The Corrosion of Steel by Aqueous Solutions
of Hydrogen Sulfide
D. R. Morris,* L. P. Sompaleanu, 1 and D. N. Veysey 2
D e p a r t m e n t of Chemical Engineering, University ol N e w B r u n s w i c k , Fredericton, N e w B r u n s w i c k , Canada
ABSTRACT
Steel corrosion has been investigated through polarization studies in aqueous H2S systems of acid pH using a rotating disk electrode cell. H2S does not
change the Tafel slopes of the anodic and cathodic processes. The anodic
curves are shifted toward more negative potentials m a i n l y due to the decrease of the reversible potential of iron, while the exchange c u r r e n t density
appears to r e m a i n unchanged. The cathodic process m a i n t a i n s the reversible
potential and the exchange c u r r e n t density of the H2S free system, but the
H+ diffusion control gradually disappears. A corrosion c u r r e n t density dependence on the H2S concentration is found which matches that obtained from
published weight-loss experiments. The product of corrosion, m a c k i n a w i t e
is essentially n o n a d h e r e n t and in certain circumstances enhances the corrosion rate. A n e w method for the compensation of ohmic overpotential in
polarization m e a s u r e m e n t s is described.
The corrosion of steel b y aqueous H2S is a significant
technical problem i n two m a j o r i n d u s t r i a l areas. I n oil
refineries and n a t u r a l gas t r e a t m e n t facilities, the
process conditions v a r y widely due to n a t u r a l causes
(1-4). Replication of the conditions for laboratory work
is difficult. The G i r d l e r - S u l p h i d e (GS) process for the
production of heavy water involves the use of H2S as
the d e u t e r i u m exchange agent. The process prescribes
the basic p r e s s u r e / t e m p e r a t u r e conditions, hence laboratory replication is simpler.
This paper presents the results of studies of the corrosion of carbon steel by deaerated aqueous solutions
of H2S at 25~176 using the potentiostatic polarization
method and u n d e r long term (up to 1000 hr) dynamic
exposure conditions.
Theory
According to the electrochemical theory of corrosion, the corrosion process takes place at a mixed potential, Ecorr b e t w e e n the reversible potentials, ErevFe
for the anodic process and ErevH for the cathodic process, in the absence of oxygen.
T h e reversible potential, ErevFe for the Fe2+/Fe electrode is given generally by (5)
ErevFe ~-~ -- 0.44 + 0.030 log l e e 2+ ]
[1]
For the p a r t i c u l a r case involving aqueous H2S solutions, the concentration of ferrous ions, [Fe 2+] is related to the first and second ionization constants of H2S,
K1 and K=, the solubility product of FeS, Ks, and the
H e n r y law constant, k, for the H2S solution. Inserting
n u m e r i c a l values of K1, K2, and Ks equal to 9.1 • 10 -s,
1.1 • 10 -12, and 3.7 • 10 -19 at 18~ respectively (6),
and k equal to 8.3 a t m (mole liter - 1 ) - 1 (7) at 18~
gives
ErevFe ~- -- 0.39 -- 0.06 pH -- 0.03 log P~2s
[2]
where PH2S is the partial pressure of H2S.
According to Hilbert et al. (8), iron with a high density of crystal imperfections dissolves by a catalyzed
mechanism (CM) whereas iron with low surface activity dissolves by a noncatalyzed mechanism (NCM).
The dissolution reaction is activation controlled with a
Tafel slope b a ~_ 0.04 V/decade for the NCM (8-10)
and b a ~_ 0.03 V/decade for the CM (11).
The influence of the ferrous ion concentration on the
reaction rate is disputed; Bockris and Reddy (12) claim
a n electrochemical reaction order, PFe2+ a
* Electrochemical Society Active Member.
1 Presen t address: Ontario Hydro, Toronto, Ontario, Canada.
2 Presen t address: Boise Cascade Limited, Newcastle, New
Brunswick, Canada.
Key words: corrosion film, H2S-H20 system, mackinawite scale.
PFe2+a ----
0 log ~
)
-- 1
d log [Fe e+ ] E,pH
[3]
where i is the c u r r e n t density. Hilbert et al. (8) state
that all authors agree that the reaction rate shows no
dependence on [Fe2+].
The electrochemical reaction order related to pH,
ppH a ~-- (~ log i/~ PH)E,Fe2+ is agreed to be (8, 12),
ppH a -- 1 • 0.1 (NCM) and pp~a = 2 +__ 0.3 (CM).
The exchange c u r r e n t densities, io A - c m -2 for the
F e / F e 2+ electrode at pH : 0 and [Fe 2+] _ 1 are reported as (8), log io -- --8.11 to --8.51 (NCM) and log
io = -10.28 to --12.7 (CM).
The influence of anions on the process of iron dissolution is not well understood. Florianovich et al. (13)
conclude that anions i n addition to O H - ions play a
direct part in the anodic reaction; an effect consistent
with a model in which specific adsorption of the anion
changes the surface area for the F e / F e 2+ exchange
has also been advanced (9, 10).
The reversible potential, ErevH for the H + / H electrode is given b y (5)
ErevH -~ -- 0.06 pH -- 0.03 log PH2
[4]
The reaction proceeds in two stages, according to the
generally accepted V o l m e r - H e y r o v s k y m e c h a n i s m (14)
H + + e- = Hads
H + -~ Hads + e -
~- H2
(Volmer)
(Heyrovsky)
[5]
[6]
The cathodic reaction of h y d r o g e n exhibits a Tafel
slope b c = 0.118 V/decade (12) which m a y v a r y
slightly depending on the degree of H2 coverage of the
electrode. The b u l k of reported Tafel slopes is b e t w e e n
0.110 and 0.120 V/decade. According to Bockris et al.
