Coupons of Corrosion in CuAlNi in the Petroleum Industry:
A valuable additional resource in electrochemical techniques
M. C. P. Cruza, R. T. Figueiredob, R. E. Souzac, P. M. M. Araújod
a
b
Faculdade Pio Décimo, Av. Tancredo Neves, 5655, Sergipe-Brasil
Instituto de Tecnologia e Pesquisa, Av. Murilo Dantas, 300, Sergipe-Brasil
c
Petrobras, Rua Acre, 2504, Sergipe-Brasil
d
Departamento de Engenharia mecânica, UFS, Sergipe-Brasil
This study evaluates the corrosive behavior of the alloy CuAlNi with Shape Memory Effect
when exposed to fluids produced in mature onshore oil fields (coupons) and in the laboratory
(immersion and Tafel curves). The resistance of the alloy to corrosion was shown to be low
when exposed to water produced in steady state with salinity levels similar to sea water in
temperatures approximate to those found in the related environment, however when the
corrosion coupon was used more severe corrosion was perceived.
Keywords: coupon, corrosion levels, CuAlNi alloy
Introduction
In the oil and gas industries there are normally occurring synergies between
physico-chemical variables, as well as variables of flow, pressure, and of erosion in the
corrosion. To isolate the effects of each of these variables would most likely lead to a
difficult interpretation of this study. A more viable strategy is the experimental simulation
of the physico-chemical environmental conditions through a laboratory experimental
design, especially to study the combined effects of these variables.
Another way in which to reliably analyze corrosion in this area is the technique by
which corrosion rates are analyzed using corrosion coupons. The standards ASTM G4
(2001) and NACE RP 0775 (2005) were created to serve as guides for these types of tests
[1,2]. While they may not completely substitute laboratory testing, field studies with
corrosion coupons are specifically designed to monitor the durability of operating
equipment as well as to evaluate alternative construction materials and determine the
effects of processing conditions that cannot be reproduced in the laboratory [3].
However, the resistance to corrosion of the alloy CuAlNi with Shape Memory
Effect will be studied through the analysis of corrosion rates in the field (corrosion
coupons), reproducing the actual processing conditions, and in the laboratory with water
produced without the actions of dissolved gasses, the effects of flow, and suspended solids
(immersion and Tafel curves)
Methodology
The objective of the laboratory tests was to study the behavior of the corrosion of
the alloy CuAlNi in fluid collected directly from onshore oil wells.
Physico-chemical composition of water produced in oil exploration
The water produced from onshore wells for oil exploration in the State of Sergipe,
Brazil, identified here by the letters A, B, C and D was monitored for pH, temperature,
pressure, flow and content of dissolved gases (CO2, H2S and O2), total salinity, and chloride
content [4]. The monitoring of temperature, pressure and flow was accomplished using the
existing on-site instruments.
Rates of corrosion in laboratory immersions
The immersion tests were conducted in water produced in the selected oil wells. The
specimens were placed in receptacles with the collected samples of unstirred water, at room
temperature, and in contact with atmospheric air (aerated) to evaluate the corrosion rates by
mass loss.
Tafel corrosion rates
The electrochemical tests consist of determining the corrosion rate by the Tafel
extrapolation method using a G-300 Gamry potentiostat, making a sweep at the rate of 2
mV.s-1, at an interval of ± 100 mV from around the potential open circuit. All of the water
samples produced were filtered before the start of the electrochemical tests to remove any
suspended particles of oil that could alter the effects of the test results. All water was tested at
room temperature, unstirred, and in contact with atmospheric air (aerated).
Corrosion rates by corrosion coupons
Fifteen (15) tangential corrosion coupons were distributed in five wells as seen in
Table 1, with the assembly of three coupons per well. Two coupons were used for evaluation
of mass loss and with the third used to characterize the type of corrosion. Before being
exposed to corrosion the coupons were abraded with glass beads, weighed and visually
inspected.
Tabela 1 - Distribuição dos cupons entre os poços.
Well
A
B
C
D
E
Cupons de corrosion
1, 2 e 3
4, 5 e 6
7, 8 e 9
10, 11 e 12
13, 14 e 15
Results and discussion
Table 2 presents the physico-chemical characteristics of the fluid produced.
