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  • Writer's pictureSteady Marine

Sacrificial Anodes Selection

Which anode do you need:

1-What shape

2-What weight

3-Zinc vs Aluminium

4-Do I need to replace the existing one?

1-What Shape: Teardrop or block shaped?

According to “Sacrificial anodes are generally cast in three basic geometric shapes: the long, slender, stand-off type; the flat-plate, flush-mounted type; and the bracelet type. In most cases, anode selection is performed by the owner, taking into account effects like sea current drag and interference with subsea interventions. If the anode type has not been specified by the owner, then the contractor selects the anode type, taking into account such factors as net anode mass to be installed and available space for location of anodes. The anode type further affects the anode utilization factorand the anode current output in relation to weight.”

2-What weight?

There is a function to calculate:

Anode Weight (lbs) = [(Wetted SurfaceArea) x (Current Density) x( Immersion)[(Energy Content) x (1000 mA/Amp)]

However, It is strongly suggested to get help from seller & vendors in terms of weight & quantity selection. So call your vendor. They’ll ask your shell expansion and tank plans to get you a more accurate anode plan.

3-Zinc or Aluminium?

“Zinc anodes protect well in salt, fair in brackish and a little in fresh. Aluminum anodes protect well in salt, good in the upper levels of brackish waters, but not so good in the lower levels of brackish, and decent in fresh. However, aluminum passifies (films over with oxide coating) quicker than zinc. If cleaning occurs on a regular basis, aluminum will work. If not, the aluminum anode can become “passified”, rendering it useless” says the

In any case- it is still a commercial decision. Price for Al and Zn differentiate from time to time. It may be wiser to pick the material according to current market rates of dry docking time.

4-Do I need to replace the existing anodes?

If you find your anodes relatively corroded, then you may need to consider replacing them with bigger sized ones.

But watch out! If they are fully corroded- like nothing remained on the supports, you may need to think twice about your anodes’ material quality. Cross-check the material certificates & class type approvals of your manufacturer in this case.

In general, anodes are advised to be renewed in every special dry dock period.

5- My anodes are shining like new! So no need to take any action?

I’d re-consider this:

“ If anode does not dissolve from the time of installation to some later time at inspection and the parent material keeps on corroding, then this anode is ineffective and should be replaced with some other more reactive or efficient anode material.

If there is no proper electrical continuity then also anode does not work properly. Because of this, the parent metal starts getting corroded instead of the anodes. Then check for proper connection between electrolytic solution and metal strips during the installation of setup” stated

This brings us to what will be a good connection?

-Check your anode’s connection type first.

-Welding type: no issues expected.

-Bolted type: you must make sure that anode’s body doesn’t have a direct contact to ship’s body / hull. Usually rubbers gaskets are used to avoid such issues.

What other recommendations do you have about sacrificial anodes replacement

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