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Cathodic protection by distributed sacriﬁcial anodes-Performance at Elevated
Temperature and in Mud
Conference Paper · April 2018
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Cathodic protection by distributed sacrificial anodes – Performance at Elevated Temperature
and in Mud
Audun Ryen+, Roy Johnsen+, Mariano Iannuzzi+*, Lars Årtun*
+ Norwegian University of Science and Technology, NO-7491 Trondheim, Norway
* General Electric Co., NO-1338, Sandvika, Norway
ABSTRACT
Thermal Spray Aluminum (TSA) can be used to reduce anode demand or to extend anode life on
projects with long design lives (i.e., 40 to 50 years). However, for subsea structures, TSA has not been
used to replace the functionality of sacrificial anodes. In conventional CP design, TSA should not
degrade while it remains connected to the CP system, draining current from sacrificial anodes, which
ensure adequate cathodic protection.
During the CORROSION 2017 conference, a new concept named CP by distributed sacrificial anodes
(DSA) was presented.1
The main principle was to convert the cathode area to anode area by
distributing anode mass over the surface of the equipment to be protected. CP by DSA is achieved by
the deposition of a single-layer metallic coating. In this work, DSA was applied by thermal spray (TS).
DSA reduces the total exposed cathode area to small defects and imparts active cathodic protection.
In previous work, the outcome of exposure testing in flowing natural seawater at 10°C was discussed.
In this paper, exposure in seawater at 50 °C and 80°C and in mud, are discussed. Freely exposed
samples thermally sprayed with DSA and conventional TSA as well as galvanic couplings between
DSA and both TSA and carbon steel were investigated.
Keywords; thermal sprayed aluminum, distributed sacrificial anode, seawater, mud, corrosion,
cathodic polarization
INTRODUCTION
Over the last 50 years, only minor changes have been made to cathodic protection (CP) design. In
this regard, optimization of the CP system has not been considered as a major cost saving opportunity
to date. For subsea applications, sacrificial anodes, combined with organic coatings, is the main
corrosion protection strategy per DNVI
RP B4012
(structures) and ISOII
1558923
(pipelines). The total
anode mass can be substantial, depending on the design life of the subsea system, the size and
complexity of the structure to be protected, and the environmental conditions. Weight increase for
subsea structures due to sacrificial anodes has not, up to now, been considered an aspect that can
I Det Norske Veritas, Veritasveien 1, 1363, Høvik, Norway.
II International Organization for Standardization, SO Central Secretariat Chemin de Blandonnet 8, CP 401
1214 Vernier, Geneva, Switzerland.
1
Paper No.
11106
©2018 by NACE International.
Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to
NACE International, Publications Division, 15835 Park Ten Place, Houston, Texas 77084.
The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

be substantially improved without designing for costly retrofit solutions. The total anode weight can be
considerable, i.e., exceed 500-600 metric tons, for structures on field developments with a long design
life, especially, uninsulated gas subsea systems.4
The weight of the CP system adds to the total
structure weight, and can put special constraints on lifting vessels and cranes. It follows, therefore,
that a lower anode mass can significantly reduce both fabrication and installation costs of the offshore
structure. Additionally, less complex lifting operations can result in safer installation campaigns.
Even if most structures are exposed subsea at a seawater temperature below 10o
C, some structures
can be exposed to higher temperatures (e.g. subsea heat exchangers) and structures close to warm
pipelines. In addition, some structures such as foundations will be exposed to mud.
TSA “State-of-the-art”
Much research has been conducted on the use of TSA as a means of corrosion protection during the
past 40 years. However, studies regarding the electrochemical properties, especially anodic response,
of TSA are limited. Salient studies at normal operation temperatures in seawater were summarized
have been published in two conference papers.1, 4
However, limited results from exposure at elevated
temperature and in mud for TSA are available.
Knudsen et.al.5
published in 2016 an article regarding TSA coatings exposed to saline mud under
cathodic protection. The main issue related to TSA surfaces exposed to saline mud under CP was the
possibility of alkalization at the TSA surface due to overly effective polarization in conditions that do
not favor electrolyte circulation. In flowing seawater, the produced hydroxides from the cathodic
reactions (oxygen or water reduction) are removed by diffusion, maintaining the pH in the passive
region.6
However, in mud, where there is limited water flow, diffusion is restricted. The combination of
TSA and CP in saline mud might, therefore, be insufficient, as it may, in fact, decrease the lifetime of
the coating. The authors also found that the corrosion rate on TSA polarized to -1100 mVAg/AgCl
increased from 10 µm/year at 10°C to 20 µm/year at 95°C after 170 days of exposure.
