We have a problem of excessive corrosion of 316L urea grade weld material in carbamate / urea service. Current data shows that the corrosion rate is 6 mm per year. These welds were done on 316L piping back in August 2007. What are the possible causes of this abnormal corrosion? Which parameters should be looked into?

[updated:LAST EDITED ON Oct-23-08 AT 10:06 PM (CDT)]Thanks Mr. Srinivasan for your reply. It is worth mentioning here that the corrosion / erosion phenomenon is not taking place in base metal of piping as we perform regular inspection of piping. It is still sound over a period of 10 years. Welds are being eroded / corroded continuously in this span and this phenomenon attacks root and hot pass more rapidly and metal loss rate then starts decreasing after root pass and hot pass are eaten up. One of the finest reason is that corrosion / erosion would be more likely to occur in the welded area where there are stress concentration points plus HAZ area. The other reason we found out was the weld material used for piping welding was the same as piping material that is 316L normal grade containing high percentage of ferrite content. Then we decide to re-weld some corroded weld joints with 316L urea grade and did so in August 2007. But we again found them corroded. The question is if for urea carbamate service 316L urea grade is also not acceptable then which material then be selected to avoid much excessive corrosion loss. We are thinking of 25Cr.22Ni.2Mo. Is there any problem with our operation / process?? Or something wrong with welding (but 100% RT conducted)? Regarding your question, corroison is taking place in all the HP circuit Reactor to Stripper, Stripper to carbamate condensers, Carbamate Condensers to Reactor, Carbamate solution from pump discharge to Carbamate Condensers, Scrubber. All HP loop WELD JOINTS are effected and PIPING BASE METAL is sound. Please advise the way to resolve this issue in the light of above information. Thanks & regards, Ashfaq >Ashfaqanwer >Urea carbamate at the discharge of Urea Reactor is highly >corrosive. When Urea Carbamate is down loaded at the down >stream with a pressure reducing valve to reduce pressure >from 200-220 bar (KSC) to 20-22 bar (KSC) it is highly >corrosive. Material like 316 L will erode-corrode easily to >this extent of 5-6 mm per year. > >Is it at the down stream of Reactor where due to high >velocity and moderate temperature of 190-200 deg cent but >pressure reduction or down loading from 200-230 bar (KSC) >to 20-22 bar (KSC)? >Welds under throttling condition will easily erode -corrode >in Urea Carbamate service. > >Another erosion -corrosion prone area is at the discharge of >first decomposer where again throttling of pressure takes >place from 20-22 bar (KSC) to about 2-3 bar (KSC) when >ammonia carbamate enters the second decomposer- i.e, >ammonium carbamate solution when throttled thru pressure >reducing valves will erode - corrode 316 L very severely to >have this 5-6 mm per year corrosion rate. > >Valves and pipes should have enough smooth flow and >throttling condition should be well monitored in Urea >carbamate or Ammonium Carbamate service from Reactor down >stream to First or Second Decomposer in Urea plants. >Hope this helps >C.V.Srinivasan Nishi Engineers Pvt Ltd India Oct 23, 2008 >E-mail: nishi@vsnl.com > > >>We have a problem of excessive corrosion of 316L urea grade >>weld material in carbamate / urea service. Current data >>shows that the corrosion rate is 6 mm per year. These welds >>were done on 316L piping back in August 2007. >> >>What are the possible causes of this abnormal corrosion? >>Which parameters should be looked into?

