What Does Calcareous Coating Mean?
A calcareous coating is a layer that consists of calcium carbonate and other salts deposited on the substrate's surface. When the surface is cathodically polarized, as in cathodic protection, this layer is the result of the increased pH adjacent to the protected surface.
In other words, it is a layer that contains a mixture of calcium carbonate and magnesium hydroxide deposited on surfaces that are cathodically protected against corrosion, due to the protected surface's increased pH adjustment.
The importance of calcareous deposits to the effective, efficient operation of marine cathodic protection systems is generally recognized by engineers and scientists concerned with cathodic protection in submerged marine environments.
A calcareous coating is also known as a calcareous deposit.
Corrosionpedia Explains Calcareous Coating
The chemistry and structure of this type of surface film depends on its nucleation and growth kinetics, which, in turn, are determined by variables like:
- Seawater chemistry
- Substrate surface condition
When calcium and magnesium are supersaturated in seawater, they form calcium carbonate (CaCO3), magnesium carbonate (MgCO3) and magnesium hydroxide Mg(OH)2. These solid products, known as calcareous deposits, promote a physical barrier against oxygen diffusion, hence decreasing the corrosion rate. The different forms of calcareous deposits have different structure and form under different parameters.
Several factors influence the formation of calcareous deposits, including:
- Sea water chemistry
There is an increasing interest within oil and gas production in placing most of the production on the seabed. In that connection, there is a need for subsea heat exchangers. Normally, materials used underwater are made of steel protected with cathodic protection. Steel is a relatively cheap material. In addition to lowering the immune state potential, use of cathodic protection leads to precipitation of calcareous deposit, due to increased interfacial pH. This layer functions as a barrier against the corrosive environment, leading to a decrease in current demand.
However, a calcareous deposit also hinders thermal conductivity, which is unwanted in connection to a heat exchanger. Thus, it is important to find a solution in which calcareous deposits do not precipitate on the surface.
The main element of calcareous deposits is magnesium (Mg). Studies have shown that Mg hinders the precipitation and growth of CaCO3, giving reason to believe that Mg-containing alloys contain less calcareous deposits.