The effect of shock deformation on the anodic polarization behavior of polycrystalline nickel in 1N sulphuric acid

Corroaion Science, 1973, Vol. 13, pp. 569 to 574. Pergamon Press. Printed in Great Britain

THE EFFECT OF SHOCK DEFORMATION ON THE ANODIC POLARIZATION BEHAVIOR OF POLYCRYSTALLINE NICKEL

I N 1N S U L P H U R I C

ACID*

C. R. CROWEand S. G. FISHMAN Naval Weapons Laboratory, Dahlgren, Virginia, U.S.A. Abstract--The results of anodic potentiostatic polarization of shocked nickel polycrystals in IN H2SO4 are reported. With increasing shock pressure, passivation and Flade potentials shift in the cathodic direction, the critical current density decreases, and the passive current density decreases. These effects are discussed in terms of substructural changes accompanying shock loading. R~sum6----R6sultats de la polarisation anodique potentiostatique de polycristaux de nickel choqu6s dans H~SO4N. L'augmentation de la pression d'impact abaisse les potentiels de passivation et d'activation et diminue les densit6s de courant critique de passivation et de passivit6. Ces effets sont discut6s compte tenu des modifications substructurales accompagnant le chargement du coup. Znsammenfassung--Die Ergebnisse der anodischen potentiostatischen Polarisation von angestossenen Nickel-Polykristallen in 1N H~SO4 werden berichtet. Mit zunehmendem Anstoss-Druck werden die Passivierungs- und Flade-Potentiale in katodischer Richtung verschoben, die kritische Stromdichtigkeit vermindert sich, und die passive Stromdichtigkeit vermindert sich. Diese Wirkungen werden in Beziehungen auf unterstrukturelle Ver/inderungert besprochen, die Antoss-Ladung begleiten. INTRODUCTION

IN R~CENT years the passivation process in nickel has received much attention.1-3 These studies have been on some particulars of the passivation mechanism based on determination of anodic polarization diagrams. In order to measure effects of anisotropy on the passivation process, Latanision and Opperhauser determined anodic polarization curves on low index nickel single crystals. 4 They suggest that crystallography and defect structure are important in the passivation process. In a recent publication Garz and H/ifke reported the results of deformation on the anodic potentiostatic polarization behavior of nickel single crystals in 0.SM NiSO4. 5 In their study, they found that the passivation potential becomes more negative and the critical current density for passivation increases with increasing plastic deformation. They also observed that the beginning of the transpassive region shifts in the cathodic direction with increasing plastic deformation. In this paper a study of the effect of shock deformation on the potentiostatic polarization of polycrystaUine nickel is reported and the results discussed in connection with changes in defect structure. EXPERIMENTAL

PROCEDURES

The 4ram × 14ram dia specimens used in this study were machined from 99.98 per cent polycrystalline Ni plate. The discs were annealed in vacuum at 900°C for 1 h prior to shock loading. The samples were shocked by planar impact using a light gas *Manuscript received 28 August 1972; in revised form 27 October 1972. 569

570

C.R. CROWEand S. G. FISHMAN

gun to total transient shear strains, Es, of 1.75, 2.56, 4.34 and 6.15 per cent. The values of Es which the specimens experienced in shock deformation were calculated, assuming uniaxial deformation from the relation e

