Galvanic Corrosion - screws, sealant etc.

[Bill D]

I'm looking at replacing all of the screws on the marker lights of my 2013 Hawk, with whatever is best to avoid galvanic corrosion.

After reading for about 1 week on galvanic corrosion, I'm more confused than ever. Some people say use stainless steel, others say do not use stainless steel fasteners on aluminum. Then again others say it's okay if there is no moisture.

When I took a couple of the screws out of my Hawk it appeared that the long marker lights were using 1
" stainless screws to hold the light assembly in place, but none of the other screws were stainless. The remainder of the screws were attracted to a magnetic. Perhaps the are galvanized. But why did some of the ones I took out have rust.

Only the 3 marker lights on the front had sealant on them. Of course more water gets to the front while driving, but shouldn't all of the screws have received the same treatment.

Anyhow. I want to know what are the best screws to use. Should sealant be used to cover the screw heads. What about the threads of the screws. RV sealant seams quite messy and it might make it difficult to get the light covers off again. Can something else be used. Are people sealing the covers as well?

 

[Rusty]

the lights have to be grounded to work....so you have a charge running through the metal grounding bar in the plastic frame....if the galvanized screws are touching the metal in the grounding base section and the aluminum siding they will corrode...see the noble chart of metals.

As far as Stainless goes..there are a number of different grades of SS fasteners....and then there's Chinese.....ugh. Use Marine grade SS fasteners and apply a small amount of regular silicon to the hole in which they are being placed...it will help to water resist the hole and the fitting, they come out easily. Covering the head is something I do on my Aluminum boat...basically to protect the head from dirt/debris/fish guts/et al.

don't seal your light covers in place.....they WILL get water in them..one way or another...and if you'll notice, most covers have a small cutout which goes at the bottom for drainage...trapped moisture will cause corrosion and light failure...a loose or sort of loose lens will allow the moisture to evaporate out and not condense

the best you could do...put a small line of regular, clear silicon over the top edge of the cover...extending from the siding the the lens...it will help to keep out "excess" water...better fixtures have thin fiber washers which help seal the fixture but allow evaporated water to release out though hot air pressure

 

[Bill D]

What type of screws does FWC use?

Why did I only find 4 stainless screws out of about 40 screws if marine grade stainless is the way to go.
What were the other screws made of?

There are so many galvanic charts. Can somebody post one that they think is a good chart to reference.

 

[kmcintyre]

Isn't galvanic corrosion a result of 2 different metals in direct contact? Think of a water heater. You get galvanic corrosion on copper that's connected directly to galvanized pipe. The sell nipples that have plastic washers so the metals don't touch. I would assume that's the same issue you have with the screws.

 

[ntsqd]

Search for a Galvanic Potential chart. Some will list the electron-volt potential of various metals in order, others just list them from most anodic to most cathodic. You can look at the two metals that you're considering using together and get a qualitative and/or quantitative idea of how incompatible or compatible they are. The further apart they are in the list, the less compatible they are. It is important to know which alloys are in question are because different alloys of the same base metal appear in different locations in the list. Most aluminum extrusions, like the square tube used in camper frames, are 6063 T6. Stainless fasteners are commonly quoted as being "18-8" stainless. Rarely is a stainless fastener made from some other alloy. In the more common alloy designation scheme that translates to being AISI 304 stainless. Note the Chromium and Nickel percentages in 304 SST: http://matweb.com/search/DataSheet.aspx?MatGUID=abc4415b0f8b490387e3c922237098da&ckck= 18% and 8% respectively.

Two different metals in dry contact won't make a battery, but add an electrolyte or even pure water and you have a battery.

EDIT: Decided to see what I could find. We hear "Mil-spec" all of the time, but I found the MIL-STD dealing with this (MIL-STD-889): http://quicksearch.dla.mil/Transient/799290190D7541E8AF3CCC418F2E185A.pdf with the latest Change Notice (for Page 4) (1993): http://quicksearch.dla.mil/Transient/13B297D5732B4F63B5F46D38986C3177.pdf Included in this document are definitions, recommended practices, etc.