(9), the exchange c u r r e n t density io is pH dependent,
0 log io/O pH = --0.5; at p H = 3, io : 1.6 • 10 -6
A-cm-L
The total a m o u n t of h y d r o g e n produced b y the cathodic reaction is e q u i v a l e n t to the metal weight loss and
m a y be used as a measure of the overall corrosion
rate. However, in certain circumstances, dangerous corrosion effects m a y occur with little weight loss due to
hydrogen e m b r i t t l e m e n t of the metal. This situation
arises w h e n the Heyrovsky m e c h a n i s m is hindered,
leading to the diffusion of H atoms into the metal and
the p h e n o m e n o n of hydrogen e m b r i t t l e m e n t . H2S is
k n o w n to promote this p h e n o m e n o n but a mechanism
for its action has not been generally accepted. Technically, a solution to the p r o b l e m has b e e n found
1228
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VoL 127, No. 6
C O R R O S I O N OF S T E E L
t h r o u g h the specification of the m a x i m u m
stress a n d / o r hardness of the steel used.
applied
Experimental
Polarization cells.NPolarization m e a s u r e m e n t s about
the corrosion p o t e n t i a l w e r e conducted w i t h two d e signs of cell, the A S T M s t a n d a r d cell (15) and a r o t a t i n g disk electrode cell (RDE). The design of the
w o r k i n g electrodes used in the two cells is shown in
Fig. 1. The a u x i l i a r y e l e c t r o d e for the A S T M cell was
located on a chord p l a n e in front of the w o r k i n g elect r o d e to i m p r o v e the s p a t i a l s y m m e t r y of the electrical
field. This a r r a n g e m e n t gave good r e p r o d u c i b i l i t y of the
e x p e r i m e n t a l results.
T h e d e t a i l e d design of the RDE cell is described
e l s e w h e r e (16). It was f a b r i c a t e d such t h a t the only
m a t e r i a l s in contact w i t h the electrolyte, aside from
t h e electrodes, w e r e glass and Teflon. Electrical contact to the r o t a t i n g shaft was m a d e v i a copper and
mercury.
The steel specimens used as the w o r k i n g electrodes
w e r e m a c h i n e d f r o m rod stock of 1020 carbon steel
and sleeved as shown in Fig. 1. The surfaces w e r e p r e p a r e d according to s t a n d a r d p r o c e d u r e s (15). A u x i l i a r y
electrodes w e r e p l a t i n i z e d p l a t i n u m (17). As the elect r o l y t e was s o d i u m chloride based, no t r e a t m e n t was
a p p l i e d to r e m o v e occluded chloride.
A KC1 s a t u r a t e d calomel r e f e r e n c e electrode was
used for all m e a s u r e m e n t s ; Eref : -{-241.5 mV SHE. The
r e f e r e n c e electrode was b r i d g e d to the L u g g i n c a p i l l a r y
w i t h cell e l e c t r o l y t e via a b e a k e r containing 1M NaC1
solution.
The e l e c t r o l y t e solutions w e r e p r e p a r e d f r o m r e a g e n t
g r a d e chemicals and twice distilled water. The solutions
w e r e NaCl based, b l e n d e d w i t h HC1 for p H control
w i t h a t o t a l c h l o r i d e c o n c e n t r a t i o n of 0.2M. The b u f f e r i n g agent used was acetic a c i d - s o d i u m acetate. Before use the solutions w e r e d e o x y g e n a t e d with b u b b l i n g n i t r o g e n for at least 30 min, followed b y HeS
/s
1229
b u b b l i n g for a n o t h e r 30 m i n w h e n required. The cells
w e r e p u r g e d w i t h n i t r o g e n before the electrolyte was
siphoned in and once wet, the electrode did not contact
oxygen. The composition of the H2S/N2 gas m i x t u r e
was controlled b y the use of c a l i b r a t e d rotameters.
A u x i l i a r y e q u i p m e n t comprised a W e n k i n g p o t e n t i o stat, a K e i t h l e y electrometer, and H o n e y w e l l digital
m u l t i m e t e r a n d a t h e r m o s t a t e d w a t e r b a t h in w h i c h
the polarization cells w e r e immersed. A d d i t i o n a l l y for
the RDE cell a speed control unit a n d a stroboscope
w e r e used.
The c u r r e n t - v o l t a g e curves w e r e o b t a i n e d using a
h a n d - s w i t c h e d potentiostatic p o t e n t i a l step e v e r y five
to ten minutes, to give at least five e x p e r i m e n t a l points
p e r c u r r e n t decade. The c u r r e n t densities o b s e r v e d
w e r e essentially stable. The b u l k of the e x p e r i m e n t s
were conducted w i t h the RDE cell at 200 rpm.
Results f r o m initial e x p e r i m e n t s w e r e o b s c u r e d b y
significant ohmic overpotentials. Compensation for the
ohmic o v e r p o t e n t i a l is u s u a l l y effected w i t h the aid of
a positive f e e d b a c k circuit (18) (FBC) in conjunction
w i t h a cell substitute circuit r e p r e s e n t i n g the e q u i v a lent of the cell circuit, including a p o l a r i z a t i o n voltage
and two resistors to satisfy the r e l a t i o n
Rt/R~ -- Rc/Ra
[7]
w h e r e Re and Rn a r e the resistors in the substitute
circuit; Rf and Rg a r e compensating resistors in the
FBC. The essential difficulty is the d e t e r m i n a t i o n of
the ratio Re/Ra. A n e l a b o r a t e m e t h o d using a r a p i d
response voltmeter, an oscilloscope, and a fast s w i t c h ing device to create transients s h o r t e r t h a n 1 msec is
r e c o m m e n d e d (18). To o v e r c o m e these difficulties an
a l t e r n a t i v e m e t h o d was devised (see A p p e n d i x ) . This
m e t h o d is fast, accurate, a n d p r o v i d e d t h a t t h e position
of the L u g g i n c a p i l l a r y m a y be controlled b y a screw
movement, does not r e q u i r e a n y e x t r a equipment. In
the p r e s e n t work, the cell did not h a v e this provision
and the c a p i l l a r y was a d j u s t e d m a n u a l l y t h r o u g h the
cell top passage. The exact position was followed with
a crossline reticle finder w i t h m i c r o m e t e r control.
Flow loop.~A flow d i a g r a m of the corrosion test
loop is shown in Fig. 2. It consisted e s s e n t i a l l y of s t a n d a r d 1 in. (2.54 cm) d i a m glass p i p e l i n e in the f o r m
ROD
PTFE S LEEV[
I
I1
~
,
3
8
E LECTRODE
f
\
9
()
Fig. I. Sectional view of working electrodes. Left: ASTM cell,
right: RDE cell.
Fig. 2. Diagram of flow loop. Legend: I. Gas outlet, 2. Header
tank, 3. Gas inlet, 4. Pump, 5. Filter, 6. Heat exchanger, 7. Location of electrodes, 8. Test sections, 9. Rotameters.