In two internal projects at Norwegian University of Science and Technology (NTNU, for its Norwegian
abbreviation), the effect of temperature on protection current density and deposition of calcareous
deposit on TSA coated carbon steel pipes for subsea heat exchangers (direct cooling) has been
investigated.7, 8
Heated pipes with internal temperature 50°C, 70°C and 90°C were exposed to natural
seawater. Both freely exposed samples at open circuit potential (OCP) or EOC and samples connected
to Al-Zn-In sacrificial anodes were exposed for 9 months. The following main conclusions were found:
• After 100 days of exposure the cathodic current density stabilized in the range 3 – 6 mA/m2
for
TSA coatings independent of temperature. The surfaces were polarized in the range of -1050
to -1070 mVAg/AgCl.
• Calcareous deposition was observed on the TSA surfaces at the three exposure temperatures
with increased volume and Mg/Ca-ratio with increasing temperature.
• Coating thickness reduction at 90°C indicated chemical degradation in addition to
electrochemical degradation.
The Distributed Sacrificial Anode (DSA) concept
In design codes such as DNV RP B4012
the TSA coating is considered as a cathode area, consuming
current from sacrificial anodes. Compared to bare carbon steel, the protection current requirement for
TSA-coated surfaces is dramatically reduced (i.e., about 90%).2
DNV RP B401 also imposes a 25%
lower protection current for TSA-coated areas than for painted carbon steel. The lower protection
current required for TSA surfaces results in a reduced total anode mass, which is particularly beneficial
for structures with long design lives.
2

Due to differences in chemical composition, the electrochemical properties of Al-Zn-In anodes differ
from those of TSA coatings. Zinc (Zn) and Indium (In) are added to the traditional aluminum-based
anode to:
Zn Shifts the pitting potential in the cathodic direction, i.e. towards more negative values. Zn has a
strong synergistic effect with small amounts of In, moving the pitting potential in cathodic direction
due to cracking and rupture of the oxide layer.9-11
In Shifts the pitting potential in the cathodic direction, preventing passivation.9,11,12
In CP by DSA, a commercial Al-Zn-In anode alloy composition is used as the precursor for the coating.
This means that the Al-Zn-In anode alloy must be available in powder or wire form for application by
conventional TS equipment with coating thicknesses up to 1000 - 1500 µm.
The other essential requirement of the concept is that the DSA-coated surfaces can be exposed either
i) without the need to connect any traditional sacrificial anode to protect the coating or the equipment
to which DSA is applied, or ii) in combination with traditional Al-Zn-In anodes, with a minor current
drain to DSA. Thus, in contrast to TSA, DSA shall act as a sacrificial anode on par with traditional Al-
Zn-In anodes in preventing corrosion of the substrate.1
Objectives
The objectives of this work were to determine and compare the effectiveness of DSA with conventional
TSA;
1. In seawater at 50°C and 80°C freely exposed at OCP and coupled to carbon steel,
2. In Mud at 10°C freely exposed at OCP, coupled to carbon steel and cathodic polarized
simulating cathodic protection.
EXPERIMENTAL
Wire manufacturing
Since wire and powder with the desired Al-Zn-In composition were not available, wire was produced
by re-casting commercial Type A High-Grade Al-Zn-In anodes, followed by wire drawing.13
The anode
alloy was melted and re-cast into 100 mm diameter bars. The bar was subsequently heated to 506°C
and extruded in one step through a 5mm die. The wire was, then, drawn in several stages with an 8-
20% reduction per step, down to a final wire diameter of 2 mm, which was the diameter required by
the thermal spray gun. After 20 successful steps, the bar had been extruded to a 300m long wire.
Thermal spray process
Both conventional TSA (Al-99.5) and DSA coatings were applied. The wires were shipped to a thermal
spray application company qualified to NORSOK M-501 System 2 requirements.14
Several 6 mm thick
600 x 600 mm plain carbon steel plates were coated. Before application, the steel plates were grit
blasted to SA 2.5 according to ISO 8501-1.15
TSA and DSA were applied using the electric arc thermal
spray technique using a 300 A gun. ISO 8502-316
was used to control surface dust, and ISO 8502-617
for controlling salt and chloride surface contamination levels. The roughness was determined
according to ISO 8503.18
No sealer was applied on top of the thermally sprayed coatings. Average
coating thicknesses were measured to 1140 – 1250 µm on seven test plates.
