DETECTION AND MONITORING OF NAPTHENIC ACID CORROSION IN A VISBREAKER UNIT USING HYDROGEN FLUX MEASUREMENTS

High corrosion rates of up to 0.8 mm/per month were detected on the Visbreaker fractionation column.  Internal examination of the column showed that the shell corrosion was confined to the vicinity of four trays situated approximately ²/₃ of the way up the column, with the highest rates being seen within the locations of the downcomers.  Examination of the corroded areas within the downcomers indicated that smooth, uniform zones of metal loss characteristic of Napthenic acid corrosion were present at these locations.  Further analyses of both the internal deposits and sections of shell plate taken from the areas of high corrosion indicated that Napthenic acids were present.  Whilst the column was on-line, hydrogen flux measurements were recorded in the areas of active corrosion in order to provide an estimate of the corrosion rate in real time, to monitor the extent of active corrosion and to assess the effectiveness of a newly-initiated corrosion inhibitor program.  The corrosion rate estimations were found to corroborate well with ultrasonic thickness measurements.  The paper considers the effect of a wide range of process variables which may have been expected to cause this unusual and severe form of corrosion in a Visbreaker, and what effect these variables may have on the corrosion rate.  In conclusion, the clearest cause of variation in corrosion-induced hydrogen flux was the application of an inhibitor intended to prevent any further acid attack, which caused flux at most corroding sites to decrease at least 10-fold.