Corrosion Behavior and Passivity of Nickel-Chromium
and Cobalt-Chromium Alloys
A. Paul Bond1 and H. H. Uhlig
Corrosion Laboratory, Department of Metallurgy,
Massachusetts Institute of Technology, Cambridge, Massachusetts
ABSTRACT
P u r e Ni-Cr alloys c o n t a i n i n g up to 29% Cr and Co-Cr alloys containing up
to 23% Cr were prepared in vacuum. Corrosion rates in sulfuric and nitric
acids, corrosion potentials, and critical c u r r e n t densities for passivity were
d e t e r m i n e d at 25~ It is concluded that specific alloying proportions of passive
compositions are better evaluated i n relation to electron configuration of the
alloy system by critical c u r r e n t densities than by potential or corrosion rate
measurements.
M u c h w o r k on the c o r r o s i o n b e h a v i o r of i r o n c h r o m i u m alloys has b e e n p u b l i s h e d a n d is u s e f u l to
a n u n d e r s t a n d i n g of c o n d i t i o n s f a v o r i n g p a s s i v i t y i n
a l l o y systems. S i m i l a r d a t a for t h e c o b a l t - c h r o m i u m
s y s t e m h a v e n o t b e e n reported. F o r n i c k e l - c h r o m i u m
alloys most of t h e p r e v i o u s w o r k i n v o l v e d r a t h e r
impure metals and often ill-defined e x p e r i m e n t a l
c o n d i t i o n s . D a t a r e p o r t e d b y R o h n (1) a n d P i l l i n g
a n d A c k e r m a n (2) i n d i c a t e t h a t as Cr c o n t e n t is i n creased, c o r r o s i o n r a t e s of t h e N i - C r alloys i n o x i d i z i n g m e d i a a r e decreased. G r u b e (3) s h o w e d t h a t
i n 1N HNO~, t h e alloys b e c o m e p a s s i v e a n d t h e i r
corrosion r a t e s r e a c h a m i n i m u m at a b o u t 10% Cr.
In the present work, relatively pure Ni-Cr and
C o - C r alloys w e r e i n v e s t i g a t e d for t h e low Cr solid
s o l u t i o n r a n g e . P a r t i c u l a r a t t e n t i o n was p a i d to t h e
q u a n t i t a t i v e a l l o y i n g p r o p o r t i o n s r e q u i r e d to a c h i e v e
passivity.
Experimental Procedure
A l l o y s w e r e p r e p a r e d f r o m h i g h - p u r i t y Cr p u r chased f r o m t h e E l e c t r o M e t a l l u r g i c a l C o m p a n y ,
c a r b o n y l Ni (99.93% Ni) f u r n i s h e d b y c o u r t e s y of
the I n t e r n a t i o n a l N i c k e l C o m p a n y , a n d h i g h - p u r i t y
Co (99.97% Co) s u p p l i e d b y c o u r t e s y of t h e Cobalt
I n f o r m a t i o n C e n t e r at B a t t e l l e M e m o r i a l I n s t i t u t e .
I n d u c t i o n m e l t i n g was c a r r i e d out u n d e r a v a c u u m
of 10 -'~ m m Hg, w i t h the m e l t c o n t a i n e d i n a " M o r g a n i t e " h i g h - p u r i t y a l u m i n u m oxide crucible. No
d e o x i d i z e r s w e r e added. S a m p l e s w e r e cast i n a h e lium atmosphere by drawing the melt into 8 m m
d i a m e t e r " V y c o r " t u b e s a n d p l u n g i n g the i n g o t s into
w a t e r . T h e h e l i u m was purified b y p a s s a g e over Mg
chips at 640~ and, i n l a t e r e x p e r i m e n t s , o v e r Ti
sponge at 800~
I n b o t h cases t h e h e l i u m was
passed t h r o u g h a t r a p s u r r o u n d e d b y l i q u i d n i t r o g e n
before entering the furnace.
H o m o g e n i z a t i o n of castings was c a r r i e d o u t at
l l 0 0 ~ for 12 h r i n silica t u b e s u s i n g a h e l i u m a t mosphere followed by a water quench. The Co-Cr
alloys, w h i c h w e r e h a r d a n d r a t h e r b r i t t l e , w e r e
used i n t h e a s - h o m o g e n i z e d c o n d i t i o n , w h i l e t h e
N i - C r ingots w e r e cold r o l l e d to a t h i c k n e s s of 2.5
9 Present
address:
Diamond
Fuze Laboratory,
Washington,
D. C.
~45T
/ Illi
I."H:tl ~ \
Cal:iillQry
Fig. 1. Cell for measuring corrosion rates by weight loss
and solution analysis.
m m , g i v e n a 1 5 - m i n a n n e a l at 1000~
and water
q u e n c h e d . C o m p o s i t i o n of each alloy w a s d e t e r m i n e d
by chemical analysis.
Corrosion rate m e a s u r e m e n t s w e r e c a r r i e d out i n
a n air t h e r m o s t a t m a i n t a i n e d at 25 ~ --+0.5~ u s i n g
2000 m l cells of t h e t y p e s h o w n in Fig. 1. L a b o r a t o r y
c o m p r e s s e d air, filtered b y m e a n s of glass wool a n d
charcoal, was b u b b l e d t h r o u g h the cells at the r a t e
of 40 m l / m i n as d e t e r m i n e d b y a f l o w - m e t e r p r o v i d e d for each cell. S o l u t i o n s w e r e p r e p a r e d f r o m
laboratory distilled water and reagent grade chemicals.
