Weldtech You have not specified the location of corrosion seen - whether it is seen at the top cone or bottom cone portion. Please specify the type of process inhibitor used in your Absorber. Also specify if the corrosion is seen at the partent metal or HAZ of cone portions. C.V.Srinivasan april 4, 2005 India >We have co2 absorber in hydrogen production plant. after >about 12 years of service, we found corrosion below the cone >portion. The type of corrosion is like very deep pits of 2 >inch dia and one inch deep. Very hard deposits present in >the pits. we found these at three places at same elevation >on the internal surface of the vessel below the cone >portion. If any one can explain the phenomenon of corrosion >and type of corrosion etc.

Weldtech 1. Cone - isolated pitting of 20 mm deep - arouond 2 Sq inches may not be related to any carbonic acid corrosion. I guess that this could be a problem related to vapor velocity is much higher compared to normal operating parameters. Absorber tower is mainly designed for certain liquid velocity so that inhibitor like V2O5 takes care of shell corrosion. If localized pitting occurs, this is most likely due to higher temperature of operation in the absorber - creating more gas velocity which will be substantially higher than liquid velocity. HIgher gas velocity will easily displace the inhibitor from doing the function. Localized pitting will start initally. Later this may leave a large surface area of the shell getting eroded and corroded. In your case, this could be the beginning of localized isolated /random pitting of 20 mm deep. Once local grooving or deep pits this will form the nuclei for further localized pitting and deep grooving corrosion. This may not be related to any carbonic acid corrosion. This should normally occur over a large surface area whenever inhibitor is swept by vapor velocity.. Liquid-vapor velocity if it exceeds the design conditions can effectively prevent inhibitor fuction. There will be a swirl type action due to localized grooving type attack on the metal.. I wonder why this occurs preferentially over a small surface area of 2 square inches. I guess your tempeature monitoring in both absorbers may be higher than design conditions. Or V2 O5 concentration is not adequate to prevent localized pitting and grooving. Your absorber temperatures at the cone may be relatively high compared to other places where vapor velocity is higher than liquid velocity due to temperature difference. Again this may not occur at local spots of 1-2 sq inches but this will be distributed over a large surface area of the cone, If the absorber had been operated at above design temperature conditions, liquid -vapor velocity effects will drastically change to effectively prevent inhibitor from functioning. 2. Did you check the concentration of V2 O5 by any chance? Also the temperature profile of the absorber at different areas of trays, top and cone portions? 3. Problem seen int he second absorber - viz., at the tray support ring weld HAZ crack could be stress related cracks. Again this may occur if vapor-liquid impingement due to higher temperature operation in the absorber. Trust this helps you to monitor temperature profile and also inhibitor concentration. C.V.Srinivasan Nishi Engineers Pvt Ltd Chennai India April 14 E-mail: nishi@vsnl.com >Dear Mr. Srinivasan, >Thank you for your reply. I am giving below the details you >needed. >1) Location: The big pits are formed on the parent metal of >the absorber, near the cone. Our absorber have one cone >only. we found this type of corrosion below the cone also. >Three spots of deep pits having dia. 1 ot 2 inches and a >depth of 20 mm. This seems to be carbonic acid corrosion. >But I could not understand, how the acid attacked at three >spots only. >2) Inhibitor used: is Vanadium Pentoxide V2O5. >3) Where: These big pits are formed on the parent metal of >carbon steel. At one place exactly on the weld also formed. >It is of irregular shaped pit of 1 to 3 inch dia. >Today we found small cracks on the weld HAZ of the tray >support rings above the cone portion in our CO2 absorber in >another unit. ( We have two similar hydrogen units). Please >explain the corrosion mechanism.