16 February 2012
Durability of metallic structures exposed to natural seawater is often linked to the efficiency of protective systems which consist mainly of cathodic protection.
Unsuitable cathodic protection may induce higher costs of installation and services, loss of performance of the structures, etc.
The design of a cathodic protection system involves many calculations whose results are dependent on the structure (materials, surface area exposed to seawater, etc) and on environmental parameters. These parameters are connected to the properties of the interface metal/seawater and are specific to the structure (alloy, surface state, coating, biofouling, etc) and the marine environment (salinity, dissolved oxygen, temperature, flow rate, etc).
The most common way to select current demand for a cathodic protection design consists in applying data available in the literature. In case of inaccurate data with regards to actual parameters, it can result in an over/under-protection of the installation and therefore an over-cost or underestimated lifetime of the cathodic protection.
The French Corrosion Institute carried out a research work with the objectives of supplying to cathodic protection designers data collected in situ, in the actual environment in which the protected structure is deployed.
The experiment was based on the deployment of six arrays which met CPC sensors from nke electronics and an environmental parameter data logger.
A CPC sensor records the coupling current between a cathode and a sacrificial anode, and the cathode potential versus a pseudo-reference electrode (pure zinc) as a function of time. A resistance set between the cathode and the anode simulates the natural circuit resistance of the cathodic protection system (eg structure plus electrolyte). Cathode materials can be made of carbon steel, stainless steel or any other material whereas and anode material can be made of zinc, aluminium indium or aluminium gallium alloys.
By immersing a set of CPC sensors for each tested material, (eg each sensor with a different resistance value and eventually equipped with different anode alloy), it is possible to build a so-called pseudo-polarization curve corresponding to the stationary state and to determine the cathodic current demand for the conventional protection potential. These pseudo-polarisation curves are integrated to various softwares which calculate cathodic protection potential and current density distribution of cathodically protected structures.
Forty-eight CPC sensors were deployed from 9 to 18 months, at 35m to 900m depth. All data were recovered successfully and the results of this research are now integrated in major oil companies’ general specifications for cathodic protection design.
A new research is ongoing which is focused on cathodic protection of stainless steel in deep sea environment. Three CPC sensor arrays will be immersed from 150m to 2200m depth. Two shallow sites are going to be monitored with array directly laid on seafloor.