Corrosion comes in many forms and is the number one enemy of any steel hulled boat. Therefore it is worth studying corrosion and knowing how to deal with it, or as Sun Tzu said “Know your enemy”.
Some enemies are easier to spot than others and rust tubercles are one of the more unusual form of corrosion found on steel boats. These marine growths look rather like an oyster stuck to the hull. Alas, no pearl inside, just an enclosed pocket containing anaerobic, often mildly acidic stagnant water which can cause localised deep pitting and pin holes in the steel plate.
Rust tubercles are easily identified by the brown exterior surface of rust (Fe2O3) and the black interior / bottom surface of magnetic iron oxide (Fe3O4). They detach easily with a scraper and once off, the pitting can be dramatic and 4mm pits or deeper are not uncommon.
My Surveyor’s Top Tip for anyone dealing with rust tubercles: Hitting them with a hammer usually results in a liberal splash of rust fragments and organic matter mixed with stagnant, acidic ‘water’ - nice. Just one of the many reasons to wear safety glasses when percussion testing steel hulls.
Thankfully dealing with rust tubercles isn’t hard, just clean them all off and check for pitting. If there is 5mm of steel thickness or more left in the pits, clean the steel back to SA 2.5 and repaint ideally with 2 pack epoxy. Where the pitting is more severe and pit depth thickness is below 4mm, localised pad welding can buy the steel plate some time. Extreme cases with 2mm or less of pit depth thickness may demand extensive remedial steel work.
They say a picture paints a thousand words so watch this video I made during a pre-purchase survey of a steel houseboat where I found quite a few rust tubercles.