Thermally Sprayed Aluminum (TSA)
Coatings for Extended Design Life of 22%Cr
Duplex Stainless Steel in Marine Environments
In this article, evaluation of sealed and unsealed thermally sprayed aluminum (TSA) for the protection of
22%Cr duplex stainless steel (DSS) from corrosion in aerated, elevated temperature synthetic seawater
is presented. The assessments involved general and pitting corrosion tests, external chloride stress corrosion cracking (SCC), and hydrogen-induced stress cracking (HISC). These tests indicated that DSS
samples, which would otherwise fail on their own in a few days, did not show pitting or fail under chloride
SCC and HISC conditions when coated with TSA (with or without a sealant). TSA-coated specimens
failed only at very high stresses (>120% proof stress). In general, TSA offered protection to the
underlying or exposed steel by cathodically polarizing it and forming a calcareous deposit in synthetic
seawater. The morphology of the calcareous deposit was found to be temperature dependent and in
general was of duplex nature. The free corrosion rate of TSA in synthetic seawater was measured to be
~5-8 lm/year at ~18 °C and ~6-7 lm/year at 80 °C.
Keywords
1. Introduction
Offshore operators are currently looking to extend the design life of offshore facilities, structures and components to improve the affordability, and to increase their availability in later years of operation. Duplex stainless
steels (DSSs), often used in offshore structures, are sus-ceptible to localized corrosion and environmentally
assisted cracking when subjected to loads and temperatures approaching their material design limits in marine environment. Conventional organic coatings (paints) provide limited mitigation because of their rapid degradation at
elevated temperature, particularly in hot risers. While the use of thermally sprayed aluminum (TSA) to mitigate the
This article is an invited paper selected from presentations at the 2012 International Thermal Spray Conference and has been
expanded from the original presentation. It is simultaneously published in Thermal Spray 2012: Proceedings of the
International Thermal Spray Conference, Air, Land, Water, and the Human Body: Thermal Spray Science and Applications,
Houston, Texas, USA, May 21-24, 2012, Basil R. Marple, Arvind Agarwal, Laura Filofteia-Toma, Margaret M. Hyland, Yuk-Chiu
Lau, Chang-Jiu Li, Rogerio S. Lima, and Andre´ McDonald, Ed., ASM International, Materials Park, OH, 2012.
S. Paul, C.M. Lee and M.D.F. Harvey, TWI, Cambridge, UK; and S. Shrestha, Keronite International Ltd., Haverhill, UK.
Contact e-mail: shiladitya.paul@twi.co.uk.
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commercially pure Al (99.5%) onto 22%Cr DSS (UNS S31803). The composition of the
materials used is given in Table 1. The DSS substrate was supplied, as heat treated to 1050-1100 °C and quenched,
by Severn Metals Ltd., UK. The material was supplied as 2000 9 1000 9 6 mm3 hot-rolled fl at plate and as-rolled
round bar of diameter 12.5 mm. The tungsten inert gas (TIG) welding process was used to produce butt-welded
samples of DSS plate. Solid DSS rods of 2.4-mm dia. (supplied by Metrode Products Ltd., UK) were used as
fi ller material. The supplied rods were designed for TIG welding of standard DSSs meeting the requirements of
UNS S3183. The nominal composition of the fi ller wire is given in able 1. It had a pitting resistance equivalent
Table 1 Chemical analyses (in wt.%) of materials used for specimen preparation
Material
DSS substrate
Welding fi ller wire
Al wire for thermal spray
B
B
0
Table 2 Spray parameters used for preparing TSA coatings
Equipment
used
528
number (PREN) of 36.3 and tensile strength and 0.2% proof stress values of 690 and 450 MPa, respectively. The
parent material had a PREN of 35.2 and 0.2% proof stress value of 575 MPa.
Surface preparation before welding included machining and cleaning with acetone. The welded plates were produced using heat input in the range of 0.72-0.97 kJ/mm and the maximum inter-pass temperature was maintained
below 200 °C. The weld was prepared in four passes, and pure argon was used for shielding and back purge.
TSA-coated glass samples were also prepared to mea- sure the free corrosion potential of TSA. The TSA coating
was applied to a nominal thickness of 250-300 lm in all cases using the parameters given in Table 2. The sealant/
topcoats used were standard commercial Al-silicone (Intertherm 50Ò) and epoxy phenolic formulations
(Intertherm 228Ò). The coated or uncoated DSS speci-mens were tested with and without welds.