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1230
J. Electrochem. Soc.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y
of a closed circuit with two parallel test sections, a
stainless steel header t a n k fitted with the (stainless
steel) cooling coil of a refrigerator u n i t (Blue M Electric C o m p a n y Model PCC 24SSA3), and a p u m p
(Crane D y n a p u m p Model 880E) incorporating a b y pass. A third parallel line incorporated a Q.V.F. heat
exchanger and a sieve plate filter to trap particles
of corrosion product. Calibrated rotameters were i n stalled u p s t r e a m of each test section; the solution flow
rate to each test section was controlled manually. Gas
connections were fitted as shown in order that the distilled water could be deoxygenated initially with n i t r o gen and s u b s e q u e n t l y saturated with H2S gas. The gas
outlet line included a trapping a r r a n g e m e n t to p r e v e n t
ingress of air. Deoxygenation of the distilled w a t e r was
achieved in the loop by p u r g i n g w i t h nitrogen over a
2 hr period; this reduced the oxygen concentration to
0.3 p p m or less. H2S gas at 1 a t m was t h e n introduced
and the solution circulated for at least 4 h r prior to
insertion of the test specimens. A continuous flow of
H2S gas was m a i n t a i n e d during the e x p e r i m e n t a l work.
The test loop was m o u n t e d on plywood a n d situated
i n a p o l y e t h y l e n e enclosed cage i n front of a w a l k - i n
fume hood. A n c i l l a r y equipment, installed to m o n i t o r
the solution properties and the corrosion potential of
the test specimens, comprised a glass pH electrode
(Fisher Scientific Model E-12), a sulfide ion electrode
(Orion Research, Model 94-16), a n d a double j u n c t i o n
reference electrode DJE (Orion Research, Model 9002). O u t p u t signals were recorded via a digital pH
meter (Orion Research, Model 701) using a strip chart
recorder (Yokagawa Company, Model LER 12A).
The pH electrode i n conjunction with the DJE was
calibrated periodically using s t a n d a r d buffer solutions
of pH 4.0, 7.0, and 11.0 at 25~ The sulfide ion electrode
in conjunction with the DJE was calibrated at 25~
using s t a n d a r d solutions of NaOH saturated with H2S
at 1 atm. The slope OEs/O log [S2-], w h e r e Es is the
potential of the sulfide ion electrode and [S 2-] is the
sulfide ion concentration, was m e a s u r e d as --26.7 m V /
decade in satisfactory agreement with the calculated
value --29.6 mV/decade.
The corrosion potential was m e a s u r e d in conjunction
with the DJE by adapting the assembly holder such
that electrical contact with a corrosion specimen was
achieved. The potential of the DJE was d e t e r m i n e d to
be +125 m V SHE at 25~
Carbon steel test specimens were m a c h i n e d from
AISI 1020 rod stock to a diameter of 0.5 in. (1.27 cm)
and an exposed l e n g t h of 1.5 in. (3.81 cm), Fig. 3. Test
specimens were prepared according to ASTM specification (17), weighed, and m o u n t e d in a test assembly
w i t h i n 30 m i n of preparation. A test assembly comprised m u l t i p l e specimens and was m o u n t e d i n an a n n u l a r geometry in the test section, Fig. 3.
Each test specimen was electrically insulated from
others i n the assembly using fiberglass i m p r e g n a t e d
Teflon spacers machined to the same dimensions as the
test specimens. The two test assemblies were centered
in the test sections using spiders. The upper and lower
assembly holders were machined from Perspex; the
lower was sufficiently long (6.7 in. 20 cm) to act as a
calming section for the inflowing solution. The upper
holder was of a n adjustable length to accommodate
changes in the overall length of the test assembly
arising from variations in the numbe~" of specimens i n stalled. The above a r r a n g e m e n t of test specimens permitted the rapid removal and replacement of specimens comprising the assembly at any time within the
overall d u r a t i o n of a n experiment. Hence the average
corrosion rate could be d e t e r m i n e d over different time
intervals at the solution flow rates prevailing in the
two test sections.
Upon removal from the loop each specimen was
washed in acetone, g e n t l y air dried, and weighed. The
corrosion product was mechanically removed b y r u b bing with acetone-soaked paper towels while rotating
June 1980
2
I
II
II
3
m
4
5
Fig. 3. Corrosion specimen and test assembly for flow loop.
Legend: 1. Top holder, 2. Test specimen, 3. Teflon spacer, 4.
Centering sllider, 5. Bottom holder.
a drill press at low speed. This was sufficient to remove
lightly a d h e r e n t corrosion product. The specimen was
again washed i n acetone, air dried, a n d weighed. In
later experiments the specimen was then cleaned i n a
special solution (25 g / l i t e r stannous chloride, 10 g / l i t e r
a n t i m o n y trioxide i n 50% hydrochloric acid). It was
found that this solution did not appreciably attack bare
m e t a l d u r i n g exposures up to 2 rain. The specimen was
i m m e r s e d for 10-20 sec in the cleaning solution at room
temperature, washed w i t h water, a n d r u b b e d with a
soft pencil eraser. This was repeated u n t i l no more
scale was visible. The specimen was then washed in
acetone, air dried, and weighed.
Samples of the corrosion product were kept u n d e r
nitrogen and sub.~equently analyzed by x - r a y diffraction using Co radiation.
Experiments were conducted i n the m a n n e r described at 25 ~ • 0.5~ using w a t e r saturated with H~S
at 1 arm with solution velocities in the range 0.1-13.3
ft/sec (0.03-4.05 msec -1) and for specimen exposure
times in the range 5-1400 hr.
Results and Discussion
The corrosion potential.--Over the pH range i n vestigated, the corrosion potential of steel, Ecorr, in
systems with and without H2S exhibited a linear relationship
E c o r r --- E % o r r ~ 0 . 0 5 9 p H
[8]
where E%orr is a constant. At pH > 4 slight c u r v a t u r e
was observed in a g r e e m e n t with other data (9, 13, 19).
For H2S-free electrolytes, E%orr had a spread of up
to 80 mV depending on the degree of agitation. With
the RDE cell, the corrosion potential at pH = 2 was
shifted from --318 mV SHE at 300 rpm to --394 mV
at rest; the largest v a r i a t i o n took place b e t w e e n rest
a n d 100 rpm.
In HzS systems, the corrosion potential becomes more
negative, due to the change in the reversible potential
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VoL 127, No. 6
CORROSION OF STEEL
of iron with HfS concentration according to Eq. [2].
The hydrogen reversible potential is unaffected b y the
HfS concentration. Further, the presence of HfS at low
concentrations reduced the corrosion potential v a r i a tion with rotational frequency; for P ~ s > 0.5 arm the
corrosion potential was i n d e p e n d e n t of frequency. The
largest variation of the corrosion potential was 60 m V
with the RDE at 2000 r p m observed as the partial pressure of H2S was raised from 0 to 1 atm; the m a j o r part
of this change occurred over the dilute range of H2S
partial pressure.
