Materials Performance

NOV 2014

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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56 NOVEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 11 BLOG Continued from The MP Blog, p. 13. The following items relate to chemical treatment. Please be advised that the items are not peer-reviewed, and opinions and suggestions are entirely those of the inquirers and respondents. NACE Inter- national does not guarantee the accura- cy of the technical solutions discussed. MP welcomes additional responses to these items. They may be edited for clarity. Corrosion in spent caustic line Q: We are facing corrosion in the bottom of a carbon steel (CS) pipe. Detailed inspection of the pipe by cutting it in two shows deep scattered pitting corrosion (5 to 25 mm in diameter) in the bottom of the pipe, resulting in frequent leakage on the bottom. The top half of the pipe is slightly affected. The line is not used regularly. We suspect that the stagnant solution in the pipe (half- f illed) and the presence of oxygen are causing the corrosion. The line operating condition is as follows: • Line service: spent caustic • Total dissolved solids (TDS): 5,000 ppm • pH: 8.3 • Temperature: 38 °C • Pipe size: 4-in ASTM A053 Can someone explain the cause of the severe corrosion in the pipe bottom and suggest remedial measures? A: It sounds like your assessment of the problem is correct. Remove oxygen or keep dry, assuming corrosion is occurring during down or idle time. Chemical oxygen scavengers such as sodium sulfte (Na 2 SO 3 ) added to residual liquid or moisture would remove oxygen. A: At pH 8.3, it is most probable that the solution is primarily bicarbon- ate, with a trace of carbonate. I would check for chloride. Te TDS may have a suspended solids component. Settling out of suspended solids, along with chloride content >100 mg/L, would explain the severity and location of the corrosion. Bicarbonate solutions are weak ly aggres- sive to steel. A: I can think of no permanent remedial measure as long as you do not use the line continually, but this advice may help: 1) stress relieve the welded joints, and 2) provide stream tracers and proper insulation to keep the caustic hot. A: Te easiest solution is to replace the system with high-density polyethylene or another nonmetallic system. Using an oxygen scavenger to treat a half-flled pipe may not be success- ful if the vapor space in the pipe is air. We have multiple locations that have spent caustic in CS piping systems. We also have some degree of corrosion in these

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