ENGINEERING

Galvanic Corrosion in Oil and Gas Production:

Part 1—Laboratory Studies✫

S.M. Wilhelm*

ABSTRACT INTRODUCTION

Galvanic corrosion between dissimilar materials was investi- Historically, galvanic corrosion was not a major engi-
gated using laboratory simulations of oil/gas production neering problem in production systems when low-
environments. Galvanic corrosion of materials used in pro- alloy steels were the primary materials of construction.
duction equipment (4130, 9 Cr, 13 Cr, 2205, 718, 825, NIC Table 1 provides a compilation of oilfield alloys used in
32, NIC 42, SM 2550, Beta-C Ti, C-276, 925) was studied in
1950. When used, chromium-containing alloys were
corrosive environments, which included sweet well produced
fluids, sour well produced fluids, heavy brine packer fluids, usually minor components of production strings. Con-
and acidizing fluids. Corrosion coupons of various geom- sequently, highly alloyed materials were not commonly
etries were used to measure corrosion rates and used in situations where they could provide large ca-
morphologies. Electrochemical measurements were per- thodic surface areas. Therefore, the frequency of
formed to determine potentials and current densities. The galvanic corrosion problems associated with coupling
experimental study found that the severity of galvanic attack grossly dissimilar materials was low prior to the advent
is a strong function of the type of corrosion products that of corrosion resistant alloy (CRA) completion technol-
form on metal surfaces. Galvanic interactions are mitigated ogy.
in produced fluids where carbonate and/or sulfide scales
The situation changed when higher alloy stainless
dominate the corrosion morphology. Carbonate scales tend
to block long-range galvanic currents and sulfide scales tend steels and nickel-base alloys were used routinely for
to short-circuit them. As a result, coupling of dissimilar mate- tubing strings and other subsurface equipment. CRA
rials in produced fluids may be less of a problem than liner and tubing strings are now prevalent as are
suspected. In more aggressive fluids, such as acidizing or highly alloyed, downhole equipment such as packers,
packer fluids where protective scales do not form, the sever- valves, hangers, etc. Thus, the opportunities for gal-
ity of galvanic corrosion is much more pronounced. In these vanic corrosion have increased dramatically due to the
situations, however, many chromium-containing materials proliferation of new oilfield metallurgy and the greater
actively corrode, and their chromium content provides short- complexity of equipment. Figure 1 shows a schematic
range galvanic assistance to dissolution, thus reducing
of typical bottomhole well completion equipment.
long-range effects provided by coupling to dissimilar materi-
als. A galvanic series was constructed based on coupon For well completion engineers, galvanic corrosion
data and electrochemical measurements for each of the four is an area of real concern because CRA completions
environments examined. are often designed for long, uninterrupted service. To
attack this problem, engineering data that would assist
KEY WORDS: electrochemistry, galvanic corrosion, nickel- prediction of galvanic corrosion were required. The re-
base alloys, petroleum production, stainless steels
ported study was conducted in an effort to provide an
engineering database for prediction and remediation
of galvanic corrosion.
✫
Submitted for publication January 1992; in revised form, April 1992. The data reported in this paper are a portion of
Presented as paper no. 480 at CORROSION/92 in Nashville, Tennessee.
* Cortest Laboratories Inc., 11115 Mills Road, Suite 102, Cypress, TX
the information generated in a joint industry research
77429. project sponsored by 14 companies (Table 2). The
0010-9312/92/000167/$3.00/0
CORROSION–Vol. 48, No. 8 © 1992, National Association of Corrosion Engineers 691
ENGINEERING

TABLE 1
Alloy Steels Used in Oil and Gas roduction in 1950
Elemental %

Steel C Mn Si Ni Cr Mo Cu

J-55 0.46 1.06 0.17 — — — —

H-40 0.25 0.91 0.13 — — — —
N-80 0.44 1.54 0.19 — — 0.23 —
Cast Steel 0.38 0.70 0.41 — — — —
Copper Steel 0.07 0.31 — — — — 0.26
Cor-Ten† 0.09 0.42 0.57 0.48 1.04 — 0.41
Yoloy† 0.09 0.66 0.21 1.84 — — 1.26
Cr-Mo-Si Steel 0.12 0.50 1.17 — 2.09 0.56 —
Croloy† 2 1/4 0.07 0.50 0.30 — 2.19 0.98 —
Croloy 5 0.11 0.47 0.31 — 5.08 0.49 —
Croloy 9 0.09 0.55 0.75 — 9.10 0.82 —
Croloy 12 0.12 0.40 0.10 — 12.00 0.05 —
11-13 Chrome 0.11 0.70 0.94 0.47 12.48 — —
3-Nickel 0.11 0.52 0.31 3.41 0.10 0.08 0.24
5-Nickel 0.10 0.52 0.16 5.00 — 0.06 —
9-Nickel 0.07 0.48 0.30 9.17 — 0.04 0.10
†
Trade name.

results of this study were originally communicated to

sponsoring companies in December 1985. In addition
to the data reported here, full-scale evaluations were
conducted as well as an investigation of galvanic influ-
ences to stress corrosion cracking. These latter topics
will be published separately.