Es.= 4/3 In (V/Vo) where 1Io and V are the specific volumes of the material in the initial and compressed states, respectively. The values of V as a function of shock pressure were obtained from the data of Berger and Fanquignon. v Specimens were prepared for corrosion testing by polishing the exposed face of the sample through 000 grit metallographic paper and subsequently cleaning them ultrasonically in a detergent solution. After cleaning, the samples were washed in distilled water, mounted in the specimen holder, and immersed in the test electrolyte 18-24 h prior to testing. All polishing procedures were performed immediately prior to immersion. Metallographic samples were polished to 0.05 ~t smoothness and electrolyticaIly etched in an acetic acid-perchloric acid solution at 5V to produce etch pits which could be observed with an optical microscope. Potentiostatic measurements were made in deaerated 1N H2SO4 which was prepared using triple distilled water and reagent grade acid. Potential measurements were made relative to a saturated calomel reference electrode immersed in a separate reservoir of the electrolyte and connected to the test cell by means of a Luggin probe. The specimen holder was similar to the one described by France s with 1 cm ~ of the sample surface area exposed to the electrolyte. Polarization was accomplished using a potentiostat driven by a stepping potentiometer which automatically changed the control potential by 2 mV every 6 s. The stepping potentiometer was programmed to perform an anodic scan over a 1.6 V range, reverse itself, and return to the initial control potential. After performing an anodic scan, the potentiostat was turned off for approximately 1 h to allow the sample to re-equilibrate and a cathodic scan was performed. Potential--current curves were automatically recorded on an x-y plotter and all testing was done at 20°C ~ 2°C. RESULTS A typical potentiostatic polarization curve showing the test procedure and indicating the polarization parameters studied is shown in Fig. 1. Reproducibility of the curves was excellent with steady-state corrosion potentials, E¢, close to the values associated with the reversible hydrogen electrode, in agreement with that reported by Greene, 9 and Myers, Beck and Fontana. 1° A correlation of Ec with shock pressure, as illustrated in Fig. 2, shows a slight shift to more active potentials at intermediate shock pressures producing a minimum in the curve. Values of critical current density, Ic, ranged from 11 to 7 m A / c m ~, with Ic decreasing slightly with increasing shock pressure as shown in Fig. 3. These values compare favorably with the 20 mA/cm 2 reported by Greene, 9 the 17 mA/cm 2 of Arnold and Vetter n and the I0 m A / c m 2 of Osterwald and Uhlig 12 for a similar environment. The values are somewhat lower than those reported by Myers, Beck and Fontana, 10 Kolotyrkin, la and Okamoto, Kobayashi, Sato and Nagayama, 14 who report values

The effectof shock deformationnickel in 1N sulphuricacid

....

571

Ni shockedot 47 kb

OJ'

~"~ ~L :~. 103I

/

{.~ 1021-//

o

~

F

R

~

~

t

i - "-E~I-- +i

-0 304 -01104

0"096 0-296 0-496 0'696 0896 Potentialvs SCE, V

I1096

FIG. I. Potentiostaticpolarizationcurvefor Ni specimenshockloadedat 47 kbars.

i _02401

-

"6

ll--~,-..~I ' ~

•

/

-0.250-

~1

--OlZ60

0

1

20

I

40

I

50

I

80

I

I00

Shocp kressure,kbors FIG. 2.

Corrosionpotentialcorrelatedwithshockpressure.

of about 100 mA/cm~. Since Ic is affected by experimental procedure and by specimen purity, the values obtained are considered reasonable. The trend of decreasing Ic with increasing deformation due to shock is directly opposite to that observed by Garz and HAfke5 in their study of the effects of deformation by cold work. This difference in trend is difficult to understand since both cold working and shock loading produce similar structures although the defect content is greater after shock deformation than after cold working to equivalent values of strain, x5 The passivation potential Epp associated with lc as a function of shock pressure

572

C.R.. CROWeand S. G. FISHMAN

•

Emp

0'08 <

::k

>

LJ o o9

0.06

m (3 oA

0104

--~

12

>,

IO

E t.)

002 & I

0

2O

I

40

I

60

S hock pressure,

I

80

I

I00

k bors

F[~. 3. Primary passivation potential and critical current density correlated with shock pressure. is also shown in Fig. 3. This curve shows a decrease in Epp with shock pressure, however in this case, the trend agrees with the observations of Garz and H~ifke. The minimum passive current density, Ip, as a function of shock pressure is shown in Fig. 4. This parameter decreases with shock pressure but the effect is most pronounced in going from the annealed to the shocked state. Figure 5 shows the effect of shock deformation on the Flade potentials, Ere and ErR. It should be noted by referring to Fig. 1 that the definitions of these parameters do not follow Flade's original definition nor do they correspond to the potential at which the minimum current density is reached. Here, the terms refer to the extrapolations to zero current density of the straight line portion of the region of negative resistance. Thus Ere refers to the potential obtained when passing from the active to the passive state and ErR refers to the potential obtained upon reactivation. The important point to be obtained from this figure is, however, that the difference E~e -- ErR is increasing. This indicates that the passive film formed on the surface of the nickel becomes more stable as the shock pressure increases. DISCUSSION The works of Garz and H/ifke 5 and Latanision and Opperhauser 4 very definitely suggest that crystallography and surface defect structure play an important role in the passivation of metals. According to the chemisorbed oxygen model of passivation, the primary source of passivation is an adsorbed film which inhibits direct activated transfer of ions from the metal into the solution. This occurs presumably by blocking

The effect of shock deformation nickel in IN sulphuric acid

251

573

zo•-

:t

15

I0

:} (..)

