INTRODCUTION Corrosion-fatigue is an important possible mode of structural damage that must be considered in the design and safety evaluation of welded tubular joints used in offshore platforms. It is an especially critical parameter for structures built in hostile and deep water environments that have been avoided in the past when the demand for petroleum products was not so great. Specifically@ the corrosion-fatigue behavior of cathodically protected@ welded carbon steel at long lives (gt; 106 cycles) and in cold seawater (~ 4 0C) is of prime interest in such environments. Because of the interest in this technical area@ Battelle-Columbus has completed a detailed and exhaustive interpretative review of it (1)*. This review covered six major areas ?C (1) information on fatigue performance and design of welded joints@ (2) corrosion of structural steel in seawater@ (3) cathodic protection of steel saltwater@ (4) carrion-fatigue of structural steel in seawater@ (5) test methods and variables for corrosion-fatigue experiments@ and (6) long-range research needs relevant to corrosion-fatigue of steel in seawater. Based upon work from this review@ a paper (2) will also be published covering item (4). In item (5)@ an exploratory test program was defined and discussed in detail. The objective of this testing was to (1) calibrate and verify test procedure@ (2) assess the most important variables affecting corrosion-fatigue of steel in seawater@ and (3) develop limited@ preliminary corrosion-fatigue data. Variables planned to be investigated in the testing included: ? Crack initiation for Nf = 105 to 107 cycles and crack growth for da/dN = 2.54 x 10-3 to 2.54 x 10-6 m/cycle (10-7 to 10-4 in./cycle) ? 20 and 4 C seawater ? 5 to 8 ppm O2@ 30 to 38 ppt salinity@ and 7.8 to 8.1 pH ? -0.783 and -0.933 V SCE* applied potentials and free corrosion (i.e.@ adequate@ over@ and under protection) ? 0.5 to 10 Hz cyclic frequency ? API 2H steel plate with AWS Class C weld@ where Nf = number of cycles to failure and da/dN = crack-growth rate. Results of this testing will be used to help outline plans for a long-range study of corrosion fatigue with the objective of developing design-oriented information on cathodically protected@ welded carbon steel in a cold seawater environment. The present report describes work that has been completed on the exploratory test program. First@ experimental procedures are briefly described. Second@ experimental results are discussed with fatigue-crack initiation and fatigue-crack propagation covered separately. Finally@ observation and conclusions are briefly summarized. * Numbers in parentheses indicate reference listed at the end of this report. * Saturated calorel electrode.
API OSAPR 4-1977 history
1977API OSAPR 4-1977 CORROSION FATIGUE OF CATHODICALLY PROTECTED@ WELDED CARBON STEEL IN COLD SEAWATER