TECHNICAL BACKGROUND

Galvanic corrosion in theory and practice has

been widely studied. References 1 through 11 provide
a thorough background into the theoretical treatment
of electrochemical corrosion caused by contact of dis-
similar materials. Efird12 has also discussed galvanic
corrosion in petroleum production environments from
an engineering viewpoint. Efird’s bibliography12 docu-
ments many case histories of petroleum production-
related galvanic corrosion.
Dissimilar materials come in contact in petroleum
production equipment in a variety of ways. Mixed tub-
ing strings are occasionally used in which the bottom
portion is CRA and the upper portion is steel. In this
situation, the corrosive environment can be either the
produced fluid or the packer fluid. More common are
CRA liners coupled to the casing. The CRA liner con-
tacts formation fluids on its outside diameter (OD) and
packer fluid on its inside diameter (ID). Galvanic corro-
sion can be experienced at the point of mechanical
coupling.
Even if tubing and casing are each homogeneous
materials, they are almost always in contact in the well
annulus. The most common situation is a CRA tubing
string in contact with steel casing. Galvanic interac-
tions become possible if a water-based packer fluid is FIGURE 1. Downhole completion equipment.
used. The method of contact lends itself not only to

692 CORROSION–AUGUST 1992

 ENGINEERING

galvanic corrosion but also to crevice attack in the oc- TABLE 2

cluded area between tubing and casing. Company Sponsorship of Galvanic Corrosion Project
Surface piping and flow lines are occasionally
1. Baker International Corporation
constructed with bimetallic (clad) pipe. Several types 2. CAMCO
of joining methods are used. Mechanical couplings 3. Canterra Energy, Ltd.
can suffer galvanic attack if the steel portion of the 4. Chevron Production Research
pipe is exposed. Similarly, defects in clad wellheads 5. Conoco
6. Mobil Research and Development Corporation
can produce the situation of a large CRA cathode 7. Nippon Kokan, K.K.
coupled to a small steel anode in the presence of pro- 8. Otis Engineering Corporation
duced fluids. 9. RMI Company
A variety of alloy interactions are common in sub- 10. Sohio Petroleum Company
11. Sumitomo Metal America, Inc.
surface equipment such as safety valves, packers, 12. Nippon Steel
mandrels, hangers, shut in valves, chemical injection 13. OAI (Vienna, Austria)
equipment, etc. It is a fairly standard practice to use 14. Arco Oil. and Gas
CRA trim (overlay) on portions of valves.
Tool manufacturers are frequently faced with not limited by electrolyte conductivity. In practice, the
compatibility questions concerning complex assem- conductivities of produced fluids (sweet gas or sour
blies constructed of several alloys, some of which may gas) can be sufficiently low to mitigate galvanic corro-
be generically “stainless” but that have substantial dif- sion. Similarly, nonconducting surface films, e.g.,
ferences in alloy content. Typical examples are CRA adsorbed hydrocarbon, corrosion inhibitors, or scale,
springs, retainer clips, seats, etc., that contact duplex, also reduce current flow and can similarly inhibit
13 Cr or 9 Cr housings. galvanic corrosion.
To mitigate galvanic attack it is often not possible The normalized galvanic corrosion rate (A/cm2)
to homogenize construction materials. Successful en- depends on the summed area of cathodes, those that
gineering depends on an understanding of the mecha- exist naturally on the material of interest and those
nisms that operate and ways to avoid corrosive that exist on dissimilar materials that are coupled to
interactions. Several important aspects are cited as the anode. Galvanic currents can originate from re-
follows. mote locations only if solution conductivity is sufficient
and continuous. A discontinuous water phase often
Heterogenous Surfaces limits the cathodic area. For example, if water and oil
Alloy surfaces should not be viewed as homoge- are produced and the water exists as droplets or small
neous entities. Inherent in their characteristics are slugs, the maximum cathodic area may be limited by
multiple solid phases that function as cathodic or an- the size of the slug or droplet.
odic locations. As an example, the oxide films that
form on most metals are cathodes.13 When dissimilar Polarization Effects
materials are electrically coupled in a corrosive envi- A rigorous treatment of galvanic corrosion must
ronment, the resulting situation cannot be viewed take into account the polarizability of both materials
simply as one material being a cathode and one an that make up a galvanic couple. The overall corrosion
anode. The ratios of cathodic and anodic areas on reaction can be controlled by the cathodic or anodic
each material change due to galvanic interactions. half reactions, either of which may be limited by solu-
Both oxidation and reduction reactions are distributed tion kinetics. Empirically, polarization effects are
on both materials to some extent. determined by measuring electrochemical polarization
The point to be stressed is that the engineer curves.11 Many corrosion reactions are constrained by
should not analyze galvanic corrosion without consid- mass transport either to the cathode, from the anode,
eration of the electrochemical polarization it produces or both.
and the recognition of the fact that polarization
changes the physical characteristics of the interfacial EXPERIMENTAL PROCEDURES
system. As a result, the nature and rate of galvanic
corrosion is seldom predicted based on a simple an- Coupon Experiments
ode/cathode potential difference model. The chemical compositions of the materials ex-
amined are listed in Table 3. The materials selected
Electrolyte for study are representative of alloys used as tubular
The type and rate of galvanic corrosion are func- goods or downhole equipment. Mechanical properties
tions of the ionic conductivity of the electrolyte. The are compiled in Table 4.
fluids examined in this study are, by design, highly The experimental design entailed autoclave
conductive; therefore, the corrosion that is observed is exposures of coupon assemblies to four static

