Ribs on floor are pitting

[Douglasdzaster]

Hello everyone,
I need some of your wisdom and advice with my first aluminum boat.
I have a 1982 Monark 16/52. I brought the motor back to life and have really been enjoying the boat. I noticed a little water the other day and decided to remove the floor ( that the boat came with) and check it out. The floor has good plywood on the boat then it’s topped with tin or aluminum (old signs) and that’s covered with a none slip material that’s holding up well. Had to laugh when I seen the signs though.
Found a small leak on one of the boats dinged spots no big deal.
I’m more concerned about when I removed the floor some of the cross members underneath are pitting. What can I do to repair and stop it from getting worse?
Also when I put the floor back what screws should I use. Several of the old ones broke off when removed. They are all corroded. I want to replace them with something that won’t react badly with the aluminum. Or can I coat some stainless screws with something?
Y’all’s help is greatly appreciated. I want to get back on the water.

 

[Douglasdzaster]

After some more research I’ve found out that stainless screws should be ok. Found the topic on Tin Boats. Also by cross members I believe the correct term is ribs. Sorry it’s my first aluminum boat.

 

[BernieP]

If the plywood that is sitting on the ribs is treated it could be causing the pitting. Certain types of wood treatment have a very bad reaction with aluminum.

 

[DaleH]

After some more research I’ve found out that stainless screws should be ok. Found the topic on Tin Boats.

Maybe, but please be AWARE that there are 'caveats' to be aware of, that might not have been mentioned in what you read.

METALS - Galvanic corrosion is when two different metals are in contact in the presence of an electrolyte. One metal will be more chemically active than the other; hence a reaction occurs. Stainless steel (SS) being a superior or more 'noble' element that aluminum, aluminum WILL give up its ions to the SS overtime in the presence of an electrolyte ... which means the tin deterioates or corrodes away.

Clearly this is exacerbated if the hull or fastener is allowed to be periodically wet (rain or condensation) or kept wet (like hull sits in water all the time) and greatly accelerated if used in saltwater (where the saltwater is a perfect electrolyte). Very pure water will not conduct electricity, so an electrolyte isn’t present. However, with unknown water quality in lakes and ponds along with acid rain and other pollutants falling from the sky in rain ... I believe it is a best practice to isolate any SS fastener from your tin boat hull. Then you know you'll never have any issues or corrosion.

So you 'can' use them, but isolate the SS from the tin. By far the simplest, fastest and cheapest method for a freshwater use hull is a layer of duct tape applied on any SS washer where it bears up against the tin and then 1-wrap on the body of the fastener. A superior method for saltwater use hulls are nylon washers against the tin (under the SS washers used for strength) and adhesive-lined heat-shrink on the body of the SS fastener. Or use TefGel ($25) goop on the SS bolt body and threads, like our US Navy does on its aluminum superstructured frigates and destroyers.

RIBS - For your pitted ribs, I'd throughly scrub any pitted area hard with a copper scrub pad wet with cheap white vinegar. Then rinse really well with water.

 

[Douglasdzaster]

Thanks for the advice it’s greatly appreciated. Sorry it’s taken so long to get back on here.The plywood is coated with either epoxy or fiberglass resin. I cleaned the ribs but didn’t use vinegar. The way I put the floor back in makes for easy removal. Two pieces held together at the seam with a strip and screws ( none in the aluminum of the boat ). When I apply the vinegar I will have to redo what I put on the ribs. I used stick on foam insulation that is used between a truck bed and camper shell.
I figured it would make a good beerier. Thinking now I need to use something that is closed cell. Don’t know why I didn’t think of vinegar at the time. I filled in the few holes with JB weld as well. The boat is fresh water only but not sure about the previous owner. I don’t have a shed for it so I leave it jacked up high so rain water can drain and keep a cover on the motor. Sounds like I need to make other arrangements. The pitting is in the front of the boat.
I basically put it back together the way it was which included a couple of old signs that I don’t know if they’re tin or aluminum or what. Could tin be causing the corrosion in the front. As far as electrolysis (which is a new subject for me so forgive this next statement if it’s dumb) My motor is pull start so no battery there. I do use a battery in the front mounted in a box for my trolling motor. It’s only in the boat when I’m using it. It was just in a tray until recently and it’s where the pitting is but no corrosion on the tin between it and the boat. I’ll post pictures of the floor the way it is tomorrow.

 

[Douglasdzaster]

If the plywood that is sitting on the ribs is treated it could be causing the pitting. Certain types of wood treatment have a very bad reaction with aluminum.

The plywood is coated with either epoxy or fiberglass resin. I put a beerier on the ribs also .
I used foam insulation that’s used between a truck bed and camper shell. It fit the ribs perfectly and has an adhesive on one side. All though I may be redoing it since I didn’t treat the aluminum with vinegar after cleaning it.
Part of the flooring was done with old signs. Tin I think. Will this cause a reaction as well?

 

[Douglasdzaster]

Well I think I’ve convinced the Admiral to find another place for all her plant stuff instead of my shop. I’ll have to hang some stuff during a major rearranging but I’m determined to get the boat in there. I used to store a 16’ bass boat in there for my sister. But we accumulate over the years. Lol