T h e changes of corrosion potential of specimens i n
the flow loop with exposure time and flow rate were
small over the ranges studied. The initial corrosion
potential of a n e w test specimen was a p p r o x i m a t e l y
--460 mV SHE changing to --450 mV after 5 h r exposure to the corroding solution at pH = 4. Thereafter
it r e m a i n e d essentially constant for exposure times up
to 6 days. A n increase of the l i n e a r velocity of the corroding solution from 0 to 10 ft/sec (0 to 3 msec - I )
after 2 h r exposure was accompanied by a change in
the corrosion potential of 1.4% i n the noble direction.
After six days exposure, a change of solution velocity
had no influence on the corrosion potential.
These results indicate that the presence of HfS progressively eliminates the strong concentration polarization of the H2S-free system.
Polarization s t u d i e s . ~ A s a p r e l i m i n a r y to the polarization measurements, the performance of the RDE cell
was e x a m i n e d by m e a s u r e m e n t s of the limiting c u r r e n t
density iL as a function of the rotational frequency, w.
Plots of i~ vs. w ~/2 were l i n e a r in accord with theory
(20). At pH -----2.1 PH2s = 0.19 atm, iL increased b y a
factor of a p p r o x i m a t e l y 1.3 relative to the value for
the H2S-free system, again indicating the progressive
e l i m i n a t i o n of concentration polarization. This is att r i b u t e d to additional h y d r o g e n discharge from H~S
by the overall reaction
2H2S -F 2 e - ----/-/2 -+- 2 H S -
[9]
Over the same HfS concentration variation but at
higher pH, Bolmer (21) obtained about one order of
m a g n i t u d e increase of iL. However, Bolmer's work was
conducted in a static system u n d e r which circumstances,
stirring by the evolved gas would have a decisive effect.
These
effects would be minimized i n the RDE cell.
The anodic polarization curves are shown in Fig. 4
and 5. Data for Fig. 4 were obtained at pH ---- 3.9-4.0;
the p a r a m e t e r is P~fs equal to 0, 0.19, 0.50, and 1.0 arm.
With the exception of data at PH~S -~ 0.5 atm, all data
were t a k e n in acetate buffered solutions. On Fig. 5 the
p a r a m e t e r is the solution pH at PH2S equal to 0 and 0.19
arm. F r o m these data, Tafel slopes are in the range
38 <: b a < 45 mV/decade in good agreement with values
reported by Bockris et al. (9).
The spacing of the curves of Fig. 4 is essentially the
same as the spacing calculated for the reversible potential of iron at p H -- 4 from Eq. [2]. This is illustrated
1231
;
i
-0"5C
a/
. ~
3, O, B
D / /
5, O, NB
.o/~
o /
~/
U3
t)
U)
:>
u/
3"95, O, B
/
w
-0.60
v/'vf~.
<
pZ
hJ
F0
O_
J"~f/t~:"
v/v
~ /
-0.70
j
/
,/*x/x
x/
3, 0"19,B
3"9, O'lg~ B
4.9~,o.~9, B
* ~ •
I'
10
I(~0
CURRENT DENSITY / ~ A c ~ 2
Fig. 5. Influence of buffering and pH on anodie polarization.
Parameter is solution pH at PH2S ~ 0 and 0.19 arm. B ~ buffered,
NB ~ no buffer.
on Fig. 4 a r b i t r a r i l y taking as reference the curve at
PH2S ~ 1 atm. Thus, variations of HfS concentration
are only causing changes in the reversible potential of
iron; the exchange c u r r e n t density remains the same.
The latter is thus calculated to be io ---- 8.9 X 10 -6
A-cm-Z, in good a g r e e m e n t with reported values for
the F e / F e 2+ electrode (8, 9).
F r o m Fig. 5, based on just two pH values, the
buffered, HfS-free solutions show an electrochemical
reaction order, p p H a - ~ -~- 1.8 consistent with the reaction
proceeding b y the catalyzed mechanism (8). At PHfs ---0.19 atm, P p H a , ~ -I-0.4 suggesting a change of mechanism relative to the HfS-free system.
The results of some cathodic polarization experiments
using buffered solutions of pH ---- 3 are presented in
Fig. 6. For the H2S-free solution, a l i m i t i n g c u r r e n t
density, iL -----1.06 m A c m -2 was observed at an overpotential of 170 mV in a g r e e m e n t with the m e a s u r e ments of Bockris et al. (9). This diffusion control is
eliminated by the presence of HfS as noted earlier.
Tale1 slopes, b c = 110 and 116 m V / d e c a d e are observed at Pnfs = 0.5 and 1.0 arm in agreement with the
theory for the discharge of the H + ion (9). The Tale1
line is insensitive to [HfS] changes in a g r e e m e n t with
previous observations (21,22); the slight shift observed at PHfs -- 0.5 and 1.0 atm is within the fluctuations due to hydrogen preconditioning of the electrode.
Polarization d i a g r a m and corrosion c u r r e n t d e n s i t y . The results of this work and those of Bockris et al. (9)
are presented in the polarization diagram, Fig. 7. It was
shown e x p e r i m e n t a l l y that the shift of the anodic
curves by a change of HfS concentration is due to the
shift of the reversible potential of iron. Hence the corrosion reaction order n =- (~ log icorr/C3 log [HfS])pE ---0.20 from geometrical considerations with b ~ _-- 0.110
V/decade and b a ---- 0.041 V/decade. At pH ---- 3 the
~~
-0.50
-o.7
Ld
0
CO
>
_~-060
o~ ~
I--0-5
o J ~ _ ~ ~-1.o o~
Z
hl
o/
-0"70
~
o/
~x ~"
-o.9
-
~ . ~
!
zI--
L
i O0
o
uJ -1.0
~.,r/J-~"
L
x~ ^
-1'1
i
~o
100
CURRENT DENSITY /mA or4 a
Fig. 4. Influence of H2S concentration on anodic polarization.
Solution pH: 3.9 to 4.0; Parameter is PH2s (atm).
!
I
0'.I
CURRENT
l
DENSITY /,~Ac,-22
I0
Fig. 6. Influence of H2S concentration on cathodic polarization,
at pH = 3. Parameter is PH2S (atm).
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1232
J. E l e c t v o c h e m . Soc.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y
I
O{)I
-o-, I
-0.2 I9
I
'
I
"
I
June 1980
I
4 ~
,o,B
W
-r
Fig. 7. Polarization diagram,
system Fe -f- H.~S -f- H20. Light
lines, from Beckris et al. (9).
