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.