2.2 Corrosion Potential (ECorr), Linear Polarization
Resistance (LPR), and Zero Resistance
Ammetry (ZRA)
The free corrosion potentials ‘ ‘ Ecorr’ ’ of the TSA- coated glass, uncoated DSS weld, the TSA-coated, and the
TSA + Al-silicone-sealed specimens were monitored in aerated synthetic seawater (pH 7.6-8.0, 18 ± 2, and
80 ± 2 °C). LPR was also used where appropriate. One set of TSA-coated and the TSA + Al-silicone-sealed
specimens were electrically connected to an uncoated DSS weld specimen giving a coating (anode) to DSS weld
(cathode) area ratio of 95:5. The galvanic current fl owing between the coating (anode) and the DSS (cathode) was
continuously recorded using a zero resistance ammeter.
2.3 Pitting Corrosion
Pitting corrosion tests were conducted on the uncoated, TSA-coated and TSA + Al-silicone-sealed DSS specimens
(50 9 50 9 6 mm3) in aerated and slowly circulated syn- thetic seawater at 80 ± 2 °C for 30 days. For the coated
specimens, a 10-mm-diameter coating holiday was intro- duced on the weld cap of the test specimen to expose the
underlying DSS surface.
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and
uncoated specimens were subjected to 3% plastic strain to induce cracking in
T
he coating before the HISC test. HISC tests were con-ucted on these pre-strained tensile specimens in synthetic
S
eawater at an ambient laboratory temperature 8 ± 2 °C). A constant load was kept at 100% of the .2% proof stress
value (575 MPa). After the initial
E
xposure of 165 days, testing on one set of specimens was
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ontinued by increasing the stress level by 25 MPa per
week. Ecorr of the coated specimens was monitored. corrosion rate dropped rapidly as protective calcareous
The
deposits formed.
uncoated specimens were under À1100 mVSCE cathodic
protection potential.
3.2 Pitting Corrosion
Detailed examination of the uncoated DSS weld spec-
3. Results
3.1 General Corrosion
3.1.1 At 18 ± 2 °C. The TSA-coated and the TSA +
Al-silicone-sealed specimens recorded an initial Ecorr of
about À700 to À750 mVSCE for the fi rst few hours, after
which the potential values began to lower signifi cantly (toward the negative value) during the fi rst 7 days. After 7 days
the TSA-coated specimen recorded a potential value of
about À1300 mVSCE, and the TSA + Al-siliconesealed specimen recorded a potential value of about
À1100 mVSCE. The potential of the TSA coating
remained
signifi cantly negative, i.e., ‘ ‘ active’ ’ and changed with
time
during the initial 90-100 days. From about 120 days until
the end of the test (235 days), the TSA-coated specimen and
the TSA + Al-silicone-sealed specimen displayed steady
Ecorr values of about À1000 and À900 mVSCE ,
respectively.
The uncoated DSS had potentials between À120 and
À190 mVSCE during the entire duration of the 40-day
test.
The potential initially decreased from À120 to À190 mVSCE
in the fi rst week of testing indicating increase in activity.
However, after the fi rst couple of weeks of testing,
the potential began to increase, fi nally reaching values
around À120 mVSCE after ~35 days. General corrosion
rate
for the uncoated DSS specimen was calculated to be
~0.06 lm/year, but such measurements are of limited use
owing to the susceptibility of DSS to pitting or localized
corrosion.
In addition, an attempt was made to monitor the corrosion rate of a TSA-coated specimen during the longterm HISC test. Measurements of the corrosion rate using
the LPR technique for the TSA-coated tensile specimen
were started after 90 days of exposure. A steady corrosion
rate of 5-8 lm/year was obtained for the TSA-coated DSS
specimen during the period of 120-160 days in static
(unaerated) synthetic seawater.
3.1.2 At 80 ± 2 °C. A stable Ecorr of the TSA coating
on glass after 15 days of immersion was about
À1050 mVSCE. A low corrosion rate was calculated for
the
TSA coating and was measured at about 6-7 lm/year
using the LPR technique. The Ecorr of the uncoated DSS
weld specimen shows a steady corrosion potential ranging
about À100 to À150 mVSCE. Steady Ecorr was observed for
both the TSA and the TSA + Al-silicone-sealed specimens
after about 20-25 days and were very similar for both
coated specimens at about À900 mVSCE. The measured
free corrosion potential values for both the TSA and the
TSA + Al-silicone systems were suffi ciently negative and
indicate that an exposed DSS area of about 5% would be
sacrifi cially protected. A higher initial corrosion rate was
recorded from the TSA coating (anode) when coupled to
about 5% area of DSS weld (cathode), but this trend
lasted only for about 2-3 h of immersion after which the
Journal of Thermal Spray Technology
3.3 Chloride SCC
A failure by cracking was recorded for the uncoated DSS weld
specimens after 14 days of exposure. Severe cracking in the center
of the specimen (weld area) was observed, and these cracks initiated
from the edge of the specimen (Fig. 3a). The cracking was mainly in
the ferrite phase, and was either transgranular or around the austenite phase.
