Reacting to your post- WHATTTT?
How come the entire boat isn't reacting? What is "purely saltwater corrosion"? Of course I've only been involved with welded aluminum since the 1970's so my experience may be too limited to compare with whoever tried to sell that steaming pile? I've never heard of, seen or experienced saltwater corrosion of marine alloy aluminum - so my malarkey meter just pegged on that phrase.
regarding soap, all that matters is the MSDS, and maybe some patch testing to see how it behaves? The photos of the soap bottle show the main cleaner is ammonium chloride and I'm not aware of that being particularly corrosive to aluminum? But I'm not well informed about the compatibility- I'd still contact the mfg and do a spot test.
As regard the images - although I'd say it was somewhat hard to tell for sure - I see initial corrosion in all of them. Maybe these coloration spots are just water stains of some kind? IT doesn't look like the builder removed the mill scale? and that porous film will promote white flower corrosion cells because it hold ambient moisture to the metal allowing crevice sites to form. This fact is denied by countless major builders- who use phrases like - "normal corrosion" but that is 100% absent from acid etched boats.
There are countless posts here showing many boats with the mill scale intact- and their corrosion while sitting on a trailer-just from the ambient air moisture!
so did the entire hull, as it ran in salt water to get South- why hasn't that metal act like the fish box?
You can get a rental or for a hundred bucks; buy a video bore scope- that will allow you to inspect any place you can feed the camera into.
https://www.amazon.com/video-borescope/ ... 0borescope
I'm kind of stumped by the surface being covered uniformly by bilge water and still pitting in sites something more commonly seen where there is a drying/wetting cycle that concentrates the out of range ph material. However, I've seen that when the water was carrying metal particles or in solution.
Galvanic corrosion is usually a site related event- for example a SS bolt in a hole drilled in the aluminum. If the SS is not passivated and coated with some dielectric paste or thread goop- the corrosion is usually at the nearest points of contact between the metals. However, I have seen examples of extreme pitting in wide areas of a bilge (the main hull area) where a wash down pump that was brass- not bronze- had created a bilge water that was corrosive- the corrosion was galvanic- different metals and wide spread because there were copper ions in the water. The pumps' impeller was not rubber but metal so the rate of transfer of metal particles into the water was apparently high enough to make that bilge copper filled?
So my remarks about galvanic corrosion are not confined to points of contact of different solid metals, there is also a good possibility of bilge water to have a galvanic action due to a metal salt in solution. The term salt here is not limited to sodium chloride salt. http://www.vopelius.com/products/metal-salts
Also a note in reply to the idea of a bilge anode. Anodic protection is based on a couple things- #1 the galvanic scale where any metal more noble (more inert) can be 'protected' by a less noble (relatively more reactive) #2 when there are electrons flowing- current- ie "stray current". Why or how is there any place for current to flow from the hull INTO the bilge water? How is it that bilge water could be a lower potential location for electrons? Why would the current (if there is some stray current) flow into the bilge water instead of off the hull into the water the boat is floating in?
Using a plating tank as an example- meaning to compare a plating tank and current flow model as a means to show corrosion in a bilge- I miss the part where the circuit is completed in the bilge? Plating tanks have two poles to their driving electrical circuit- I can't see them in a boat bilge?
Outside the hull, where the ocean/lake/river is an infinite lower potential that allows any current to dissipate- I can see an anode being able to 'give up' or 'sacrifice' molecules as the site of the current flowing off the hull into the water- but I'm lost finding that model applying to the bilge? Maybe you understand this anode in the bilge idea better than I do, and perhaps I've just not grasped the fundamental idea? But I can't see the bilge being a site where the metal of the boat flowing into the bilge water - and that seems required to make the anode idea applicable??
Not sure I'm much help but we're at least exploring this condition of your boat's fish hold with more than one set of experiences to help solve your problem.
Cheers,
Kevin Morin
Kenai, AK