Galvanic Reaction Help

[Ganglin]

I'm pretty handy but know nothing about this topic - trying to do each step in the restoration completely as we go along:

- replaced a seal on an access door and started on another today - noticed the rivets holding the hinge to the door (refrigerator) are starting to rust - can only guess that the hinges have been taken off for some reason in the past and the rivets were replaced with steel ...

- went back and looked at the first door and it's the same - didn't notice it because someone had touched up the rivet heads with silver paint - looked at the trunk hinge at it's the same... GRRRRRRR.....

- will leave the first door as is for now but the second door will have the rivets drilled out and replaced with aluminum rivets so we don't have to do it again later

SO QUESTIONS:

1. the nuts that hold the access door locks in place are against a panel of aluminum and they are reacting - nuts rusted and white crust on aluminum skin - can I replace the nuts with stainless and stop the reaction or do I need a bushing of some type - the bolts are stainless and appear fine.....

2. all of the internal screws holding such things as the drape brackets and bed support channels were zinc coated and OK for 35 years old - as we do projects we are pitching those and replacing them with stainless screws - will this cause a reaction..?

3. does anyone have a source for 1/8th rivets that have a shaft less than 1/4" long.....

3. is the reaction only caused when moisture is present...?

Thanks.... Gary

[boatdoc]

Hi Gary: This is definetely difficult subject which I will attempt to explain in laymans terms the best I can. In reality it is electronic engineers nightmare. Your problem can be solved if you can do some study on affects of electrolisys encompasing dissimilar metals. There can be galvanic reaction as well as electrolisys driven reaction. Metals of diferent composition can react to contact with each other for many diferent reasons, or under number of specific conditions. In any case, a less noble metal becomes a looser. First let's look at [stray currents] within the trailer. Especially in a 12 Volt circuits grounding is very important. [In case of 110 Volt, you have direct ground and neutral vire, thus eliminating the grounding issue]. Ideally, each ground in a 12 Volt circuit should be direct to the battery. If the ground circuit is weak, the current will seek next best ground whatever it may be. Have you ever tried to get running lights on the tralier to work with a bad ground? Each time you turn different circuit on, something else comes on when it not supposed to. Unless you provide a good direct ground from your trailer to the tow wehicle you cannot solve the problem. This is exactly why I am against grounding a 12 volt system to trailer chassis instead of direct ground. Any loose ground connection will have a different level of effect, pending on the amerage demand in that circuit.

Atomic structure is different in all metals. This is why some metals are less conductive than others. Also, the moisture content can accelerate the process of galvanic or electrolisys damage between disssimilar metals. In the presence of stray currents. The moisture will try to brigde weak connections adding to corrosion in less noble materials. Steel rivets may not neccesarily be the victim of stray current but they will rust in presence of moisture and oxygen. Steel rivets do not belong in aluminum chassis anyway. However, any electrical charge no matter how small will have a increased effect on connections between dissimilar metals. To start with, the aluminum rivet in a aluminum chassis eliminates dissimilar metal reaction. With that out of the way, bear in mind that there are different grades of aluminum as well. You get what you pay for. [There is lot of cheap stuff comming from the third world countries]. When the application requires stronger connection than a aluminum rivet can offer, good grade of Stainless Steel rivet should be used. Caution should be used when using SS rivets in thin aluminum skin. Stainless Steel is much stronger than aluminum, and the mushrooming end of the rivet can pull right thru the thin skin. If access is possible to the back side, use proper ID SS washer for back up. SS will not be affected by oxidation since it is more noble material. Less oxidation the less damaging galvanic effect on less noble metal such as aluminum.

Secondly, keep galvanized materials away from aluminum. Galvanizing in a presence of high moisture content can produce oxidation, which in turn will destroy aluminum. Keep steel rivets out of your AS and you will be fine.
Good luck, "Boatdoc"

[Ganglin]

Thanks for the great reply. Took the steel rivets out tonight and replaced them with aluminum - then resealed the access door. Also took off the nuts from the latch and replaced them with stainless. If I understand correctly the stainless should not react to the aluminum that it's in contact with?

Not only concerned on this project but have been replacing every screw we removed in the interior (drape channels - bed frame rails) where we've done repairs with stainless. Not that I knew this was correct - just didn't like the look of the original zinc coated screws and never wanted to mess with them again.

