Every mast stands on a step. A deck-stepped mast sits on a fitting on deck. A keel-stepped mast goes through the deck and stands on a step low in the boat, usually right above the bilge, where water drips down the mast and collects.
That's where aluminium sits against wet steel or stainless, and the heel of the mast corrodes out of sight.
What a surveyor checks
Lift the sole and look at the heel and the step. Practical Sailor says the first clue is a powdery white deposit round the edge of a fitting. Look for pitting, bubbling paint, cracks and swelling where the heel meets the step, and probe anything suspicious. Check the drain holes are clear and the step isn't standing in water.
In the report
"Mast heel, keel-stepped, heavy white corrosion and pitting round the base where it sits in the stainless step, step standing in bilge water. Clean back and assess wall thickness; clear the drains and isolate the heel from the step." See mast step corrosion.