt.~, Pb Fe Cd Cu AI Si Zn
0.002 0.002 0.003 0.0003 0.001 0.001 Balance
Prior to coupling, the zinc samples were ground on abrasive paper to a 600 grit finish, measured, and weighed. Nylon nuts and bolts were used to join the couples. The mild steel was descaled and free of rust. Laboratory corrosion cell couples were exposed at atmospheric pressure for periods up to four weeks, with duplicate couples for each time examined. The zinc samples were subsequently lightly rinsed in distilled water, dried, weighed, and mounted on SEM stubs for examination of corrosion film structure and morphology by X-ray diffraction (XRD) and SEM, respectively. Film structures throughout the study were obtained using a Norelco diffractometer and CuKa radiation. Laboratory pressure cell tests
Zinc-steel cells were investigated in the laboratory as a function of equivalent ocean depth using a stainless steel pressure chamber (6 in. dia. x 12 in. long). A glass liner insulated the couples from the walls. The chamber was filled with sea water to within ½ in. from the top and the couples were suspended from the cover plate. Desired equivalent depth pressures up to 200 ft were obtained by pressurizing with argon gas, chosen because of its low sea water solubility. The zinc samples were ground, measured, and weighed prior to testing. Exposures at equivalent depths of 50, 100, 150 and 200 ft for times up to 14 d were made. After the desired exposure they were rinsed, dried, weighed (to obtain the sample weight gain), and mounted on SEM stubs; or alternately rinsed, dried, scrubbed, and weighed (to obtain the metal weight change). Ocean exposure arrays
Actual ocean exposure tests were also conducted, consisting of exposure of couples at various depths and for various times with the primary aim of developing film structures and corrosion morphologies wliich could be compared with laboratory control samples. Couple preparation was identical to that of laboratory test samples. Monterey Bay was chosen as the test site as it afforded
Corrosion product morphology found on sacrificial zinc anodes
379
proximity and provided desired water depths. In addition, oceanographic data was readily availablq (Table 2). The ocean exposure location was 3 km west of Moss Landing, California Power Station (U.S. Coastal and Geological Survey Chart No. 5402). TABLE 2. OCEANOGRAPHICDATA FOR MONTEREY BAY SEA WATER (DATA PROVIDED BY U.S. NAVAL POSTGRADUATE SCHOOL OCEANOGRAPHY DEPARTME~rr, 1970 SURVEY AT 60 m DEPTH)
Salinity Oxygen Temperature
32.8-33 parts per thousand 1.33 ml/I. 8°C
Microacopic examination of corroMonproduct morphology The method of examining corrosion films by metallographic sectioning requires extreme care during the mounting and polishing operations in order to preserve the surface structure, and in the case of delicate or fragile structures, it is particularly difficult to avoid damage during these operations. For this reason, an observation method requiring no sample handling would be ideal. Direct observation by optical microscopy is found to be unrewarding for rough surfaces due to lack of depth of focus. Scanning electron microscopy provides several orders of magnitude improvement in depth of focus while also having considerably higher magnifications. This technique allows direct study of the morphology of corrosion products which may then be correlated with models of the corrosion processes in order to interpret observed corrosion behavior. In this study, a Cambridge $4-10 scanning electron microscope (SEM) was used to study the corrosion product morphology. Photomicrographs of the surfaces of samples exposed under the various corrosive conditions were obtained. Surface qharging by the electron beam was observed to varying degrees, but was generally not a serious problem. All samples were able to be examined without deposition of a conductive coating at an accelerating potential of 20 KV.
