On most production sailing yachts the ballast keel is a separate casting of iron or lead, bolted on. The bolts run up through the hull bottom into the bilge, where nuts and backing plates spread the load into the floors.
They corrode where nobody can look: inside the joint between keel and hull, where seawater sits against the metal. Clean nuts in the bilge prove little, as Practical Sailor points out, because the damage is usually lower down.
What a surveyor checks
The keel-to-hull joint on the haul-out, for cracks and rust weeping. A keel smile, an open crack at the front or back of the joint, is the classic sign of a keel working on its bolts. In the bilge, the nuts and backing plates for crumbling or wasting, and a nut sitting lower on its bolt than it should. After a hard grounding, the floors and stringers round the keel too.
Practical Sailor notes that pulling a bolt is often the most practical check, so a surveyor recommends it when the signs justify it.
In the report
"Keel to hull joint, open crack about 2 mm at the forward end with rust weeping. Bilge nuts sound. Recommend drawing a bolt for inspection." See keel bolt corrosion.