Abstract
Knowledge of the hydrogen diffusion coefficient DH in metals susceptible to hydrogen embrittlement is a critical input to models and integrity software, since it directly governs the rate at which hydrogen transport occurs. A review of the literature shows that there is significant discrepancy in published values of DH, even for the same steel grade. For example, for X65 pipeline steel, reported values of DH range from 10-5 cm2/s to 10-7 cm2/s1, which is a variation by two orders of magnitude. Due to this large variability in measured DH values, it is safer to measure DH directly on a portion of the metallic structure of interest, as this would enable the subsequent model/software to make more accurate predictions. Unfortunately, the standard Devanathan-Stachursky measurement technique, as described in ISO 170812 requires a coupon made of the material of interest and is therefore “destructive” in nature. Due to the age of operating assets, it is no longer possible to acquire the same steel for testing and needless to say, cutting a piece of the asset is out of the question.
The present paper discusses a new in-house methodology and apparatus for nondestructive field measurement of the hydrogen diffusion coefficient (diffusivity) of metallic structures (e.g., pipelines, tanks, vessels). The proposed technique makes it possible to carry out in-situ non-destructive measurement of hydrogen diffusion coefficient on any metallic equipment while in operation. In addition to the probe setup, a methodology for processing the acquired electrochemical data and deriving accurate hydrogen diffusivity values has been developed using numerical simulation analysis.