Your aircraft operates on D/c Direct/Current...Aluminum is a perfect metal for "Anode/Cathode" environment. Corrosion loves The Cathode side which is the Airframe Negative-Side of the Battery Current Flow.
The Negative side where corrosion develops creates "Resistance" this resistor on the ground can be anywhere from 1 Ohm to Infinity...This resistor creates a "Voltage-Drop" which shows up as Intermittent Radios and other Panel Gages.
When the Resistive Corrosion is developed at the Starter...a Voltage Drop is created causing insufficient voltage/amperage... ...The result is slow turning during starts. Mechanics miss-diagnosing the problem and a NEW Starter is purchased needlessly.
To compound the aggravating problem, it takes years for these and other symptoms to surface. Whereas when a Spark-Plug or Magneto fails those failure-symptoms shows-up immediately.