Thermally Sprayed Aluminum Research Articles

An Investigation of Corrosion Behaviors of Thermally Sprayed Aluminum (TSA) at Elevated Temperatures Under Thermal Insulations and Autoclave Immersion Conditions

ABSTRACT Thermally Sprayed Aluminum (TSA) protects against internal and external corrosion in many industrial applications. Even though TSA coating has been the subject of many studies, there is still a need to gain better insights into the degradation mechanisms of the TSA especially under immersion conditions and moisture-saturated thermal insulations. This study addresses the corrosion behavior of TSA in a CUI simulation setup (per ASTM G189-07) and autoclave immersion. The corrosion tests were conducted for three and four days under isothermal wet (IW) and cyclic wet (CW) conditions. Linear polarization resistance (LPR) scans were conducted during both (i.e., CUI simulation and autoclave immersion tests) to better understand the corrosion behaviors of TSA coating. Following corrosion testing, thorough microstructural examinations were conducted employing confocal laser microscopy, 3D topography, scanning electron microscopy (SEM), and Energy dispersive spectroscopy (EDS) to understand the microstructural and tribological changes resulting from corrosion testing. TSA coating under the insulation showed significant degradation via flashing moisture and active dissolution of iron at the insulation-metal interface. Unlike immersion conditions, the wear of TSA due to flashing moisture under thermal insulation created the crevices that caused the active corrosion of the steel substrate.

Growth Kinetics of Calcareous Deposits on Thermally Sprayed Aluminum Coatings in Natural Seawater

The formation of calcareous deposits on thermally sprayed aluminum (TSA)-coated surfaces immersed in natural seawater and connected to a cathodic protection system is a recurring issue. There is at present limited knowledge on the calcareous deposits’ formation kinetics on TSA in natural seawater following prolonged exposure at different temperatures. In this study, bare 25Cr super-duplex stainless steel (SDSS) and TSA-coated SDSS tube coupons (with and without a silicone-based sealer) were exposed to natural seawater under potentiostatic cathodic protection (CP) and open-circuit potential (OCP) conditions at different temperatures (i.e., 20°C, 35°C, 60°C, and 80°C) for 1.5 y. The coupons were extracted at various intervals to construct growth kinetics curves under various test conditions. The deposits were subsequently examined using scanning electron microscopy, energy dispersive spectroscopy, and x-ray diffraction. For all of the specimens, the deposits on the exposed surfaces increased with exposure time. Fewer deposits were seen under CP conditions than OCP at low temperatures (20°C and 35°C), whereas more deposits were observed with CP than those at OCP at high temperatures (60°C and 80°C). Last, a recommended protection potential of −0.90 VSSC (VAg/AgCl) and cathodic current density values of −8 mA/m2 and −15 mA/m2 were obtained based on the findings of this study.

The effect of bird droppings on the corrosion of steel and aluminum used in offshore applications

AbstractEngineering materials in service are frequently exposed to bird droppings (BDs). In this study, the interaction has been studied of BDs from chicken with a carbon steel, AISI 316, 22% Cr duplex stainless steel (DSS), 25% Cr super duplex stainless steel (SDSS), carbon steel with a thermally sprayed aluminum (TSA) coating, and aluminum alloy EN AW 6082. Samples with and without BDs are exposed to a cyclic salt spray test for up to 692 h (29 days). Electrochemical experiments in the presence of BDs were conducted in a syringe‐based droplet cell. Characterization of the corrosion products by infrared (IR) and Raman spectroscopy shows no build‐in of organic components into the corrosion products in the samples exposed to BDs. In general, BDs affect the anodic dissolution mechanism of metals and increase the critical pitting potential. For carbon steel, a component of the BDs acts as an anodic inhibitor, leading to passivation of the surface and decreasing corrosion rates by 1–2 orders of magnitude. For 316, BDs increase the critical pitting potential but may facilitate crevice‐like initiation of localized corrosion. There are negligible effects of BDs on the corrosion of SDSS, and no evidence either for adverse effects on DSS. TSA shows an increased corrosion rate by half to one order of magnitude with BDs, but its ability to galvanically protect a steel base material is not compromised. AW 6082 shows decreased corrosion rate, no difference in mass loss, but increased pitting in the presence of BDs. Overall, these model experiments may only be seen as the start of a more systematic understanding of the interaction of BDs with engineering materials.

