cathodic protection of ship hulls and calcareous deposit

[Sridhar K]

Dear All,

In one of our ships, heavy calcareous deposit was found in the gap between the propeller shaft and the bearings. After a series of efforts failed to solve the problem, the entire ICCP system was shut down in order to rule out the possibilty- whether excessive cathodic protection viz., overprotection is leading to calcareous deposits at the location. To our surprise, the entire hull potential was found to be -850 mV to 870 mv vs.Ag/AgCl i.e, the hull is protected without ICCP cathodic protection ON. Can any CP experts in this forum could give any probable reasons for the above behaviour. Would greatly appreciate feedback.

Thanks and warm regards

Sridhar

 

[LittleInch]

It's called polarisation or polarization.

It takes time to build it up and will slowly lower. How fast I can't find out but it will slowly dissipate after you turn the ICCP off.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EdStainless]

And what will happen is that due to flow and other factors some places will drop faster. To the point that they become anodic to the rest of the hull and begin actively corroding.

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P.E. Metallurgy, consulting work welcomed

 

[Sridhar K]

To LittlEInch

Thanks for the info. However in our case the steel is protected even after 25 days of switching off ICCP and after a short sailing of the ship as well.

 

[Sridhar K]

To,
EdStainless (Materials)

Thanks for the info. Is it possible that a bare steel of the ship's hull due to paint degradation can dissolve and protect the other regions of the hull as the potential difference between the two will be very small and hence the driving voltage

with warm regards

sridhar

 

[TugboatEng]

Yes, steel is anodic to exposed underwater components such as your bronze propeller, stainless steel shafting, copper nickel heat exchangers, etc... The welds on the steel hull are also anodic to the steel hull plating.