An Active-Path-Dissolution Model for Corrosion-Fatigue Cracking of Drill-Pipe Steels in Offshore-Drilling Environments
by
Hung-Yuan Hsieh
F. V. Lawrence
Department of Civil Engineering
Abstract
The corrosion-fatigue life of drill-pipe steels in simulated offshore-drilling environments was predicted using an analytical model. This model, named the Active-Path-Dissolution (APD) model, simulates a sequence of three corrosion-fatigue processes: chemical notching process, microcrack development, and macroscopic crack growth. The first two processes are combined with mechanical fatigue to simulate corrosion-fatigue crack initiation. A series of fatigue-life simulations for a variety of corrosion-fatigue conditions were carried out. The effects of fluid aeration, electrolytic conductivity (η), electrochemical potential (Ebulk), mean stress and stress ratio (R) effects, fluid temperatures (T), material yield strengths (Sy), pre-existing notch (Kt) effects, and test frequencies (f) were studied to provide data to validate the predictions of the APD model. Experience with the APD model suggests that microcrack development is the dominant period in the corrosion fatigue life.
The APD model partitioning of the corrosion-fatigue cracking into three processes was confirmed by observations of the electrochemical potential (Ebulk) and displacement (Δe) during corrosion-fatigue tests. Observations of current/potential transient phenomena under various waveforms of cyclic loading corroborated that film rupture/film formation induced active-path dissolution is responsible for promoting crack initiation. The SEM micrographs of crack-initiation sites on fractured surfaces provided evidence suggesting the occurrence of vigorous anodic dissolution and galvanic coupling effect within a crack, particularly when the crack size is in microscopic level. The corrosion rate (Tafel’s behavior) and corrosion-fatigue resistance of an API-S135 drill-pipe steel were also studied experimentally in a simulated offshore drilling environment to verify the proposed APD model. No effect of fluid aeration or the addition of a polymeric inhibitor on the corrosion-fatigue system investigated was observed.
Comparison of experimental results with the APD model simulations showed satisfactory agreement. Both the APD model and the experimental results suggest that altering the electrochemical potential (Ebulk) by impressing current or by galvanic coupling with Ion-Vapor-Deposited (IVD) aluminum is an effective and practical means of controlling the corrosion-fatigue phenomenon.
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