Galvanic Corrosion, specifically stainless steel and aluminum

[wolfee]

I have to say this turned out to be much more of a research task then I thought it would be. After reading countless posts via Google about galvanic corrosion, the key factor that always comes back to mind is the fact the two metals have to be in contact with each other. So when securing a transom to an aluminum boat or bolting the motor to a transom, I've always read to use stainless steel. Unfortunately, finding SS screws/bolts, nuts and washers all in the correct sizes is pretty much an online order situation and a decent amount of money. My question is this: why not just use galvanized screws/bolts, nuts and washers and just put a plastic washer between the aluminum and galvanized, just like they suggest for SS? In both circumstances the two metals shouldn't touch each other, so why bother with all the expense of SS? I've done construction with galvanized nails and have torn down construction with galvanized nails and for the most part the nails are not rusted and are in pretty good shape. Please let me know if I'm missing something (other than possibly the look of SS vs. galvanized, but heck it's gonna be painted).

 

[dingbat]

Re: Galvanic Corrosion, specifically stainless steel and aluminum

I have to say this turned out to be much more of a research task then I thought it would be. After reading countless posts via Google about galvanic corrosion, the key factor that always comes back to mind is the fact the two metals have to be in contact with each other.

Your missing the fact that the materials don't have to be in physical contact. The moisture in the air is enough to cause "contact".

Your nails didn't rust because their exposure to oxygen was limited by the wood. Remove the nails from the wood and expose them to the "chemical soup" we call the atmosphere for a while and they will rust

 

[Thalasso]

Re: Galvanic Corrosion, specifically stainless steel and aluminum

Your missing the fact that the materials don't have to be in physical contact. The moisture in the air is enough to cause "contact".

Your nails didn't rust because their exposure to oxygen was limited by the wood. Remove the nails from the wood and expose them to the "chemical soup" we call the atmosphere for a while and they will rust

Not necessarily true. If the fastners are isolated from each other, this will stop the galvanic corrosion. Notice the isolating material between the cross members to the main frame. (galvanized to aluminum). The bolts used are not galvanized


Aluminium's corrosion resistance - Aluminium Design
Aluminium's corrosion resistance - Aluminium Design

 

[dingbat]

Re: Galvanic Corrosion, specifically stainless steel and aluminum

The corrosion of dissimilar metals in close proximity of each other is inevitable. Isolating the materials only slows the process.

To a trained eye (I) see at least 3 examples of corrosion that picture.
1. The flat washer already shows sign of pitting and the plating has been attacked.

2. The aluminum itself has been attacked. Note the white spots on the surface progressing to the left of the bracket and underneath.

3. The zinc on the galvanized bracket has been eaten pretty good as well. Notice the ""granular" look of the galvanized bracket materiel immediately surrounding the washer. This is a sign that the coating has leached a substantial percentage of it's zinc content.

Need I go on?

 

[Thalasso]

Re: Galvanic Corrosion, specifically stainless steel and aluminum

Yes please do. Because your trained eye isn't 20/20
#1 the trailer is 7yr's old
#2 that isn't corrosion on the flat washer (paint)
#3 The white stuff on the aluminum by the brkt is aluminum and boat polish that was not removed

Did you bother to read the link? Apparently these people have a good idea how to and what to use

 

[metriccrescentwrench]

Re: Galvanic Corrosion, specifically stainless steel and aluminum

...why not just use galvanized screws/bolts, nuts and washers and just put a plastic washer between the aluminum and galvanized, just like they suggest for SS....

Probably because the plastic washer couldn't handle the crush load that would be imparted when the bolt is properly torqued.

Also, it's not necessarily galvanic corrosion that will bite you with non-stainless fasteners. Good old fashioned rust/oxidation/corrosion will do a number on bolts over a number of years if allowed to stay in contact with moisture. A previous owner of my Checkmate tried to go cheap with bolts for mounting the jackplate. Here's what I found when I pulled them (and the photo doesn't do the situation justice, that bolt was rusted nearly 2/3 through):

 

[dingbat]

Re: Galvanic Corrosion, specifically stainless steel and aluminum

Yes please do. Because your trained eye isn't 20/20
#1 the trailer is 7yr's old
#2 that isn't corrosion on the flat washer (paint)
#3 The white stuff on the aluminum by the brkt is aluminum and boat polish that was not removed

Did you bother to read the link? Apparently these people have a good idea how to and what to use

Ok, whatever you say. The obvious loss of zinc in the galv coating is not the result of corrosion, its age. The gold irradiate on the hardware that is slowly being eaten away is really paint. The classic signs of aluminum corrosion, the white streaking and pitting, is really just left over wax.
The "these people" you refer too are industry colleagues.

FWIW: No one in their right mind would recommend, or even consider, isolation as a viable means of controlling bimetallic corrosion on the boat. Too much electrolyte floating around for a high probably of success...... Pun intended :lol:

 

[minuteman62-64]

Re: Galvanic Corrosion, specifically stainless steel and aluminum

I'll leave the technical arguments to the experts. On the practical side, everything I've read (quite a bit over the past couple of years while bringing my boat back to life) indicates (when fastening aluminum) the first choice is aluminum and second is SS.

 

[metriccrescentwrench]

Re: Galvanic Corrosion, specifically stainless steel and aluminum

Your missing the fact that the materials don't have to be in physical contact.

It's been awhile so I went back and looked at a chemistry text. For galvanic corrosion, yes, the two dissimilar metals have to be electrically connected, plus there has to be an electrolytic bridge between them (in a lead acid battery the electrolyte is dilute sulfuric acid, on a boat engine it's sea water). And like a battery, current doesn't flow until you have an electrical connection between the two battery posts, and it's when that current flows is when you have galvanic corrosion.

Your nails didn't rust because their exposure to oxygen was limited by the wood. Remove the nails from the wood and expose them to the "chemical soup" we call the atmosphere for a while and they will rust

I've done construction with galvanized nails and have torn down construction with galvanized nails and for the most part the nails are not rusted and are in pretty good shape. Please let me know if I'm missing something

What you both are talking about here is oxidation and not galvanic corrosion.

 

[bobb729]

Hi all just reaching out to the forum as a life line now as insurance, boat manufacture and marina have all but given up on my corrossion problem. I have a 2006 Starcraft that upon getting the boat out of storage found corrossion poking thru the hull from the inside out. If i were to start tracking this down does anyone know how I can start?

 

[Ned L]

You should really start a new new thread specifically about your problem rather than dredging up a three year old post. We might be able to offer some reasonable suggestions, but would need a Lot more information.

 

[jbcurt00]

Yes, please start a new topic about your boat and its corrosion Bobb.

Thanks
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