Aluminium protects itself with a thin oxide skin. Corrosion starts where that skin can't re-form: under debris, in crevices, against dissimilar metals, and wherever current leaves the hull. David Pascoe notes aluminium is extremely vulnerable to crevice corrosion and that stray current does serious damage with very little current.
How a surveyor checks it
Under the floors. Powerboat & RIB Magazine NZ says the biggest share of the boats it repaired had deposit attack, from leaves and pine needles left under the floor. Floors screwed to a sub-frame and treated plywood are warning signs: the treatment leaches onto the aluminium and starts pitting. What looks like small corrosion where the floor meets the hull can be the tip of the iceberg.
Fittings and fastenings. Stainless deck fittings, bronze washers and alloy don't make happy partners, the magazine says. Look for lifting paint, white powder and pitting around every through-hull, transducer, bracket and screw.
The electrical system. The same source names the metal hull used as an electrical earth as the most common cause of stray current. Every circuit should be two-wire and insulated, with no part of the system grounded to the hull's fittings.
The reference cell. Steve D'Antonio's method: unplug the shore power cord, since switching it off leaves the ground connected. With a multimeter on DC volts, connect a silver/silver chloride electrode to the negative lead and the hull or bonding system to the positive. Hang the electrode over the side in still water without touching the hull. Then plug the shore power back in and read again. If the reading changes, there's a problem.
Thickness. Where pitting is found, measure what's left of the plate before deciding the grade.
What decides the grade
Metal left. Pitting through or nearly through the plate is an A. Active pitting, crevice corrosion at fittings or readings outside the protection range are B. Spent anodes or unisolated fittings on a hull that's sound are C.