CORROSION–Vol. 48, No. 8 693

ENGINEERING

TABLE 3(a)
Materials for Coupon Experiments
Generic
Name C Mn S Ni Cr Mo Others

4130(A) 0.31 0.46 0.018 0.03 0.88 0.20 —

9 CR(A) 0.13 0.48 0.010 0.057 8.27 0.96 CU-0.11
420(A) 0.38 0.36 0.010 0.37 12.70 0.05 Cu-0.10
2205(A) 0.02 1.83 0.003 5.82 22.33 2.77 N-0.23
718(A) 0.04 0.13 0.0002 53.20 18.18 3.12 Cb+Ta-5.25
Ti-0.95
BC-Ti(B) 0.02 — — — 5.90 4.10
Al-3.4
V-8.2
Zr-3.1
(A)
Balance Fe
(B)
Balance Ti

TABLE 3(b)
Additional Materials for Electrochemical Experiments
Generic
Name C Mn S Ni Cr Mo Others

925† 0.020 0.62 0.001 40.95 22.20 2.74 Ti-2.11

N-32† 0.030 2.50 0.030 29.50 26.00 3.00
N-42† 0.030 0.40 0.002 43.31 23.16 2.75 Ti-0.79
Al-0.03
G-3 0.014 1.35 <0.002 balance 21.99 6.80 Cb+Ta-1.92
Co-2.27
Fe-18.47
†
Trade name.

TABLE 4
Mechanical Properties of Materials
Yield Tensile
Generic Strength Strength % UNS
Name Manufacturer MPA/ (ksi) MPA/(ksi) Elongation No.

925† Inco Alloys 765/111 1131/164 28 N09925

718 Teledyne Allvac 876/127 1227/178 30 N07718
BC-Ti RMI 1179/171 1241/180 9 R56260
G-3 Haynes 882/128 951/138 24 N06985
22 Cr NKK 876/127 944/137 19
N-42† NKK 944/137 1000/145 15 N08042
N-32† NKK 924/134 958/139 16 N08032
†
Trade name.

environments that simulated fluids encountered in oil/ and four times the specimen area. This resulted in
gas production. Coupon assemblies are depicted in specimen couple area ratios of 1:1, 1:2, and 1:4, re-
Figure 2. A coupon assembly consisted of a “speci- spectively. Viewed conversely, the “couple” alloy in a
men” and a “couple” of variable dimension. The as- joined configuration can be treated as a “specimen”.
sembly was joined by tetrafluoroethylene (TFE) bolts, When rates are calculated in this fashion, the speci-
nuts and washers. Specimen materials were predomi- men couple area ratios are 4:1, 2:1, and 1:1,
nantly 4130, 9 Cr, or 420. Couple materials were pre- respectively. The bar graphs (Figures 3 through 15)
dominantly 9 Cr, 420, 2205, 718, or Beta-C Ti. This that provide galvanic corrosion data describe the area
distinction is useful in deciphering the data tables. ratios in this manner.
The corrosion rates of 4130, 9 Cr, and 13 Cr were Coupons were weighed before and after exposure
measured using a specimen of dimension 3.81 by to obtain weight loss from which corrosion rates were
1.27 cm (Figure 1). Materials suspected to be cath- calculated. The equation used to calculate corrosion
odes were then connected, having areas equal, twice, rates was:

694 CORROSION–AUGUST 1992

 ENGINEERING

—Solutions were not agitated throughout the test

period.
—At the end of the test period, the autoclaves were al-
lowed to cool, gases were removed, and the
autoclaves were sparged with N2.

Electrochemical Experiments
Electrochemical measurements of galvanic cur-
rent and freely corroding potential were measured in
an autoclave. The methods for constructing the envi-
ronments were identical to those described for coupon
experiments. The autoclave was 5-L total volume,
having electrically insulating feed through for eight
working electrodes and a centrally located reference
port. The reference electrode was a remotely situated
and pressure-balanced Ag/AgCl/0.1 M KCl that was
calibrated (daily) to calomel (SCE).
Open-circuit potentials were measured over 24 to
48 h at temperature for each electrode until steady-
state readings were achieved.
Current flow between pairs of electrodes was
measured using a zero-resistance ammeter. Thirty-six
individual measurements were required in each envi-
ronment. Currents were measured until steady state
(±5 percent over 1 h) was achieved. At the completion
of a galvanic measurement, electrodes were allowed
FIGURE 2. Coupon test assemblies. to return to their previously measured Ecorr prior to sub-
sequent testing. Currents reported in the tables are
not normalized to electrode area; however, electrode
areas were equal and are reported in the data tables.
mpy = 82.8W
DAT Cylindrical electrodes were employed, using shrink fit
tubing to mask the gas/solution interface.
where: The acidizing environment for electrochemical
W = weight loss (mg) tests was reacted acid (approximately 1% HCl) as op-
D = density (g/cm3) posed to the neat acid (15%) used in coupon
A = coupon surface area (cm2) exposures (Table 5).
T = test duration (h)
The test environments for coupon experiments RESULTS
are shown in Table 5 along with test durations. All
coupon exposure tests were conducted in 5-L (total Coupon Experiments
volume) autoclaves. Duplicate coupon assemblies Corrosion rates were generated for the condition
were used for each specimen/couple combination. of freely corroding metals. The coupon configuration
The general procedures for conduction an auto- for baseline experiments was identical to experiments
clave experiment were as follows: using coupled coupon assemblies. Data for freely cor-
—Coupon assemblies were fixed on a roding metals (each coupled to itself) are compiled in
polytetrafluoroethylene (PTFE) “tree” that was placed Table 6. Data, generated using coupon assemblies
in the autoclave along with the test solution (previously coupled to dissimilar metals, were used to calculate
deaerated). the increase or decrease in baseline corrosion rate
—The autoclave was sealed and leak-tested with N2. (∆R) caused by the galvanic couple.
—A soft vacuum was pulled on the test solution. The change in corrosion rate (∆R) as a percent-
—The solution was deaerated by bubbling N2 through age of the baseline rate is a useful indicator of the
the solution for 1 h. severity of galvanic interactions. Percentages greater
—The test gas (CO2 and/or H2S) was admitted to the than 50 percent increase at 2:1 specimen-to-couple
autoclave by bubbling through a dip tube over an ex- ratio constitute a rough line of demarcation between
tended period. The autoclave was then heated to test moderate (or low) galvanic acceleration of corrosion
temperature. and more severe attack.