Numerals indicate solution pH.
Heavy lines, this work. Numerals
indicate solution pH and PH2s
(arm), respectively. B - - buffered, N.B. - - no buffer.
<
0'6
~
:3, I'0, B
-08
3~O, NB
Z
W
0
12.
-09
IOFE
I
ICOPR
iL
I
10 -6
I
10-5
CURRENT
i0-4
10-3
i0-2
DENSITY 1 Acre "a
corrosion c u r r e n t density, {tort is given b y
z~
icorr = 3.1 X 10 -4 PH2s0'20 A - c m -2
[10]
This is the form of dependence reported by Bartonicek
(23), based on weight-loss experiments. Since the data
obtained in this work reflect initial corrosion rates,
whereas those of Bartonicek are averaged over the 6
hr d u r a t i o n of his experiments, direct comparison is
not possible. For this reason his corrosion rates are
a p p r o x i m a t e l y 85 and 38% smaller at P~2s = 1 atm
and 10 -4 atm, respectively, t h a n values calculated from
Eq. [10].
Based on a limited n u m b e r of experiments, the exp e r i m e n t a l finding that ~E : ~ E r e v F e might be true only
over a n a r r o w pH range. A more general expression
m a y be obtained considering that H2S causes not only a
change of E r e v F e b u t also of the exchange c u r r e n t
density ioFe. I n this case, again from geometrical considerations
1
(
"
)
n = b e + ba- 0.03 + b a alog $oFe .
[11]
a log [H2S]
Besides including the previous case, this expression
produces smaller or larger n depending on the sign
of the derivative.
Rate o f corrosion.--A s u m m a r y of the corrosion rate
data obtained from the flow loop as a function of the
solution velocity and exposure time are presented i n
Table I and illustrated in Fig. 8. The time average
corrosion rate r is defined by the expression
r:
re~so
[12]
,~176
I
o
06
~,
.~.
o
5
3
~
1
o
_~
o
~
o
10
IO0
EXPOSURETIME O/h
1000
Fig. 8. Time-average corrosion rate of AISI 1020 steel. P ~ S :
1 atm, temperature ~ 25~ Legend: see Table I. G Solution
velocity = 3.0 msec-1, ,'~ solution velocity = 3.1 msec-1.
where m is the mass of metal ionized in the exposure
time o and s is the surface area of the specimen exposed to the corroding solution. A power law relation
b e t w e e n r and O of the form
r-.- ,~O'r
[13]
is assumed where ~ and ~/ are constants. Values of
and 7 are given in Table I together with the correlation coefficients as a measure of the fit of the data
to the assumed equation. Owing to the large scatter
of data, only those p e r t a i n i n g to line 6 are shown i n
Fig. 8.
T i m e - a v e r a g e corrosion rates yere generally i n the
order range 10-100 g i n - 2 d a y - l , (equivalent to 0.5-5
m m p e n e t r a t i o n per year), the lower order values
Table I. Summary of experimental results. Corrosion of AISI 1020 steel by the H2S Jr H20 system at 25~
Solution
V e l o c i t y / m s e c -1
Reynolds n u m b e r
Experimental data
No. o f d a t a p o i n t s
0.60
8530
0.98
13,800
24
16
2
Line No., F i g . 8
1
T i m e a v e r a g e c o r r o s i o n r a t e , p a r a m e t e r s o f Eq. [13]
~ / g m-2 d a y -1
7
Correlation coefficient
32.3
-- 0.22
-- 0.63
Instantaneous corrosion rate, Eq. [14]
r~v/g m-2 daY -~
13.5
Standard deviation, ~
9.0
C o r r o s i o n p r o d u c t r e m o v a l p a r a m e t e r P, Eq., [16]
P ~ a, a l l d a t a
P ~ ~, t i m e s p a n
0 t o 100 h r
0.83 • 0.15
0.78 -----0.20
1.91
27,000
6
3
2.32
32,800
3.05
43,700
4.05
57,400
14
4
16
6
ll
5
45.3
-- 0,33
-- 0.61
69.9
-- 0.32
- 0.96
34.8
-- 0.07
-- 0,24
44.0
+ 0.02
+ 0.09
14.4
12.1
15.3
11.7
23.4
69.8
54.2
35.6
0.90 • 0.07
0.87 -~ 0.07
0.89 ~ 0.03
0.89 -*- 0.03
--
0.84 -- 0.13
0.73 + 0.13
0.97 • 0.03
0.95 -~ 0.04
79.7
-- 0.18
-- 0,47
34,2
103
0.98 -*- 0.02
--
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Vol. 127, No. 6
CORROSION OF STEEL
corresponding to long term exposure (0 > 102 hr) at
solution velocities less than about 2 msec -1. At solution velocities i n excess of about 2 msec -1, corrosion
rates generally retained initially high values and in
certain instances exceeded these values. This observation will be discussed later. The "initial" corrosion rate observed at O ~ 1 hr (values of ~ listed in
Table I) averaged 50 __+ 19 gm -2 day -1, in satisfactory
a g r e e m e n t with the value 75 gm -2 day - I calculated
from Eq. [10]
The corrosion rate r defined as
r =
1 (m2--ml)
$
[14]
02 - - O1
w h e r e ml and m2 are the masses of metal ionized d u r ing the exposure times O1 and 02 were also calculated.
Neglecting changes in the surface area of the specimen, the value r approximates the i n s t a n t a n e o u s corrosion r a t e at the m e a n exposure time O---- (O1 + 02)/2.
Due to the large scatter of data, the values r show
wide variations; the results are included i n Table I
as the average value, ravg together with the s t a n d a r d
deviation. These data emphasize the increasing corrosion rate with increasing solution velocity though the
t r e n d is somewhat obscured at high flow rates by the
l a r g e s t a n d a r d deviation.
The corrosion product release rate, r' is a m e a s u r e
of the rate of r e m o v a l of metal from the specimen. It
is defined b y the expression
[15]
= (ra - - 0 . 6 4 f ) / s O
where f
be FeS,
time O.
m a y be
is the mass of the co--o~ion ~)roduct, taken to
a d h e r i n g to the specimen after the exposure
F r o m the definitions of r and r'__ the ratio P
defined
P - - r ' / r ~ 1 -- (0.64f/m)
[16]
with the Iimiting values, P ---- 0 i n which case all the
corrosion product remains on the specimen, and P : 1
i n which case all the corrosion product is removed.