Table II. Physico-chemical characteristics of the fluid produced
Parameter
Well A
Well B
Well C
Well D
Well E
Salinity
Chloride (1)
32,7 ± 5,3
16,04 ± 0,02
38,9 ± 3,4
22,67 ± 1,13
22,1 ± 3,3
12,75 ± 1,21
8,4 ± 2,3
5,09 ± 1,39
10,6 ± 3,3
5,47 ± 2,18
Total Iron (1)
25,00 *
18,40 ± 3,96
4,46 ± 1,56
2,43 ± 2,59
0,54 ± 0,41
(1)
(1)
**
**
**
77,40 ± 69,86
13,0 ± 11,0
H2S (1)
Sulfides (1)
3,43*
3,23*
7,25 ± 2,48
6,84 ± 2,35
11,66 ± 2,09
10,95 ± 1,92
25,38 ± 11,82
23,92 ± 11,18
14,29 ± 2,97
13,45 ± 2,80
Sulfates (1)
***
***
100,00 *
230,00 *
25,00 *
pH
7,12 ± 0,37
7,31 ± 0,45
7,33 ± 0,38
7,00 ± 0,54
7,68 ± 0,12
Alkalinity (1)
794,30 ±
73,96
1,4
± 0,4
781,35 ± 49,80
252,58 ± 40,81
275,39 ± 238,86
3,9 ± 0,8
8,8 ± 0,6
0,6 ± 0,4
226,56 ±
85,27
9,5
± 1,2
Temperature (4)
38,7 ± 3,7
42,5 ± 3,3
118,3 ± 8,3
74,1 ± 23,4
132,5 ± 3,5
Pressure (5)
TSS (2) (6)
11,7 ± 2,1
61,87 ± 11,98
12,8 ± 2,6
44,35 *
8,8 ± 0,6
26,38 ± 4,84
12,7 ± 2,3
19,27 ± 0,09
4,0 ± 0,0
21,72 ± 5,63
CO2
Velocity
(3)
* Only one measurement performed; ** Values changed by the content of total above 1,0 mg.L-1; *** Not
measured. (1) In g.L-1; (2) In mg.L-1; (3) m.s-1; (4) In oC; (5) In kgf.cm-2; (6) Total suspended solids.
It can be observed that the water produced in mature oil fields presents a high
distribution of concentrations and suspended solids as well as wide variations in temperature
and pressure. It is a complex electrochemical fluid with a unique balance, and as such is
difficult to replicate in laboratory settings.
In table three the corrosion rates of the alloy are presented, in water produced in the
laboratory as well as in the field. It can be seen that the laboratory results are smaller than
those produced in the field, which may be explained by the absence of dissolved gasses (CO2
and H2S), the lack of the effects of temperature, or also by the fact of the electrolyte being
stagnate, with no mass transfer and without the erosive effects of suspended solids. In the
presence of all the above mentioned effects, the corrosive coupons more closely represent the
reality of the system in operation.
Table III – Corrosive rates obtained at 25 oC through immersion tests in the laboratory and
in the field by the Tafel extrapolation method.
Corrosion Rates (mm.year-1)
A
B
C
D
E
Field immersion
0,20
0,16
0,13
0,11
0,22
0,0091
0,0095
0,0021
0,0030
-
0,00015
0,00016
0,00036
0,00006
0,00006
Laboratory immersion
Tafel
The coupons 01, 02, 04, 05, and 06 showed pitting corrosion. The coupons 07, 09, 10, 13, and
14 showed uniform corrosion. It may be observer, however, that the coupons in temperatures
above 70o C presented predominantly uniform corrosion [5].
Conclusion
The corrosion rate of the alloy CuAlNi is drastically reduced when immersed in water
produced without the presence of corrosive gasses (CO2 or H2S); quantities of movement,
especially with flow velocities greater than 8 m.s-1; suspended solids, whose effects on the
corrosion rates of the alloy are noticeable even in flow velocities lower than 2 m.s-1. For
these reasons, the use of corrosion coupons is recommended in future corrosion studies in the
Petroleum Industry.
References
1 - AMERICAN SOCIETY FOR TESTING AND MATERIAIS. Standard Guide for Conducting Corrosion
Tests in Field Applications, ASTM G4. West Conshohocken, 2001. 9p.
2 - NATIONAL ASSOCIATION OF CORROSION ENGINEERS. Recommended Practice for Preparation,
Installation, Analysis, and Interpretation of Corrosion Coupons in Oilfield Operations, NACE RP 0775.
Houston, TX, 2005. 16p.
3- DEAN, S. W. In: Service Monitoring. In: Metals Handbook, v. 13: Corrosion, 9 ed., American Society for
Metals, 1987.
4 - EATON, A. D.; CLESCERI, L. S.; RICE, E. W.; GREENBERG, A. E. Standard Methods for the
Examination of Water and Wastewater, 21 ed. Washigton: American Public Health Association, 2005.
5 - FONTANA, M. G.; GREENE, N. D. Corrosion Engineering. NY: McGraw-Hill, 1967;