CORROSION–Vol. 48, No. 8 695

ENGINEERING

TABLE 5
Environments
Environment Description

A Sweet Well Production Fluid - Aqueous brine (5% NaCl) in equilibrium with 1,200 psia CO2 at 150°C. Test duration 30
days.
B Sour Well Production Fluid - Aqueous brine (25 % NaCl) in equilibrium with 1 psi H2S and 1,200 psi CO2 at 200°C. Test
duration 30 days.
C Packer Fluid - Deaerated CaCl2 brine (12 lb/gal), 400 psi CO2 without inhibitor at 175 C. Test duration 30 days.
D Concentrated Acidizing Fluid - 15 % HCl + inhibitor, 120°C. Test duration 12 h.
E Reacted Acidizing Fluid - 1 % HCl + inhibitor, 120°C. Electrochemical test only.

assists dissolution of the iron matrix when 9 Cr and 13

Cr materials do not passivate.
Galvanic corrosion of 4130 is illustrated in Figure
3, which graphs ∆R and %∆R for the various couple
materials (9 Cr, 13 Cr, 2205, 718, Ti). The designation
of “P” above a corrosion rate bar indicates that pitting
was observed on the “specimen” coupon at the rate in-
dicated on the graph. General corrosion rates were
calculated based on the total area of the coupon. Pit-
ting rates were calculated based on the measured
depth of pits.
Galvanic corrosion of 4130 was not overly obvi-
ous due to the formation of ferrous carbonate scales
on the coupons. On a percentage basis, the galvanic
FIGURE 3.Galvanic corrosion of 4130 in 5% NaCl, 1,200 psia influence appeared significant; however, the corrosion
CO2 150°C. rates were low. The mean deviation of replicate
baseline coupons was on the order of 1.5 mpy. There-
fore, the baseline rate was between 2.0 and 5.0 mpy.
The general corrosion rate, measured in unbuffered
carbonic acid solutions, likely depended on the kinet-
ics of carbonate scale deposition.
Passive alloys (2205 and 718) tended to increase
the general corrosion rate of 4130, as expected. Cou-
pling to active alloys (9 Cr and 13 Cr) had a mixed
effect. Coupling to titanium produced no galvanic ef-
fect. Titanium (actually the TiO2 surface oxide) is a
poor cathode in this environment.
Figure 4 illustrates the change in rate for 9 Cr pro-
duced by coupling to 4130, 13 Cr, and 718. The data
infer that 9 Cr was anodic to each of these materials;
however, the greatest percentage change occurred
when 9 Cr is coupled to 13 Cr, which was also active.
FIGURE 4. Galvanic corrosion of 9 Cr in 5% NaCl, 1,200 psia Figure 5 indicates that 13 Cr was anodic to 4130,
CO2, 150°C. slightly cathodic to 9 Cr, and clearly anodic to passive
alloys 2205 and 718.

Sweet Well Brine Sour Well Brine

The sweet well brine consisted of 5% NaCl in The solution investigated, containing H2S (1 psia)
equilibrium with 1,200 psia CO2 at 150°C. Corrosion and CO2 (1,200 psia) in equilibrium with 25% NaCl at
rates for 4130, 9 Cr, and 13 Cr were 3.5, 7.4, and 17.7 204°C (400°F), differed in severity from the sweet well
mpy, respectively (Table 6). The data demonstrate the produced brine primarily in the fact that protective car-
principle that, when active, 9 Cr and 13 Cr corrode at bonate scales did not form. This was evident from the
a more rapid rate than low-alloy steels. This is likely corrosion scale morphology observed at the comple-
caused by microgalvanic effects, i.e., that chromium tion of the coupon exposures.

