Rust under the paint from Factory?

[ShadowsPapa]

For "galvanic corrosion" to occur, you must have two dissimilar metals, AND conductive electrolyte solution - something like WATER, must connect the two metals on a regular basis.
Just two metals without the water or conductive "solution" will not produce galvanic corrosion. And - the place where this happens is where the metals meet. The most severe attack happens at the JOINT - where the metals MEET each other. Areas away from the joint aren't impacted greatly.
Hmmm - that hinge doesn't fit, and it's not aluminum.
The ALUMINUM will be eaten first as it's more active than most other metals - such as iron, nickel, etc.

In short, where aluminum meets steel, for there to be galvanic corrosion, there must be an "electrolyte" such as water, and the most active metal will be eaten away faster than the more noble metal. Steel will last, aluminum will be eaten away.
You can see evidence of that in the cars I've worked with - the aluminum sill plates are eaten away while the steel is hardly touched.
Similar for aluminum parts bolted to steel - the aluminum will be pitted, maybe even have a white powdery appearance, the steel - not so much. It won't bubble.
When paired with other metals such as steel, aluminum is the more active or anodic, while the steel or iron, copper and so on are cathodic - less active so won't rust or corrode like the aluminum.
Of the common metals used, only zinc and magnesium are more active or anodic than aluminum.

I do plating, de-rusting using electrolysis, etc. so have an idea which metals will go first.
If those hinges are steel or iron of any sort and that hood is aluminum, then the destruction is on the wrong part for galvanic corrosion. And there's no water present unless his hood is constantly wet. The proof would be removing the hinge and checking the aluminum. If it's not harmed, then......
For the hinge to go first, it would HAVE to be zinc or magnesium, possible, but quite expensive to use those materials over cast steel of some sort.

I hear there is magnesium in Jeeps - isn't that used in the tail gate? Or is that mistaken info?

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[BillG]

For "galvanic corrosion" to occur, you must have two dissimilar metals, AND conductive electrolyte solution - something like WATER, must connect the two metals on a regular basis.
Just two metals without the water or conductive "solution" will not produce galvanic corrosion. And - the place where this happens is where the metals meet. The most severe attack happens at the JOINT - where the metals MEET each other. Areas away from the joint aren't impacted greatly.
Hmmm - that hinge doesn't fit, and it's not aluminum.
The ALUMINUM will be eaten first as it's more active than most other metals - such as iron, nickel, etc.

In short, where aluminum meets steel, for there to be galvanic corrosion, there must be an "electrolyte" such as water, and the most active metal will be eaten away faster than the more noble metal. Steel will last, aluminum will be eaten away.
You can see evidence of that in the cars I've worked with - the aluminum sill plates are eaten away while the steel is hardly touched.
Similar for aluminum parts bolted to steel - the aluminum will be pitted, maybe even have a white powdery appearance, the steel - not so much. It won't bubble.
When paired with other metals such as steel, aluminum is the more active or anodic, while the steel or iron, copper and so on.
Of the common metals used, only zinc and magnesium are more active or anodic than aluminum.

I do plating, de-rusting using electrolysis, etc. so have an idea which metals will go first.
If those hinges are steel or iron of any sort and that hood is aluminum, then the destruction is on the wrong part for galvanic corrosion. And there's no water present unless his hood is constantly wet. The proof would be removing the hinge and checking the aluminum. If it's not harmed, then......
For the hinge to go first, it would HAVE to be zinc or magnesium, possible, but quite expensive to use those materials over cast steel of some sort.

I hear there is magnesium in Jeeps - isn't that used in the tail gate? Or is that mistaken info?

Tailgate hinges on JL are magnesium for sure... not sure about where or if it’s used on JT.

 

[Willys2Gladiator]

Looks like the hinges just wernt properly finished. Not smooth out before painting.

I have seen other things in my days that look identical to that from not proper prep of raw part once manufactured.

 

[Dewyaw]

It’s not though, I deal with galvanic corrosion all of the time, same, deal with it daily, it’s not “rust” but the paint will fail as you know....that is 100% the onset of galvanic corrosion. JMHO, I’m not getting in a pissing contest with you, but I’ve seen this in this exact application already and that’s exactly what it was...it’s not as uncommon as we’d like it to be ( in wranglers/gladiators) unfortunately.

What are the two dissimilar metals there to cause the galvanic corrosion?

 

[Dewyaw]

Whatever the f*** it is, it needs to be fixed by the dealership...

 

[ShadowsPapa]

What are the two dissimilar metals there to cause the galvanic corrosion?

I can't say that there are in this case - as I don't know the exact metalurgy of the hood portion of the hinge.
Assuming the hood itself is aluminum, I know the body portion of the hood hinge is a ferrous metal - contains iron, in other words, it's a steel alloy.
The hood part of the hinge is NOT ferrous - it is NOT steel. It is possibly aluminum or a magnesium alloy (magnesium ain't cheap)
It could be aluminum or a magnesium alloy, but it's NOT steel.

I also observe that there are isolators (insulators) around the bolts (which are steel), and likely between the hood sheet metal (aluminum) and the hood side of the hinge, then it is painted as a unit on the body.
That being said, the two metals are not in contact, and, the paint should prevent water from getting between the parts.

If you have done plumbing (as I have) or electrical work - you'd have likely seen where copper pipe was joined to the older galvanized steel pipe - and they used a special fitting that had a plastic insulator to keep the copper from touching the steel pipe directly. Since there is always water in the pipes, the two metals would cause a galvanic reaction - electrolysis - which would quickly cause the galvanized steel pipe to be eaten away.
Also note that in home wiring, there are alerts about using aluminum wire in certain devices - if you clamp aluminum wire into a copper connector, the aluminum wire won't last - if exposed to humidity/water/moisture. If it's dry, not so fast, but wet -it goes fast.

For galvanic corrosion to occure, you must have 3 things - a more active metal, one that is more anodic, less noble, and a more passive or nobile metal - one that is cathodic compared to the othyer, they must be in contact with an electrolyte - which can be water.
If any of those are not present, it just won't happen. So if the metals are separated by an insulator, it won't happen, and if they aren't in constant or frequent contact with water - meaning both of them - it won't happen.

Anyone can check with Google and "check my info".

More info on prevention -


Galvanic Corrosion Prevention
Select metals/alloys as close together as possible in the galvanic series.
Avoid unfavorable area effect of a small anode and large cathode.
Insulate dissimilar metals wherever practical.
Avoid threaded joints for materials far apart in the galvanic series.

Magnesium and aluninum are right next to each other in the charts, so the case for a reaction is small.

Reactions happen at the point of contact, not 2" away.
I have a stack of sill plates (aluminum" that are good examples of that - eaten away where it contacted the steel, but still nice just an inch away from the points of contact.

 

[jeepjonah]

Are the hinges even steel? Try a magnet on them.... door hinges are aluminum or magnesium I think.

Good idea, Bill. Thanks. I tried a magnet as suggested and it did not stick. So I assume not rust as not steel?

 

[ShadowsPapa]

Good idea, Bill. Thanks. I tried a magnet as suggested and it did not stick. So I assume not rust as not steel?

Rust doesn't form like that anyway.
As I stated in my post above, the cowl side of the hinge is steel.
The part on the hood is not. It's either aluminum or an alloy.
The bolts are steel so are insulated from the hood part of the hinge.

 

[BillG]

Good idea, Bill. Thanks. I tried a magnet as suggested and it did not stick. So I assume not rust as not steel?

Shouldn’t be rust.... but paint adhesion still isn’t right, whatever the reason. Warranty should cover you, so it’s just a headache.... not the end of the world.

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