S p e c i m e n s of N i - C r alloys for c o r r o s i o n r a t e d e t e r m i n a t i o n s w e r e cut as r e c t a n g u l a r c o u p o n s m e a s u r i n g a p p r o x i m a t e l y 3 c m long, 0.5-1 cm wide, a n d
0.2 c m thick. A hole 0.3-0.4 cm i n d i a m e t e r w a s
d r i l l e d n e a r one e n d so t h a t t h e s a m p l e could b e
h u n g f r o m a glass hook. T h e s e s p e c i m e n s w e r e
a b r a d e d smooth, f i n i s h i n g w i t h 600 g r i t silicon c a r bide wet polishing paper. After washing with dist i l l e d w a t e r a n d w i p i n g dry, s p e c i m e n s w e r e i m m e r s e d in b o i l i n g b e n z e n e for 5 rain. This was followed b y p i c k l i n g i n hot 6N H~SO4. P i c k l i n g w a s
c o n t i n u e d u n t i l t h e s u r f a c e w a s sufficiently etched
to r e v e a l the m i c r o s t r u c t u r e of t h e metal. A f t e r
pickling, samples were thoroughly washed with
distilled w a t e r a n d d r i e d i n a desiccator.
488
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Vol. 107, No. 6
CORROSION
BEHAVIOR
Corrosion r a t e s w e r e m e a s u r e d b y c o l o r i m e t r i c
a n a l y s i s of a l i q u o t s of t h e test s o l u t i o n i n w h i c h
specimens were immersed. By periodic sampling,
c o r r e s p o n d i n g w e i g h t loss was c a l c u l a t e d as a f u n c t i o n of time. F o r Ni the d i m e t h y l g l y o x i m e m e t h o d
(4) was used, a n d for Co t h e t h i o c y a n a t e m e t h o d
(4). C o n v e n t i o n a l g r a v i m e t r i c w e i g h t loss m e a s u r e m e n t s w e r e also m a d e a n d c h e c k e d w i t h w e i g h t
losses c a l c u l a t e d f r o m a n a l y s i s of t h e solution. A l l
r u n s w e r e c o n t i n u e d for sufficient t i m e to r e a c h a
c o n s t a n t corrosion rate.
E l e c t r o d e s for p o l a r i z a t i o n m e a s u r e m e n t s w e r e
s i m i l a r i n size a n d shape to t h e c o r r o s i o n r a t e specim e n s . N i c k e l w i r e w a s silver s o l d e r e d to a s t e m
w h i c h w a s m a c h i n e d on t h e electrode. A Teflon
g a s k e t b e t w e e n the electrode a n d a P y r e x t u b e w a s
c o m p r e s s e d b y m e a n s of a n u t a n d m a c h i n e screw
s i l v e r s o l d e r e d to the n i c k e l w i r e , i n o r d e r to e x clude the electrolyte from contact with any metal
other than the electrode proper.
Potential measurements were made using a porta b l e p o t e n t i o m e t e r i n series w i t h a n e l e c t r o n i c p H
m e t e r e m p l o y e d as a h i g h r e s i s t a n c e g a l v a n o m e t e r .
A n Ag-AgC1, 0.1N KC1 electrode w a s t h e r e f e r e n c e
electrode. C o n s t a n t p o l a r i z a t i o n c u r r e n t was s u p p l i e d b y 30 d r y cells i n series w i t h a n a d j u s t a b l e
resistance.
C r i t i c a l c u r r e n t d e n s i t i e s for p a s s i v i t y w e r e d e t e r m i n e d i n a cell fitted w i t h t w o a u x i l i a r y P t electrodes a n d a r r a n g e d so t h a t the a l l o y electrodes
could be p i c k l e d w i t h i n the cell w i t h 1:1 H=SO4,
w a s h e d , a n d t h e test e l e c t r o l y t e i n t r o d u c e d w i t h o u t
e x p o s u r e to air. Test s o l u t i o n a n d d i s t i l l e d w a t e r for
washing were contained in 5-gal carboys connected
to t h e cell b y glass t u b i n g . D e a e r a t i o n was a c c o m plished by prepurified grade nitrogen which had
b e e n passed o v e r copper t u r n i n g s at 450~
The
L u g g i n c a p i l l a r y of t h e salt b r i d g e filled w i t h t h e
test s o l u t i o n was p l a c e d a d j a c e n t to the alloy electrode.
C r i t i c a l c u r r e n t d e n s i t i e s for p a s s i v i t y w e r e m e a s u r e d b y two m e t h o d s . F o r N i - C r alloys, the " i n d i r e c t " m e t h o d w a s used. This consisted of a p p l y i n g
a series of c o n s t a n t anodic c u r r e n t s s l i g h t l y a b o v e
t h e critical c u r r e n t d e n s i t y a n d m e a s u r i n g the t i m e s
r e q u i r e d to r e a c h t h e p a s s i v e p o t e n t i a l . C u r r e n t d e n sity i w a s t h e n p l o t t e d a g a i n s t t h e r e c i p r o c a l of t i m e
t for p a s s i v i t y i n accord w i t h t h e r e l a t i o n i - io =
K/t, w h e r e io is t h e c r i t i c a l c u r r e n t d e n s i t y (5, 6).
T h i s r e l a t i o n is l i n e a r for c u r r e n t s n o t e x c e e d i n g t h e
c r i t i c a l c u r r e n t d e n s i t y b y m o r e t h a n a factor of 2
or 3, a n d c a n t h u s be e x t r a p o l a t e d to r e c i p r o c a l
t i m e ---- 0 to o b t a i n it. T h e v a l u e K r e p r e s e n t s t h e
n u m b e r of c o u l o m b s p e r u n i t a r e a r e q u i r e d to
achieve passivity.
