The corrosion inspection of a cathodically protected structure can be conducted by measuring:

• Electrochemical potentials (CP)
• Electric field strengths (FG)

The electrochemical potential of a steel member is a parameter defining the protective level produced by the Cathodic Protection system installed on the structure. It is commonly accepted that a properly protected structure must have a potential level between -800mV and -1050mV if zinc sacrificial anodes are installed, and between -800mV and -1100mV for aluminium anodes (ref. Ag/AgCl). An impressed current system can be adjusted to adequately protect the structure and keep the potential in the -1000mV region, although depending on positioning of the anodes this system does not always give even protection across the entire jacket.

If the potential reading is more positive than -800mV the point of measurement is considered to be under-protected creating a possible environment for corrosion action on the steel surface. On areas of potentials more negative than -1050mV/1100mV, there could be the danger of hydrogen embrittlement of the steel surface occurring. Thus weakening the steel member and leading to possible failure.

For seabed members under anaerobic conditions (buried), it is generally accepted that a potential more negative than -900mV is required to achieve satisfactory protection.

Valuable information about the properties of the cathodic protection can be obtained by examination of the potential profiles over the structure and especially at anodes. If the anode has a high current output, there will be a significant potential drop (electric field) in the seawater close to the anode indicating a high current drain to protect the nearby structural components. A low current output will give a low potential profile along the attachment member and will indicate a probable stable polarisation condition.

The anode usually has to be polarised in a more positive (anodic) direction to give a high current output. The potential of unpolarised zinc or aluminium in seawater is in the order of -1050 mV and -1100 mV respectively.

Anode potentials which are considerably more positive than this, indicate a high current output, however, it is possible to observe, in some cases, less negative anode potential with low current output due to passivation of the anode material.

The potential profiles and field strength readings obtained over structural members give a good indication of the corrosion protection level. The potential gives general information while the field gradient can give more detail on anode activity and areas requiring higher current distribution i.e. at nodes, pile sleeves, conductor guide frames etc. Also from field gradient readings, anode current output can be calculated and remaining anode life can be estimated.

The accuracy of the potential readings is controlled by strict calibration of the half-cells against a calomel electrode.

Therefore from the close relation between potential and field gradient measurements, it is possible to compute the level and distribution of potential and current density over the structure giving a general representation of the efficiency and effectiveness of the CP system.

Typical results can be summarised as follows:

• A well-protected structure is expected to have:
• Low electric field strength, and thus, low anode current output
• General potential levels between -900mV and -1050mV against Ag/AgCl

• High field strengths and higher current output at anodes
• Large drops in the potential profiles at the anode due to high current output
• High field strength readings at local areas of high steel concentration.
• General potential levels more positive than -850mV

Estimation of remaining anode life can be adopted from numerical calculations, however it is possible to utilise a computer modelling technique to determine more accurate current density distribution and anode outputs.

iicorr have a sophisticated suite of programmes for accurate CP data analysis and have access to computer modelling of pipelines and structures, drawing from a large database of information compiled from numerous corrosion inspection surveys.

In order to ensure continued accuracy of results, the measuring electrode should be regularly calibrated against a saturated calomel reference electrode every 24 hours in accordance with DNV Procedure RP B403, March 1987.

It should be noted that the largest changes in CP levels are normally measured within 1m from the pipe surface, i.e. there may often be very little difference in Cathodic Potential between 2m and 5m. Strict cell calibration procedures should be maintained according to DNV RP B403.