TSA and DSA characterization prior to testing
Adhesion strength: before exposure, pull-off adhesion tests as per ASTM D454119
were conducted on
a thermally-sprayed steel plate specially prepared for adhesion testing.
3

Cross section analysis: the thickness and overall morphology of TSA and DSA samples were also
determined in cross section by optical and scanning electron microscopy (SEM).
Chemical analysis
The as-received anode, the re-cast bar, the drawn wire and the final DSA coating were characterized
by inductively coupled plasma mass spectrometry (ICS-MS). To characterize the DSA layer, the
carbon steel substrate had to be mechanically removed.
Testing in natural seawater and mud
Test materials
Seawater exposure
Round samples with diameter Ø27 mm were cut by water jetting from the TSA and DSA coated plates.
During immersion, only the TSA/DSA coated surface was exposed to seawater. Carbon steel
10x10x10 mm (giving the same seawater exposed area as the TSA and DSA samples) cubes were
used as the sample to be protected.
Exposure in mud
Samples with size 50 x 50 x 6 mm were used both for DSA, TSA, and bare carbon steel samples.
Only one surface (50 x 50 mm) was exposed to mud, while the other surfaces were coated with a two-
part epoxy coating.
The CS specimens for seawater exposure and all samples for mud exposure were attached to M3-
rods made from stainless steels UNS S31600 to secure metallic connection. The rods were covered
with a 1 mm thick shrinkable polymer tube to prevent contact with the electrolyte during exposure.
Seawater and mud
Seawater was taken from the Trondheimsfjord at a depth of 80 m and transported to the laboratory.
The seawater was replaced every 2 weeks. The seawater had a pH ≈ 8.1±0.1.
The mud was taken from the shoreline in the Trondheim region.
Test set-up for seawater exposure
The experiments were conducted in two custom made glass beakers (containers). Each container
had eight entrance points, where the samples were attached by using rubber gaskets and plastic
screw caps. The screw cap had holes to ensure metallic contact on the back side of the sample.
In this way, only one side (i.e., surface) of the test samples were exposed to seawater. Seven
test samples were connected to each container through the entrance points. One entrance point
equipped with a tube was used for controlling the pH level of the seawater throughout the
exposure period, and to exchange the seawater.
Air pumps were included in the experiment design, ensuring i) movement, and ii) oxygen
saturation of the seawater.
Seawater was replaced every 2 weeks to prevent the samples from being exposed to the same
electrolyte throughout the exposure period. This made the experiments to some extent more realistic.
Before filling into the containers, seawater was preheated to the actual test temperature.
Figure 1 shows a schematic view of the test-set up. A total of ten test samples were mounted in each
container; i) three DSA samples, ii) two TSA samples, iii) two Anode samples, and iv) three CS
specimens. The following measurements were done at the two temperatures:
4

1. One DSA, TSA, and Anode sample were galvanically coupled with three separate CS-
coupons.
2. One DSA, TSA, and Anode sample were freely exposed to natural seawater.
3. One DSA sample was freely exposed and used for obtaining polarization curves in the
beginning of exposure (1 day) and towards the end of exposure. This DSA sample was also
used for weekly linear polarization measurements.
The potential was measured versus a silver/silver-chloride (Ag/AgCl) reference electrode with the
reference electrode installed in a beaker outside the seawater container — kept at room temperature
— connected through a salt bridge. The galvanic current was measured as a potential drop across a
resistor. The resistor was adjusted to give a maximum potential drop of 2 mV.
The containers were placed on separate heating plates with temperature control through a
temperature sensor attached to a control unit. To secure a surface temperature of 80°C parts of the
container had to be insulated to prevent excessive heat loss to the environment. In addition, the
seawater temperature was kept at 83°C. The temperature on the exposed surface (external) was
measured with a thermocouple.
Figure 1: Test set-up for seawater tests at 50o
C and 80o
C.
