SAS Flight 1209 from Copenhagen (CPH) to Aalborg (AAL) was uneventful. On approach to Aalborg, the landing gear was selected down. When the landing gear extension sequence was completed, the cockpit indication for the landing gear indicated two green lights and one red light. The two green lights indicated that the left main landing gear and the nose landing gear were down and locked (safe). The red light indicated that the right main landing gear was not locked (unsafe). A go-around was initiated. An alternate landing gear extension procedure was conducted but the cockpit indication for the right main landing gear remained unsafe. A visual inspection of the landing gear was performed. The cabin was prepared for an emergency landing and the passengers were briefed. The aircraft touched down on the left main landing gear, followed by the right main landing gear, and shortly thereafter the right main landing gear collapsed. The aircraft then departed the runway to the right and came to rest on the lower fuselage and right wing tip. Investigators found that, due to severe corrosion of the threaded connection between the right MLG retraction/extension actuator piston rod and rod end, the separation of the actuator piston rod and rod end caused the malfunctioning of the right MLG. When selecting the landing gear to down position, the landing gear was released from the landing gear up-lock hook. Due to the separation of the rod end from the actuator piston, the right MLG extended in an undamped free fall condition. The kinetic energy cause the failure of the stabilizer brace link joint lugs. This failure rendered the stabilizer brace incapable of safety locking the right MLG in down position. FACTORS: 1. There were no specified inspection tasks for inspection of the MLG retraction/extension actuator and rod end either in the MRB’s report or in the Maintenance Requirement Manual in so far as “L”, “A” and “C” checks. 2. The right and left MLG retraction/extension actuator piston and rod end were made of noble martensitic stainless steel and the less noble 4340 steel material, respectively. 3. Severe corrosion in the threaded connection between the right MLG actuator rod and rod end. 4. Separation of the right MLG retraction/extension actuator from the actuator piston rod end. 5. The right MLG stabilizer joint lugs failed.