An outboard is an aluminium casting that lives in salt water. It corrodes from the outside, where salt and stray current attack the leg, and from the inside, where salt stays in the cooling passages after every trip. Yamaha says corrosion attacks an outboard every day, even in fresh water.
How a surveyor checks it
Anodes. Find all of them: Yamaha puts them on the lower unit just above the prop and on the bottom of the engine bracket. BoatUS warns some are hidden, such as under the cavitation plate. Replace at about half gone. Look at what's left too. Steve D'Antonio notes anodes that seem to last forever, or grow weed, are dormant and protecting nothing, often because of poor contact.
Paint on anodes. Yamaha says never paint or cover them. Antifouling on an anode is a common find.
The leg and bracket. Bubbling paint, white powdery corrosion and pitting, especially around the water intake, trim tab and cavitation plate.
Under the cowl. Salt crust on the powerhead and green or white corrosion on electrical connections.
The tell-tale. A weak stream can mean salt build-up in the cooling passages. Yamaha notes salt water crystallises above 170 degrees and restricts them.
The routine. Ask how it's flushed. Yamaha's advice is 15 minutes of fresh water after each trip.
What decides the grade
Metal lost. Corrosion through a casting, or seizing steering or tilt, is an A. Spent anodes and active corrosion on the leg are B: the outboard is being eaten and the anodes aren't stopping it. Painted anodes and a missing flushing routine are C.
If the anodes are going fast, or the leg is corroding with good anodes, suspect stray current and test with a reference electrode. See stray current corrosion.