ICP Data: In-Situ Data Collection for Cathodic Protection Design

The 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 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 (e.g. EN 13174, DNV RP B401). 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 and Ifremer carried out a joint industrial project entitled “ICP-DATA”, with the objectives of supplying to cathodic protection designers data collected in situ, in the actual environment in which the protected structure is deployed:

Sensors arrays have been launched and recovered at six different sites.  The deployment depths were from 35 m to 900 m.

The measurements are based on the deployment of a CPC sensor which records the coupling current between a cathode and a sacrificial anode, and the cathode potential versus a pseudo-reference electrode as a function of time. A resistance set between the cathode and the anode simulates the natural circuit resistance of the cathodic protection system (e.g. structure plus electrolyte). Cathode materials were carbon steel and stainless steel and anode material were zinc, aluminium indium, aluminium gallium alloys and iron.

First, the effect of temperature and pressure on calcareous deposit properties was put into evidence and was useful to explain the different pseudo polarisation curves obtained at the different sites.

When comparing the results of this work to standard and recommended practices for CP of carbon steel structure, 3 points can be noticed:

-        At all sites, the recorded initial current demand was higher than the recommended practice.

-        The recorded steady state current demand for tropical area was highly lower than the recommended practice.

-        The current demand of a site classified as Arctic area was the half of the recommended practice.

When considering the CP of stainless steel structures,

-        The initial current was independent of the location.

-        The steady state current demand to maintain a potential at around -600 mV/AgAgCl depends strongly on the environment.

-        The current demands tent to decrease with time in tropical and arctic zones, but increased in sub-tropical area.

-        The current demand to maintain a stainless steel structure at -950 mV/Ag.AgCl does not depend on the location.

When considering the recorded performances of galvanic materials,

-        Capacities of both aluminium alloys were lower than theoretical values.

-        The performances of magnesium alloy matched the theoretical values.