Development of a superhydrophobic and bactericide organic topcoat to be applied on thermally sprayed aluminum coatings in offshore submerged components

Marine environment is a very aggressive working atmosphere, where metals are subjected to corrosion and biological attacks, among other phenomena. The protection of steel components and structures has been achieved by means of protective coatings, including coatings with antifouling characteristics to avoid microbiological corrosion. In this work, an organic topcoat usually applied on top of Thermally Sprayed Aluminum (TSA) coatings has been functionalized. The paint was provided with superhydrophobic and antibacterial properties by the addition of SiO2 nanoparticles and biocide agents to its composition. Three biocide agents were evaluated, and the most promising one was selected. The evaluations performed comprised wettability tests, bacterial activity assessment, ecotoxicity tests, adhesion measurements in aqueous marine environment, and weathering aging tests to evaluate the corrosion protection ability of the coatings for its use in aggressive atmospheres. The paint containing a 25% wt of SiO2 and a 1.5 wt% Cu2O presented superhydrophobicity and antibacterial characteristics, with high adherence, low toxicity, and an adequate performance on the weathering aging tests.

Evaluation of Protective Coatings for High-Corrosivity Category Atmospheres in Offshore Applications.

The interest in renewable energies obtained from the resources availed in the ocean has increased during the last few years. However, the harsh atmospheric conditions in marine environments is a major drawback in the design of offshore structures. The protective systems that are employed to preserve offshore steel structures are regulated by several standards (ISO 12944, NORSOK M-501), which classify the corrosivity category of offshore installations as C5-M and Im2. In this work, three coatings employed in offshore components protection have been evaluated according to these standards by performing weathering aging tests in different climatic cabinets. The coatings studied were a thermally sprayed carbide coating with an organic sealant (C1), a thermally sprayed aluminum (TSA) coating with an organic topcoat (C2), and an epoxydic organic coating reinforced with ceramic platelets (C3). The only coating that reached the higher categories in all the tests was the C2 coating. The C1 coating presented ferric corrosion products coming from the substrate in some of the tests, and blistering was detected in the C3 coating.

Corrosion, wear and tribocorrosion performance of a thermally sprayed aluminum coating modified by plasma electrolytic oxidation technique for offshore submerged components protection

Thermally Sprayed Aluminum (TSA) coatings have been widely employed to protect steel components from corrosion in marine environment. However, TSA is highly damaged in transport operations, due to the low wear resistance of aluminum, leading to a rapid coating deterioration during final service. In this work, the TSA properties have been improved by Plasma Electrolytic Oxidation (PEO) technique. The response of the newly generated TSA/PEO duplex system was investigated by means of sliding wear tests, electrochemical corrosion tests, and tribocorrosion tests in synthetic seawater. The study was completed with several characterization techniques, including SEM and confocal microscopy, and X-ray diffractometry.

The Effect of Temperature and Local pH on Calcareous Deposit Formation in Damaged Thermal Spray Aluminum (TSA) Coatings and Its Implication on Corrosion Mitigation of Offshore Steel Structures

This paper is based on experimental data and provides better understanding of the mechanism of calcareous deposit formation on cathodically polarized steel surfaces exposed to synthetic seawater at 30 °C and 60 °C. The study comprises measurement of the interfacial pH of thermally sprayed aluminum (TSA) coated steel samples with and without a holiday (exposing 20% of the surface area). Tests were conducted at the corrosion potential for up to 350 h. It was experimentally determined that the local pH adjacent to the steel surface in the holiday region reached a maximum of 10.19 and 9.54 at 30 °C and 60 °C, respectively, before stabilizing at about 8.8 and 7.9 at the two temperatures. The interfacial pH on the TSA coating at 30 °C was initially 7.74 dropping to 4.76 in 220 h, while at 60 °C it increased from pH 6.41 to the range pH 7.0–8.5. The interfacial pH governed the deposition of brucite and aragonite from seawater on the steel surface cathodically polarized by the TSA. This mechanism is likely to affect the performance of TSA-coated offshore steel structures, especially when damaged in service.

Can Thermally Sprayed Aluminum (TSA) Mitigate Corrosion of Carbon Steel in Carbon Capture and Storage (CCS) Environments?

Transport of CO2 for carbon capture and storage (CCS) uses low-cost carbon steel pipelines owing to their negligible corrosion rates in dry CO2. However, in the presence of liquid water, CO2 forms corrosive carbonic acid. In order to mitigate wet CO2 corrosion, use of expensive corrosion-resistant alloys is recommended; however, the increased cost makes such selection economically unfeasible; hence, new corrosion mitigation methods are sought. One such method is the use of thermally sprayed aluminum (TSA), which has been used to mitigate corrosion of carbon steel in seawater, but there are concerns regarding its suitability in CO2-containing solutions. A 30-day test was carried out during which carbon steel specimens arc-sprayed with aluminum were immersed in deionized water at ambient temperature bubbled with 0.1 MPa CO2. The acidity (pH) and potential were continuously monitored, and the amount of dissolved Al3+ ions was measured after completion of the test. Some dissolution of TSA occurred in the test solution leading to nominal loss in coating thickness. Potential measurements revealed that polarity reversal occurs during the initial stages of exposure which could lead to preferential dissolution of carbon steel in the case of coating damage. Thus, one needs to be careful while using TSA in CCS environments.