Values of P with the s t a n d a r d deviation ~ are i n cluded i n Table I both for all the data at a given
solution velocity and for data obtained i n the time
span 0-100 hr exposure. It is evident that in all experiments, the corrosion product is largely removed
by the flowing solution (P --> 1). At low solution
velocity ( < ,,,2 msec -1) there is a suggestion of
greater r e t e n t i o n of corrosion product p a r t i c u l a r l y in
the first 100 h r exposure. Thus the lower corrosion
rates observed m a y be due to a protective action of
the corrosion product film.
X - r a y diffraction m e a s u r e m e n t s of samples of the
corrosion product revealed the presence of m a c k i n a w i t e
(Fel+~S). P y r i t e (FeS2) was not present. No significant
differences in the n a t u r e of the products were observed
for either static or dynamic exposure conditions.
Metallographic e x a m i n a t i o n of cross sections of specimens showed a n increasing extent of surface roughness
w i t h increasing exposure time to the corroding solution. The surface roughness is defined as the s t a n d a r d
deviation of the surface from the m e a n surface line observed with a microscope. The results obtained at the
solution velocity of 3.1 msec -1 are pre.~ented in Table
II. Similar m e a s u r e m e n t s were made of the scale roughTable II. Surface examination of steel specimens exposed to
H2S + H20 solutions. Solution flow rate: 3.1 msec - 1
DuraUon of
exposure/hr
Surface
rough-
ness//J.m
Scale
rough-
~ess//zm
Scale
thick-
ness/#m
l
0
260
98O
1.9
6.2
33
8.9
28
26
55
1233
ness and thickness which also increased with increasing
time of exposure. However the scale was observed to be
discontinuous o n the surface due, presumably, to occasional and r a n d o m r e m o v a l b y the solution. The metal
then exposed would exhibit a larger a p p a r e n t corrosion rate t h a n a fresh specimen due to the increased
surface area with increasing roughness. Evidently, at
high solution velocities ( > ~ 2 msec -1) the scale is
more readily sloughed off leading to e n h a n c e d a p p a r e n t
corrosion rates i n some instances.
These results complement those of Tewari a n d Campbell (24, 25) who studied the corrosion of carbon steel
using the rotating disk technique. I n the l a m i n a r flow
regime at 22~C, P H 2 S "-- 1 arm, these authors reported
corrosion rates in the order of 10 gm -2 day -1, and concluded that the corrosion rate was controlled by the
rate of the chemical reaction b e t w e e n m a c k i n a w i t e and
hydrogen ion and by the transport of FeSH + ions to
the bulk solution. At 120~ PH2S -- 15.8 arm i n the
transition and t u r b u l e n t flow regimes, corrosion rates
in the order range 20-200 gm -2 day were observed. T h e
authors also noted that the mass of iron sulfide scale
r e m a i n i n g on the steel specimen (,-, 200 gm -2) was a p p r o x i m a t e l y i n d e p e n d e n t of the rotational frequency
a n d exposure time indicating that the corrosion product was continuously removed from the specimen: In
the work reported here, i n the fully t u r b u l e n t regime
(Re > 8500) the corrosion product is largely removed
(P --> 1) as noted earlier. The mass of product r e m a i n ing on the specimens varied in the order range 6-60
gm -2 for exposure times of 10-1000 hr.
The n a t u r e of the various iron sulfides a n d their i n fluence on the corrosion of steel have been reviewed by
Smith and Miller (26) in which they point out that
the formation of iron sulfides will in c e r t a i n conditions
not give protection to the metal, but will contribute a
n e w corrosion m e c h a n i s m to such systems. Thus they
quote the work of King (27) who showed that the
presence of m a c k i n a w i t e depressed the potential required for cathodic protection of steel below that norm a l l y considered adequate. Earlier, Greco and his coworkers (28) found that with increasing concentration
of H2S, increasing amounts of m a c k i n a w i t e [identical
to Kansite (29)] relative to other sulfides were formed
and that the formation of m a c k i n a w i t e was accompanied by an increasing corrosion rate. Meyer e t a l (30)
found the initial corrosion product to be m a c k i n a w i t e
tarnish, which provided some protection to the u n d e r lying metal. After some 200 hr exposure to the corroding solution the m a c k i n a w i t e t a r n i s h grew into a
thicker m a c k i n a w i t e scale a n d was accompanied by an
increasing corrosion rate. S u b s e q u e n t behavior depended on the presence or absence of sodium chloride
in the solution; in the absence of NaC1, the outer layers
of mackinawite transformed to pyrrhotite and pyrite,
which provided protection to the metal. They proposed
that the m a c k i n a w i t e tarnish, consisting of discrete
crystallites consolidated into a disjointed permeable
scale leading to increased electronic conductivity consequent u p o n a n increased defect density. Similar results regarding the influence of NaC1 were found by
Ewing (3i). This proposal is supported by the work of
Mara and Williams (32) who found in p a r t i c u l a r that
mackinawite was an excellent anodic and cathodic depolarizing agent.
F u r t h e r evidence regarding the influence of the
morphology of the scale is provided by the work of
Macdonald e t al. (33), who studied the corrosion of
steel in the presence of wet elemental sulfur u n d e r
aerobic and anaerobic conditions. Corrosion rates were
found to increase r a p i d l y after an induction period
which was found to be a f u n c t i o n of the initial pH of
the system, the particle size of sulfur, and the presence
of oxygen. In p a r t i c u l a r they noted that the onset of
catastrophic corrosion was accompanied b y the formation of H_~S and m a c k i n a w i t e a n d a shift in the corrosion potential to more positive values. They postulated
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1234
J. EIectrochem. Soc.: E L E C T R O C H E M I C A L S C I E N C E AND T E C H N O L O G Y
a m e c h a n i s m i n which the m a c k i n a w i t e acted as a
catalyst of the corrosion process.
It is evident from the above discussion that iron sulfide is u n r e l i a b l e as a protective scale; on the contrary
the formation of m a c k i n a w i t e m a y promote the corrosion process. Some evidence i n support of the concept
of a n induction period noted by Meyer et al. (30) and
b y Macdonald et aL (33) was obtained in this work.
This is shown in Fig. 8 i n which the t i m e - a v e r a g e corrosion rates at the solution velocities of 3.0 and 3.1
msec - i are included. These two sets of data are combined in the line 6 of Fig. 8. A significant increase in
the time average corrosion rate is observed after a n
exposure time of the order 200 hr. Data obtained at
other solution velocities showed the same phenomenon.
However, the scatter of data is such as to preclude
q u a n t i t a t i v e evaluation.
