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

Abstract

The calcareous deposits formed under commonly applied cathodic protection techniques on coated and uncoated carbon steel samples immersed in Guanabara Bay for 150 days, simulating ships and pipelines, were investigated. Potentials were monitored throughout the analysis, and deposits were examined by visual inspection, optical microscopy, energy‐dispersive X‐ray spectroscopy, and X‐ray diffraction. Beyond calcite, aragonite, and brucite, which are crystals commonly reported in laboratory tests or under confined space conditions, herein, also calcite‐II and calcite‐III were identified as intermediate crystalline structures. Furthermore, it was demonstrated that biological activity also plays an important role, forming calcareous tubular structures within the deposit.

Understanding biofouling and contaminant accretion on submerged marine structures

“…Although calcareous deposition may afford some benefits in corrosion protection and structural reinforcement, its presence may introduce structural and operation issues typical of hard fouling, such as interfering with heat exchange surfaces, impediment of moving parts, and the obstruction of components such as sensors, connecters, and interfaces 1,9 . More research is needed across different environmental marine settings into the adhesion mechanics/dynamics and composition of calcareous deposits formed under cathodic protection, along with whether the observed protective effects is maintained in the long term, and how biological activity interacts with or influences the formation of calcareous material 11,47 .…”

“…The effect of coatings in cathodic protection systems Cathodic protection is seldom used alone to protect marine structures from corrosion, with coatings used in tandem to improve the longevity and effectiveness of the protective system 63,64 . The coatings also serve to prevent the build-up of fouling material, such as calcareous deposition or biofouling, with a study showing a large reduction in fouling on panels with antifouling coatings compared to uncoated panels, regardless of whether the surfaces had galvanic or impressed current cathodic protection 11 . Zinc based coatings have been trialed as a more environmentally friendly alternative, with these coatings appearing to inhibit the formation of calcareous scales through the elimination of brucite and the promotion of hydrated forms of calcium carbonate over aragonite, likely due to the release of zinc cations interfering with the formation process 65 .…”

“…In calcium carbonates, the stability, solubility, and resistance to shear forces is dependent on the polymorph and morphology of the deposited type 2,81,82 . As the structure of calcium carbonate can be modified by the introduction of inorganic salts 47,65 or from biological activity 4,11 , additives or coatings could be used to facilitate the formation of more amenable forms that are easier to remove or cause less impediment. Analysis of interactions with carbonate polymorphs from biotic and abiotic sources are also essential to understanding key connections between fouling and the marine environment, such as how in co-occurrent calcareous deposition from cathodic protection and biocalcifying bacteria interact 4 , preferences of (micro)organism attachment to calcareous surfaces 10 , as well as solubility and adhesion of different polymorphs to submerged surfaces 83 provides vital information needed to assess and combat hard fouling.…”

“…Calcareous deposition and marine growth often co-occur on submerged surfaces, and there is a need to understand how the biotic and abiotic sources interact to produce broader countermeasures against marine fouling 10 . This is crucial as there may be potential differences in adhesion dynamics on surfaces with cathodic protection where the formation of the initial conditioning film is calcareousbased 2,4,11 , compared to the organic material-based film in typical biofouling successional models 9,12 .…”