After 23 days, no cracking was observed in the TSA- coated
specimens (Fig. 3b). The TSA + epoxy-painted specimens, however,
showed signs of degradation of the painted layer.
3.4 HISC
3.4.1 HISC at 575 MPa Tensile Stress. None of the specimens failed
during 165 days of exposure at 575 MPa (100% of 0.2% proof
stress). The loading regime is shown in Fig. 4.
A visual examination of the cross section of the uncoated DSS
specimen (with applied cathodic protection (CP) potential at À1100
mVSCE) show a uniform layer (about 20-30 lm thickness) of
calcareous deposit on the entire surface of the test specimen and
this fi lm has been detached at several locations (Fig. 5a). Within the
fi rst few days of testing the surface gets partially covered with
hemispherical growths, reminiscent of the fl orets of a caulifl ower,
above an apparently smooth layer of Mg-rich deposit (Fig. 5b). EDS
traces collected from the calcare- ous layer confi rmed a two-layer
structure—a thin inner
layer rich in Mg and O and an outer layer comprised
primarily of Ca, O and C (Fig. 5c). Detailed examination
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imen surface displayed evidence of numerous small, shallow
pits. These pits were seen mostly on the surface away from the central
weld cap region. The central weld region of the uncoated specimen
had a thin fi lm. Pitting or any type of corrosion attack was not seen
on the
central, exposed weld (holiday) region of the TSA-coated samples with
or without sealant. The central weld cap region of all the coated
specimens had a thick layer of white corrosion product/calcareous
deposit (Fig. 2a).
The transverse cross sections of the specimens did not display any
visible corrosion attack of the weld after 30 days of immersion. The
cross-sectional image of the TSA-coated specimen in Fig. 2(b) shows
a uniform layer of corrosion product (15-25-lm thickness) on the
entire TSA surface. The gray contrast phase adjacent to the TSA
in Fig. 2(c) was aluminum oxide-based corrosion product as indicated
by the presence of Al and O peaks from the EDS traces. The brighter
contrast phase is believed to be a CaCO3 scale (Fig. 2d). No
measurable TSA disbondment was obtained at or near the holiday
region of the TSA- coated or the TSA + Al-silicone-sealed specimens.
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Fig. 2 TSA-coated pitting corrosion specimen showing (a) exposed weld area, (b) cross section along the holiday, and EDS traces of
(c) Al-rich and (d) Ca-rich regions
Fig. 3 Transverse cross sections of (a) uncoated DSS and (b) TSA-coated DSS after chloride SCC tests
Fig. 4 Loading sequence used in the HISC test
The TSA-coated and the TSA + Al-silicone-sealed
specimens show white corrosion products on the coating
surface, but no visible cracks. The TSA layer has retained
a good bond with the DSS substrate (Fig. 5d). No visible
crack was found in this specimen after 165 days of exposure at a tensile stress of 575 MPa (proof stress) as any
crack initially formed was fi lled by corrosion product when
exposed to seawater (Fig. 5d). Similar observations were
also found in TSA + Al-silicone-sealed specimens.
3.4.2 HISC at 669-695 MPa Tensile Stress. The
uncoated DSS specimen failed within 2 h of increasing
load to 669 MPa and showed a primary and several secondary hydrogen stress cracks (Fig. 6a). This image also
shows some evidence of calcareous deposit on the DSS
surface. This, however, was very thin when compared with
the TSA-coated specimens.
The TSA-coated specimens (with and without Al-silicone sealant) were subjected to a higher tensile stress of
695 MPa for an additional 48 days. No failure of the
specimens was observed, and these specimens were
unloaded and photographed (Fig. 6b and c). These images
show that at high tensile stress levels (typically 695 MPa),
the coatings suffer from severe cracking. Both specimens
show some white-colored corrosion product on the surface. Some black globules were also seen on the surface of
the TSA + Al-silicone-sealed specimen. EDX analyses on
Fig. 5 SEM images of uncoated DSS showing (a) cross section, (b) surface, (c) EDS traces of regions 1 and 2, and (d) TSA-coated DSS
after HISC testing at 575 MPa
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of the cross section revealed very fi ne cracks in the
uncoated DSS specimen. These fi ne cracks are the initial
stages of HISC. As the cracks have only started to form, it
is diffi cult to see if they preferentially propagate along the
ferrite or the austenite phase.