F o r t h e C o - C r alloys, t h e " d i r e c t " m e t h o d w a s
u s e d (7). This consisted of a p p l y i n g a g i v e n c u r r e n t
a n d o b s e r v i n g t h e p o t e n t i a l . T h e lowest c u r r e n t
w h i c h p a s s i v a t e d t h e electrode w a s t a k e n as t h e
critical c u r r e n t d e n s i t y . A t c u r r e n t s b e l o w t h e c r i t i cal, p o l a r i z a t i o n w a s c o n t i n u e d u n t i l t h e p o t e n t i a l
c h a n g e d at a r a t e less t h a n 2 m v / m i n . T h e t w o
m e t h o d s give c o m p a r a b l e v a l u e s for i~, as was d e m o n s t r a t e d b y s i m i l a r m e a s u r e m e n t s for t h e F e - C r
alloys (8).
OF
Ni-Cr ALLOYS
]
489
jA
1 4 0 'La
[,
I
9
D
A
41
i
I
A
[3
120 ----
OI00
O
2" 8 0 - g
l.lt
O,OIN
(o)-
Hz S04
60
40
20-
4
8
12 16
20
Wt Per Cent Chromium
0
24
28
Fig. 2. Corrosion rotes of Ni-Cr olloys in Qeroted H.2SO~,
25~ 4-dey runs.
C o r r o s i o n p o t e n t i a l s w e r e also m e a s u r e d i n t h e
a b o v e cell. T h e electrodes w e r e p r e p a r e d i n the
s a m e w a y as s p e c i m e n s for the c o r r o s i o n r a t e d e t e r minations.
Experimental Results
NickeZ-chromium.--Corrosion r a t e s w e r e m e a s u r e d i n s e v e r a l m e d i a . I n d e a e r a t e d 1.1N ( 5 % )
H2SO~, 25~
10 d a y r u n s , c o r r o s i o n r a t e s w e r e low,
b e i n g of t h e o r d e r of 1 mdd, a n d sho.wed l i t t l e v a r i ation with chromium content.
I n Fig. 2 t h e r e s u l t s of 4 - d a y r u n s i n a e r a t e d 1.1,
0.1, a n d 0.01N H~SO4 a r e shown. It c a n be seen t h a t
t h e c o r r o s i o n r a t e s of the a c t i v e alloys a r e all t h e
s a m e w i t h i n e x p e r i m e n t a l error, t h e s t a n d a r d d e v i a t i o n b e i n g 6.3 m d d as d e t e r m i n e d f r o m replicates.
H o w e v e r , alloys c o n t a i n i n g m o r e t h a n a critical
a m o u n t of Cr show a corrosion r a t e of p r a c t i c a l l y
zero, c o r r e s p o n d i n g to the p a s s i v e c o n d i t i o n . T h e s e
p a s s i v e alloys s o m e t i m e s s h o w e d a n i n i t i a l w e i g h t
~o||f
I
I
i
/ i/ ~ I
I
i
11
--
~15.1 $8.1, 29.4
\
Per Cent
Chromlum
0
J
2
r
I
I
4
6
8
WI. Per Cenl Chromium
k
I0
12
Fig. 3. Corrosion rQtes of Ni-Cr alloys in HNO,, 25~
day runs.
l-
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JOURNAL OF THE ELECTROCHEMICAL SOCIETY
490
Ioss as d e t e r m i n e d b y c o l o r i m e t r i c a n a l y s i s , a f t e r
w h i c h a n y f u r t h e r c h a n g e w a s v e r y slight. I t s h o u l d
be n o t e d t h a t h i g h e r a c i d c o n c e n t r a t i o n s h i f t s t h e
p a s s i v i t y l i m i t to h i g h e r c h r o m i u m c o n t e n t s .
F i g u r e 3 s h o w s c o r r o s i o n r a t e s of t h e s e a l l o y s in
1.0 a n d 0.1N HNO~ as d e t e r m i n e d b y o n e - d a y runs.
Confidence l i m i t s ( 9 5 % ) w e r e c a l c u l a t e d b y s t a n d a r d s t a t i s t i c a l m e t h o d s ( 9 ) . I t c a n b e seen t h a t t h e
i n i t i a l C r a d d i t i o n s h a d l i t t l e effect on t h e c o r r o s i o n
rate, causing only a slight decrease. Further additions c a u s e d a m a r k e d i n c r e a s e , w i t h a p e a k o c c u r r i n g a r o u n d 7% Cr. T h i s t y p e of b e h a v i o r w a s also
o b s e r v e d b y R o h n (1) a n d P i l l i n g a n d A c k e r m a n
( 2 ) , w h i l e G r u b e (3) f o u n d o n l y a h o r i z o n t a l a r r e s t
b u t n o t a p e a k . T h e t r a n s i t i o n f r o m a c t i v i t y to
p a s s i v i t y is q u i t e a b r u p t , c o m i n g b e t w e e n 7.1 a n d
8.9% Cr in 0.1N HNO~, a n d b e t w e e n 8.9 a n d 11.7%
Cr in 1.0N HNO3.
