Hull corrosion

[Bern0]

I recently blacked the hull of my boat. Beforehand however, I had to grind out rusty craters on the bottom of the boat.

 

Some of these were filled with a soft silver substance which disappeared under the grinder.

 

I was wondering what this was and why it this was sometimes present, and other times, rust.

 

Thanks

[Alan de Enfield]

1 minute ago, Bern0 said:

I recently blacked the hull of my boat. Beforehand however, I had to grind out rusty craters on the bottom of the boat.

 

Some of these were filled with a soft silver substance which disappeared under the grinder.

 

I was wondering what this was and why it this was sometimes present, and other times, rust.

 

Thanks

 

Pictures would be good, but It may well have been MIC (very nasty stuff) and not got rid of easily.

 

 

Boat owners and yards know all about rust. There is endless literature on electro-chemical and galvanic corrosion – all under the general heading of ‘rust’. But there are other types of corrosion which closely resemble (but are not) rust in the conventional sense about which little is known by boat owners and by many yards. This is a corrosion caused by microbiological action which is can occur on boat hulls, particularly those lying in canals or rivers containing high levels of chemicals or decaying vegetable matter.

 

Microbially Induced Corrosion (MIC) is a highly unpredictable process but under the influence of micro-organisms, corrosion processes can be rapid, happening in a matter of months compared to the years it would take for ordinary abiotic corrosion to reach serious proportions. This phenomenon is well known in the oil, gas, water and mining industries but is little understood in the steel boating world.

 

MIC frequently occurs in areas with high nitrate content in the water – this particularly pertains to arable regions of the canal network and particularly to canals and rivers on the east side of the UK and where there is intensive crop farming using non organic chemical fertilizers with consequential phosphate, sulphate and nitrate run-off into the watercourses. Marinas fed by rivers are another risk area and, in salt water environments, it is well known that harbour muds are highly contaminated by sulphides produced by these creatures.

 

Sulphide films are, by their very nature, highly corrosive and the identification of such very obvious. It is usually found under muddy and slimy surfaces, sometimes even behind paint coatings and a very careful visual inspection is necessary to locate it. It is not discoverable by non-destructive testing such as ultrasonic thickness measurement, eddy current testing or the magnetic method familiar to most marine surveyors. The bacteria are often found inside oxidised welds or in areas which contain physical defects such as porosity, overlap or lack of penetration. The microbes leading to this condition can both cause corrosion from beneath existing coatings or seek out pinpricks in the steel coating and cause the reaction to occur from the outside.

 

MIC bacteria can be present under previous blackings and is not eradicated by simple pressure washing. Unless correctly treated, MIC can continue to thrive beneath the coating, emerging as major pitting

 

More here (with pictures)

 

 

MICROBIOLOGICAL-CORROSION.pdf

[Tony Brooks]

I thought active galvanic corrosion left silver pits.

[Alan de Enfield]

Just now, Tony Brooks said:

I thought active galvanic corrosion left silver pits.

 

I was working on his statement "a soft silver substance" rather than "a shiny silver surface"

[Bern0]

Thanks for info. The stuff was also shiny.

 

I'm hoping this damage mainly occurred before I got the boat, as it was in a marina with no galvanic isolator installed for a few years. 🤞

[Tony Brooks]

Just now, Bern0 said:

Thanks for info. The stuff was also shiny.

 

I'm hoping this damage mainly occurred before I got the boat, as it was in a marina with no galvanic isolator installed for a few years. 🤞

 

In that case once the electrical potential was removed I would think the base of the pits would rust. If it stayed silver and was a deposit then I fear Alan may be correct.

[Bee]

9 hours ago, Alan de Enfield said:

 

I was working on his statement "a soft silver substance" rather than "a shiny silver surface"

I think you are right, I'm not an expert by any means but I have stared into many pits over the years. I always assume that a shiny silver pit is 'active' and is probably something electrical.  Paint seems to snuff the thing out. The other soft silvery greenish stuff I think is the microbial activity and I think can spread under filler. I've seen filler used on more shapely sea boats to fair curvy bits and if the filler falls off or is hacked off  there is sometimes corrosion underneath.  Hauling out every 3 - 4 years seems to be about normal for narrowboats these days - it used to be somewhere between every 5 years to 'never' but I am starting to think every couple of years might be more sensible just to check.  This makes a wide boat quite an expensive proposition. I have laid under a Peniche and thought 'stuff that' when faced with half an acre of pitted steel.  At the end of the day a decent film of good paint is probably the best protection and frequent checking.