Tag archives: submerged metals

Dezincification

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….

Fair winds.

dezincification, prop blade, antifouling, tubular worm
Dezincified prop blade

NDT – Galvanic Corrosion Inspections

Galvanic Corrosion
Galvanic Corrosion Aluminium Saildrive Strut

Our surveyors perform galvanic corrosion potential surveys to assess, diagnose and troubleshoot galvanic and stray current problems on a boat, yacht or dock.

Please contact us today for a for a free quotation, or please ask for a sample report.

(See also our post about dezincification.)

With a electric potential inspection, we are able to assess the following:

  • Measuring a boat’s hull potential

The hull potential for a vessel is a function of its hull material and its underwater metals. Whether it is an aluminum, steel, fiberglass or a wooden vessel, the hull potential shows if the vessel’s underwater metals are protected against corrosion or not.

  • Verifying the boat’s bonding system

Sacrificial anodes such as zinc anodes attached to the hull of the vessel’s hull provide corrosion protection to underwater metals including but not limited to propeller shafts, metal struts, rudder posts, through-hull fittings, seacocks, stern tubes etc. via the bonding system. We are able to verify each component’s good connection to the vessel’s bonding system.

  • Checking the boat’s galvanic isolation

When a boat’s AC shore cable is connected to a dock’s power pedestal, it is possible for the vessel to galvanically couple to other boats and further underwater metals in the vicinity. We are able to determine the galvanic currents to and from the boat and comment on the possible measures.

  • Locating stray currents on the boat

The most insidious form of marine corrosion is electrical leakage that causes electrolytic corrosion, also known as stray current corrosion. It is able to quickly consume vessel’s sacrificial anodes and critical underwater metal appurtenaces. We are able to detect its existence and pinpoint its cause. It is good practice to have the stray current property of a boat inspected regularly – before the hull starts “to boil” or the submerged fittings start to get pitted or dezincified.