C o r r o s i o n r a t e s w e r e also d e t e r m i n e d in 1N
H~SO, to w h i c h w a s a d d e d 25 g of h y d r a t e d f e r r i c
s u l f a t e p e r liter. I n t h i s case o n l y o n e r u n w a s m a d e ,
a l l s a m p l e s b e i n g p l a c e d in t h e s a m e cell. T h e d a t a
of T a b l e I s h o w t h a t this s o l u t i o n is m u c h m o r e
strongly corrosive toward the two alloys which rem a i n e d a c t i v e t h a n is 1.0N HNOs. T h e i n i t i a l m a j o r
c r i t i c a l p a s s i v e c o m p o s i t i o n is s h i f t e d to l o w e r C r
c o n t e n t , n o w l y i n g b e t w e e n 4.2 a n d 6.6% Cr. T h e s e
corrosion rates were determined by the conventional
w e i g h t loss m e t h o d only, since t h e h i g h c o n c e n t r a t i o n of i r o n in t h e s o l u t i o n i n t e r f e r e s w i t h t h e c o l o r i m e t r i c d e t e r m i n a t i o n of n i c k e l . H o w e v e r , since in
o t h e r o x i d i z i n g m e d i a s t u d i e d , t h e s e alloys, w h e n
in t h e a c t i v e c o n d i t i o n , e x h i b i t e d a c o n s t a n t c o r r o sion r a t e f r o m t h e b e g i n n i n g of t h e r u n , it s e e m s
r e a s o n a b l e t h a t t h i s w a s also t r u e in t h i s case. T h e
a c t i v e a l l o y s w e r e l e f t in t h e s o l u t i o n 6 hr, t h e
o t h e r s for 2 d a y s .
C o r r o s i o n p o t e n t i a l s of N i - C r a l l o y s in d e a e r a t e d
( H ~ - s a t u r a t e d ) 1.1N H~SO, s h o w e d no s y s t e m a t i c
v a r i a t i o n w i t h c h r o m i u m c o n t e n t and, in fact, t h e
p o t e n t i a l s of t h e s e a l l o y s w e r e n o t f a r r e m o v e d f r o m
t h e p o t e n t i a l of t h e r e v e r s i b l e h y d r o g e n e l e c t r o d e .
C o r r o s i o n p o t e n t i a l s in 0.1 a n d 1.0N HNO:., a r e s h o w n
in Fig. 4. T h e r e is a m o r e o r less c o n t i n u o u s e n n o b l i n g of t h e c o r r o s i o n p o t e n t i a l w i t h i n c r e a s i n g
chromium content with a slight peak being observed
in t h e r e g i o n of 4 - 6 % Cr.
Critical current densities for passivity were det e r m i n e d for t h e s e a l l o y s in N ~ - s a t u r a t e d 1.1N H.~SO~
b y t h e i n d i r e c t m e t h o d . T h e r e s u l t s a r e s h o w n in
Fig. 5 a n d 6. T h e s e c u r v e s h a v e s i m i l a r s h a p e , b u t
w i t h t h e a b s o l u t e v a l u e s of c r i t i c a l c u r r e n t s c o n s i d e r a b l y l a r g e r in 1.IN t h a n in 0.01N H~SO,. It a p p e a r s t h a t in b o t h cases t h e c r i t i c a l c u r r e n t d e n s i t y
d e c r e a s e s s h a r p l y as Cr is i n c r e a s e d to t h e v i c i n i t y
of 14%, a f t e r w h i c h t h e d e c r e a s e in c r i t i c a l c u r r e n t
d e n s i t y b e c o m e s m u c h less steep. C o u l o m b s p e r
s q u a r e c e n t i m e t e r for p a s s i v i t y in 1.1N H2SO, r a n g e
f r o m 0.3 for h i g h Cr a l l o y s to 2.0 f o r l o w Cr alloys.
o.3~0
:
.
2
I
J
~
~ N HN03
June 1960
[
]
0,1
5~
Z
0
--
d
-O.t
[.ON H N 0 3 ~ ' ~
Q
__
; -0.2
uJ
Noble
-0.3-- I
0
5
I0
WI.
Per
15
20
Cent Chromium
25
30
Fig. 4. Corrosion potentials of Ni-Cr alloys in HNO3, 25~
C o b a l t - c h r o m i u m . - - I n c o n t r a s t to t h e N i - C r s y s t e m , t h e C o - C r a l l o y s do n o t h a v e a s i n g l e p h a s e
s t r u c t u r e . P u r e c o b a l t i t s e l f in t h e a n n e a l e d c o n d i t i o n is a m i x t u r e of h e x a g o n a l c l o s e - p a c k e d a n d
f a c e - c e n t e r e d c u b i c p h a s e s , a n d t h i s b e h a v i o r is r e flected in t h a t of t h e alloys. T h e r e is s o m e d i s a g r e e m e n t as to t h e C o - C r p h a s e d i a g r a m (10, 11), b u t
t h e r e is a g r e e m e n t t h a t at t h e a n n e a l i n g t e m p e r a t u r e of l l O 0 ~ all a l l o y s u s e d in t h i s w o r k w o u l d be
in a s i n g l e p h a s e field 9 Also, W e v e r ( i 0 ) r e p o r t e d
that x-ray data revealed only the face-centered
cubic a n d h e x a g o n a l c l o s e - p a c k e d p h a s e s in a l l o y s
of 20 a n d 30% Cr q u e n c h e d f r o m 1000~
Since t h e
4
0
-
,
~
/
I
j
!
36 ~
~24
c
N2o
~
o
o
(6
3~2
~ a
~ 4
0
4
~
~ 0
S
12
16
20
Per Cenf Chrornlurn
.~"~
i
24
0"
28
Fig. 5. Critical current densities for passivity of Ni-Cr alloys
in N,~-saturated 1.iN H,~SO4,25~
k
1
1
I
1
[
2,0
~l,o
--
_
g
_0.5
__
Table I. Corrosion rates of Ni-Cr alloys in 1.0N H.~SO4plus 25 g
Fe=(SO4)~'9H20/liter, aerated, 25~
o
% Cr
todd
1.8
8259
4.2
6950
6.6
8.9
6
9
11.7
15.1
18.i
0
0
0
5
io
Wt
J5
Per
Cenl
20
25
Chromqum
3O
Fig. 6. Critical current densities for passivity of Ni-Cr alloys
in N2-saturated 0,01N HsSO4, 25~
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Vol. 107, No. 6
CORROSION BEHAVIOR OF Ni-Cr ALLOYS
t e m p e r a t u r e of the t r a n s f o r m a t i o n is r e l a t i v e l y low
(500~176
and the e q u i l i b r i u m t w o - p h a s e field
b e t w e e n the h i g h - and l o w - t e m p e r a t u r e modifications is n a r r o w , it a p p e a r s t h a t v e r y l i t t l e diffusion
w o u l d t a k e place d u r i n g quenching. Therefore, it is
p r o b a b l e t h a t the two phases p r e s e n t in the alloys as
tested h a d the same chemical composition.
