During flight, the N2 and NR suddenly accelerated to 110 percent. The pilot eventually shut down the engine and made an autorotation. An open field was selected for a landing site; however, it was muddy with standing water. The pilot decided on a full stop landing rather than a slide-on landing to avoid a nose over. During touchdown, the main rotor blades flexed downward and severed the tail boom. An investigation revealed there was sand and aluminum hydroxide corrosion in the turbine governor, PN 2524706-3. Also, some corrosion and foreign material was found in the fuel control unit, PN 2524644-1. Both units had 2.5 hrs of operation since they were installed on this aircraft. An inspection of the governor showed that the PG channel in the drive body was completely blocked. This blockage would have resulted in an engine overspeed of up to 112 percent. The governor had been salvaged from a helicopter that had been in an accident in Brazil. The corrosion was indicative of long term exposure to the environment.