Potentiostatic measurements Anodic polarization studies were performed using a Model 200A TRW research potentiostat Prior to testing, the samples were ground to a 600 grit finish and drilled and tapped to fit the sample holder. The ASTM G5-71/7 test procedure was followed, with the exceptions of the sea water electrolyte, and one minute time intervals between readings. The primary purpose of the potentiostatic investigations was to accelerate and control corrosion of samples so that direct correlation could be made with the SEM and XRD studies. EXPERIMENTAL RESULTS AND DISCUSSION
Morphology of anodic films T h e c o r r o s i o n p r o d u c t f o r m e d on polycrystalline zinc d u r i n g a n o d i c electrocrystallization in sea w a t e r when c o u p l e d with steel is f o u n d to be zinc oxide (ZnO), as shown by X - r a y diffraction p o w d e r p a t t e r n s o f the in place a n o d i c film. T h e hexagonal Z n O films typically a d o p t a m o r p h o l o g y consisting o f n u m e r o u s small plates (see Fig. 1). These single crystal plates are 10-100 ~m in size (average dia.) with a particle density o f a p p r o x . 106 plates/cm ~ o f base m e t a l surface. This structure, which is p o r o u s o n a fine scale, is desirable with respect to sacrificial a n o d e a p p l i c a tion, as it allows continuous access of the electrolyte to the base anode metal, and
therefore allows electrons to be continuously provided to the cathode. No evidence was found of a compact sealing layer of any kind adjacent to the anode surface, in natural couples at r.t. and atmospheric pressure; this is seen clearly in Fig. 2. The appearance of the substrate and the nature of the crystal morphology indicates that the film is the result of a dissolution-precipitation type mechanism. Maintenance of a porous film of this sort should lead to desirable linear corrosion
380
J. PERKINSand R. A. BORNHOLDT
kinetics for sacrificial anodes. However, under certain external conditions of potential, pressure, solution chemistry, and time, these structures may be envisaged as sealing off the surface, leading to effective passivation. This is demonstrated in Fig. 3 for a couple identical to that of Figs. 1 and 2 exposed at 16 m depth for 200 h, resulting in a dense compact anodic film and loss of cathodic protection. This structure probably represents the manifestation of a polarity reversal in the zinc-steel couple;8,9 comparison of the film structure with that of a sample subjected to a complete potentiostatic anodic sweep followed by a cathodic sweep (Fig. 4) lends credence to this conclusion. It is apparent that for a wide range of anodic potentials, the crystallization of ZnO on zinc involves anodic dissolution and oriented three-dimensional nucleation of discrete crystallites of quite small size. The crystallite plates tend to be oriented such that growth occurs primarily normal to the base metal surface, as typified in Figs. 1 and 2. X-ray diffraction patterns of the in-place films verify this crystallographic texture, exhibiting, in particular, a pronounced decrease in the hexagonal ZnO basal plane peak intensity. Sideplates are observed to nucleate from the broad faces of plates, evidently having specific crystallographic orientations with respect to the ZnO plate surface on which they originate (see Fig. 5). Even at plate intersection points (see Figures for example), no plate thickening whatsoever is demonstrated, indicating the pronounced lack of growth tendency perpendicular to broad plate faces. It is also evident that the substrate polycrystalline zinc alloy exerts virtually no influence on the orientation of the initial or subsequent ZnO crystallites. It may be reasonably deduced from the vivid development of hexagonally-shaped plates, some of which are remarkably regular (Fig. 6), that the crystals grow with basal (0001) ZnO planes parallel to the exposed broad faces of the plates. Growth proceeds almost exclusively by lateral extension, corresponding to growth perpendicular to other, more rapid-growing crystallographic planes. These are clearly either the (I 120) or (1070) ZnO prism planes. Electrochemical considerations Potentiostatic polarization experiments indicate no tendency for the zinc anode material to passivate in stagnant sea water. A typical anode sweep is shown in Fig. 7. Examination of the anodic corrosion product from several points on the anodic polarization curve indicates that the average plate diameter increases with coulombic input, as expected (1.25, 2.5 and 5.0 ~tm for 3, 30 and 300 C, respectively), and that the nucleation frequency also increases for greater anodic polarization (see Fig. 8).
Electrochemical considerations relating to film nucleation and growth processes are complex under the conditions examined. Generally speaking, crystal morphology in dissolution-precipitation crystallization will be dependent on the average applied surface potential, and, of course, in local variations in this potential. As the ZnO erystallites deposit on the surface, the average surface activity changes, as does the base metal current density. The specific changes in these important conditions will depend upon the growth form and surface nucleation density of the crystallites, and vice versa. Thus, when an experiment is conducted at constant average current density, the base metal overpotential can be expected to change markedly as the new
FIc. I. Scanning electron micrograph of an array of ZnO crystal plates formed on zinc exposed in a zinc-steel couple at room temperature and atmospheric pressure for 100 h. FIG. 2. The appearance of the zinc substrate surface at an early stage of crystal development indicates a dissolution-precipitation mechanism. FIG. 3. Compact film formed in sea exposure, at 16 m depth for 216 h. FIG. 4. Film morphology developed by anodic-cathodic potentiostatic sweep in sea water.
FIG. 5. Sideplates are observed at various crystallographic orientations to the main ZnO plate on which they originate; the plate intersections are free of thickening; exposure at room temperature and pressure for 100 h. FIG. 6. Many of the ZnO plates have extremely regular hexagonal profiles; exposure at atmospheric pressure for 100 h. FXG. 8. Increased nucleation frequency evident at greater anodic polarization exposure at 0.9 V, 100 mA/cm'-' for 5 rain.