696 CORROSION–AUGUST 1992

 ENGINEERING

TABLE 6
Baseline Corrosion Rates

Sweet Well Produced Fluid (a)

Area Localized
Specimen Ratio W(mg) Rate (mpy) Corrosion
S C S:C S C S C S C

4130 4130 1:1 139.1 127.1 3.6 3.3 CR** CR

9 Cr 9 Cr 1:1 249.9 270.0 7.8 7.1 CR CR
13 Cr 13 Cr 1:1 759.7 572.5 20.2 15.2 — —
2205 2205 1:1 1.9 2.1 0* 0 — —
718 718 1:1 2.1 2.4 0 0 — —
BC-Ti BC-Ti 1:1 2.3 1.8 0 0 — —

Sour Well Produced Fluid (b)

Area Localized
Specimen Ratio W(mg) Rate (mpy) Corrosion
S C S:C S C S C S C

4130 4130 1:1 253.7 192.1 13.3 10.0 PT(A) PT

9 Cr 9 Cr 1:1 236.5 224.9 12.6 12.0 — —
13 Cr 13 Cr 1:1 246.7 241.3 13.0 12.7 PT(B) PT
2205 2205 1:1 5.5 4.0 0.3 0.2 — —
718 718 1:1 4.4 4.2 0.2 0.2 — —
BC-Ti BC-Ti 1:1 1.8 1.3 0.1 0.1 — —

Brine Packer Fluid (c)

Area Localized
Specimen Ratio W(mg) Rate (mpy) Corrosion
S C S:C S C S C S C

4130 4130 1:1 102.4 102.9 5.4 5.4 CR CR

9 Cr 9 Cr 1:1 104.9 90.6 5.5 4.8
13 Cr 13 Cr 1:1 98.4 102.0 5.2 5.4
2205 2205 1:1 4.1 4.4 0.2 0.3
718 718 1:1 2.1 2.4 0.1 0.1
BC-Ti BC-Ti 1:1 1.7 1.8 0.2 0.2

Acidizing Fluid (d)

Area Localized
Specimen Ratio W(mg) Rate (mpd) Corrosion
S C S:C S C S C S C

4130 4130 1:1 84.0 65.5 1.4 1.1 PT**(A) PT(A)

9 Cr 9 Cr 1:1 990.9 1023.4 17.2 17.8
13 Cr 13 Cr 1:1 3315.9 3397.3 58.0 59.4
2205 2205 1:1 5673.3 5587.8 98.5 97.0
718 718 1:1 10.4 10.1 0.2 0.2
BC-Ti BC-Ti 1:1 455.4 463.5 13.9 14.1 SC+ SC+
* Less than 0.1 mpy
** CR = Crevice Corrosion; PT = Pitting
S - Sample
C - Couple
(A)
- Pitting rate = 20 to 40 mpy
(B)
- Pitting rate = 10 to 20 mpy

CORROSION–Vol. 48, No. 8 697

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There exists a complex set of galvanic interac-

tions in sour systems. The surface corrosion products
on iron-base materials (FexS1–x’Fex S2–x) are electronic
conductors (as opposed to semiconductors or insula-
tors) and have a galvanic influence to the dissolution
rate. The effect of sulfide products on coupling to dis-
similar materials is to short-circuit the galvanic current
originating at longer distances. Therefore, the effects
of couple area were less pronounced, as discussed
below.
The baseline rates of 4130, 9 Cr, and 13 Cr were
similar at approximately 10 to 12 mpy. Alloys 2205,
718, and Ti were passive, having essentially zero cor-
rosion rate.
Figure 6 shows the change in rate for 4130. Re-
FIGURE 5. Galvanic corrosion of 13 Cr in 5% NaCl, 1,200 psia ductions in rate in excess of 50 percent were pro-
CO2, 150°C. duced by coupling to 9 Cr and 13 Cr. Twenty to 50
percent increases were produced by coupling to pas-
sive alloys 2205, 718, and Ti. No substantial depen-
dence on couple area was found. The data indicate
that 4130 is cathodic to 9 Cr and 13 Cr and anodic to
passive alloys in these sour brines.
Nine Cr was anodic to 4130, 13 Cr, and 718 (Fig-
ure 7); 4130 and 13 Cr produced a stronger interaction
with 9 Cr than 718; 13 Cr was anodic to 4130, 2205,
and 718 but cathodic to 9 Cr (Figure 8). Interestingly,
the effect of coupling 13 Cr to passive alloys 2205 and
718 was significantly more pronounced than when 718
or 2205 were coupled to 4130 or 9 Cr.

Heavy Brine Packer Fluid

The brine packer fluid examined was 12 ppg
CaCl2 in equilibrium with 400 psi CO2 at 200°C. This
environment constitutes a more “theoretical” system in
FIGURE 6. Galvanic corrosion of 4130 in 25% NaCl, 1 psia that scales that affect the normal galvanic interactions
H2S, 1,200 psia CO2 200°C. that derive from dissimilar materials do not form. In ad-
dition, corrosion in high-temperature brines is limited
by the cathodic reaction, i.e., reduction of water,
which, in turn, is sensitive to cathodic area.
The baseline corrosion rates of 4130, 9 Cr, and
13 Cr were all essentially identical at 5 to 6 mpy. As
mentioned, this equivalency of rate reflected the limita-
tion of the cathodic reaction. No localized corrosion
(pitting) was observed on any of the coupons or
couples in this environment.
The galvanic corrosion of 4130 is shown in Figure
9, which demonstrates that 4130 was anodic to 9 Cr,
13 Cr, 2205, 718, and Ti. The latter three alloys were
passive in this environment. The change in rate,
caused by coupling to dissimilar materials, was sub-
stantial. Rate increases in excess of 400 percent were
recorded for coupling 4130 to 2205 in a coupon-to-
couple ratio of 1:4. The increases in rate that were
caused by increasing couple area are regular, mean-
FIGURE 7. Galvanic corrosion of 9 Cr in 25% NaCl, 1 psia ing that rates are proportional to couple area. This ob-
H2S, 1,200 psia CO2, 200°C.
servation held for all of the materials examined.