Thanks for sharing some key informations. We have checked the ferrite number of as welded 316L normal grade which has come out to be 8 which is quite a high number. What impact does it cause on corrosion rate? Are there any welding precautions to control ferrite content in weldments? Regarding sandvik 2RE69, we have also decided for this material to be used next time as a welding material. What do you say about purity of Ar gas for purging as that was 99.5% pure at that time. Is it sufficient purity? What can insufficient purging impart on the quality of weld? I have learnt that heat tints can be formed (oxidation - coloration of weld) due to insufficient purging. But during radiography we did not find any defect. This discussion is really helping me in explroing new dimensions. >Asfaqanwer >Why this affects welds - only - in 361 L urea grade - is >possible due to choice of welding electrode with ferrite >content > 1%. please examine. >Ferrite content in urea grade 316 L welded material >(especially welds / HAZ) should be lower than 1%. Please >examine from this angle or the other reason higher velocity >or higher temperature in the system. > >Sandvik 2 RE 69 or its equivalent is a well proven material >for urea carbamate/ ammonium carbamate service for > 20 >years. >We have used this with success in urea service for > 20 >years in ammonia first stage decomposer tube where problems >of similar erosion - corrosion (not on welds) but on tube >itself > >Hope this helps >C.V.Srinivasan Nishi Engineers Pvt Ltd India Oct 24, 2008 >E-mail: nishi@vsnl.com >>Thanks Mr. Srinivasan for your reply. >> >>It is worth mentioning here that the corrosion / erosion >>phenomenon is not taking place in base metal of piping as we >>perform regular inspection of piping. It is still sound over >>a period of 10 years. >> >>Welds are being eroded / corroded continuously in this span >>and this phenomenon attacks root and hot pass more rapidly >>and metal loss rate then starts decreasing after root pass >>and hot pass are eaten up. >> >>One of the finest reason is that corrosion / erosion would >>be more likely to occur in the welded area where there are >>stress concentration points plus HAZ area. The other reason >>we found out was the weld material used for piping welding >>was the same as piping material that is 316L normal grade >>containing high percentage of ferrite content. Then we >>decide to re-weld some corroded weld joints with 316L urea >>grade and did so in August 2007. But we again found them >>corroded. The question is if for urea carbamate service 316L >>urea grade is also not acceptable then which material then >>be selected to avoid much excessive corrosion loss. We are >>thinking of 25Cr.22Ni.2Mo. Is there any problem with our >>operation / process?? Or something wrong with welding (but >>100% RT conducted)? >> >>Regarding your question, corroison is taking place in all >>the HP circuit Reactor to Stripper, Stripper to carbamate >>condensers, Carbamate Condensers to Reactor, Carbamate >>solution from pump discharge to Carbamate Condensers, >>Scrubber. All HP loop WELD JOINTS are effected and PIPING >>BASE METAL is sound. >> >>Please advise the way to resolve this issue in the light of >>above information. >> >>Thanks & regards, >> >>Ashfaq >> >>>Ashfaqanwer >>>Urea carbamate at the discharge of Urea Reactor is highly >>>corrosive. When Urea Carbamate is down loaded at the down >>>stream with a pressure reducing valve to reduce pressure >>>from 200-220 bar (KSC) to 20-22 bar (KSC) it is highly >>>corrosive. Material like 316 L will erode-corrode easily to >>>this extent of 5-6 mm per year. >>> >>>Is it at the down stream of Reactor where due to high >>>velocity and moderate temperature of 190-200 deg cent but >>>pressure reduction or down loading from 200-230 bar (KSC) >>>to 20-22 bar (KSC)? >>>Welds under throttling condition will easily erode -corrode >>>in Urea Carbamate service. >>> >>>Another erosion -corrosion prone area is at the discharge of >>>first decomposer where again throttling of pressure takes >>>place from 20-22 bar (KSC) to about 2-3 bar (KSC) when >>>ammonia carbamate enters the second decomposer- i.e, >>>ammonium carbamate solution when throttled thru pressure >>>reducing valves will erode - corrode 316 L very severely to >>>have this 5-6 mm per year corrosion rate. >>> >>>Valves and pipes should have enough smooth flow and >>>throttling condition should be well monitored in Urea >>>carbamate or Ammonium Carbamate service from Reactor down >>>stream to First or Second Decomposer in Urea plants. >>>Hope this helps >>>C.V.Srinivasan Nishi Engineers Pvt Ltd India Oct 23, 2008 >>>E-mail: nishi@vsnl.com >>> >>> >>>>We have a problem of excessive corrosion of 316L urea grade >>>>weld material in carbamate / urea service. Current data >>>>shows that the corrosion rate is 6 mm per year. These welds >>>>were done on 316L piping back in August 2007. >>>> >>>>What are the possible causes of this abnormal corrosion? >>>>Which parameters should be looked into?