Test set-up for mud
The mud was stored in a plastic container with dimensions approximately 500 x 400 x 800 mm (W x
H x L). These tests were run in the seawater laboratory with fresh seawater continuously flowing
through the container on top of the mud. Test samples were embedded in the mud with a distance
from the top of the sample to the interface between the samples and seawater of 30 mm. Inlet
seawater temperature was 10±2°C, and pH ≈ 8.1±0.1. Figure 2 shows the set up before the samples
are embedded in the mud.
In total, 11 samples were embedded in mud: Four DSA samples, four TSA samples and three CS
samples. The top of the samples was embedded 30 mm below the mud/seawater interface. The
sample setup was arranged in the following manner:
• One DSA and TSA sample were galvanic coupled with two separate CS samples. The distance
between the samples was 45 mm.
• One DSA and TSA sample were polarized potentiostatically to -1100 mVAg/AgCl, to simulate CP
conditions.
• One DSA, TSA and CS sample were freely exposed in mud.
5

• One DSA and TSA sample were freely exposed in mud and used for weekly linear polarization
measurements. These samples were also used for obtaining polarization curves towards the
end of exposure.
Figure 2: Samples before embedding in mud, which is covered by seawater.
Procedure for electrochemical measurements
Linear polarization (LP)
The polarization resistance (RP) was estimated by the linear polarization method. Samples were
polarized 20 mV below the OCP for five minutes while monitoring the resulting cathodic current.
Samples were, then, disconnected and left at the OCP for 10 minutes. After the stabilization period,
the specimens were polarized 20 mV above EOC while monitoring the resulting anodic current.
Polarization curves
Anodic polarization curves were obtained in a 3-electrode array at a potential scan rate of 600 mV/h.
Samples were polarized from -1200 mVAg/AgCl to -600 mVAg/AgCl.
Post-exposure characterization
Samples were rinsed in distilled water and acetone immediately after exposure. Subsequently, the
specimens were examined for pitting and cracks by both optical microscopy (OM) and SEM. The
samples were washed for three minutes in 65 % HNO3 (later called Nitric Acid) and rinsed in distilled
water to remove corrosion products on the surface.20
RESULTS
Wire production and thermal spraying
Detailed information about the wire production and the outcome of the thermal spraying process is
described elsewhere.1
Analysis of the chemical composition of the anode material and the DSA coating
showed a measurable difference in Zn-content. In this regard, the Zn content decreased from 4.8 wt%
in the anode material to 2.6 wt% in the as deposited DSA coating. The In content was also reduced
slightly after TS, but the value in the DSA coating corresponds to the required In content in NORSOK
M-50313
and DNV RP B-401.2
The Zn content was, however, lower than accepted in the Al-Zn-In anode
material specification, but remained above the 2.5 wt% limit established by NORSOK M-503 and DNV
RP-B401.2
Open circuit potential measurements in seawater
Figure 3 (left) shows the OCP values for freely exposed DSA and TSA samples at both 50°C and 80°C
in seawater. Figure 3 (right) illustrates the direct correlation between the change in pH and the
corresponding EOC evolution at 80°C. As can be seen from the curves, the pH dropped from 8.1 (fresh
seawater) to 7.0 during two weeks of exposure. During the same period, OCP increased up to 140
mV with lower increase with increased exposure period. For seawater at 50°C, the increase in
6

potential was < 10 mV with a pH reduction from 8.1 to 7.8. The potential dropped immediately when
new fresh seawater (at the actual test temperature) replaced the “old” seawater.
Galvanic couplings in seawater
Samples of DSA, TSA, and anode material were coupled to CS samples with a 1:1 area ratio. Figure
4 (left) shows the development in galvanic current density, while Figure 4 (right) shows the coupled
potential, which was only measured from day 32 and until the end of the exposure period.
The galvanic current started at high values, but dropped quickly during the first 10-20 days to more
stable values that remained constant through the remaining exposure period. The same stability after
30 days of exposure can be seen in galvanic coupling potential. One exception is the DS:CS couple
at 50°C where the galvanic potential during day 32 – 52 changed from -1000 mVAg/AgCl to -1050
mVAg/AgCl and back to a stable value of approx. -960 mVAg/AgCl close to the value at 80°C.
Figure 3: Seawater exposure - Left: OCP of DSA and TSA at 500
C and 800
C. Right:
Development of OCP and change in pH for DSA at 800
C. Arrows indicate
replacement of seawater.
