Hello all, just took some disturbing pictures in a vessel. I have black "spyder" ajoining the welds and along places where the bottom dish end plate was rolled. http://img24.photobucket.com/albums/v72/edouard_g/Bottom_dish_head_joint_small.jpg http://img24.photobucket.com/albums/v72/edouard_g/Spider_web_on_bottom_dish_head_seam_small.jpg But other welds on the same sheet seem ok, http://img24.photobucket.com/albums/v72/edouard_g/Coil_support_base_small.jpg note the roughness of the plate. What's going on here then? must cut and paste the pic url (sorry, don't know how to post pics) Regards, Edouard.

These pics come from a pressure vessel made of 304L in which chelates are made (EDTA liquid 40%) by saponification (with Sodium Hydroxide 25%). temp always less than 120°C. the vessel is pressurised (with Ammonia!). The phenomenon is related to the welding methods as not all welds have these. here is a nice one: http://img24.photobucket.com/albums/v72/edouard_g/baffle_anchor_small.jpg I think this vessel pretty much has run its race. Just would like to know what caused it, caustic cracking proper or bad welding practice. eD

In my opinion, you should proceed to a PMI on the corroded sheet. On your picture, I can see that just one of the two welded sheets is corroded. Apart from it, I do not know exactly what is EDTA, but it must a corroded localy the welds, and because of lack of protective layer caustic attack could occured. Hope it will help