Controlling the Components of Calcareous Deposit to Protect Steel Structures from ALWC with DC and Pulse Current

Accelerated Low Water Corrosion (ALWC) is a particularly severe form of microbiologically-induced aggressive localised corrosion, which is most commonly found close to the level of the Lowest Astronomical Tide (LAT). To mitigate, the calcareous film, a deposit of CaCO₃ and Mg(OH)₂ produced on steel structures that are cathodically protected, has been studied as a corrosion control coating, inexpensive and friendly to environment. This film has two functions: reducing the current demand from the CP system and acting as a protective coating. The properties and endurance depend on film structure, so when deposition are appropriately controlled, it could provide long-term corrosion protection for steel in seawater. In this work, calcareous deposits formed under impressed current cathodic protection (ICCP) of both DC and pulse current in artificial seawater were analyzed in the combination of modelling to point out methods to control calcareous film quality; and the relative proportions of brucite (Mg(OH)₂), calcite and aragonite (CaCO₃) on applied potential.

Key words: ALWC, low water corrosion, calcareous deposit, brucite, calcite, aragonite, endurance, cost effectiveness, quality, DC, pulse current.