A Straightforward Method for Predicting Corrosion in Galvanically Coupled Piping Systems

摘要

Some piping systems used in deep water oil production, including risers and flow-lines, or those used for seawater piping, are galvanically coupled. For oil production risers or flow-lines, a portion of steel pipe may be coupled to another portion, clad internally with a nickel-based alloy such as Alloy 625(1> (UNS No. N06625); a steel pipe may be coupled directly to a titanium stress joint at a touch-down zone. With a conductive electrolyte flowing inside the piping, the galvanic coupling between two dissimilar metals can elevate the corrosion of the less noble metal (steel) pipe and increase the hydrogen charging into the more noble metal (e.g., nickel alloy or titanium) pipe. The distribution along the pipe's longitudinal direction of the galvanic potential and current has traditionally been predicted through numerical modeling by using as model inputs the polarization curves of the two metals or the Tafel equations derived from the polarization curves. This numerical simulation, however, requires the use of a computer software code, a skill not often mastered by those who rarely conduct such an analysis. This paper reports a straightforward four-step procedure that needs only the use of an Excel spreadsheet (ES) to acquire the same results. Results obtained from this ES method are actually more reliable. These ES results are compared with those from known numerical and analytical methods.