Repairing coatings offshore can be up to 100 times the cost of the initial coating, and NACE International estimates that the total cost of marine corrosion worldwide is between $50-80 billion every year. Source: Maritime Industry. 2018. Maritime Industry. [ONLINE] Available at: https://www.nace.org/Corrosion-Central/Industries/Maritime-Industry/.
With proper planning, ship-owners can ensure that their ships are operating at maximum performance and maintain cost efficiency while preserving the condition of their assets. If poor surface preparation is the cause of deterioration, then the only solution is to remove the paint and start again. It pays to get it right the first time.
Two types of corrosion particularly relevant for the marine industry are pitting corrosion and bacterial corrosion.
How to prevent corrosion
Preventing corrosion requires elimination or suppression using two principal methods, cathodic protection and coatings. Generally, cathodic protection systems are used in conjunction with coating systems.
The objective with cathodic protection is to suppress the electrochemical reaction taking place. Under normal corrosive conditions, current flow from the anode results in a loss of metal at the anodic site which results in the protection of the metal at the cathodic site.
Protection can be provided by making the structure you wish to protect cathodic, using two methods:
1. Sacrificial anodes:
When two dissimilar metals are immersed in seawater, the metal with the lowest electrical potential will suffer the greatest corrosion. For example, the corrosion rate of mild steel can be controlled by connecting it to zinc as it will then become the anode and corrode. In this example, the zinc anode is referred to as a sacrificial anode because it is slowly consumed (corrodes) during the protection process.
Another use of zinc as a sacrificial anode is when steel is coated with the zinc; either in the form of galvanization or metalisation or in a paint which contains high levels of active zinc.
2. Impressed current systems:
A vessel hull can be made cathodic by using a direct current source. An impressed current is applied in the opposite direction to cancel out the corrosion current and convert the corroding metal from anode to cathode. In this example, the negative terminal of DC is connected to a pipeline to be protected. The anode is kept in to increase the electrical contact with its surrounding environment.