Problem mixing galvanized and stainless components?

[gustafson]

I don't collect photos of galvanic corrosion. You clearly win. I am humbled.

Don't want you to be humbled, just rational

 

[Donkey Hotey]

I'm being completely rational. I was trying to share some simple guidelines about how the galvanic scale is used. You want to turn it into a million cases of "it depends." You're not wrong but, the exceptions do not make the rules.

Not everybody has a materials engineer on speed dial, that can sign off on the design if the proper masses of materials are met, along with interface area and controlled moisture. Maybe you'd like to point out how he could install a power supply and do active anodic protection and install whatever he wants?

 

[seiner]

My fishing boat is 80000lbs of 5086 aluminum, steel, and some, not much stainless.
I only use fasteners made of grade 316ss on the exterior to prevent(reduce) galling(using good practices clean, never seize, hand tools only, ect). my plumbing for refrigerated seawater is all welded 308ss,,, and I use hot dipped galvanized fasteners on the flanges because they are 100% reliable, never gall, never seize up and get taken apart fairly often and never cause or have corrosion. In the situation that you are describing I would use 316ss fasteners if available at a reasonable cost or go strait to hot dipped galvanized steel.

 

[JasonPAtkins]

Thanks for the responses.

Not to add complication to the discussion, but in this case I'm using stainless precisely because of salt-water proximity. The brackets and fasteners are to secure a timber-frame truss over the front porch of this little guest bungalow (red), which is right by a finger inlet of ocean water (though we're 30km from the open ocean).


There's no salt spray like the cliffs of the PNW so the salt isn't in the air much, the tide just calmly goes in and out with no waves. However, it will be exposed to monsoon-season driven rain water.

Parenthetically, one of the bungalow's uses will be to host visiting teachers for our trade school, which is being built on the same site (blue arrow) in case any of you close to retirement are looking for an opportunity to teach for a few weeks or a couple months in west Africa while the winter is crappy back where you live! We're a non-profit, so it's a volunteer gig, but pretty fulfilling, and at least it's warm!

 

[boslab]

Generally I agree with the exception of fog not causing corrosion, I’d does, in fact we seed fog in the accelerated corrosion chambers in work, it’s wet and an electrolyte, galvanic corrosion needs an electrolyte to form the cell,
Stainless is an oddity in that it’s own oxide forms the cell, so even with no dissimilar metal it can be induced to pitting,
Mark

 

[gustafson]

I'm being completely rational. I was trying to share some simple guidelines about how the galvanic scale is used. You want to turn it into a million cases of "it depends." You're not wrong but, the exceptions do not make the rules.

Not everybody has a materials engineer on speed dial, that can sign off on the design if the proper masses of materials are met, along with interface area and controlled moisture. Maybe you'd like to point out how he could install a power supply and do active anodic protection and install whatever he wants?

Again, if you are rationally applying engineering principles you should have no problem proving me wrong by showing examples of how similar installations have failed.
Instead you bring up airframes which are not a rationally equivalent situation

every time, I mean every time this comes up on any forum, someone pipes up with the dreaded galvanic corrosion without actually thinking it through

yes it is a thing

no, outside of certain situations involving immersion in water, or, usually, copper, it is a distant secondary issue.

 

[Donkey Hotey]

Again, if you are rationally applying engineering principles you should have no problem proving me wrong by showing examples of how similar installations have failed.
Instead you bring up airframes which are not a rationally equivalent situation

every time, I mean every time this comes up on any forum, someone pipes up with the dreaded galvanic corrosion without actually thinking it through

yes it is a thing

no, outside of certain situations involving immersion in water, or, usually, copper, it is a distant secondary issue.

Cool story bro.

I told you that I have no pictures and I'm not going to spend a bunch of calories hunting around for some so I can win some imaginary jousting contest with you. I'm trying to help the OP and anyone not already familiar with the concept of galvanic corrosion. As I've posted over and over: you're right in very specific use cases and managing those cases is more than is appropriate for this discussion.

In the case of what the OP is doing, the best choice would be to not mix materials, if the stainless fasteners meet the structural requirements and other fasteners haven't already been purchased. If he already has buckets of hot dipped galvanized screws, it might pay to do a wet coating of some kind during installation to keep moisture out of the joints and just run with it.

 

[gustafson]

OR it might pay to never ever worry about it again

Corrosion risks with galvanised steel and stainless steel in contact​

Galvanised steel in contact with stainless steels is not normally considered to be a serious corrosion risk, except possibly in severe (marine type) environments.

 

[triumph406]

in case any of you close to retirement are looking for an opportunity to teach for a few weeks or a couple months in west Africa while the winter is crappy back where you live! We're a non-profit, so it's a volunteer gig, but pretty fulfilling, and at least it's warm!