K r S h n k e and Masing (12) studied the corrosion
p o t e n t i a l and anodic a n d cathodic p o l a r i z a t i o n b e h a v i o r of f a c e - c e n t e r e d cubic and h e x a g o n a l closep a c k e d Co in 0.1 and 2.0N HC1. They found the
two c r y s t a l l i n e modifications to b e h a v e identically.
One of us also found t h a t b o d y - c e n t e r e d and facec e n t e r e d cubic 18-8 stainless steels corrode at essent i a l l y the same rates in several m e d i a (13). F r o m
this it a p p e a r s r e a s o n a b l e to conclude t h a t the b e h a v i o r of the Co-Cr alloys w a s not g r e a t l y affected
by the presence of two different c r y s t a l l i n e forms.
M e t a l l o g r a p h i c e x a m i n a t i o n showed t h a t the m i c r o s t r u c t u r e s of all these alloys are s i m i l a r and t h a t the
observed changes in e l e c t r o c h e m i c a l b e h a v i o r do not
c o r r e s p o n d to any change in m i c r o s t r u c t u r e .
Corrosion rates of the C o - C r alloys in 0.1 and 1.0N
HNO~ a r e shown in Fig. 7. Time of test r a n g e d from
hours to days d e p e n d i n g on w h e t h e r the corrosion
r a t e was high or low. U n i f o r m corrosion was obs e r v e d in all cases. P u r e Co has a corrosion r a t e
about 10 times h i g h e r t h a n p u r e Ni, and the active
alloys also show h i g h e r rates t h a n do t h e N i - C r a l loys. The shapes of the corrosion r a t e - c o m p o s i t i o n
curves also differ for the two alloy systems in t h a t
the i n i t i a l Cr a d d i t i o n causes a m a r k e d decrease
in corrosion of the Co alloys and no m a x i m u m is
found. There is, however, a d i s c o n t i n u i t y in slope
in t h e region from 2 to 4% Cr, s i m i l a r to the more
p r o n o u n c e d change in t h e N i - C r system. The onset
of p a s s i v i t y in HNO~ begins at about 12% Cr, since
alloys containing m o r e Cr corroded at a r a t e of 0
todd in 0.1 N HNO~ and 0 to 7 m d d in 1.0N HNO.~.
Corrosion p o t e n t i a l s of these alloys in 1.0N HNO~
a r e shown in Fig. 8. A p r o n o u n c e d shift t o w a r d
l
I
J
k
0.2
,•-•$•
I
491
P
I
r
_
1
9
o>
z
0,1
@
@
o
-0, I --
N~
I
5
Wl,. Per
I
I
I0
I5
20
Cent Chromium
Fig. 8. Corrosion potentials of Co-Cr alloys in 1 .ON HNOs,
25 ~
m o r e noble p o t e n t i a l s is o b s e r v e d as Cr content is
increased, b u t it is not possible on t h e basis of these
d a t a alone to d r a w a n y conclusions r e g a r d i n g c r i t i cal composition limits for p a s s i v i t y .
F i g u r e 9 is a plot of critical c u r r e n t densities
w i t h composition, o b t a i n e d b y the d i r e c t method.
P u r e Co, as w e l l as alloys containing up to 6.2% Cr,
h a v e v e r y high critical currents, w h i l e a s h a r p drop
occurs in t h e v i c i n i t y of 8% Cr, followed b y a l e v e l ling off again. These d a t a give a clear i n d i c a t i o n of
a critical composition in the v i c i n i t y of 8% Cr, as
well as indicating t h a t for alloys of this or h i g h e r
Cr content t h e critical c u r r e n t d e n s i t y ( ~ 1 m a / c m ~)
is low enough for s e l f - p a s s i v a t i o n of t h e alloys to be
possible in s t r o n g l y oxidizing media.
Discussion
Nickel-chromium
alloys.--In d e a e r a t e d 1.1N
H2SO~ t h e corrosion of Ni a n d N i - C r alloys a p p e a r s
I
I
E
[
-
I
6
xlO 4
I0(
~E
5
D
5
E
1,0
O, I N
N
alO
HNO 3
HN03
9
Z
~
-
=
- ~ _ _ ~
I0
5
Wt
b
15
Per Cent Chromium
ol
20
c~
Fig. 7. Corrosion rates of Co-Cr alloys in HNO~, 25~
4
S
WI
12
Per Cent
16
20
24
Chromium
Fig. 9. Critical current densities for passivity of Co-Cr alloys in N~-saturoted 1.0N H~SO~, 25~
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492
JOURNAL
OF THE ELECTROCHEMICAL
to be l a r g e l y u n d e r a n o d i c control. This is suggested
b y t h e fact t h a t the corrosion p o t e n t i a l s a r e n o t far
r e m o v e d f r o m p o t e n t i a l s of t h e r e v e r s i b l e h y d r o g e n
electrode. T h i s m e a n s t h a t t h e local cathodes a r e
o n l y s l i g h t l y polarized.