 

[Bill D]

On one thread on this forum I read that FWC uses:

C1022-grade steel with a Climaseal organometallic polymer coating over base coat of non-electrolytically applied zinc

Although a mouthful, this makes sense to me.

Steel = strong screw (not to far from aluminum on the galvanic table)
zinc = sacrificial to the aluminum (so the aluminum does not corrode, the zinc does)
Climaseal = minimizes risk to zinc coating while installing the faster in addition to corrosion resistance



Now giving that the zinc coating is sacrificial it makes sense that these screws get replaced periodically.
I want to change them all, because I am not the original owner of my Hawk. I want to ensure all the screws are correct and fresh. As you can see, I had to do a repair. This may have been my doing from changing a ground screw to a plain metal screw.

Initially I noticed a bit of corrosion underneath the marker light and as you can see when I took the light off, it was pretty bad. I fabricated some aluminum checker plate and sealed with RV sealant (not silicone).
I went to a specialty fastener shop and ended up taking stainless steel screws home. That is what I am using right now.

I really just want to replace the screws once and even if I have to do it again in a few years I'm fine.
I'm personally not comfortable with stainless because I know it wins over aluminum. I'd rather replace screws than repair more aluminum on my Hawk.

 

[ckent323]

FWIW: NTSQD is correct on this (see his post above), Two different metals in dry contact won't make a battery, but add an electrolyte or even pure water (humidity, fog, rain) and you have a battery which leads to corrosion.




Here are some useful links:

http://www.airforums.com/forums/f4/is-stainless-steel-corrosive-to-aluminum-49955.html


https://www.fastenal.com/content/feds/pdf/Article%20-%20Corrosion.pdf


https://www.galvanizeit.org/hot-dip-galvanizing/how-long-does-hdg-last/in-contact-with-other-metals



Read these carefully before using stainless with Aluminum.

Seems to me that Aluminum (for sure - same galvanic potential), Zinc plated or Galvanized steel fasteners are a better choice for attaching to the Aluminum frame of the camper.



Regards,
Craig

 

[ntsqd]

Seeing the damage done above makes me question the very common practice of grounding lights like that thru the camper's frame. Which sends some voltage right thru that "battery." I think that I'd want a dedicated grounding system that does not include the camper's frame in it. That might be over-the-top, but I'd sure feel better about it.

 

[Vic Harder]

Seeing the damage done above makes me question the very common practice of grounding lights like that thru the camper's frame. Which sends some voltage right thru that "battery." I think that I'd want a dedicated grounding system that does not include the camper's frame in it. That might be over-the-top, but I'd sure fell better about it.

Totally agree. Kind of lame to use the frame.

For those folks who have redone their campers or seen them during construction, is it possible to "fish" more wires? Or use the existing ones to pull new ones in?

Vic

 

[Seth]

...For those folks who have redone their campers or seen them during construction, is it possible to "fish" more wires? Or use the existing ones to pull new ones in?...

From the minor projects I've done, it looks like you would have to remove some skin to get access to fish in the frame. On my older (97) camper there are holes drilled through the square frame tubing for wires. Newer campers the same?

No wire pulls with old wires or fish tape, I think.


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[Vic Harder]

Bill, can you explain what we are looking at in this picture?

I suspect the blue arrow points to the power wire, and the big hole... is that corrosion damage?

 

[cwdtmmrs]

If it is a concern, why not use aluminum fasteners?

aluminumfastener.com

cwd

 

[Bill D]

The blue arrow is indeed where the power wire comes through a factory drilled hole.
The big hole on the far right is indeed loss of metal due to galvanic corrosion. This is where a stainless steel screw was used to mount the light to the camper.
The two screw holes to the right of the power cable are a ground screw hole and a mounting hole.
There is galvanic corrosion near these two holes as well, but not nearly as bad.

The red junk is just reminant Tuck tape, that I used to seal the hole overnight. Ignore it.