Figure 4: Seawater exposure - Left: Development of galvanic current density for DSA, TSA
and anode at 500
C and 800
C. Right: Development of galvanic coupling potential.
Polarization curves after 65 days of exposure in seawater
Figure 5 shows polarization curves on samples that had been exposed at OCP for 65 days in seawater
at 50°C and 80°C. Figure 5 (left) suggested that the DSA and anode samples had similar active anodic
behavior at 50°C, while the TSA specimens showed a pseudo-passive behavior. The OCP was most
negative for the DSA sample, while it is slightly more positive for the TSA sample relative to the anode.
7

Figure 5 (right) shows that while the anode sample had an active anodic behavior at 80°C, the DSA
sample showed a passive behavior up to -875 mVAg/AgCl, which was taken as breakdown potential of
the alloy. In contrast, the TSA sample had a general pseudo-passive behavior similar to the behavior
at 50°C. The OCP was slightly more negative for the anode sample compared with the DSA, while it
was more positive for TSA.
One important observation: The polarization curves at 80°C were measured 4 days after replacement
of seawater with pH 7.6 according to Figure 3 (right).
Figure 5: Polarization curves after 65 days in seawater at OCP - Left: Temperature 50o
C.
Right: Temperature 80o
C.
LP measurements of DSA at OCP in seawater
Figure 6: Corrosion rates from LP measurements on DSA samples at OCP in seawater at 50°C
and 80°C.
LP measurements on DSA samples were done once a week at 50°C and 80°C. Figure 6 shows the
calculated corrosion rates based on the Stearns-Gary equation and measured RP values. Anodic and
cathodic Tafel slopes were taken from the polarization curves. As seen in Figure 6, the corrosion rates
on DSA at OCP dropped quickly from values in the 35-70 µm/year range after 1 day to 10 µm/year
after a week, ending below 5 µm/year at both temperatures after 63 days of exposure.
8

Summary of results
The test results referred above are summarized in Table 1. The table contains in addition
corresponding values in seawater at 10°C from the work performed by Quale et.al.1
These data are
measured in continuously replenished seawater.
Table 1:
Overview of some important data from tests in seawater at 10, 50 and 800
C.
DESCRIPTION DSA TSA Anode
Temperature [o
C] 10 50 80 10 50 80 10 50 80
OCP [mVAg/AgCl] -1000 -1017 -10104)
-935 -979 -9364)
-1090 -982 -10104)
Pitting potential [mVAg/AgCl] -1010 -1025 -875 -600 > -500 > -500 - -1014 -1023
bA [mV/dec] 1301)
90 85 2501)
160 115 861)
- -
bC[mV/dec] -2151)
-105 -80 -2151)
-95 -125 -1631)
- -
Corr.rate at OCP [µm/y]3) 28.21)
5.5 2.9 14.51)
1.5 4.0 14.6 1.0 7.0
Coupling pot. [mVAg/AgCl] -10002)
-960 -970 -8222)
-820 -800 -10902)
- -
Galv.curr dens. [mA/m2
] 4.82)
48 98 4.62)
22 66 13.02)
8 230
1)
Exposed for 30 days, 2)
Area ratio anode/cathode 10:1, 3)
From pol.curves, 4)
pH ≈ 7.6
Exposure in mud
Figure 7 (left) shows the development in OCP for freely exposed samples of DSA, TSA, and CS during
the exposure period. For DSA and TSA the potential increased during the first 2 days of exposure
before it quickly dropped down to the lowest potential, reached within the first 10-15 days of exposure.
This corresponded to a potential of -1090 mVAg/AgCl for DSA and -920 mVAg/AgCl for TSA. While the
potential on TSA slowly increased ending up with -880 mVAg/AgCl after 63 days of exposure, the OCP
on DSA increased rapidly again and reached a stable value close to -980 mVAg/AgCl after 25 days. The
potential on CS had a slow increase throughout the test period ending up with an OCP of -710 mVAg/AgCl
after 63 days.
Figure 7 (right) shows the anodic polarization curves of DSA, TSA, and the cathodic polarization of
CS measured at the end of the exposure period.
The development in galvanic coupling potential and galvanic current density is shown in Figure 8.
After an initial value of -900 mVAg/AgCl and, -730 mVAg/AgCl for DSA respective TSA, the potential dropped
quickly to -1080 mVAg/AgCl before it increased to -940 mVAg/AgCl during the next 20 days for DSA. For
TSA, the potential also dropped during the first 20 days, but ended at a stable value of -850 mVAg/AgCl.