Conclusions
1. I n the presence of H2S, the corrosion potential of
steel becomes more negative. This change is due to the
change in the reversible potential of iron with change
of H2S concentration according to Eq. [2].
2. The presence of H2S does not change the Tafel
slopes of the anodic and cathodic processes w i t h i n the
investigated pH domain.
3. The exchange c u r r e n t density of the anodic process is unaffected b y H2S.
4. The cathodic process m a i n t a i n s the reversible potential and the exchange c u r r e n t density of the H2S
free system, b u t the H + diffusion control gradually
disappears with increasing H2S concentration.
5. The necessity for ohmic overpotentiaI correction
has been emphasized and a new method for compensation is described.
6. A corrosion c u r r e n t dependence on H2S concentration has been found which agrees with that found
from published weight-loss experiments.
7. The product of corrosion, mackinawite, is essentially nonadherent, p a r t i c u l a r l y at solution velocities in
excess of about 2 msec -1, e q u i v a l e n t to a Reynolds
n u m b e r of about 28,000.
8. The presence of m a c k i n a w i t e scale in certain circumstances m a y enhance the corrosion rate above the
initial value obtained from polarization measurements.
This is in accord with earlier observations.
Acknowledgment
Thanks are due to Atomic E n e r g y of Canada Limited
for financial support of this work.
Manuscript s u b m i t t e d Oct. 24, 1979; revised m a n u script received Dec. 31, 1979.
A n y discussion of this paper will appear in a Discussion Section to be published in the December 1980
JOURNAL. All discussions for the December 1980 Discussion Section should be submitted b y Aug. 1, 1980.
APPENDIX
Compensation of Ohmic Voltage Drops in the Polarization
Experiments
As noted earlier, the results of initial polarization
experiments were obscured b y significant ohmic overpotentials; a compensation method necessitating substantial additional e q u i p m e n t is described by the lOOtentiostat m a n u f a c t u r e r s (8). Britz has recently reviewed the general problem (34). I n what follows, a
simple method of compensation is described suitable
u n d e r conditions of stable c u r r e n t density; this method
requires little extra equipment. The method is e n u m e r ated as follows in conjunction with Fig. 9 and Ref. (18).
1. Select the polarizing voltage of the feedback circuit (FBC) as usual with no c u r r e n t through the loop
cell-FBC. The potentiometer for Rg is t u r n e d to Rg ~ 0.
2. Place the tip of the Luggin capillary at the norm a l l y required distance S from the electrode.
3. Select the control voltage Ei on the potentiostat
control. When the c u r r e n t density is stable, at ii, the
s
relation holds
DOUBLE
J u n e I980
~..~LUGGIN
I-"',.CAPILLARY
I
~l~ "
E LECT RODE
y
~
2s
,
S
.
.
.
.
i
# _ _
.
cell=O
']ideal
.
.
Icell= II
9 uncomp,
.
.
.
Icell=l 2
Cell: ii
i= Jcell~R-c
Icellmll
qc~
9 ideal
'T
Fig. 9. Principle of new method for compensation of ohmic
overpotentiol.
~l + l i r a = Ei
[A-17]
where ~ is the process overpotential u n d e r study and
iRa = Ea is the ohmic overpotential to be compensated.
4. Change the Luggin tip location from S to 2S. To
this corresponds a n electrolyte resistance 2Ra. The current density a n d the overpotential change to new
values
~2 + i2 2Ra -- E1
[A-18]
5. Compensate the FBC to a v a l u e Rgt which ret u r n s the c u r r e n t to ii. Then one has
~i + ii 2Ra : E1 § i Rgi
[A-19]
The last t e r m is a compensating voltage produced by
the c u r r e n t i of the FBC over the resistor R~l. S u b tracting Eq. [17] and [19], one obtains
liRa = iRgl
[A-20]
The total resistance of the FBC is selected to be m u c h
larger t h a n that of the cell so the c u r r e n t i is small
relative to/ceil
i "- (Rc/Rf) ioeli
[A-21]
I n view of Eq. [7], Eq. [20] and [21] are identical;
hence Rgi is the value which will compensate the ohmic
overpotentiai due to Rn corresponding to the distance S.
6. R e t u r n the Luggin capillary from 2S to S. The
a d j u s t m e n t is now complete and the control voltage
matches the process overvoltage, ~ since
Re
'~8 -F isRa -~ E1 ~- i3 R--[Rgl -- E1 -F i3Ra [A-22]
or, for a n y other change of E, ~ : E.
The above method was found to be fast a n d accurate
i n the polarization experiments described.
REFERENCES
1. R. V. Comeaux, Corrosio,n (Houston), 11, 189t
(1959).
2. C. B. Hutchinson and W. B. Hughes, ibid., 17,
514t (1961).
3. J. Gutzeit, Mater. Prot., 7, 20 (1968).
4. R. H. Hausler and N. D. Coble, A P I Proc., Division of Refining, 52, 586 (1972), Met. Abstr., 6,
35-0282 (1973).
5. M. Pourbaix, "Atlas of Electrochemical E q u i l i b r i a
in Aqueous Solutions," Pergamon, New York
(1966).
6. R. C. Weest, Editor, "Handbook of Chemistry and
Physics," 58th ed., CRC Press. Inc. (1977).
7. J. H. Perry, Editor, Chemical Engineers Handbook,
3rd Ed., McGraw Hill Book Co., Inc., New York
(1950).
8. F. Hilbert, Y. Miyoshi, G. Eichkorn, and W. J.
Lorenz, This Journal, 118, 1919 (1971).
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Vol. I27, No. 6
1235
CORROSION OF STEEL
9. J'. O'M. Bockris, D. Drazic, and A. R. Despic,
Electrochim. Acta, 4, 325 (1961).
10. J. J. Podesta and A. J. Arvia, ibid., 10, 171 (1965).
11. K. E. Heusler, Z. Elektrochem., 62, 582 (1958).
12. J. O'M. Bockris and A. K. N. Reddy, "Modern
Electrochemistry," P l e n u m Press, New York
(1970).
13. G. M. Florianovitch, L. A. Sokolova, and Y. M.
Kolotyrkin, Electrochim. Acta, 12, 879 (1967).
14. K. J. Vetter, "Electrochemical Kinetics," A c a d e m i c
Press, N e w York (1967).
15. A S T M Standards, S t a n d a r d No. GS-72.
16. L. S a m p a l e a n u , M.Sc. Thesis, U n i v e r s i t y of N e w
Brunswick, (1978).
