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Problem mixing galvanized and stainless components?

JasonPAtkins

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Sep 30, 2010
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Guinea-Bissau, West Africa
I'm curious, does anyone know if there's some kind of corrosion problem mixing stainless and galvanized?

In my specific application I'm making tie plates to mount timber frame trusses to masonry columns. What I have in stock for making the brackets is stainless plate, and have some hot dipped carriage bolts handy. Would that interface cause corrosion faster than either an all-galv or all-stainless solution?
 
i think I would buy some stainless bolts cause they would look cool
Ignore 90 percent of the conversation anywhere about galvanic corrosion as mostly[mostly] only affects things in water 100 percent of the time, or extreme cses like copper and aluminum
 
If outside I would do all stainless.

The only points of contact will be the threads and underneath two washers. Use some gray anti-seize on those small areas.
 
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i think I would buy some stainless bolts cause they would look cool
Ignore 90 percent of the conversation anywhere about galvanic corrosion as mostly[mostly] only affects things in water 100 percent of the time, or extreme cses like copper and aluminum
Sorry, bit offtopic. It’s not once or twice i’ve seen banjo coupling attached to cylinder/reservoir with copper washer in between where the aluminium has corroded very badly around the washer. I don’t get it why do they do such rookie mistakes not knowing the very bad galvanic corrosion with this combination. Mostly I’ve seen this in motorcycles.
 
Sorry, bit offtopic. It’s not once or twice i’ve seen banjo coupling attached to cylinder/reservoir with copper washer in between where the aluminium has corroded very badly around the washer. I don’t get it why do they do such rookie mistakes not knowing the very bad galvanic corrosion with this combination. Mostly I’ve seen this in motorcycles.
copper washers are the standard for banjos even on aluminum masters and calipers
they work for 50 years or so, unless they are submersed in water.
not a rookie mistake, they know exactly what will happen
thus my point about listening to concerns
 
Zinc and stainless are fairly close by on the electrochemical series, there won’t be a problem, like unistrut with stainless bolts , as supplied by firms like hilti, I’d be happy ( my girder trusses use galv and stainless screws and washers.
In salt water stainless can be cut in half by a Ty wrap or elastic band due to the cell causing pitting corrosion, amazing to see.
We did this experiment in college, 316 stainless plate with a rubber band round in a beaker of 2% salt water, the plate got severed in one term, a plain mild steel was virtually untouched so stainless isn’t always the best thing.
Mark
 
Zinc and stainless are fairly close by on the electrochemical series, there won’t be a problem, like unistrut with stainless bolts , as supplied by firms like hilti, I’d be happy ( my girder trusses use galv and stainless screws and washers.
In salt water stainless can be cut in half by a Ty wrap or elastic band due to the cell causing pitting corrosion, amazing to see.
We did this experiment in college, 316 stainless plate with a rubber band round in a beaker of 2% salt water, the plate got severed in one term, a plain mild steel was virtually untouched so stainless isn’t always the best thing.
Mark
Thanks, that's what I wanted to know. Obviously aesthetically I'll eventually use stainless fasteners to go with the stainless brackets, but wanted to know if the galv fasteners until I can arrange the stainless would attack the brackets in the meantime.
 
You asked a good question. Despite some claims to the contrary, any exposure to rain, fog, etc, can cause very bad corrosion between dissimilar metals.

Are you familiar with the galvanic series? Dissimilar metals are going to behave like a battery. Having metals that are far apart in voltage potential is bad. Better versions of this chart are all over the web but, this link will probably survive over time:


You generally want to choose metals that are closer together on the scale. That minimizes the reaction.

Another strategy is to buffer the materials with a sacrificial material. That's the logic behind using zinc plating on steel fasteners when installed with aluminum. You'll see that aluminum and steel are somewhat close but, by zinc plating the steel, the zinc becomes the sacrificial material and will corrode first.

If you use zinc plated steel fasteners in stainless hardware, you'll see that they are further apart than steel and aluminum. The zinc will be the first to corrode. Unfortunately, over many years and enough environmental moisture, the steel will also be eaten away by the stainless once the zinc is used up. If you compare their relative voltage potential, the zinc is going to be very reactive with the stainless until it's used up. This may also cause localized pitting and deterioration of the parent materials.

For the longest life, it would be better to use all steel in this application, or all stainless hardware in your stainless plates.
 
You asked a good question. Despite some claims to the contrary, any exposure to rain, fog, etc, can cause very bad corrosion between dissimilar metals.

Are you familiar with the galvanic series? Dissimilar metals are going to behave like a battery. Having metals that are far apart in voltage potential is bad. Better versions of this chart are all over the web but, this link will probably survive over time:


You generally want to choose metals that are closer together on the scale. That minimizes the reaction.

