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: firstname.lastname@example.org. c d m c c t s 2 s c c c ( i c c w a t u e t e s T w t c d a 2 2 P U Peer Reviewed 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. M F c 2 C m u T e s b t ± r m s m t c p u a 2 C c s p Peer Reviewed 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 C 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 Peer Reviewed 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. Peer Reviewed 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 Peer Reviewed 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.