698 CORROSION–AUGUST 1992

 ENGINEERING

Titanium did not provide substantial acceleration of the

galvanic rate due to the poor cathodic ability of the
TiO2 surface film.
Nine Cr was cathodic to 4130 but anodic to 13 Ce
and 718 (Figure 10). Its baseline rate was roughly
doubled when coupled to 718 in a 1:4 specimen-
to-couple area ratio. However, 13 Cr was cathodic to
4130 and 9 Cr and anodic to 2205 and 718 (Figure
11). Rates were proportional to couple area.

Acidizing Fluid
The acidizing fluid for coupon experiments con-
sisted of 15% HCl with a common acid inhibitor for
steel. Test temperature was 120°C. In strong acid at
elevated temperature, all of the materials tested cor-
roded actively. Referring to the baseline data compiled FIGURE 8. Galvanic corrosion of 13 Cr in 25% NaCl, 1 psia
in Table 6, the corrosion rates ranged from 0.2 mpd H2S, 1,200 psia CO2, 200°C.
for 718 (70 mpy) to 97 mpd for 2205 (36,000 mpy).
The inhibitor had little effect on corrosion of alloys that
contained chromium because it was formulated spe-
cifically for carbon and low-alloy steels.
Corrosion rates for predominately iron chromium
alloys (9 Cr, 13 Cr, 2205) were proportional to chro-
mium content. On a microscopic scale, the chromium
atoms serve as cathodes and, thus, provide localized
galvanic acceleration. Similarly for 2205, there exists a
slight discrepancy in alloy content between the ferrite
and austenite grains; hence, a galvanic interaction on
a microstructural scale assisted dissolution of ferrite.
Postexposure examination of 2205 revealed that the
ferrite was preferentially attacked.
Figure 12 illustrates the galvanic corrosion (∆R)
for 4130, which was largely unaffected by coupling to
other alloys. Because of the influence of the inhibitor,
it was difficult to draw definite conclusions on the mag- FIGURE 9. Galvanic corrosion of 4130 in 12 ppg CaCl2, 400
nitude of galvanic influences to 4130. The galvanic psia CO2, 200°C.
corrosion rates of 4130 were low compared to other
alloys.
Nine Cr (Figure 13) was cathodic to 4130 (inhib-
ited) and 718, in spite of its (9 Cr) higher general cor-
rosion rate. On the other hand, 13 Cr (Figure 14) was
cathodic to both 4130 and 9 Cr on a macroscopic
scale but anodic to 2205 and 718. The macroscopic
galvanic interaction for 9 Cr, 13 Cr, and 2205 followed
a compositional prediction that did not coincide with
the magnitude of (baseline) corrosion rates exhibited
by each alloy.
Alloy 2205 (Figure 15) exhibited accelerated cor-
rosion rates when coupled to 4130 and 13 Cr. The ef-
fect of couple area suggests 2205 was anodic to
(inhibited) 4130 and cathodic to 13 Cr; however, the
fact that the corrosion rates of both materials were
greater when coupled together may reflect factors
other than galvanic influence. The corrosion rate of ti-
tanium is not significantly affected by coupling to 4130 FIGURE 10. Galvanic corrosion of 9 Cr in 12 ppg CaCl2, 400
(Figure 16). psia CO2, 200°C.

CORROSION–Vol. 48, No. 8 699

ENGINEERING

Electrochemical Measurements
Galvanic currents and potentials are compiled in
Tables 7 through 10 for the environments described in
Table 5. The data are arranged so that the most noble
potential is in the first (top) row and the most active
potential is in the bottom row. Potentials were mea-
sured in mV vs Ag/AgCl, 0.1 M KCl, 25°C. Galvanic
currents were recorded in mA and were not normal-
ized to the electrode area. The electrode areas for
each environment are indicated in the respective table
legends. In Tables 2 through 10, a negative value sig-
nifies that the material in the row was cathodic to the
material in the column. Likewise, a positive value indi-
cates the alloy in the corresponding row was anodic to
the material in the corresponding column.
FIGURE 11. Galvanic corrosion of 13 Cr in 12 ppg CaCl2, 400 The galvanic currents were measured after only a
psia CO2, 200°C.
few hours of environment exposure. For this reason,
they do not necessarily correlate with coupon experi-
ments that are essentially averages of over a 30-day
time period. It is suspected that iron carbonate or sul-
fide precipitation on passive alloys, acting as cathodes
in galvanic couples, served to reduce current flow over
time.
In general, the sign and magnitude of galvanic
current clearly categorizes alloys as active or passive.
Typically, the current that flowed between two passive
alloys was very low. This means that substantial gal-
vanic interactions between dissimilar nickel-base
CRA’s or between austenitic stainless steels and
nickel-base alloys are not likely. The exception to this
observation was the acidizing solution in which fairly
large galvanic currents flowed between different CRA
materials. In these situations, the various alloys are all
active with complex galvanic interactions depending
FIGURE 12. Galvanic corrosion of 4130 in 15% HCI (inhib- on subtle compositional differences.
ited), 120°C. The galvanic current and potential measurements
correlated well with the coupon experiments. In the
sweet well environment, galvanic measurements con-
firmed that 9 Cr and 13 Cr are anodic to 4130 when
they are not passivated. This also held true for the
sour well environment. The potentials of passive alloys
in both the sweet well fluid were essentially equal (ex-
cept titanium).
Titanium exhibits noble potentials in all the envi-
ronments but serves as a poor cathode because of its
TiO2 surface film. This general statement holds even
for acid solutions in which the titanium corrodes
actively.

