The pilot of the tail wheel-equipped experimental airplane had just touched down for a full stop landing when the airplane started veering to the left. Although the pilot, who had about 420 flight hours in the airplane, applied rudder and brake inputs to correct the airplane's alignment, the airplane did not respond. The pilot then added power to help correct the veering, but the airplane departed the left side of the runway and impacted a runway/taxiway sign. A post-accident inspection of the tubular steel left gear leg revealed that it had partially failed along a corroded weld line, and had bent backwards from its normal position. Corrosion/rust was present on about one inch of the fracture face, and along a wider area of the interior wall of the gear leg adjacent to the fracture. The fracture itself continued around about 70 percent of the circumference of the tubular leg.