On a keel-stepped boat, the mast goes through the deck and stands on a step at the bottom of the boat, usually right above the bilge. Salt water drips down the mast, collects round the step and sits against aluminium, often next to a stainless or steel fitting. The heel corrodes quietly where nobody looks.
How a surveyor checks it
Lift the sole and look at the heel and step. Practical Sailor says corrosion at the heel is probably the most common problem with keel-stepped aluminium spars, and 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. Probe anything suspicious.
Check the heel sits square on the step without gaps, the drain holes are clear, and the step isn't standing in water. Note what the step is made of and what it's bolted to.
What decides the grade
Remaining metal and how the mast sits. Corrosion through the wall, cracks or a heel that's settled off the step is an A. Heavy pitting on sound metal is a B. Light powder on a dry step is a C.