Ashok Kakkar Your problem of circumferential crack - close to stack support plate - it appears more likely to be SCC related than galvanic problem. Aluminium brass is susceptible for SCC - even at ambient temp depends on the supply - whether you had procured as quarter hard, half hard and full hard condition. Naphtha comes in three stages from refined product viz., Ligtht Naphtha, Medium Naphtha, Heavy Naphtha. Light Naphtha contains : Hydrocarbons,Isobutane, Butane, Isopentane, Normal Pentane and other mercaptans. Medium Naphtha contains in addition to aromatics, naphthene and paraffin%. Heavy Naphtha contains in addition to aromatics naphthene and paraffin. Percentages of aromatics and other constitutens vary from each refined product you get from different refined plants. If you had procured more sour (H2S content being higher) and with either medium or heavy naphtha in your plant, possibility of SCC due to Sulphide or some traces of ammonia in paraffin (If paraffin content exceeds 40-45% in the crude processed in the refinery) may contribute to your circumferential cracking seen in your tubes. Also check if the failure pattern is identical in all the tubes failed. If the failure pattern had shown a distinct pattern of failure close to stack support plate, examine closely if virbation induced failure in the top layers of tubes. Vibration induced failure is mostly flow related also. From the description you had given, it appears that your tubes had shown distinct failure pattern of SCC. An analysis of the failed tube, analysis of the naphtha from aromatics content (Vol%), Iso butane, normal butane, Iso pentane, normal butane, other mercaptans + naphthene % and paraffin volume % would throw more light on the likely mechanism of failure to get circumferential tube cracks. To my mind, this is not related to galvanic coupling related failure and i think you have diagnozed this cause and ruled out correctly that galvanic coupling is not the likely cause ( it appears to me). Even small traces in ppm levels of contaminants of ammonia present in paraffin % or a higher percentage of sour crude refined with H2S content exceeding limits can make your Al Brass tube susceptible to failure at ambient temperature. Trust this is of help to you C.V.Srinivasan Nishi Engineers Pvt Ltd India Feb 13, 2006 E-mail:nishi@vsnl.com >Any idea what are the constituents of Naptha ? I was >thinking just in case there was some halides or ammonia, >there could be some element of SCC as well. >I would tend to rule out galvanic corrosion theory without >seeing any evidence of corrosion in that area.