5-

0

I

]

I

I

I

20

40

60

80

IO0

Shock

FIG. 4.

pressure,

k bars

Minimum passive current density correlated with shock pressure. 036 I-.-.._.__

\

• ~ 0.32

•

EFp, increosing V

•

EFR,decreasing V

0.2B >

°2*K (J u')

0.20 o

g o Q-

0'16

012

0"08 0

I 20

I 40 Shock

I 60

pressure,

I 80

I I00

k bors

Fio. 5. The effect of shock pressure on the passivating and reactivating Flade potentials. kink sites on the surface which in turn causes a reduction in the reaction rate. If this model is correct, the implications are that the most active sites, such as surface kinks and those areas adjacent to dislocation intersections at the surface, are the first to passivate. Also the ease with which the entire surface becomes passive should depend upon the density o f active sites.

574

C.R. CROWEand S. G. l~sm,t ~

It seems likely, therefore, that the shock pressure dependence of passivation observed in this study can be explained in terms of the defect structure caused by shock loading the Ni. In order to obtain an indication of defect density intersecting the surface of the specimens, the samples were etched to show the dislocation distribution using the etch pitting procedure developed by Akhtar and Teghtsoonian. 16 Figure 6(a) shows that etching the annealed specimens resulted in little etch pitting in the interior of the grains, indicating that few dislocations intersected the surface. In Figs. 6(b) and (c) the etched surfaces of specimens shocked at 47 kbars and 108 kbars, respectively, show increasing etch pit density with increasing shock pressure in conformity with a number of investigations showing that shock loading produces dense tangles of dislocations in the metal, is,x7 This indicates that substantially more dislocations intersect the surface of these specimens and thus it would be expected that film nucleation would occur more easily. This would have the effect of lowering all potentials associated with the passivation process, as observed. It can further be noted by comparing Figs. 2 and 3 that the average rate of increase of corrosion rate in the active region (i.e. between Epp and Ec) increases with increasing shock pressure. This is also to be expected since defects are preferential sites for corrosion on an active metal surface. CONCLUSIONS Potentiostatic polarization tests on shock deformed Ni polycrystals indicate that with increasing shock pressure passivation and Flade potentials shift in the cathodic direction, the critical current density decreases, and the passive current density decreases. These effects are consistent with the effects of substructure on the polarization parameters as implied by the adsorbed oxygen model of passivation. Acknowledgements--The authors wish to express their appreciation to Drs. W. Mock and W. H.

Holt for shock loading the specimens, to Dr. M. F. Rose for helpful discussions and to Dr. B. Z. Hollmann for translating the abstract. REFERENCES 1. G. OKAMOTOand N. SATO,Trans. Japan Inst. Metals 1, 16 (1960). 2. T. J. GROMOeOYand L. L. S ~ , Electrochim. Acta 11, 895 (1966). 3. A. K. N. REDDY,B. RAO and J. O'M. Bocgatls, d. phys. Chem. 42, 2246 (1965). 4. R. M. LATANISIONand H. OPPEm-L~,USER,Corrosion 27, 509 (1971). 5. I. GARZ and U. HArKE, Corros. Sci. 11, 329 (1971). 6. H. K.RF...~ELand N. BROWN,,J'. appL Phys. 38, 1618 (1967). 7. J. BEI~,GERand C. F^r~GUIGNON,Compendium of Shock Wave Data, Section A-1 (Ed. M. van Thiel), UCRL-50108 (Vol. 1), 39-5 (June 1966). 8. W. D. FRANCE,J. electrochem. Soc. 114, 818 (1967). 9. N. D. GREENE, First International Congress on Metallic Corrosion, pp. 113-117. Butterworths, London (1961). 10. J. R. MYERS,F. H. BECKand M. G. FOWrANA,Corrosion 21, 277 (1965). 11. K. Am~OLDand K. J. VETrER, Z. Elektroehem. 64, 407 (1960). 12. J. OSTERWALDand H. H. UHUO,J. electroehem. Soe. 108, 515 (1961). 13. Y. M. KGLO3~aX.tN,Z. Eleetroehem. 62, 664 (1958). 14. G. OKAMOTO,H. KOBAYASm,N. SATOand M. NAGAYAMA,J. electrochem. Soe. Japan 25, 199 (1957). 15. M. F. ROSE,T. BERGERand M. C. INMAN,Trans. AIME289, 1998 (1967). 16. A. AmXrARand E. TEotrrsooNXAN,J. appl. Phys. 42, 4285 (1971). 17. M. F. Rose and T. L. B ~ o ~ , Phil. Mag. 17, 1121 (1966).

[] FIG. 6.

Etch pits on samples electrolytically etched to provide an indication of defect density (200 x ). (a) annealed; (b) 47 kbars; (c) 108 kbars.