TWO WITNESSES HEARD A LOUD NOISE FROM THE ACFT AS IT WAS IN FLT. THEY LOOKED UP & NOTED THE LEFT WING OF THE ACFT HAD FAILED. SUBSEQUENTLY, THE ACFT SPUN TO THE GND & CRASHED IN A WOODED AREA. AN EXAM OF THE WRECKAGE REVEALED THE LEFT FORWARD LIFT STRUT HAD FAILED, APRX 6 INCHES FROM WHERE IT WAS ATTACHED TO THE FUSELAGE. A METALLURGICAL EXAM REVEALED THE INSIDE OF THE STRUT HAD BECOME CORRODED & THE STRUT HAD FAILED IN AN AREA WHERE ITS WALL HAD CORRODED TO A KNIVE EDGE (LESS THAN 0.001 INCH). THE THICKNESS OF THE NON-CORRODED PORTION OF THE WALL WAS 0.38 INCH. A CHEMICAL TEST OF THEINSIDE SURFACE SHOWED THE PRESENCE OF A CORROSION INHIBITOR (VAL-OIL) IN THE NON-CORRODED AREAS. HOWEVER, NO INHIBITOR COULD BE FND WHERE SEVERE CORROSION HAD OCCURRED. TWO FAINT LINES RESEMBLING WATER MARKS WERE FND IN THE STRUT. LOG BOOKENTRIES SHOWED THAT AD 77-03-08 & PIPER SVC BULLETIN #528B HAD BEEN COMPLIED WITH. A TOXICOLOGY CHECK OF THE PLT'S BLOODWAS POSITIVE FOR ALCOHOL (0.04%), BUT THERE WAS NO INDCN THAT THIS WAS RELATED TO THE STRUT FAILURE.