More Info on Corrosion Protection of Concrete Hulls      

A zinc anode at work  |  photo and post by Stan Barbarich

During the recent FHA Virtual Annual Meeting, a panel of people who work in building and maintenance of reinforced concrete hulls were asked to address the use of sacrificial anodes to protect hulls. They were kind enough to give their valuable time to frankly express their opinions on the subject. Those opinions, based on each person’s logic and experience, were not supportive of the use of sacrificial anodes, primarily, as I understood it, because all the rebar in our hulls is only tied, not welded together. The conclusion was that anodes are only capable of protecting the one specific piece of rebar to which they are attached, leaving the rest of the rebar to corrode. I’m adding some further information, from different sources, for your consideration.

During the period from 2005 to 2008, failure of some localized areas of a number on concrete of hulls generated a large number of opinions about the causes of failure, but no confirmed facts. This was sufficient to motivate FHA board to spend $4,200.00 to fund a formal study by CORRPRO, a top international concrete engineering firm, to determine scientifically whether sacrificial anodes, which have long been applied to bridges around the world, and were already being installed on a number of hulls here, could perform effectively on floating home hulls.

The board asked me to manage the process. The engineering team conducted extensive, in-the-water measurements on 17 hulls (one or two on every dock in all marinas) with anodes attached, which measured the actual current flow through the rebar. On my home, a more extensive study was conducted using 2 new sets of anodes (that I personally purchased): first using zinc and then replacing them with aluminum, the study automatically recorded current flow for 50 hours. All the engineers’ measured findings were totally supportive of the protective value of anodes on our hulls, and found that aluminum was the preferred choice, especially due to much lower cost.

The methodology and the results are readily available for you to read on the FHA website.

The engineering study (in plain English) and CORRPRO’s formal evaluation, and my report to the board and the community are both there and are titled:

Cathodic Protection Evaluation and Preliminary Design Survey (PDF)
Hull Protection Assessment Results (PDF)

One point that may not have been perfectly clear in these documents was that the measurements were done by the engineers applying a constant current to the rebar on a front corner of the barge and viewing an ammeter reading of the rebar on the diagonal rear corner. This clearly showed a constant current flow throughout the barge’s rebar, establishing that the rebar conducted sufficient current to protect it, and that welding the rebar together was therefore not necessary.

Last point: The engineers proposed, but never actually expected to be paid to design, a specific system for our hulls, because their own studies showed that what we were already doing was sufficient. All this hopefully gives you enough information to decide whether you want to add passive protection to your hull.