Dezincification can be recognized by the metal colour, shine and texture. In the image below, for example, the section of a bronze propeller blade with distinctly dezincified areas, recognizable by their pinkish colour, can be noticed. Black areas are rests of antifouling paint. White residues are housings of tubular worms and barnacles.
Dezincification can be limited to the surface or may well extend into the metal body.
The marks on one of the dezincified areas on this prop blade have been inflicted by the surveyor’s pocket knife tip. Blows by the pocket knife in the sound areas, in contrast, have left no marks whatsoever! Dezincification compromises the bronze metal’s mechanical properties significantly and will lead to failure of submerged metals and, if left undetected, to heavy, subsequent damages. Usually, dezincification will slowly develop over time, but this is not a firm rule: A short circuit somewhere on board may lead to a quick failure of one fitting over short time, say few days.
For example, a short in a recently replaced deck light on one spreader may lead to power the mast, which, over the forestay may put the bowsprit’s waterstay under voltage. As a result the waterstay fitting starts to dezincify. This example is a real case and it required a lot of effort and wit to narrow down and eventually find the shortened deck light.
If one dezincified metal appurtenance is identified in the submerged hull, one should inspect the vessel for further damages, including damages to metal bonding and undesired galvanic shorts to one or the other metal fitting. A galvanic corrosion survey with a reference electrode may be required.
The author of these lines distinctly remembers the failed skin fitting on his own boat, which he plugged with his finger until somebody around heard him shouting for help….