Corrosion product morphologyfound on sacrificialzinc anodes
383
1.0
o
0.5
tO
17
=>
.o_ -I-
/
R I7
-0.5
~o fl_ -I.0
~
...__.v . / v - v ' v ~ v ' ~
v
Io -z
FIG. 7.
v
, I0-' C u r r e n t density,
II0 mo/cm 2
I2 10
Typical anodic polarization curve for zinc in sea water obtained using one minute time intervals between steps.
crystals form on the surface. In view of potentiostatically controlled experiments conducted in this study, changes in surface potential can be expected to affect nucleation frequency significantly. In practice, as anodic electro-crystallization proceeds, there is, of course, a change in actual exposed base metal surface area, giving an increased current density (assuming a significantly higher resistance for the deposited crystals). This is accompanied by a change in the average surface activity of the electrode, corresponding to the difference in activity between the original surface and the surfaces of the electro-crystallized particles; this will be reflected by a change in the average exchange current density, I o, of the electrode. In the present study, it is observed that in the range of conditions experienced (with the exception of certain actual sea exposures, where severely different environmental conditions were experienced), no pronounced changes in crystal morphology (other than size and nucleation frequency) are caused by electrochemical variations (potential, current density) for anodic electro-crystallization of ZnO on zinc in sea water. However, in general, ZnO crystals grown on zinc electrodes in this study are observed to be oriented with primary growth directions distributed near the surface normal direction, i.e. along field lines; few plates are observed to align anywhere near parallel to the base metal surface. This observation is considered to reflect an influence of electric field orientation and resulting concentration profiles on nucleation events, with much less influence on growth mechanisms. A final comment regarding electrochemical considerations is that a number of previous indirect studies of anodic film formation on zinc and other materials have perhaps overemphasized the importance of film resistance. As is obvious from the present direct observations of film morphology, such measurements will have limited significance if there exists a high degree of microscopic porosity over wide ranges of electrochemical conditions.
384
J. PERKINSand R. A. BORNHOLDT S U M M A R Y AND C O N C L U S I O N S
The pronounced growth deficiencies in the c-axis direction and pyramidal normal directions of the hexagonal ZnO structure for anodically electro-crystallized ZnO crystals is most fortunate with regard to avoiding development of a coherent sealing anodic film on sacrificial anode materials. Unlike many materials subject to corrosion conditions, the development of dense coherent films, in which growth (and current passage) is limited by solid state diffusional processes, is not desirable in the case of corrosion of sacrificial anodes. This study has led to the following specific conclusions regarding anodic ZnO crystallization. 1. The anodic film on unbiased zinc-steel couples in sea water at room temperature is zinc oxide (ZnO). 2. The morphology of anodically electro-crystallized ZnO is a porous film consisting of many discrete single crystal plates. 3. The individual ZnO plates in anodically electro-crystallized films are on the order of 10-100 ~m diameter, with a diameter-to-thickness ratio of approx. 50 : 1. These are present with a density of c a . 106 plates per cmL 4. Individual anodic ZnO plates have hexagonal basal planes parallel to the broad plate surfaces. The plates have a hexagonal outline with blunt edges, indicating growth predominantly perpendicular to prism planes. 5. No surface ledges or spirals are observed on anodic ZnO crystals, indicating that defect growth mechanisms are not operative. Growth occurs largely by plate broadening, with limited thickening. Growth is apparently by a dissolution-precipitation mechanism. Acknowledgements--This work was supported by the Office of Naval Research (ONR), Washington, D.C.
REFERENCES 1. R. B. TEELand D. B. ANDERSON,Corrosion 12, 53 (1956). 2. B, H. TYrELLand H. S. PREISER,ASNEJ., p. 701, November (1956). 3. K. HtmER, 3". eleetrochem. $oe. 100, 376 (1953). 4. H. FRY and M. WHrrAK~R,7. eleetroehem. Soe. 106, 606 (1959). 5. R. F. AsH'rONand M. T. HEPWORTH,Corrosion 24, 50 (1968). 6. U.S. Military Specification MIL-A-18001H, Anodes, Corrosion Preventive, Zinc; Slab, Disc, and Rod Shaped, 28 June (1968). 7. ASTM Standard G5-71, Standard Reference Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements. 8. R. B. HOXENGand C. F. PRUTTON,Corrosion 5, 330 (1949). 9. P. T. GILBERT,3". eleetroehem. Soe. 99, 16 (1952).