As shown in Figure 8 (right), the galvanic current followed the change in galvanic coupling potential.
However, the two coatings behaved differently. On DSA a quick increase in galvanic current density
was seen from day 1 to day 3 reaching a max. value of 42 mA/m2
. After 20 days, the current density
was 10 mA/m2
and ended up with 5 mA/m2
after 63 days. For TSA the current density slowly increased
from a start value of 1 mA/m2
and ended up with a value close to 6 mA/m2
at the end of the test period.
One DSA and one TSA sample was cathodically polarized to -1100 mVAg/AgCl during the mud exposure
test. At the end of the exposure period, 10 mA/m2
and 14 mA/m2
were measured on DSA and TSA,
respectively. Table 2 shows a summary of the results from exposure in mud.
9

Figure 7: Exposure in mud with temperature 10°C. Left: Potential development on freely
exposed samples. Right: Polarization curves on DSA, TSA and CS after 63 days.
Figure 8: Exposure in mud with temperature 10°C for galvanic couplings of DSA/TSA and CS
with area ratio 1:1. Left: Potential development. Right: Galvanic current density.
Table 2:
Summary of results from testing of DSA, TSA and CS in mud covered with seawater at 100
C.
DESCRIPTION DSA TSA CS
OCP [mVAg/AgCl] -978 -890 -709
Pitting potential [mVAg/AgCl] -900 -625 -
bA [mV/dec] 70 180 -
bC[mV/dec] -140 -110 -225
Corrosion rate at OCP [µm/y] 10.0 7.2 24.5
Coupling potential [mVAg/AgCl] -916 -851 -
Galvanic current density [mA/m2
] 5.5 6.1
Cathodic current density at -1100 mVAg/AgCl [mA/m2
] 10 14 -
Post examination of samples
Exposure in seawater
Figure 9 illustrates the test samples at the end of the exposure period in seawater at 50°C and 80°C.
Figure 9 (left) shows the samples exposed at 50°C. Increased amounts of corrosion products on the
galvanic coupled samples compared with the freely exposed samples is visible. Small signs of possible
crevice corrosion on the coupled DSA and TSA samples under the gasket/blue silicone gel were also
noticeable. After cleaning twice with Nitric Acid severe pits were visible on the anode in the galvanic
10

coupling. The coupled TSA sample showed crevice corrosion beneath the rubber gasket/silicone. No
visible corrosion or degradation was seen on the freely exposed samples.
Figure 9 (right) shows the samples exposed at 80°C. Significant amounts of corrosion products are
seen on the galvanically coupled samples compared with the freely exposed samples. For the galvanic
coupling both DSA and TSA samples have suffered from significant crevice corrosion, beneath the
rubber gasket/silicone. The corrosion products on some samples had some amount of a brownish
color, indicating corrosion products from iron. After cleaning twice with Nitric Acid significant
degradation of coating on galvanic coupled DSA and TSA samples were observed.
Cross section examination of selected galvanic coupled samples as shown in Figure 10 documented
that i) severe crevice corrosion under the gasket/silicon for both DSA and TSA at 80°C, but minor
effects at 50°C, and ii) some delamination observed on the DSA coating.
Figure 9: After exposure in seawater – Left: Samples at 50o
C. Right: Samples at 800
C.
Figure 10: Cross section of selected samples after exposure in seawater.
Some samples were also examined in SEM to document change in coating thickness. The outcome
is reported in Table 3.
Table 3:
Coating thicknesses for DSA and DSA at 50o
C and 80o
C in seawater.
TEMP.
[o
C]
SAMPLE
DSA – BEFORE DSA - AFTER TSA – BEFORE TSA - AFTER
Min [µm] Ave [µm] Min [µm] Ave [µm] Min [µm] Ave [µm] Min [µm] Ave [µm]
50
OCP
900
1000-
1100
905 950-1100
135 200-400
106 240-400
Coupled 904 950-1100 1242)
180-400
80
OCP 975 1000-1100 143 200-400
Coupled 9601)
975-1100 1803)
200-400
1)
175-250 µm in crevice, 2)
0-40 µm in crevice, 3)
0 µm in crevice
11

Exposure in mud
Figure 11 shows the samples after exposure in mud for 63 days. After the removal from the mud,
significant amount of mud was attached to the rough surfaces of the DSA and TSA samples. A strong
smell of sulfur was generally noticed immediately after removal.
