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Bleach (sodium hypochlorite) attacking steel hull boat

Q. I'm scared. I live on a static 1929 houseboat that once carried coal. The hull was inspected 20 years ago when I bought it and lots of small corrosion pin holes were NOT found (presumably from the acids leaching from the coal plus general old age). We over-plated the rusty old metal with new steel thus essentially creating an inaccessible void between the two skins, then treated the rust with some stuff that supposedly cures rust, then liberally coated the whole thing with thick bilge grease ... worked a charm for 20 years with not so much as a teaspoon of water evident.

Then some friends (?) visited while I was away and decided to disinfect the loo and stainless steel soil tank by pouring a litre or two of bleach down the toilet ... I thought nothing more about it. Then I noticed a bit of a pong every so often. Then my water leak detector suddenly auto shut off the incoming mains potable water supply. I looked in the bilge and found 2 inches of water and, curiously, quite a lot of grease globs. Huh, they must have spilt a load of water on the floor and didn't tell me, thought I. No matter, I'll just vac it up.

But the water came back. Hmm ?, a wash machine supply pipe (or whatever,) must be leaking. Nope. Damn, the deck above must be, but it hasn't rained lately, and anyway water isn't coming through any ceiling. Surely the hull hasn't suddenly sprung a leak? So I shut off all the incoming potable water supply and waited. Nope, the hull isn't leaking (yet) and nor is the soil tank (yet). After a great deal of head scratching I found the soil pump WAS leaking behind its panelling AND the wash machine at the other end of the boat WAS leaking but only while it was in operation due to a partially blocked pipe that caused the standpipe to overflow for a few seconds. Fixed all of that and no more water in the bilge. Phew!

My problem (yes, I've finally got there.) is that I assume there will be a dilute residue of bleach still in the void between the old and new hulls which I can't access. So I Google ''Does bleach dissolve grease? ''Oh yes'' Does it affect steel?'' Oh yes, mega!'' Does it affect stainless steel? ''Oh, yes, mega !''

Rats !!. So how do I neutralise it forever? ''Flood it with several gallons of Vinegar [in bulk on eBay or Amazon (adv.)], or Sodium metabisulfite [on eBay or Amazon (adv.)] or sodium hydrosulfite [on eBay or Amazon (adv.)], which is the more aggressive 'cousin' of bisulfite..... (and I get it from where?) then scrupulously scrub and rinse it all away -- (which of course I can't do because I can't access the void between the two steel skins short of drilling a hole in the bottom of the boat :) So, what d'ya think? Do I need to lose sleep over all this or will the counter measures really stop the bleach in its tracks and stop it endlessly munching its way south over the years ?
Thanx for listening and for any advice given.

Alan Harris
Hobbyist - London UK
October 6, 2023

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