Existing guidance on the design and construction of offshore installations does not define the optimum level of cathodic protection (CP) and notes that the effectiveness of CP has not yet been proven for structural steels with a yield stress above 400 MPa.
There is clearly a need to establish the optimum and safe working limits of CP in realistic environments if higher strength steels are to be more widely employed in offshore structures. Corrosion fatigue endurance tests have been conducted on welded T-joints of a quenched and tempered steel, with a yield strength of 500 MPa, over a range of cathodic protection potentials between −850 and −1000 mV Ag/AgCl.
Testing was at constant amplitude in artificial seawater at 5°C using a frequency of 0.2 Hz. Fatigue crack initiation and growth were monitored by an automated ACPD technique. The results were compared to those obtained from normalised steel (350 MPa yield stress). In addition fatigue crack growth rates and thresholds were determined, using standard fracture mechanics techniques, under equivalent environmental conditions, to further define the effects of cathodic protection potentials.
The results showed that whilst the fatigue endurance of both higher and lower strength steels was reduced by 50% under free corrosion conditions, the effectiveness of CP was found to vary between the two steels with both applied stress and level of potential having an influence. In particular stresses over 100 MPa and potentials more negative than −950 mV Ag/AgCl could be non-conservative.
It is considered that additional constraints should be included within the HSE Guidance Notes and BS7608 restricting stresses and cathodic protection potentials for high-strength welded joints.