check the battery voltage with a digivoltometer. should be 12.6 or around there for a good battery. so check the starter voltage...while cranking the battery will register somewhere around 10.83 volts on a good battery. if the battery and voltage are low check the connections to the starter....if those are good then i would suggest that you replace the starter.
btw pouring a can of soda on a corroded battery will eat the acid right off