Figure 11: Photo of DSA and TSA samples after 63 days of exposure in mud. (V vs. Ag/AgCl)
The coating on all samples appeared to be in overall good condition. The coupled CS surfaces and
freely exposed CS surface also appeared to be in good condition i.e. no brown corrosion products
were observed.
All samples were cleaned with Nitric Acid to try to remove all deposits after exposure on the surface.
The surfaces saw some change in color. The polarized DSA and TSA samples appear significantly
darker compared with the unexposed samples, however no significant degradation was observed. The
freely exposed samples appear to be in good condition, with no discoloration.
Similar to the samples exposed to seawater, the coating thickness on selected samples was examined
in SEM. No significant reduction in coating thickness before and after exposure was measured.
DISCUSSION
Effect of temperature
Table 1 summarizes the results from exposure in seawater at 50°C and 80°C. In addition, comparable
results in seawater at 10°C from an earlier test program are included.1
One main objective with the
current work was to examine the effect of elevated temperature on the electrochemical properties of
DSA and TSA in seawater. The summary presented in Table 1 suggests that increased temperature
had a limited effect on the OCP both for DSA and TSA. For the Anode, however, it seems that an
increase from 10°C to 50°C/80°C resulted in an increase in OCP in the order of 80-100 mV.
The anodic polarization curve combined with the OCP is an indication of how effective the coating is
as a sacrificial anode. For the TSA coatings, the pitting or breakdown potential increased with
increasing temperature. In other words, the coating showed a passive behavior at higher potentials
with increased temperature. Consequently, the current output from the TSA coating is limited by the
passive current up to potentials above the OCP of CS. For DSA, the pitting potential was less affected
up to 50°C. However, passivation was seen at 800
C up to -825 mVAg/AgCl. Breakdown occurred at
more positive potentials and currents correleate well with the anodic curve of the anode material. In
this regard, the conventional AlZnIn anode samples showed no passivation even at 800
C.
12

Corrosion rate at OCP is an important parameter for the lifetime of a TS coating. Based on the data
in Table 1, the corrosion rate at OCP does not change dramatically when increasing the temperature
from 10°C to 80°C for neither DSA nor TSA. Even if there seems to be a bigger change for the anode
material, the corrosion rate was still below 10 µm/year. In the corrosion community, some people have
indicated that chemical degradation can/will increase at higher temperatures.8
This type of non-
Faradaic degradation cannot be quantified using electrochemical techniques. Examination of the
coating thickness reduction during the exposure period is one possibility to find the total coating
reduction and, in that way, estimate the contribution from chemical degradation. However, as seen
from Table 3, the coating thickness varied across the examined surfaces and it was not possible to
quantify the change in thickness as a function of the temperature. This is true for both DSA and TSA
coatings.
Crevice corrosion beneath the gasket was observed on galvanic coupled samples of DSA and TSA at
80°C and for TSA at 50°C. On samples exposed at OCP, crevice corrosion was not observed on any
samples. This shows that temperature and potential are important factors for initiation of crevice
corrosion and that TSA is more susceptible to crevice corrosion than DSA.
The galvanic couplings indicated that DSA was more effective than TSA as an anode when connected
to CS. In this test, a 1:1 area ratio between the coating and CS was used. If using DSA or TSA as a
sacrificial coating in practice, the area ratio TS:CS > 10 is normally used during the design. The
positive observation from this test program is that DSA maintain a potential down to -960 mVAg/AgCl
even at 80°C. For TSA, the corresponding potential was -800 mVAg/AgCl. The resulting current
densities, with a 1:1 area ratio, were somewhat higher on DSA than TSA. This is due to the effective
anodic properties of DSA keeping the low potential and the active anodic behavior. The effect of this
is development of a better protective calcareous deposit in areas with damaged coating on DSA
protected areas compared to TSA. Calcareous deposits will further reduce the current density
requirement from the cathodic surface (damaged area). Visual observation of the CS samples after
exposure showed that samples connected to DSA had a thicker and denser layer of calcareous
deposits at all test temperatures compared to samples connected to TSA. The more positive potential
on a TSA coated surface can give a poorer protective calcareous deposit requiring a higher protection
current through the exposure period. The overall effect is a faster TSA coating consumption compared
to a DSA coating.
