The pilot with one passenger on-board the airplane was practicing landings to familiarize the passenger with the airplane. During one of the landings, the airplane landed "firm." After noticing added noise in the cabin and the "seats being jarred” they taxied back to the hangar. The pilot reported that a fuselage tube had failed due to a "pre-existing weakened condition." The Federal Aviation Administration (FAA) Inspector who examined the airplane reported that the fuselage tube displayed evidence of corrosion.