Another strategy is to buffer the materials with a sacrificial material. That's the logic behind using zinc plating on steel fasteners when installed with aluminum. You'll see that aluminum and steel are somewhat close but, by zinc plating the steel, the zinc becomes the sacrificial material and will corrode first.

If you use zinc plated steel fasteners in stainless hardware, you'll see that they are further apart than steel and aluminum. The zinc will be the first to corrode. Unfortunately, over many years and enough environmental moisture, the steel will also be eaten away by the stainless once the zinc is used up. If you compare their relative voltage potential, the zinc is going to be very reactive with the stainless until it's used up. This may also cause localized pitting and deterioration of the parent materials.

For the longest life, it would be better to use all steel in this application, or all stainless hardware in your stainless plates.
Sorry, don't mean to be pedantic, but....

no no no no no

Everything you say is absolutely true.

However.........

In reality, as opposed to in practice, this is less of a problem


no, fog does not cause galvanic corrosion

never use steel outside because you are worried about galvanic corrosion, plain old fashioned corrosion will rear its ugly head long before the galvanic table comes into play

personal experience[feel free to chime in with your own PERSONAL experience]
drip edge on the water table trim in my last house, small overhang shaded by trees. Heating guy ran the oil line for the small Monitor heater up against the trim. Large mass of copper, up against small mass of aluminum, rotted the aluminum away where there was direct contact.

Why don't those copper washers rot away on every British car brake system?
Mass
Direction

LArge mass of aluminum of the master cylinder is in fact losing material whenever there is water present. However the loss of material is ALWAYS the aluminum to the copper, and the voltage potential is relative to the mass. Thus the few grams of copper will never rot away the pound of aluminum in a human time scale


Outside of salt water, worry less about galvanic corrosion
 
In reality, as opposed to in practice, this is less of a problem

no, fog does not cause galvanic corrosion
Well, you're splitting hairs here soooo...

No, fog won't cause galvanic corrosion but, accumulating moisture out of the fog will. So if surfaces sweat and collect droplets in foggy weather, they can galvanically corrode. Also: you claim that the absense of it being salt water means it won't happen. Again, we're splitting hairs here because that can come in many forms of other contaminants in the surrounding structure getting onto the joints. Nearby paint, roofing materials or contaminants in the air can mix with the water, accelerating the process.

never use steel outside because you are worried about galvanic corrosion, plain old fashioned corrosion will rear its ugly head long before the galvanic table comes into play

Very true, which is why it should be all stainless or all steel with zinc. As was also pointed out, most stainless is crap structurally speaking. If there was an easy answer, everyone would use that answer.
Why don't those copper washers rot away on every British car brake system?
Mass
Direction

LArge mass of aluminum of the master cylinder is in fact losing material whenever there is water present. However the loss of material is ALWAYS the aluminum to the copper, and the voltage potential is relative to the mass. Thus the few grams of copper will never rot away the pound of aluminum in a human time scale
Yes, the mass can alter the reaction. Again, we're getting off into never never land. I was trying to give him some guidelines to live by, not a university level course on material properties. You're not wrong but, your answer dismisses some simple design principles to use when making these choices.
 
No not never never land

reality

Mass and Direction

For instance, in a very wet environment:

stainless screws aluminum plate, not really a problem
mass
direction
stainless plate, aluminum screws
problem
mass direction


In Salt water, sure, stainless screws might rot a hole in aluminum plate

But
we do not live in the ocean

and it would be and interesting race to see if mild steel screws would rot away in salt water before stainless screws dissolved the aluminum
 
Okay, well that's not how it works in airplane land. Airplanes don't get submerged in sea water and they 100% live by these rules. Even airplanes that never see coastal conditions pay very careful attention to galvanic corrosion, even on interior structure that is wet installed, sealed and submerged in fuel.
 
Common places actual people run into galvanic corrosion:

galvanized nails in copper flashing, big no no
[mass direction water]
galvanized screws in CCA PT decking[mass direction water]
 
Okay, well that's not how it works in airplane land. Airplanes don't get submerged in sea water and they 100% live by these rules. Even airplanes that never see coastal conditions pay very careful attention to galvanic corrosion, even on interior structure that is wet installed, sealed and submerged in fuel.
Because falling from the sky
 
If sea water is the only place this matters, why do fresh water aluminum boats and outboard motors have sacrificial zinc or magnesium anodes? Why does a household water heater have a zinc anode? Hey, it's not salt water, right?
 
airplane
goes through dewpoint every single flight
weight savings so primary that failure of a single fastener can cause catastrophic failure
unlike anything earthbound

picture please of stainless fastener rotting galvy plate on dry land
 
airplane
goes through dewpoint every single flight
weight savings so primary that failure of a single fastener can cause catastrophic failure
unlike anything earthbound

picture please of stainless fastener rotting galvy plate on dry land
I don't collect photos of galvanic corrosion. You clearly win. I am humbled.
 








 
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