API REPORT 4 Corrosion Fatigue of Cathodically Protected, Welded Carbon Steel in Cold Seawater

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API REPORT 4 Document Information:

Title
Corrosion Fatigue of Cathodically Protected, Welded Carbon Steel in Cold Seawater

American Petroleum Institute

Publication Date:
Feb 11, 1977

Scope:

ABSTRACT

A series of four fatigue-crack initiation and two fatigue-crack propagation experiments were conducted on welded ABS DH32 steel plate in seawater and with cathodic protection. At a hot-spot strain range of 0.001 (stress ratio of 0.1), the seawater environment had no significant detrimental effect on cyclic life at both 20 and 4 C with −0.85 V Cu/CuSO4 cathodic protection and 20 C under freely corroding conditions. Increased levels of cathodic protection caused slightly increased crack growth rates for stress-intensity-factor ranges below 12 ksi/in. at stress ratio of 0.1 and in seawater at 20 C. For stress-intensity-factor ranges between 15 and 20 ksi/in., crack-growth rates were constant and cathodic protection was slightly beneficial under the same conditions. Reducing the seawater temperature to 4 C caused an increase in crack-growth rate only at 0.5 Hz and with overprotection (−1.00 V Cu/CuSO4). It had little effect with overprotection at Hz or at both 10 and 0.5 Hz with no protection or adequate protection.

Results of these experiments will be used to help establish procedures and plans for a long-range (3 to 4 years) program on developing corrosion-fatigue information on structural steel in seawater for application to design of welded tubular joints in offshore structures.

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