DISCUSSION

Corrosion rates, calculated from the galvanic cur-

rents in Tables 7 through 10, are compiled in Table
FIGURE 13. Galvanic corrosion of 9 Cr in 15% HCl (inhibited), 11. The relative magnitudes of the rates correlate well
120°C.
with coupon experiments. The absolute magnitude of

700 CORROSION–AUGUST 1992

 ENGINEERING

TABLE 7
Galvanic Currents and Potentials
Sweet Well Produced Brine
Galvanic Currents (mA/8 cm2)
Couple/ Potential,
Specimen mV (2) BC-Ti N-42 718 28 2205 4130 13 Cr 9 Cr

BC-Ti -110 -0.002 0.001 -0.002 -0.003 -0.001 -0.008 -0.010

N-42 -158 0.002 -0.003 ±0.001 0.002 -0.010 -0.710 -0.690
718 -158 -0.001 0.003 0.000 -0.003 -0.005 -0.540 -1.000
28 -158 0.001 ±0.001 0.000 -0.001 -0.021 -0.280 -0.350
2205 -166 0.003 -0.002 0.003 0.001 -0.041 -0.180 -0.190
4130 -167 0.001 0.010 0.005 0.021 0.041 -0.770 -0.770
13 Cr -246 0.008 0.710 0.540 0.280 0.180 0.770 -0.226
9 Cr -264 0.010 0.690 1.00 0.350 0.190 0.770 0.226

TABLE 8
Galvanic Currents and Potentials
Sour Well Produced Brine
Galvanic Currents (mA/8 cm2)
Couple/ Potential,
Specimen mV (2) Alloy G 925 BC-Ti 28 718 2205 4130 9 Cr

Alloy G -281 -0.009 0.001 -0.010 -0.006 -0.012 -0.150 -0.610

925 -281 0.009 0.002 0.029 -0.005 -0.011 -0.131 -0.550
BC-Ti -284 -0.001 -0.002 -0.018 -0.016 0.004 -0.055 -0.228
28 -284 0.010 -0.029 0.018 0.065 0.029 -0.138 -0.920
718 -289 0.006 0.005 0.016 -0.065 0.010 -0.250 -1.400
2205 -299 0.012 0.011 -0.004 -0.029 -0.010 -0.100 -0.800
4130 -367 0.150 0.131 0.055 0.138 0.250 0.100 -5.300
9 Cr -405 0.610 0.550 0.228 0.920 1.400 0.800 5.300

TABLE 9
Galvanic Currents and Potentials
Heavy Brine Packer Fluid
Galvanic Currents(A) (mA/8 cm3)
Couple/ Potential,
Specimen mV (2) BC-Ti 718 N-42 N-32 2205 13 Cr 9 Cr 4130

BC-Ti -93 -0.002 -0.002 -0.004 -0.005 -0.030 -0.438 -1.160

718 -139 0.002 -0.005 -0.004 -0.002 -0.310 -0.850 -0.170
N-42 -155 0.002 -0.005 -0.003 -0.002 -1.350 -0.960 -0.250
N-32 -156 0.004 0.004 0.003 -0.005 -0.011 -0.500 -0.527
2205 -165 0.005 0.002 0.002 0.005 -0.430 -0.500 -0.105
420 -283 0.030 0.310 1.350 0.011 0.430 -0.039 -0.049
9 Cr -289 0.438 0.850 0.960 0.500 0.500 0.039 0.190
4130 -293 1.160 0.170 0.250 0.527 0.105 0.049 -0.190

TABLE 10
Galvanic Currents and Potentials
Acidizing Fluid
Galvanic Currents(A) (mA/4 cm2)
Couple/ Potential,
Specimen mV (2) BC-Ti N-32 G-3 718 925 13 Cr 9 Cr 4130

BC-Ti +370 -0.22 -0.20 -0.29 -0.23 -0.62 -0.78 -0.89

N-32 +349 0.22 4.80 1.12 2.36 -18.2 -25.7 23.2
G-3 +30 0.20 -4.80 -1.27 -3.72 -27.2 -28.3 -29.9
718 +10 0.29 -1.12 1.27 -0.83 -9.80 -19.3 -9.60
925 -4 0.23 -2.36 3.72 -0.83 -37.4 -35.3 -37.8
420 -182 0.62 18.2 27.2 9.80 37.4 -13.4 -9.20
9 Cr -202 0.78 25.7 28.3 19.3 35.5 13.40 4.00
4130 -210 0.89 23.2 29.9 9.60 37.8 9.20 -4.00

