Protection of hot subsea risers by using thermally sprayed aluminium

Abstract

PurposeIn 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/approachTwin 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).FindingsThe 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/valueThe 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.