O n t h e o t h e r h a n d , i n a e r a t e d s u l f u r i c acid t h e
c o r r o s i o n r a t e s of t h e active alloys a p p e a r to be
c o n t r o l l e d b y diffusion of dissolved o x y g e n to the
m e t a l surface. This is i n d i c a t e d b y t h e fact t h a t the
c o r r o s i o n r a t e s of the active s p e c i m e n s are e s s e n t i a l l y i n d e p e n d e n t b o t h of alloy c o m p o s i t i o n a n d
acid c o n c e n t r a t i o n . It is also i n d i c a t e d b y c o r r e s p o n d e n c e of t h e c o r r o s i o n r a t e (140 todd = 5.3 x
10 -~ a m p / c m ~) to the l i m i t i n g diffusion c u r r e n t d e n sity for o x y g e n , i.e., i n accord w i t h t h e e q u a t i o n i =
(DnFc/~), w h e r e D - = 2 x l 0 ~ cmVsec, n = 4, F =
96,500 c o u l o m b s / e q u i v . , c = 2.5 x 10 -~ m o l e s dissolved O J c c , a n d ~ the t h i c k n e s s of t h e s t a g n a n t
diffusion l a y e r is a s s u m e d e q u a l to 0.035 cm. The
l a t t e r v a l u e of 8 is c o n s i s t e n t w i t h o b s e r v e d values.
T h a t the corrosion r a t e s of the active alloys should
be a n o d i c a l l y c o n t r o l l e d i n d e a e r a t e d acid a n d cat h o d i c a l l y c o n t r o l l e d i n a e r a t e d acid is i n n o w a y
s e l f - c o n t r a d i c t o r y . I n d e a e r a t e d acid, t h e cathodic
process is r e d u c t i o n of h y d r o g e n ions to gaseous h y d r o g e n , w h i l e in a e r a t e d acid the cathodic r e a c t i o n
i n v o l v e s r e d u c t i o n of dissolved o x y g e n b y h y d r o g e n
ions to f o r m water. T h e s t a n d a r d free e n e r g y c h a n g e
for t h e l a t t e r r e a c t i o n is m u c h g r e a t e r t h a n t h a t of
the f o r m e r , so t h a t in a e r a t e d s o l u t i o n t h e difference
b e t w e e n t h e open c i r c u i t a n o d e a n d c a t h o d e p o t e n tials is m u c h g r e a t e r t h a n i n d e a e r a t e d solution.
T h u s a v e r y l a r g e c u r r e n t w o u l d b e n e c e s s a r y to
p o l a r i z e t h e local a n o d e s to t h e o p e n c i r c u i t cathode
p o t e n t i a l s , a n d long b e f o r e this h a p p e n s the l i m i t i n g
diffusion c u r r e n t for o x y g e n is r e a c h e d a n d b e c o m e s
t h e r a t e - d e t e r m i n i n g step.
I n all t h e o x i d i z i n g m e d i a used i n this w o r k t h e
Ni-Cr alloys e x h i b i t e d a critical composition, t h a t
is, alloys c o n t a i n i n g less t h a n t h e c r i t i c a l a m o u n t of
c h r o m i u m for p a s s i v i t y corroded r e l a t i v e l y r a p i d l y
c o m p a r e d to those c o n t a i n i n g m o r e t h a n t h e critical
p e r c e n t . T h e t e r m passive, as used in this discussion,
is l i m i t e d to the case of a m e t a l w h i c h is p o l a r i z e d
to a p o t e n t i a l m o r e n o b l e t h a n its F l a d e p o t e n t i a l
i n t h e g i v e n e n v i r o n m e n t a n d has a n a c c o m p a n y i n g
c o m p a r a t i v e l y low corrosion rate. I n t h e case of
those alloys w h i c h are p a s s i v e u n d e r a g i v e n set of
conditions, it can b e a s s u m e d f r o m p r e v i o u s d i s c u s sions (7, 14-16) t h a t t h e local anodic c u r r e n t p r o d u c e d b y corrosion of t h e a c t i v e a l l o y e x c e e d e d t h e
critical c u r r e n t d e n s i t y for p a s s i v i t y . I n this w a y
t h e low corrosion r a t e of t h e p a s s i v e state follows
in s e q u e n c e the h i g h e r corrosion r a t e of t h e active
state.
It a p p e a r s to be a g e n e r a l r u l e t h a t w h e n e v e r
the c u r r e n t d e n s i t y e q u i v a l e n t to t h e corrosion r a t e
a p p r o a c h e s the critical c u r r e n t d e n s i t y p a s s i v i t y is
e s t a b l i s h e d . T h e p r e s e n t d a t a b e a r out this r e l a tionship. T h e corrosion c u r r e n t d e n s i t i e s a r e a l w a y s
less t h a n t h e critical c u r r e n t d e n s i t i e s to a n e x t e n t
d i c t a t e d b y t h e a n o d e - c a t h o d e a r e a ratio, a low ratio
a c c o u n t i n g for a g r e a t e r o b s e r v e d difference b e t w e e n t h e t w o v a l u e s t h a n a h i g h ratio.