CORROSION–Vol. 48, No. 8 701

ENGINEERING

electrochemical rates are generally greater than those

obtained with coupons.
The laboratory experiments reported here pro-
vided insight into the complexities of galvanic corro-
sion in petroleum production applications. One basic
question was which material in a particular galvanic
couple was the anode and which one was the cath-
ode. Such assignments could not be accomplished
based strictly on composition.
One method to assign anode/cathode (A/C) iden-
tities was to examine the magnitude of baseline (not
coupled) corrosion rate data for each material. Typi-
cally, in activation-controlled kinetics, the material that
exhibits the greater weight loss corrosion rate will pos-
sess the more negative potential.
FIGURE 14. Galvanic corrosion of 13 Cr in 15% HCl (inhib- However, alloys that exhibited microgalvanic inter-
ited), 120°C. actions (9 Cr, 13 Cr, 2205) often produced high corro-
sion rates in spite of their higher alloy content. For
example, the highest rate of dissolution in the carbonic
acid solution (sweet well brine) was 13 Cr; however, in
the electrochemical tests, it did not exhibit the most
negative potential. Similarly, 2205 in acid corroded ex-
tremely rapidly; yet, when coupled to other alloys, it
acted as a cathode. Note that these comparisons
were complicated by the fact that coupon exposures
were averages, i.e., the average dissolution rate over
an extended period, while electrochemical rates were
essentially a “snapshot” of the rate at a particular point
in time.
One may also assign A/C identities by examina-
tion of the effect of couple material and area on
baseline rate. If an alloy’s baseline rate was increased
by coupling to a second material and if that increase
was proportional to couple area, one could conclude
FIGURE 15. Galvanic corrosion of 2205 in 15% HCI (inhib- that the couple material was a cathode. Difficulties
ited), 120°C. arose when two active materials were coupled. If one
active material contained chromium and the other did
not, microgalvanic effects could dominate. Likewise, if
surface film formation (FeCO3 or FeS) dominated,
then the galvanic interaction could serve to prevent or
assist film formation on either anodes or cathodes (or
both) and rate changes were less predictable.
For example, in sweet well coupon experiments,
when 9 Cr was coupled to 13 Cr, 13 Cr was clearly ca-
thodic to 9 Cr (Figure 4), but both are cathodic to
4130. The precipitation of FeCO3 on 4130 controlled
the corrosion morphology exhibited over time. Like-
wise, 13 Cr was a better cathode for 9 Cr than a pas-
sive alloy, such as 718, in spite of the greater potential
difference measured in electrochemical experiments.
Electrochemical measurements gave a clear indi-
cation of A/C identities at a particular moment in time.
However, the current that flowed was not necessarily
(exponentially) proportional to the potential difference,
FIGURE 16. Galvanic corrosion of Titanium in 15% HCI as would be the case for activation-controlled kinetics.
(inhibited), 120°C.
When mass transfer controls the kinetics, area ratios

702 CORROSION–AUGUST 1992

 ENGINEERING

TABLE 11 a particular couple, were individually active. It is likely

Galvanic Corrosion Rates that corrosion product formation was strongly influ-
Measured Electrochemically enced by galvanic interactions.
❖ The galvanic corrosion rate (∆R) was not necessar-
Sweet Well Brine (mpy)
ily proportional to the potential difference (∆Ecorr) of two
Specimen Couple Couple Couple alloys measured independently.
718 2205 BC-Ti ❖ Microgalvanic effects dominate some corrosion
4130 0.3 2.3 0.1
processes. In low pH environments (sweet well brine
9 Cr 56.3 10.1 0.6 and acidizing fluids) where iron plus chromium alloys
13 Cr 30.4 10.7 0.5 (9 Cr, 13 Cr, 2205) do not passivate, it was observed
that the corrosion rates were proportional to chromium
content. Also, the duplex alloy 2205 exhibits similar
Sour Well Brine (mpy)
behavior because of a galvanic interaction between
Specimen Couple Couple Couple austenite and ferrite.
718 2205 BC-Ti ❖ Galvanic corrosion will not occur when passive al-
loys are coupled together, as long as both materials
4130 14.1 5.6 3.1
9 Cr 78.8 45.0 12.8 remain passive. Localized corrosion on alloys such as
2205, for example, was not observed when coupled to
CaCl2 Packer Fluid (mpy) a variety of other noble alloys. Electrochemical mea-
surements indicated that current flow between passive
Specimen Couple Couple Couple
alloys that have substantial compositional difference
718 2205 BC-Ti
was, in all cases, very low except in acids. Therefore,
4130 9.6 5.9 65.3 it is not expected that substantial galvanic interactions
9 Cr 47.8 28.1 24.6 will affect performance when nickel-base CRA or
13 Cr 17.4 24.1 1.7
stainless alloys are in electrical contact in oxidizing en-
vironments.
Acidizing Fluid (mpd)
❖ In reducing environments such as acidizing fluids,
Specimen Couple Couple Couple compositional differences can produce substantial
718 Alloy G BC-Ti galvanic interactions between nickel-base or stainless
4130 3.0 9.2 0.2 alloys.
9 Cr 6.0 8.8 0.2 ❖ The severity of galvanic corrosion depends on the
13 Cr 3.0 8.4 0.3 corrosiveness of the environment. Packer fluid corro-
(A)
Zit K sion is strongly affected by galvanic interactions.
mpy =
nFd Moreover, the distribution of weight loss in packer flu-
where Z = atomic weight (g/mol); i = current density (A/cm2); ids tends to be localized close to the metal junction,
t = time (3.154 x 107 s/y); n = no. of electrons (Fe → Fe2+); which could lead to perforation under adverse
F = Faraday (96,500 c/mol); d = density (g/cm3);
circumstances.

are critical, and the distribution of current (and the re- REFERENCES
sultant metal loss) can be highly localized. It was ob- 1. F.L. LaQue, Marine Corrosion, Causes and Prevention (New York, NY:
served in the packer fluid tests that corrosion of 4130 John Wiley and Sons, 1975), chapter 6.
2. R. Baboian, et al., “Galvanic and Pitting Corrosion—Field and Labora-
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