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.
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.
Purpose In spite of the high interest in thermally sprayed aluminium (TSA) to protect offshore structures, pipelines, risers, etc., data correlating the exposure of TSA-coated steel in hot seawater to its performance are lacking. No information is available on the performance of TSA in hot seawater, especially when damaged. This paper aims to address this knowledge gap and report the corrosion performance of damaged TSA on carbon steel exposed to boiling synthetic seawater for different periods (up to 5,000 h). It also attempts to understand the mechanism of formation of calcareous deposits in boiling synthetic seawater by using various characterisation tools. Design/methodology/approach Twin wire arc spray was used to coat carbon steel specimens with commercially pure aluminium. Holiday was drilled on the coated surface to expose the underlying steel, amounting to 4 per cent of the specimen surface area. These specimens were then exposed to boiling synthetic seawater for different periods: 2, 68, 188, 335 and 5,000 h. During exposure, the potential was monitored (vs standard calomel electrode [SCE]) and linear polarisation resistance (LPR) method was used to calculate the corrosion rate (CR). After exposure, these specimens were analysed by using characterisation tools such as X-ray diffraction (XRD) and scanning electron microscope (SEM)/energy-dispersive X-ray spectroscopy (EDX). Findings The TSA-coated steel samples with 4 per cent holiday showed a CR of approximately 0.008-0.015 mm y−1 and a stable potential (Ecorr) around −800 mV (SCE), when exposed to boiling synthetic seawater for approximately 5,000 h. Microstructural characterisation established that the deposits comprised brucite [Mg(OH)2] only (this is very different to what has been reported at ambient temperatures). The stable potentials obtained after 5,000 h suggest that the brucite layers provided effective corrosion mitigation. Thus, TSA seems to show the potential to protection hot carbon steel risers, even when damage exposing the external steel surface to the seawater is present. Originality/value The mechanism of calcareous deposit formation has been studied for many years, but most of the previously reported studies concentrate on low or ambient temperature behaviour. The reported studies mainly concentrate on the application of cathodic protection, rather than the effect of TSA. This is the first journal paper where the behaviour of damaged TSA in boiling synthetic seawater has been reported. The formation of brucite-only layer in the damaged area adjacent to the steel surface in boiling synthetic seawater is novel. And the mechanism of formation of this layer has been explained in the paper.
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