Narrative:

Flight on jul/fri/06; B767-300. We were scheduled to fly aircraft to ZZZ1 but canceled due to a discovery on walkaround by the ZZZ mechanics. I was in operations flight planning when the first officer told me the mechanics wanted to talk to me about something they found on their walkaround. Since I had the write-ups I knew of a deferral item of delaminating of the aircraft skin below the aft lavatory drain vent. The item had been deferred for 300 hours until it should be re-inspected. That appeared to be 2 weeks prior and I approximated it could have flown 10 hours a day and could be 1/2 way through its deferral period. Nothing else in the paperwork looked like it might be an item the mechanics could be concerned about; unless something new has cropped up. A mechanic arrived and said they were concerned that the interim repair done on the delaminating deferral appeared to be failing and that they had taken a peek behind what they told me was tape and discovered a caked material and when touched with a screw driver; powder fell away. They obtained a camera and were taking pictures to send to maintenance control for evaluation. I stayed in operations until the mechanics arrived with the pictures. After further discussion and review the pictures did look like corrosion. A beige area below the aft lavatory drain vent appeared behind what looked like a blue piece of tape. The mechanics even captured a picture of a dust spray of powder where they had touched it with a screw driver. The mechanics said they would feed the pictures into a computer and forward them to maintenance control but needed to go to another computer location as the feed failed in operations. I then told the first officer I would go the aircraft and inform my crew. I was told boarding is being held off until maintenance control's review. I also wanted to have a better look at the airplane as the pictures appeared to be enlarged. Once at the aircraft; standing below the aft lavatory drain vent (there are 2 together and it was the vent on the right) I observed what appeared to be an inch square piece of tape (blue in color; possibly painted) slightly pulled away from the skin of the airplane. Behind it there was a small area approximately 1 inch in diameter of beige packed cake like substance. It did have the appearance of corrosion but was solid and did not have any breaks; cracks; or holes. It appeared to me to be surface corrosion but could possibly have depth but was not visible to me at that inspection. The review by maintenance control did pull the aircraft OTS and our flight canceled. We were eventually reassigned to fly an extra flight to accommodate our passenger with an aircraft that would be ferried from ZZZ1. That aircraft would leave in the morning and should be in ZZZZ2 sometime saturday night for our turn to fly it back for a sunday morning arrival. Saturday night in operations; I asked the mechanics about aircraft and they said it was going to be ferried for further inspection and repair.

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Original NASA ASRS Text

Title: A B767-300 CAPT RPTS HIS SCHEDULED FLT BEING CANCELED DUE TO TECHNICIANS' DISCOVERY OF SKIN CORROSION AT AFT LAVATORY DRAIN VENT.

Narrative: FLT ON JUL/FRI/06; B767-300. WE WERE SCHEDULED TO FLY ACFT TO ZZZ1 BUT CANCELED DUE TO A DISCOVERY ON WALKAROUND BY THE ZZZ MECHS. I WAS IN OPS FLT PLANNING WHEN THE FO TOLD ME THE MECHS WANTED TO TALK TO ME ABOUT SOMETHING THEY FOUND ON THEIR WALKAROUND. SINCE I HAD THE WRITE-UPS I KNEW OF A DEFERRAL ITEM OF DELAMINATING OF THE ACFT SKIN BELOW THE AFT LAVATORY DRAIN VENT. THE ITEM HAD BEEN DEFERRED FOR 300 HRS UNTIL IT SHOULD BE RE-INSPECTED. THAT APPEARED TO BE 2 WKS PRIOR AND I APPROXIMATED IT COULD HAVE FLOWN 10 HRS A DAY AND COULD BE 1/2 WAY THROUGH ITS DEFERRAL PERIOD. NOTHING ELSE IN THE PAPERWORK LOOKED LIKE IT MIGHT BE AN ITEM THE MECHS COULD BE CONCERNED ABOUT; UNLESS SOMETHING NEW HAS CROPPED UP. A MECH ARRIVED AND SAID THEY WERE CONCERNED THAT THE INTERIM REPAIR DONE ON THE DELAMINATING DEFERRAL APPEARED TO BE FAILING AND THAT THEY HAD TAKEN A PEEK BEHIND WHAT THEY TOLD ME WAS TAPE AND DISCOVERED A CAKED MATERIAL AND WHEN TOUCHED WITH A SCREW DRIVER; POWDER FELL AWAY. THEY OBTAINED A CAMERA AND WERE TAKING PICTURES TO SEND TO MAINT CTL FOR EVALUATION. I STAYED IN OPS UNTIL THE MECHS ARRIVED WITH THE PICTURES. AFTER FURTHER DISCUSSION AND REVIEW THE PICTURES DID LOOK LIKE CORROSION. A BEIGE AREA BELOW THE AFT LAVATORY DRAIN VENT APPEARED BEHIND WHAT LOOKED LIKE A BLUE PIECE OF TAPE. THE MECHS EVEN CAPTURED A PICTURE OF A DUST SPRAY OF POWDER WHERE THEY HAD TOUCHED IT WITH A SCREW DRIVER. THE MECHS SAID THEY WOULD FEED THE PICTURES INTO A COMPUTER AND FORWARD THEM TO MAINT CTL BUT NEEDED TO GO TO ANOTHER COMPUTER LOCATION AS THE FEED FAILED IN OPS. I THEN TOLD THE FO I WOULD GO THE ACFT AND INFORM MY CREW. I WAS TOLD BOARDING IS BEING HELD OFF UNTIL MAINT CTL'S REVIEW. I ALSO WANTED TO HAVE A BETTER LOOK AT THE AIRPLANE AS THE PICTURES APPEARED TO BE ENLARGED. ONCE AT THE ACFT; STANDING BELOW THE AFT LAVATORY DRAIN VENT (THERE ARE 2 TOGETHER AND IT WAS THE VENT ON THE R) I OBSERVED WHAT APPEARED TO BE AN INCH SQUARE PIECE OF TAPE (BLUE IN COLOR; POSSIBLY PAINTED) SLIGHTLY PULLED AWAY FROM THE SKIN OF THE AIRPLANE. BEHIND IT THERE WAS A SMALL AREA APPROX 1 INCH IN DIAMETER OF BEIGE PACKED CAKE LIKE SUBSTANCE. IT DID HAVE THE APPEARANCE OF CORROSION BUT WAS SOLID AND DID NOT HAVE ANY BREAKS; CRACKS; OR HOLES. IT APPEARED TO ME TO BE SURFACE CORROSION BUT COULD POSSIBLY HAVE DEPTH BUT WAS NOT VISIBLE TO ME AT THAT INSPECTION. THE REVIEW BY MAINT CTL DID PULL THE ACFT OTS AND OUR FLT CANCELED. WE WERE EVENTUALLY REASSIGNED TO FLY AN EXTRA FLT TO ACCOMMODATE OUR PAX WITH AN ACFT THAT WOULD BE FERRIED FROM ZZZ1. THAT ACFT WOULD LEAVE IN THE MORNING AND SHOULD BE IN ZZZZ2 SOMETIME SATURDAY NIGHT FOR OUR TURN TO FLY IT BACK FOR A SUNDAY MORNING ARR. SATURDAY NIGHT IN OPS; I ASKED THE MECHS ABOUT ACFT AND THEY SAID IT WAS GOING TO BE FERRIED FOR FURTHER INSPECTION AND REPAIR.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.