SOCIETY
June 1960
Based on t h e fact t h a t the p a s s i v e film on Fe, Cr,
a n d C r - F e alloys is e q u i v a l e n t to a b o u t 0.01 coul o m b / c m " (8) or less, t h e v a l u e of 0.3 to 2 c o u l o m b /
c m ~ for N i - C r alloys i n d i c a t e s t h a t a n o d i c d i s s o l u t i o n of the a l l o y precedes p a s s i v i t y . T h e s a m e is t r u e
of i r o n the v a l u e for w h i c h is a b o u t 1 coulomb/cm ~,
a n d as F r a n c k (6) showed, a n i n s u l a t i n g film of
p e r h a p s FeSO, f o r m s first, f a v o r i n g high c u r r e n t
d e n s i t i e s w i t h i n pores of this film n e c e s s a r y to
a c h i e v e the p a s s i v e state. This s i t u a t i o n is also b o r n e
out b y a r e l a t i v e l y low i n i t i a l slope of p o t e n t i a l vs.
t i m e at c o n s t a n t a p p l i e d c u r r e n t d e n s i t y for N i - C r
alloys p r e c e d i n g a r a p i d fall of p o t e n t i a l to the
n o b l e p a s s i v e v a l u e , w h e r e a s for s t a i n l e s s steels
w h i c h p a s s i v a t e w i t h o u t i n i t i a l salt film f o r m a t i o n ,
the p o t e n t i a l fall is i m m e d i a t e , a n d c o u l o m b s for
p a s s i v i t y are a d i r e c t m e a s u r e of the a m o u n t of
p a s s i v e film s u b s t a n c e on the surface.
T h e r a t h e r w i d e v a r i a t i o n i n t h e a m o u n t of Cr
n e e d e d to p a s s i v a t e these alloys i n different m e d i a is
a r e s u l t of t h e d i f f e r e n t corrosion rates, a n o d e - c a t h ode area ratios, a n d critical c u r r e n t d e n s i t i e s w h i c h
p r e v a i l in t h e s e media. I n t h e case of a e r a t e d s u l f u r i c acid, o n l y t h e critical c u r r e n t s a n d n o t t h e corrosion r a t e s of t h e a c t i v e alloys w e r e r e d u c e d as the
acid c o n c e n t r a t i o n was reduced. Decreases i n acid
c o n c e n t r a t i o n c o r r e s p o n d i n g l y l o w e r e d t h e critical
a m o u n t of Cr r e q u i r e d for p a s s i v i t y b e c a u s e c r i t i c a l
c u r r e n t d e n s i t y decreases w i t h i n c r e a s i n g Cr. Since
t h e critical Cr c o m p o s i t i o n s fall in a r e g i o n w h e r e
critical c u r r e n t d e n s i t y is c h a n g e d o n l y s l o w l y w i t h
Cr c o n t e n t , s m a l l c h a n g e s i n t h e critical c u r r e n t
d e n s i t y for p a s s i v i t y b y m e a n s of d e c r e a s i n g acid
c o n c e n t r a t i o n s h i f t e d the critical Cr c o n t e n t m a r k edly.
I n n i t r i c acid, corrosion r a t e s as well as critical
c u r r e n t s w e r e raised as acid c o n c e n t r a t i o n was i n creased. T h e i n c r e a s e in corrosion r a t e is p r o b a b l y
d u e to i n c r e a s e in c o n c e n t r a t i o n of n i t r a t e ion, w h i c h
is r e d u c e d at t h e local cathodes, w h i l e i n c r e a s e in
h y d r o g e n ion c o n c e n t r a t i o n i n c r e a s e s o b s e r v e d c r i t i cal c u r r e n t densities. T h e o b s e r v e d i n c r e a s e i n Cr
n e c e s s a r y to p r o d u c e p a s s i v i t y i n these alloys on
i n c r e a s i n g acid c o n c e n t r a t i o n i n d i c a t e s t h a t the
h i g h e r critical c u r r e n t d e n s i t y b r o u g h t a b o u t b y i n creased acid c o n c e n t r a t i o n o u t w e i g h s the effect of
i n c r e a s e d corrosion c u r r e n t . B y a d d i n g f e r r i c ions
to 1N H~SO4, a corrosion r a t e h i g h e r t h a n t h a t in
1N HNO3 w a s o b t a i n e d at a p p r o x i m a t e l y the s a m e
h y d r o g e n ion c o n c e n t r a t i o n , so t h a t the critical
a m o u n t of Cr w a s decreased f r o m a b o u t 12 to 6.6%
C r as is seen f r o m d a t a of T a b l e I a n d Fig. 3.
I n r e t r o s p e c t , all t h e d a t a show t h a t p a s s i v i t y
l i m i t s for t h e s e alloys as o b t a i n e d f r o m corrosion
d a t a reflect n o t o n l y the basic p r o p e r t i e s of the alloy
system, b u t also d e p e n d s t r o n g l y on the e n v i r o n m e n t . I m p o r t a n t a r e t h e n a t u r e of t h e a n i o n , d e p o l a r i z e r concentration, s u r f a c e pH, d e g r e e of s t i r ring, etc. This is e s p e c i a l l y t r u e of a s y s t e m such as
N i - C r , b e c a u s e critical c u r r e n t d e n s i t i e s do n o t v a r y
as s h a r p l y w i t h c o m p o s i t i o n as t h e y do i n %he C r - F e
alloys as s h o w n , for e x a m p l e , b y d a t a of K i n g a n d
U h l i g (8).