 

[Vic Harder]

wow. That's a LOT of corrosion. Has anyone else ever experienced something this bad? Or was there maybe a very strange thing going on here and this is a "one-off"?

 

[longhorn1]

Has FWC been contacted? Hopefully they can address this so no one else has a problem. jd

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[Bill D]

FWC has been contacted. They said use zinc plated screws. Do not use stainless.

Anyhow. I'll use zinc plated screws and change them out every couple years as the coating gets sacrificed.

I mentioned previously. I may have changed a zinc plated screw out to a plain metal hex head screw a year ago (due to a poor ground connection), thus starting the corrosion. That being said, the 2 screws holding the front and rear center marker lights are stainless, so they may have been the source of the problem as well.

 

[longhorn1]

Your repair with the aluminum plate is awesome. Sorry that that happened. jd

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[Bill D]

Your repair with the aluminum plate is awesome. Sorry that that happened. jd

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Thanks. I actually like the way it looks. Just the right amount of rugged bling, and if anyone steals my camper I'll know right away. It's honestly not that big of a deal. I just don't want to see any more of it. I know we are enthusiasts, engineering types etc. but part of the reason I bought a FWC is it is pretty much made up of off the shelf parts and I can do most of the repairs myself.

Hopefully this thread will help educate owners of aluminum campers as to the importance of using the proper fasters. I know I've learned a lot through this process. I'm going to stick with zinc plated and do periodic replacement.

 

[resto]

I also had found the information online about SS and galvanic corrosion confusing. I did use SS steps and latches on my build but took precautions as I was made aware by FWC of the bad experiences they had with ss screws in the past. At a local fastener shop I asked their opinions on SS screws and aluminum with regards to galvanic corrosion. They were quite clear SS screws and aluminum can be used successfully together and that they had been supplying screws to aluminum boatbuilders for use on their windshields with no problems. They told me there was more to it than simply looking at noble metal charts as the screw manufactures were using treatments to avoid this problem.

So I did an experiment that I included in my build on expedition portal (link to my build - 4WheelResto ). In that experiment I tested various screws. While I am not a scientist I believe certain SS screws pose little or no threat while others will cause damage.

Incidentally, the SS latches I used, have aluminum bases - the manufacturer assured me they have had no problems due to galvanic corrosion and are warrantied. They also said it was how they treat the SS, and not as much the type of SS.

I used zinc plated screws supplied by FWC on my build, but would use SS screws approved by a supplier I trust.

As well as getting it right which screws don't cause galvanic corrosion, remember SS screws are more prone to snapping off during installation, and getting out the broken screw can be very frustrating.

SS screws at your own risk - resto

 

[ntsqd]

That touches on two things. First is that the only way some SST fasteners can be "safe" while others aren't requires some sort of barrier plating or coating. Unless this barrier is a different color there is no way to know which is which. And the barrier must be very robust to survive the install intact.

Second thing is that 18-8 alloy SST fasteners are about SAE Grade 2 in strength (when I have to use them I design around them being SAE Grade 0) and are extremely malleable. Meaning that it is very easy to strip out their drive socket if a slot, Phillips, or hex drive. They're go in fine the first time, it's the third+ time that they start to give up. And as noted they're easy to twist off. This usually happens when they're locked in place by some corrosion.

I'd rather not risk it myself. Standard practice at my work had been to use SST socket, button, and flat head screws/bolts on all production tooling. I've found that I can order alloy steel with zinc plating instead. Far stronger and more importantly to work, less likely to strip out the drive socket. I'm slowly converting the other designers & maintenance to use these fasteners instead. The plating is not at all like the ugly galvanized gate & similar hardware. It looks much like a typical cadmium plated bolt.

A footnote, if the aluminum is anodized, including in the threads, then that is enough of a dielectric barrier to stop the process. The problem is that unless whom ever tapped the part used an H13 spec tap the threads will be undersize after anodizing, which will require that a tap be run thru to chase the threads. At which point you're back to bare aluminum in the threads.