Thermally Sprayed Aluminum Coatings for the Protection of Subsea Risers and Pipelines Carrying Hot Fluids

This paper reports the effect of boiling synthetic seawater on the performance of damaged Thermally Sprayed Aluminum (TSA) on carbon steel. Small defects (4% of the sample’s geometric surface area) were drilled, exposing the steel, and the performance of the coating was analyzed for corrosion potential for different exposure times (2 h, 335 h, and 5000 h). The samples were monitored using linear polarization resistance (LPR) in order to obtain their corrosion rate. Scanning electron microscopy (SEM)/energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) were used for post-test characterization. The results showed that a protective layer of Mg(OH)2 formed in the damaged area, which protected the underlying steel. Additionally, no coating detachment from the steel near the defect region was observed. The corrosion rate was found to be 0.010–0.015 mm/year after 5000 h in boiling synthetic seawater.

Corrosion of Cathodically Polarized TSA in Subsea Mud at High Temperature

Thermally sprayed aluminum (TSA) is not immune to corrosion when polarized by sacrificial aluminum anodes. However, the corrosion rate is usually low resulting from a protecting oxide on the TSA surface, keeping the TSA passive. A certain cathodic reaction rate is found on the TSA, producing hydroxide. The hydroxide may elevate the pH on the surface of the TSA to a range where aluminum is corroding actively. When the TSA is exposed to seawater, the hydroxide will be transported away from the TSA surface by diffusion and convection, and the pH is maintained within the passive range. However, in mud there is no convection and the diffusion is limited. This may result in accumulation of hydroxide at the TSA/mud interface and activation of the TSA. The cathodic polarization may therefore result in decreased lifetime of the TSA in mud. Corrosion of cathodically polarized TSA in mud has been studied at various temperatures and potentials. It was found that polarization of the TSA to −1.2 VAg/AgCl resulted in in...

Mitigating Localized Corrosion Using Thermally Sprayed Aluminum (TSA) Coatings on Welded 25% Cr Superduplex Stainless Steel

Thermally sprayed aluminum (TSA) coating has been increasingly used for the protection of carbon steel offshore structures, topside equipment, and flowlines/pipelines exposed to both marine atmospheres and seawater immersion conditions. In this paper, the effectiveness of TSA coatings in preventing localized corrosion, such as pitting and crevice corrosion of 25% Cr superduplex stainless steel (SDSS) in subsea applications, has been investigated. Welded 25% Cr SDSS (coated and uncoated) with and without defects, and surfaces coated with epoxy paint were also examined. Pitting and crevice corrosion tests, on welded 25% Cr SDSS specimens with and without TSA/epoxy coatings, were conducted in recirculated, aerated, and synthetic seawater at 90 °C for 90 days. The tests were carried out at both the free corrosion potentials and an applied cathodic potential of −1100 mV saturated calomel electrode. The acidity (pH) of the test solution was monitored daily and adjusted to between pH 7.5 and 8.1, using dilute HCl solution or dilute NaOH, depending on the pH of the solution measured during the test. The test results demonstrated that TSA prevented pitting and crevice corrosion of 25% Cr SDSS in artificial seawater at 90 °C, even when 10-mm-diameter coating defect exposing the underlying steel was present.

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 μm/year at ~18 °C and ~6-7 μm/year at 80 °C.

Effects of Mercury on Thermally Sprayed Aluminum Coatings

The quantification of liquid metal embrittling effects of mercury on thermally sprayed aluminum was investigated. The program consisted of heat treatments at 250 and 300°C of thermally sprayed (aluminum on lead) cylinders with mercury additions of 0.25, 0.50, 1.00, and 3.00wt.%. The cylinders were then tested to determine the adhesion/cohesion strength of the thermally sprayed bond. The Pb/Al interfaces were evaluated using energy dispersive spectroscopy (EDS). The results of the testing indicate that there was no noticeable embrittling effects of the mercury, up to 3.0wt.% Hg in the Pb matrix. Published by Elsevier Science Inc.

Thermal Spray for Corrosion Control: A Competitive Edge for Commercial Shipbuilding

Thermal spraying of steel with aluminum to protect it from corrosion is a technology that has been proven to work in the marine environment. The thermal spray coating system includes a paint sealer that is applied over the thermally sprayed aluminum; this extends the service life of the coating, and provides color to the end product. The thermal spray system protects steel both through the principle of isolation (as in painting) and galvanization (as in galvanizing). With this dual protection mechanism, steel is protected from corrosion even when the coating is damaged. The thermal sprayed aluminum coating system has proven to be the most cost-effective corrosion protection system for the marine environment. Until recently, however, the initial cost of application has limited its use for general application. Today a new arc spray technology has reduced the application cost of thermal spraying aluminum to below that of painting. Commercial shipbuilders could use this technology to enhance their market position in the marine industry.