T h e v a r i a t i o n of critical c u r r e n t d e n s i t y w i t h alloy
c o m p o s i t i o n a p p e a r s to be t h e i m p o r t a n t f u n d a -
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Vol. 107, No. 6
CORROSION
BEHAVIOR
m e n t a l p r o p e r t y of a l l o y s c a p a b l e of b e c o m i n g
passive. T h e s h a p e of t h e c r i t i c a l c u r r e n t - c o m p o s i tion c u r v e is f o u n d to be n e a r l y t h e s a m e in 1.1 a n d
0.01N H~SO4 (Fig. 5 a n d 6) e v e n t h o u g h t h e c r i t i c a l
c u r r e n t s a r e m u c h s m a l l e r for t h e l o w e r a c i d c o n c e n t r a t i o n . I n b o t h cases a c h a n g e in s l o p e o c c u r s or
a p r a c t i c a l m i n i m u m is r e a c h e d in t h e v i c i n i t y of
14% Cr. This is t h e figure w h i c h it w o u l d p r o b a b l y
be m o s t p r o f i t a b l e to c o m p a r e w i t h t h e p r e d i c t i o n of
a c r i t i c a l c o m p o s i t i o n at 8.2% Cr m a d e b y one of us
(17) b a s e d on e l e c t r o n c o n f i g u r a t i o n in t h e a l l o y
system. Other than the simplified assumptions made
in t h e t h e o r e t i c a l c a l c u l a t i o n , d i f f e r e n c e b e t w e e n
t h e o r y a n d o b s e r v a t i o n m a y also be a r e s u l t of e x p e r i m e n t a l f a c t o r s , s u c h as a p o s s i b l e d i f f e r e n c e b e t w e e n s u r f a c e c o m p o s i t i o n f r o m b u l k c o m p o s i t i o n of
t h e a l l o y s c a u s e d b y p r e f e r e n t i a l c o r r o s i o n of one of
the alloy components.
Cobalt-chromium alloys.--The b e h a v i o r of t h e
C o - C r s y s t e m w a s s i m i l a r to t h e N i - C r s y s t e m , as
m i g h t b e e x p e c t e d . D i f f e r e n c e s in s h a p e of t h e c o r r o s i o n - c o m p o s i t i o n c u r v e s in n i t r i c a c i d i n d i c a t e t h a t
Cr a l l o y e d w i t h Co, e v e n in s m a l l a m o u n t s , d e c r e a s e s t h e c o r r o s i o n r a t e (Fig. 7), w h i l e in t h e
N i - C r s y s t e m , Cr m a r k e d l y i n c r e a s e s t h e c o r r o s i o n
r a t e j u s t s h o r t of t h e p a s s i v e c o m p o s i t i o n ( F i g . 3).
This m a y b e r e l a t e d to t h e p o s s i b i l i t y t h a t Co is a
b e t t e r c a t a l y s t t h a n Ni for r e d u c t i o n of NO~-, w h i c h
in t u r n is c o n s i s t e n t w i t h a m u c h h i g h e r c o r r o s i o n
r a t e of p u r e Co t h a n p u r e N i in HNO~. T h i s b e i n g t h e
case, if C r is an i n t e r m e d i a t e c a t a l y s t , a n a d d i t i o n
of C r to Co w i l l i n c r e a s e c a t h o d i c p o l a r i z a t i o n of
C o - C r a l l o y s w h i l e d e c r e a s i n g it for N i - C r alloys.
T h e effect of Cr on t h e c r i t i c a l c u r r e n t d e n s i t i e s of
t h e C o - C r a l l o y s (Fig. 9) w a s m u c h m o r e p r o n o u n c e d t h a n in t h e N i - C r s y s t e m (Fig. 5, 6). I n t h i s
r e s p e c t , b e h a v i o r of C o - C r a l l o y s is s i m i l a r to t h e
b e h a v i o r r e p o r t e d for F e - C r a l l o y s ( 8 ) . T h e c r i t i c a l
c u r r e n t d e n s i t i e s f o r t h e Co a l l o y s c o n t a i n i n g m o r e
t h a n 10% C r w e r e a b o u t t h e s a m e or s o m e w h a t
493
OF Ni-Cr ALLOYS
l o w e r t h a n t h o s e of t h e c o r r e s p o n d i n g N i - C r a l l o y s ,
b u t t h e l o w C o - C r a l l o y s a n d p u r e Co h a d m u c h
h i g h e r c r i t i c a l c u r r e n t d e n s i t i e s . F o r this r e a s o n , t h e
c r i t i c a l c o m p o s i t i o n is c l e a r l y i n d i c a t e d as close to
8% Cr. This is in r e a s o n a b l e a g r e e m e n t w i t h t h e
6.2% Cr p r e d i c t e d b y t h e e l e c t r o n c o n f i g u r a t i o n
t h e o r y (17).
Acknowledgment
The authors are grateful
search by the Office of Naval
for support
Research.
of this re-
Manuscript received Dec. 3, 1959. This paper was
prepared for delivery before the Houston Meeting, Oct.
9-13, 1960.
Any discussion of this paper will appear in a Discussion Section to be published in the December
1960
JOURNAL.
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Dissolution of Nickel in Acid Ferric and Ceric Solutions
Phoebus M. Christopher and Cecil V. King
Department of Chemistry, New York University, New York, New York
ABSTRACT
Dissolution rates of nickel cylinders rotated in acidified solutions of FeCl,
and Ce (SO4)2 have been measured. The rates are transport-controlled, but the
Ni surface becomes extremely rough, and first-order constants are obtained
only if a uniform degree of roughness is maintained during runs. The values
of the rate constants then depend on the structure of the metal, e.g., whether
it has been annealed, mechanically worked, etc.
The rate of dissolution of nickel in ferric alum
solutions was measured by V a n N a m e and his coworkers (i), w h o found it to be approximately the
same as that of several other metals in the same reagent. The authors regarded the dissolution to be
controlled by diffusion, or by convective-diffusive
transport, of ferric ion to the metal surface~
The present study shows that a polished nickel
surface becomes
very rough in acidified ferric and
ceric solutions, more so in the former than in the
latter. This is at least partly due to preferential dissolution at grain boundaries
and at flaws in the
mechanically
worked
metal. It indicates that, while
the cathodic reaction (reduction of ferric or ceric
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