Pitting

[Alan de Enfield]

13 minutes ago, blackrose said:

I personally wouldn't put bleach on a grit blasted hull as you're more likely to be adding contaminants to the surface and the last thing you want after you've grit blasted is to wash the hull with water and start it rusting.

 

 

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.

 

....................If a hull is found with evidence of microbial attack, it is necessary to deal with it to try to prevent it recurring. A simple solution is for the whole area to be washed with copious amounts of high pressure fresh water. When dry the area affected should be coated with a strong bleaching agent (sodium hypochlorite) diluted 1:4 with water and left for twenty four hours. Afterwards a second high pressure fresh water wash is necessary followed by recoating. This will probably remove around 90% of the microbes but the only real solution is to blast back to bare steel and to treat any inaccessible areas such as tack-welded rubbing strakes as best one can with the bleach solution before applying the next stage of the coating process. The main problem is that the microbes can continue to live beneath the existing paint coatings and once sealed in with a fresh blacking, the lack of oxygen and light is the perfect environment for them to thrive leading to a risk of corrosion from the inside out. No coatings are entirely proof against a microbial attack from the exterior. Minute pinpricks, mechanical damage below the waterline are all opportunities for the microbes to penetrate the steel and commence the process from the outside in..

 

WARNING SODIUM HYPERCHLORITE IS HIGHLY CAUSTIC AND TOXIC. IT MUST BE TREATED WITH GREAT CARE AND RUBBER GLOVES, WELLINGTON BOOTS AND EYE SHIELDS ARE ESSENTIAL

Edited January 25 by Alan de Enfield

[rusty69]

28 minutes ago, blackrose said:

Rusty69 of this forum had his hull pit welded last summer. I saw the result which looked pretty good. 

 

Oh, don't remind me. Never again.

 

So, our boat is an early 80s boat built with 6mm base and sides. We have owned it and lived aboard for 25 years, and regularly used standard bitumen. The last survey indicated pitting nearing 2 mm in places, so taking the nominal steel thickness close to the 'uninsurable' bracket. That is not to say that we couldn't get third party only insurance, which is not dependent on a survey, and it would probably have outlasted us, and survived another 40 years. For peace of mind, however,and to stave off the inevitable over-plating, we decided to DIY epoxy the hull (sides only), and take the opportunity to have the worst of the pitting addressed at the same time. We did not grit blast the hull, but used a very high pressure washer, and extensive elbow grease (see my other thread that is floating about somewhere) and lots of colourful language.

 

We basically went around the hull marking up with the worst of the pits as a 1 or 2, and got a welder in. He spent around 5 hours completing them all, and as there was still daylight, I gave him a further list of 3's to tackle as well. The cost was well below 500 quid for a 70 foot boat. I can't remember how many pits now, but probably well over 100. He offered to weld them and leave them, or weld them and grind them flush. I chose the former,being a tight B@@@@td, and though they look unsightly in the pics, once they were covered in the epoxy and the boat refloated they are not noticeable unless you have a snorkel.

 

We did not tackle the base plate, which, though has some pitting, is not as severe.

 

Good luck.