Exposure in mud
Table 2 summarizes the outcome from the exposure tests in mud. After 63 days of exposure there is
a difference in OCP in the order of 90 mV between DSA and TSA. This is in the same order as the
potential difference in seawater. With the specimens embedded in mud with a depth of 30 mm below
the mud/seawater interface, the mud was probably “saturated” with seawater. Compared to measured
OCP in seawater, the values in mud were 20-40 mV more positive.
When galvanic coupled the potential difference between the two coatings are in the same range as
the value in seawater. This is also valid for the galvanic current density.
The samples in mud were also exposed to cathodic polarization at -1100 mVAg/AgCl. According to Table
2 cathodic current densities in the range 10-14 mA/m2
was measured after 63 days of exposure. These
values are in the same order as described in DNV RP B401 for exposure of TSA in seawater at
temperatures below 250
C.
CONCLUSIONS
Based on the outcome of the test program the following conclusions were made:
13

1. Increased seawater temperature from 10o
C to 80o
C had a limited effect on OCP both for DSA
and TSA.
2. For the TSA coatings, the pitting or breakdown potential increased with increasing temperature;
i.e. the coating showed a passive behavior at higher potentials with increased temperature.
3. For DSA, the pitting potential was less affected up to 50°C, but sign of passivation was seen
at 80°C.
4. Corrosion rate at OCP did not change dramatically when increasing the temperature from 10°C
to 80°C for neither DSA nor TSA and remained < 10 µm/year.
5. Crevice corrosion was observed on galvanic coupled samples of DSA and TSA at 80°C and
for TSA at 50°C when connected to CS.
6. Chemical degradation at 80o
C was not documented.
7. When exposed to mud OCP on DSA was ≈ 90 mV more negative than for TSA.
8. Polarization behavior of both DSA and TSA in mud was similar to what was measured in
seawater.
ACKNOWLEDGEMENT
This work has been executed as part of a MSc thesis project at the Norwegian University of Science
and Technology. GE Oil & Gas have financially and technically supported the work. Their contribution
is highly appreciated.
REFERENCES
1. G.Quale, R.Johnsen, L.Aartun, M.Iannuzzi; “Distributed Sacrificial Cathodic Protection – A new
Cost Effective Solution to Prevent Corrosion on Subsea Structures”. NACE International
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(USA).
2. DNV-RP-B401, "Cathodic Protection Design" (Høvik, Norway: Det Norske Veritas, 2010).
3. ISO 15589-2, "Petroleum, petrochemical and natural gas industries -- Cathodic protection of
pipeline transportation systems -- Part 2: Offshore pipelines" (Geneva, Switzerland:
International Organization for Standardization, 2012).
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6. S.M. Moon, S.I. Pyun, Corros. Sci. 39, (1997): p. 399-408.
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Temperature on Protection Current Requirement and Calcareous Development. MSc thesis,
Department of Engineering Design and Materials, NTNU, 2014.
8. R. Johnsen, Test project for Statoil. December 2015.
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10. C.B. Breslin, L.P. Friery, W.M. Carroll, Corros. Sci. 36, (1994): p. 85-97.
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13. NORSOK M-503, "Cathodic Proctection" (Lysaker, Norway: Standards Norway, 1997).
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Norway, 2012)
15. ISO 8501-1, "Preparation of steel substrates before application of paints and related products
-- Visual assessment of surface cleanliness -- Part 1: Rust grades and preparation grades of
14

uncoated steel substrates and of steel substrates after overall removal of previous coatings"
(Geneva, Switzerland: International Organization for Standardization, 2007).
-- Tests for the assessment of surface cleanliness -- Part 3: Assessment of dust on steel
surfaces prepared for painting (pressure-sensitive tape method)" (Geneva, Switzerland:
International Organization for Standardization, 1992).
- Tests for the assessment of surface cleanliness - Part 6: Extraction of soluble contaminants
for analysis - The Bresle method." (Geneva, Switzerland: International Organization for
Standardization, 2006).
-- Surface roughness characteristics of blast-cleaned steel substrates -- Part 1: Specifications
and definitions for ISO surface profile comparators for the assessment of abrasive blast-
cleaned surfaces." (Geneva, Switzerland: International Organization for Standardization,
2012).
19. ASTM D4541-09e1, "Standard Test Method for Pull-Off Strength of Coatings Using Portable
Adhesion Testers" (West Conshohocken, PA: ASTM International, 2009).
20. ASTM G1-03, "Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test
Specimens." ASTM International, 2011).
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