AN AIRBUS 300-600 WAS REMOVED FROM SVC DUE TO AFT FUSELAGE PRESSURE BULKHEAD LAP JOINT SPLICE RIVET CORROSION. FOUND BY CAPT ON PREFLT INSPECTION.

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ON PREFLT OF AIRBUS 300 24 RIVETS FOUND CORRODED AT REAR PRESSURE BULKHEAD AT BOTTOM. VENT DRAIN WAS OPERABLE. CORROSION WAS FROM INSIDE OUT ON EACH RIVET. ALL 24 RIVETS IN SAME AREA. PAINT IN BTWN RIVETS WAS INTACT NOT AFFECTED. PAINT WAS CLEAN FROM RECENT CHECK. UPON INSPECTION FROM INSIDE OUT/INSIDE TAIL SECTION; FLUID WAS FOUND POOLED IN BOTTOM OF FUSELAGE AT BOTTOM WHERE REAR PRESSURE BULKHEAD MEETS FUSELAGE. EXPLANATION FROM MAINT SUPERVISORS WAS THAT THIS WAS AN 'ANTI CORROSION' TREATMENT. LIQUID HAD NOT SECURED CORROSION. ALSO; BLACK SUBSTANCE WAS EXPLAINED AS A SOLID PLASTIC POURED IN TO PROVIDE LEVELING TO MAKE SURE CONDENSATION AND OTHER FLUIDS POURED OUT THE DRAIN VENT. OBVIOUSLY; SOLID HAD BROKEN LOOSE DUE TO FLEXING OF THE FUSELAGE AND CORROSIVE FLUIDS HAD SEEPED INTO 24 OR MORE RIVETS WORKING OUT TO EXTERIOR OF FUSELAGE. CORROSION WAS FROM RIVETS OUTWARD; NOT FROM CORROSION FROM EXTERNAL FLUID LEAKS. NO HYDRAULIC LEAKS FOUND DURING INSPECTION OF TAIL FROM INSIDE OUT. ACFT WAS TAKEN OUT OF SVC AFTER REFUSED BY CAPT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE CAUSE OF THE RIVET CORROSION IS CORROSIVE FLUIDS NOT DRAINING PROPERLY FROM THE LOWER INTERIOR SKIN SURFACE AFT OF THE PRESSURE DOME AND ABOVE THE FUSELAGE SKIN PRESSURE BULKHEAD LAP SPLICE. THE RPTR SAID DRAIN MAST APPEARS TO HAVE BEEN ADDED TO DRAIN THIS FLUID; BUT IS INEFFECTIVE AS THE MATERIAL ADDED TO THE INTERIOR SKIN TO ALLOW THE FLUID TO DRAIN IS BROKEN OUT. THE RPTR STATED MAINT MUST PUT IN PLACE A MORE FREQUENT CORROSION INSPECTION AND TREATMENT PROGRAM TO ELIMINATE SKIN OR LAP JOINT FAILURE DUE TO FASTENER CORROSION. THE RPTR SAID THIS IS A RECURRING PROB ON THE FLEET.