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

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.

Effect of seawater constituents on the performance of thermal spray aluminum in marine environments

“…Values obtained in artificial seawater over the same period of time are presented here as well. The behavior of TSA in artificial seawater has been previously reported looking at different environmental parameters such as temperature, pH or position within the offshore structure (atmospheric, splash, tidal, or submerged) . Comparing these with data obtained in the present studies, corrosion rates at the end of the test are of the same order of magnitude.…”

Prediction of Thermal Spray Coatings Performance in Marine Environments by Combination of Laboratory and Field Tests

“…Fluctuations of potential values in this initial stage correspond to changes on the surface of the coatings. Upon immersion, the air-formed oxide layer dissolves and the new aqueous oxide layer begins to form [18,19]. The formation of the aqueous oxide layer leads to passivation of the coatings, which helps to reduce the self-corrosion.…”

“…Their nature, nucleation and growth depend on potential, current density, time of exposure, pressure, temperature and seawater chemistry [17]. The effect of the ions (present in seawater) on the performance of TSA revealed the formation of the calcareous deposits bilayer on artificial defects at ambient temperature [18], as well as at 60 • C [19].…”

Calcareous Deposits Formed Under Long‐Term In Situ Cathodic Protection Tests

“…Supporting Information for this article can be found under DOI: 10.1002/ceat.202000546. This section includes additional references to primary literature relevant for this research 31–35.…”