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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. Offshore pipelines and marine structures use CP to supplement the corrosion protection afforded by the structure coating and prevent corrosion at coating defects.

Since the early 19070's, the BathyCorrometer® has been the industry-standard device for carrying out underwater inspection of cathodic rust protection systems. Buckleys has developed a range of cathodic protection inspection equipment alongside the BathyCorrometer to suit a variety of cathodic protection survey procedures, such as our UCP1A and UPC1B underwater cathodic protection survey probes which are ideal for mounting to ROV's (Remotely Operated Vehicles), and our marine survey kits which enable topside cathodic protection inspections of jetties, marinas and other fixed structures.

About 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. We apply energy to create iron from rust and 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. 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; Fe2OH2O.

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: 12O2 + H2O+2e →2OH-.

The Historical Background

The principles of cathodic protection were clearly understood and concisely expressed by Sir Humphry Davy as long ago as 1824. In a paper presented in that year to the Royal Society, of which he was then President, he described a series of experiments he had carried out at the request of the Royal Navy who were concerned at “the rapid decay of the copper sheeting of His Majesty’s Ships of war and the uncertainty of the time of its duration”.

In the course of this paper he wrote:

"Copper is a metal only weakly positive in the electrochemical scale; and, according to my ideas, it could only act upon sea water when in the positive state; and, consequently, if it could be rendered slightly negative, the corroding action of sea water upon it would be null; and whatever might be the differences of the kinds of copper sheeting and their electrical action upon each other, still every effect of chemical action must be prevented, if the whole surface were to be rendered negative. . . .

A piece of zinc as large as a pea, or the point of a small iron nail, were found fully adequate to preserve forty or fifty square inches of copper; and this, wherever it was placed, whether at the top, bottom, or in the middle of the sheet of copper, and whether the copper was straight or bent of made into coils. And where the connection between different pieces of copper was completed with wires, or thin filaments of the fortieth or fiftieth of an inch in diameter the effect was the same; every side, every surface, every particle of copper remained bright, whilst the iron or the zinc was slowly corroded."

Davy also investigated the impressed-current system, using a voltaic battery, but he did not consider this to be practical in service conditions. Although the beginnings of cathodic protection date therefore from 1824 this important method of protecting metals – particularly steel – from corrosion was neglected for over a century.

Cathodic protection theory & design

There are two types of cathodic protection systems;

  • Galvanic (sacrificial) anode systems
  • Impressed current anode systems

Both systems achieve the same effect and are compatible with each other.

Sacrificial anode CP system

  • Anodes are connected directly to structure to be protected.
  • The anode corrodes (or sacrifices itself) and the current produced flows to the structure surface and removes the oxygen dissolved in the water.
  • Magnesium or zinc anodes are used onshore.
  • Offshore, the anodes are zinc but generally aluminium/zinc indium alloy anodes.
  • Anodes are provided with a steel core which is welded to the structure to be protected. 

Cathodic protection theory & design

Marine impressed current CP systems consist of the following:

  • Power source (transformer rectifier) These provide a DC current and voltage to an impressed current anode and are often powered by an AC supply
  • Impressed current anode these allow current to flow but do not suffer rapid degradation. Examples are platinum coated titanium or mixed metal oxide (MMO) coated

Impressed current CP systems are used on ships, jetties and in some cases as retrofit CP systems for offshore platforms.

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.

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