A question about pitting

[GlenBlk]

Having spoke to a few surveyors recently I am wondering what other peoples experience is with the speed of pitting on a modern steel boat or a newly over plated boat that is plugged into shore power or moored next to other boats that are plugged in?

Is it true a hull can be completely shot in less than 18 months?

Could pitting happen as quick as 6 months?

I ask as I am researching galvanic isolators.

 

thanks.

[BEngo]

Having spoke to a few surveyors recently I am wondering what other peoples experience is with the speed of pitting on a modern steel boat or a newly over plated boat that is plugged into shore power or moored next to other boats that are plugged in?

Is it true a hull can be completely shot in less than 18 months?

Could pitting happen as quick as 6 months?

I ask as I am researching galvanic isolators.

 

thanks.

Stray potentials from shore power continue to be a corrosion driver. How fast depends on how well the boat is protected.

 

It is extremely unlikely that a complete hull would last less than 18 months, unless there is a significant shore-power fault or it's never been properly painted and is in brackish or salt water. Areas of localised loss of steel can happen in that time, if the conditions happen to be right. Explaining what the 'right' conditions might be is a whole corrosion text book!

 

Pitting of unprotected steel, or of an area less well protected than those around it, can certainly happen in 6 months, whether or no there is a shore power fault.

 

A galvanic isolator (or an isolation transformer) is just an additional item in the protection armoury. The most important continues to be ensuring that the steel is protected from contact with any electrolyte that can complete a corrosion cell. Once that barrier is broken corrosion will occur at the point of failure unless something else (like an anode) prevents it. Stray voltages from shore power will speed it up, but they don't 'cause' the corrosion.

 

N

[Bloomsberry]

I got some old copies of canal boat magazine from a railway gala ( sad isn't it ! ) and in one copy (Aug 2001) there is an article about a 2 year old narrowboat that was already showing signs of corrosion and had 1.5 mm pits already.

The owners rarely used the electrical hookup , had the electrics tested and confirmed that the anodes were working correctly.

In the end they had the pits welded up and 2 pack epoxy applied. They said they were going to run an article the following year to determine if the epoxy worked.

Anybody got an August 2002 edition of Canal Boat ?

[thorne lass]

The only influence shore power has on pitting is that the earth connection provides the other link in the cell, the copper bit between the plates. Shore power does not cause electrolytic corrosion. other boats do not cause electrolytic corrosion to your boat. If you have electrolytic pitting it is due to stray DC currents on your boat making your boat the anode to another boat or metal structure. Depending on the strength of the driver electrolytic corrosion can cause very severe damage in a very short time, but that is rare.

 

Most pitting is not electrolytic, but galvanic, very often due to the presence of mill scale on the plating. Galvanic pitting is much less virulent and will subside as the mill scale falls off and differential plating potentials normalise.

[casper ghost]

I got some old copies of canal boat magazine from a railway gala ( sad isn't it ! ) and in one copy (Aug 2001) there is an article about a 2 year old narrowboat that was already showing signs of corrosion and had 1.5 mm pits already.

 

This isn't that unusual, bad pits can happen within the first few years, it's because the pits are in an area of poor steel, where there is too much carbon. Pitting usually slows once that bit has gone.

Casp'