Abstract
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.
Highlights
Carbon steel is used in offshore structures where corrosion, mainly caused by the constituents of seawater, is often exacerbated by fluctuating temperature and oxygen availability [1,2]
This study aims to address this knowledge gap and, for such purpose, experiments were designed to acquire local pH data on thermally sprayed aluminum (TSA) coatings and steel surfaces cathodically polarized by a TSA coating, while both considering exposure to seawater at 30 ◦ C and 60 ◦ C
The results show that the conditions for the precipitation of compounds were governed by the environment created in this study as the local pH reached values higher than the required minimum (Figure 13)
Summary
Carbon steel is used in offshore structures where corrosion, mainly caused by the constituents of seawater, is often exacerbated by fluctuating temperature and oxygen availability [1,2]. The calcareous deposits are beneficial to the structure since they act as a barrier to diffusion of dissolved oxygen [5]. Seawater is mildly alkaline (pH 7.8–8.3) and it is reported that a minimum pH of 9.5 and 7.5 is required at 25 ◦ C for the deposition of Mg(OH) and CaCO3 layers, respectively [5,6,7,8,9]. A higher pH is required for the deposition of Mg(OH) since seawater is usually unsaturated with Mg(OH) , while surface seawater is supersaturated with CaCO3 [10]
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