Subsea expertise

Buckleys BathyCorrometer® and reference electrodes are used to determine the effectiveness of marine cathodic protection (CP) systems and confirm the structure corrosion status.

CP is an electrochemical means of corrosion control on marine structures.

On offshore pipelines and marine structures CP is used to supplement the corrosion protection afforded by the structure coating and prevent corrosion at coating defects.

About iron corrosion

Rust is the common name for a very common compound, iron oxide.

Iron oxide (chemical formula Fe2O3) is common because iron combines very readily with oxygen. Energy is applied to create iron from rust. This energy is released when iron converts back to rust.

Corrosion in general

Most metals are more stable as their oxide than as the metal itself.

Energy is applied when converting iron oxide to iron and that energy is stored in the iron.

The iron will try and revert back to its natural lowest energy state and that is the driving force for the corrosion process; so iron will try and revert to its naturally occurring state: iron oxide.

On a freely corroding structure anodes and cathodes are distributed around the structures. Anodes have a positive charge whereas cathodes have a negative charge. Very small anodes and cathodes form next to one another because of small surface differences.

The anodic and cathodic areas are constantly changing as corrosion films form or are removed. This results in a uniform metal loss.

General corrosion is a significant threat to marine structures.

 

Anode and cathode reactions

Hydroxide  produced at the cathode reacts with ferrous ions in a near neutral solution to form ferrous hydroxide.

Ferrous hydroxide is further oxidised to form red brown rust; Fe2O3.H2O.

Depending on the supply of oxygen, other forms of rust may also form Fe3O4 (magnetite) and y-Fe2O3.

Typically rust is a combination of all three types, the proportion of each depends on the supply of oxygen. In environments with a lack of oxygen the cathodic reaction is: 2H+  +2e-→2Hads → H2.

Atomic hydrogen can enter the steel, molecular hydrogen (H2) cannot.

 

The corrosion process

  • Rust is formed at the anode and the steel surface corrodes;
  • At the cathodic sites, oxygen is removed from the water;
  • Without oxygen, corrosion cannot happen;
  • When CP is applied the entire surface of the structure to be protected becomes the cathode of an electrochemical cell;
  • The only reaction is then removal of dissolved oxygen.

Cathodic protection

Cathodic protection is an electrochemical means of corrosion control.

The object of cathodic protection is to ensure that the whole of the material under protection is made the cathode of an electrochemical cell.

The metal under protection now behaves as a cathode so the reaction: Fe→Fe2+ + 2e

cannot proceed and is replaced by the cathodic reaction: 1/2O2 + H2O+2e→2OH-.

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