A recent corrosion inhibitor recommendation for a low water cut gas condensate line was conducted by the use of weight loss measurements aided by the use of white light interferometry. Weight loss was studied using a standard rotating cage methodology. To assess localized attack on the coupons both as pitting and preferential weld corrosion (PWC), white light interferometry was utilized and the 3D scans that were produced were then used to assess both tendency for pitting corrosion as well as the depth of attack across the weldment regions.
Machined coupons from the pipeline material were provided by the operator to a set geometry to be used in a rotating cage apparatus. After preparation for corrosion testing, the coupons were prescanned then exposed to both inhibited and chemical-free synthetic fluids for periods of 120 and 500 hours. The tested coupons were then cleaned of corrosion products and rescanned using interferometry to assess any corrosion that had occurred.
From initial pre-test coupon geometries, the average weight loss results, and the depth of attack, individual corrosion rates for the weldment regions were calculated to assess the PWC in a complementary manner to using electrochemical techniques for determination of PWC. In addition to corrosion rate data, 3D profiling allowed visual assessment of preferential weld attack that is not possible using segmented weld electrodes, where individual electrodes are isolated and material lost due to the machining process. Evidence of localized corrosion in the form of pitting was captured across the weight loss coupon during the scanning process.
The technique allows for the following:
Examination of whole weld sections with no material wastage between metallurgical zones during coupon machining
Collection of supplementarydata to compare to electrochemical testing of welds
Providing greater power for spatial assessment of PWC from 3 dimensional interferometry images
This paper discusses the white light interferometry technique and results in detail, as well as demonstrating the pros and cons to utilizing non-destructive measurements, when compared to existing methodologies, for assessing localized corrosion.