17. A S T M Standards, S t a n d a r d No. G3-68.
18. O p e r a t i n g Manual, W e n k i n g potentiostat Model
No. 68.
19. F. K. N a u m a n and W. Carius, Arch. Eisenhuettenwes., 30, 283 (1959).
20. A. C. Riddiford, Adv. EIectrochem. Electrochem.
Eng., 4, 47 (1965).
21. P. W. Bolmer, Corrosion (Houston), 21, 69 (1965).
22. B. Leboucher, Rev. Inst. Fr. Pet., 1~ (1963).
23. R. Bartonicek, Proc. 3rd Int. Congr. Met. Corros,
Moscow, 1, 119 (1969).
24. P. H. T e w a r i and A. B. Campbell, Can. J. Chem.,
57, 188 (1979).
25. P. H. Tewari, M. G. Bailey, and A. B. Campbell,
Corros. Sci., 19, 573 (1979).
26. J. S. S m i t h a n d J. D. A. Miller, Br. Corros. J.,
10, 136 (1975).
27. R. A. King, Ph.D. Thesis, University of M a n chester (1971) [quoted b y S m i t h and Miller,
Ref. (26) ].
28. E. C. Greco and J. B. Sardisco, Proc. 3rd Int.
Congr. Met. Corros., Moscow 130 (1969).
29. C. Milton, Corrosion (Houston), 22, 191 (1966).
30. F. H. Meyer, O. L. Riggs, R. L. McGlasson, and
J. D. S u d b u r y , ibid., 14~ 109t (1958).
31. S . P . Ewing, ibid., 11, 497t (1955).
32. D. D. M a r a and D. J. A. Williams, Br. Corros. J.,
7, 94 (1972).
33. D. D. Macdonald, B. Roberts, and J. B. Hyne,
Corros. Sci., 18, 411 (1978).
34. D. Britz, J. Electroanal. Chem. Interracial Electrochem., 88, 309 (1978).
Mechanism of Passivity Breakdown of
High Purity Cadmium
M. G. Alvarez and J. R. Galvele*
ComisiSn Nacional de Energ~a AtSmica, Departamento de Materiales, 1429 Buenos Aires, Argentina
ABSTRACT
The passivity b r e a k d o w n of high p u r i t y c a d m i u m in NaC1, Na2SO~ and K I
aqueous solutions was studied. The pitting potential was m e a s u r e d by p o t e n t i o static polarization techniques, surface scratching techniques, and galvanostatic
techniques. The effect of p H and aggressive anion concentration on the pitting
potential was investigated. The p a s s i v i t y b r e a k d o w n potential of c a d m i u m in
1.0M NaCI solution (pH 11 and 12.5) and in 0.5M Na2SO4 solution (pH 9-12.5)
was found to be due to localized acidification on the m e t a l - s o l u t i o n interface.
No p i t t i n g potential was found for c a d m i u m either in 1.0M K I solution (pH 11)
or in 1.0M NaC1 solution (pH 9). T h e r m o d y n a m i c considerations showed that
no stable oxide film is f o r m e d in those solutions.
The p r e s e n t w o r k is p a r t of a r e s e a r c h p r o g r a m in
w h i c h the passivity b r e a k d o w n m e c h a n i s m of several
high p u r i t y metals and b i n a r y alloys was studied in
the presence of different electrolytes. The results obtained in the case of high p u r i t y a l u m i n u m (1-5).,
high p u r i t y zinc (6), high p u r i t y iron (7, 8), and
b i n a r y A1-Cu (2, 9), A1-Mg (10), and A1-Zn (10)
alloys h a v e been published. These results suggested
t h a t in all the systems so far studied the pitting
potential was the potential above which localized
acidification could be m a i n t a i n e d on the m e t a l - s o l u t i o n
interface (11-13). The purpose of the present w o r k
is to prove that such a m e c h a n i s m can be a p p l i e d to
the passivity b r e a k d o w n of high p u r i t y cadmium.
The l i t e r a t u r e about p i t t i n g of c a d m i u m is not
a b u n d a n t . According to K a d y r o v et al. (14) c a d m i u m
e x h i b i t s pitting in NaC1 solutions and in NaC1 plus
N a O H solutions at --0.50V(NHE). A u g u s t y n s k i (15)
m e a s u r e d the p i t t i n g p o t e n t i a l of zinc, cadmium, a n d
m a n g a n e s e in b o r a t e buffered solutions with different
aggressive anions. This a u t h o r found that in all cases
the pitting potential was close to the n o r m a l equilibr i u m potential of the m e t a l w i t h its ions.
In the p r e s e n t w o r k the anodic behavior and p a s sivity b r e a k d o w n of c a d m i u m were studied in a l k a l i n e
solutions of the following salts: NaC1, Na2SO4, and KI.
The effect of the p H and the aggressive anion con* Electrochemical Society Active Member.
Key words: metals, a,,ode, corrosion, passivity.
centration on the pitting potential was also investigated. The results obtained i n d i c a t e d the pitting p o t e n tial changes with the composition of the solution in
the w a y p r e d i c t e d by the t h e o r y (11-13). This came
to confirm that w i t h this metal, too, localized corrosion a p p e a r e d above the pitting potential as a result
of localized acidification on the m e t a l - s o l u t i o n i n t e r face.
Experimental
Specimens 1 m m thick w e r e p r e p a r e d b y h o t - r o l l i n g
8 m m d i a m 99.999% c a d m i u m rods from "ColnbrookBuck," England. I n t e r m e d i a t e etchings with HNO3
10% ( v / v ) were m a d e in the p r e p a r a t i o n sequence
of the samples to minimize m e t a l contamination. The
h o t - r o l l e d m a t e r i a l was cut into r e c t a n g u l a r 20 m m
long X 10 m m wide coupons which were a n n e a l e d
for 2 h r at 270~ in argon and furnace cooled. A copper
w i r e l e a d was soldered to one of the faces of the
samples. The soldered face and the edges of the
samples were covered w i t h an epoxy resin cured at
70~ l e a v i n g a free surface of about 1 cm 2 exposed.
Before the m e a s u r e m e n t s the samples w e r e chemically
polished for 1 min in a solution of 5 cm 3 HNO3 conc
plus 5 cm 3 H~,O2 (100 vol.) plus 30 cmS ethanol, at
room t e m p e r a t u r e .
The anodic b e h a v i o r of high p u r i t y c a d m i u m was
studied in the following solutions: (i) 1.0M NaC1
(pH 12.5, 11.0, 9.0, and 3.0); (it) 2.5, 0.25, 0.1, 0.025,
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