Corrosion

[phill]

What is this corrosion?

 

The pictures are before and after pressure washing so no abrasive action. The grey areas are heavily pitted clean steel, they have no coating of any kind and flash rust in 10 minutes.

 

 

Sorry I don't seem to be able to access the rest of the imagea, they are in the gallery, links below;

 

 

 

 

As you can see in the second set, the rust is quite lumpy, comes off easily through pressure washing. This is a boat that has never had a mains hookup or any 240V installed so seems unlikely to be galvanic corrosion through a faulty hookup.

Edited July 5, 2014 by phill

[peterboat]

They look like the tabs that attach anodes to the hull very strange

 

Peter

[BEngo]

That is perfectly normal corrosion (rusting) of steel in fresh water. The pressure washing has not been particularly effective in some shots.

 

Steel plate is not absolutely all the same all over so some bits are more anodic than others and eventually as the paint fails they start to corrode. Welding bits to it makes that worse. The sort of rust that forms underwater is slightly different to the sort that forms in air, ( to do with the way the iron oxide is hydrated IIRC) and it is much softer and easily removed by a pressure washer. The grey spots are clean steel and as you have noticed they rust very quickly unless you can get them dry almost immediately.

 

Bitumen will go over them nicely as soon as they are dry, even if they have gingered a bit. If feeling really enthusiastic you could wire brush them just before painting but really that sort of effort is best spent on the waterline where the rust will be the harder sort and need more mechanical effort to remove.

 

N

[Bobbybass]

I had corrosion like this. In my 'case' it was caused by the way the Travelpower was 'earthed' through the hull.

 

I didn't find this out until I sold the boat.

 

As I was doing the 'hand over' the surveyor / engineer...Steve Hand..(very respected in the Midlands ) .was changing the connections.

 

When I asked him about it...he said..."It was originally common to connect the Travelpower like this..but it is the wrong way round and it makes the hull 'live'. He carried on to say that there had been some fatalities..although rare..but brought about by meeting up with a boat connected the other way around ".

 

I said..."But you surveyed it for the BSS last year..!!."..but he said..it is not a BSS fail. ?

He was only changing it..because during the buyers survey he had noted the specialised corrosion.. and knew it needed changing.

 

He said that...with the Travelpower connected the wrong way (which apparently is that way for most of them) it causes that type of corrosion. In my case..I had accelerated corrosion on the sides like you have....and heavily pitted...Swiss cheese base plate.

 

If you do not have a Travelpower..maybe it's a good idea to check your hull earthing etc.

 

Bob

[phill]

There's no hull earthing or 240v system at all. There's no 12v connection either.

Nor is it moored near any other boats with hookup.

Hence the mystery.

[mark99]

Looks like to me that patch is acting as an anode to your Hull . The patch being an Inferior steel compared to the hull steel.

Edited July 5, 2014 by mark99

[ditchcrawler]

Looks like to me that patch is acting as an anode to your Hull . The patch being an Inferior steel compared to the hull steel.

My thoughts as well, dissimilar metals

[Phil Ambrose]

There's no hull earthing or 240v system at all. There's no 12v connection either.

Nor is it moored near any other boats with hookup.

Hence the mystery.

When you say there is no 12v connection do you mean you have no 12v circuits on the boat other than the starter battery? I ask because galvanic corrosion is not just limited to 240v or shoreline hook up, it can and does occur with 12v leakage

Phil

[Bobbybass]

Looks like to me that patch is acting as an anode to your Hull . The patch being an Inferior steel compared to the hull steel.

 

Yes..I'd definatley say some kind of electrolytic corrosion...even though there is no apparent electrical connection...

 

Don't forget that sea going boats have small anodes bolted all over their outdrives...and these dissolve even with no apparent electrical circuit. A simple differential in the metals can cuase this.

 

I think I'd maybe fix a small anode of some kind onto the patch...maybe just weld a single bolt to it and fix a small sacrificial plate.

 

Bob