[boatdoc]

Hi Ganglin; There is no problem with zinc screws providing that they are in wood where they are isolated from contacting the aluminum. The zinc oxidation in presence of any moisture will damage aluminum over period of time. Most good grades of stainless steel will seldom oxidize. Grounding effect of stainless is much better because of the hardness of the rivet over the aluminum. It makes a better, cleaner conection. Any weak connections will be attacked by electrolisys trying to bridge bad grounding connections. It will remove less noble material and try to deposit it on opposing material that is more noble. Although it will not destroy noble metal but it will turn less noble into dust particles by chemically breaking it down. Flow of free electrons within any metal creates minute static charges which can be destrucive to areas of the metal that is only partially grounded. Hope this is of some help to you. "boatdoc"

[Ganglin]

Ok - that would explain the zinc coated screws only being rusted slightly where they had gotten damp - but having a white powder on them when pulled from contact with aluminum. Also, why the access door lock bolts (stainless) were fine but their steel nuts in contact with a thin aluminum plate had rust and white powder had built up.

Presume the aluminum skin would have lost that battle.

Will keep replacing all screws with stainless as we go along. What really dirves me crazy - pardon while I vent second - is that Airstream dealers replace screws and rivets with hardware that will rust. The trunk doors were redone by a dealer in the mid 90's. Even Airstream does this. The rock guards we have are fastened to the body by aluminum brackets but then they used a steel screw to hold them into the bracket. It's rusted of course and the replacement screw is like $11.00 - the bracket was $11.50.

OK - I feel better now. Thanks so much for the good information boatdoc - you have been a huge help..!!!

Gary

[Krazyjohnny]

We typically use 409 stainless in the fab business for food grade applications and clean rooms. It is typically specified that way. Good luck.

[Ganglin]

How do I know I'm getting 409 stainless..? I feel lucky to have an old fashion hardware store available that has multiple stainless fittings including screws that are painted white/brown on the head that match the Airstream.

Thanks much for your input...!!!!

[boatdoc]

Hi Ganglin: Stainless Steel grades go by designated Series number. Starts with 300 grades and goes into 400 series. The higher the number, the higher nickel content, the higher nobility of metal. Certain food handling utilities and medical application require minimum of 400 series SS. However, in AS you will be fine with domestic produciton 300 Series SS fasteners. Because aluminum is less noble metal when comparing it to SS, it will not sustain any damage. It was the aluminum that turned into white dust which you found under zinc or steel screws caused by metal less noble to aluminum. It was most likely the dissimilarity of metals that caused galvanic reaction. "Boatdoc"

[boatdoc]

Hi Ganglin; They do it for the same reason why some cheap aluminum boat manufacturers use zinc screws in decks of aluminum boats where the conditions to which the boat is exposed to is hundred times harsher than AS.
Reason? they save a quarter penny per screw. Guess what the screws hold on to-- aluminum! Electrolisys in any moving water have plenty of fun. "Boatdoc"

[w7toc]

I found a PDF http://www.ssina.com/view_a_file/fasteners.pdf on galvanic reaction. According to the PDF 410 stainless steel is not recommended with aluminum. It shows 300 series SS and steel at being about the same corrosion wise. I would sure like to have more real life experience here.

Rick

[dwightdi]

Many years ago I worked as the materials engineer for Outboard Marine Corporation which is the parent company for Johnson and Evinrude Outboard Motors. They use aluminum diecastings held together with 300 series stainless steel fasteners. We sold the same outboards into all marine enviroments all over the world. We did alot of corrosion testing. Galvanic couples really cause the most trouble in conditions where the moisture is very conductive (such as salt water). 400 series stainless ishould really called stain resistant rather than stainless. We used 400 series in high strength applications because it can be heat treated to a higher strength. Galvanized steel coated fasteners were used in areas that were not subject to the salt water and there was alot of oil or paint to protect them. Galvanized or zinc plating is sacraficial and will corrode away and protect the steel from corroding. It is good for a specific period of time dependent on the thickness of the coating and the severity of the enviroment.

If you insulate electrically between the differing metals the galvanic corrosion will not occur. Likewise we also sold (as optional equipment) electronic protection systems which would impose a charge on the units and protect them from corrosion. We also installed protective zinc (sacrifical ) trim tabs or anodes to protect the units. We also advised against using any copper alloys (bronze propellers) in contact with the units as they made things alot worse, as brass or bronze forms an excellant battery when in contact with Aluminum and the aluminum becomes the sacifical anode for the copper alloy.

[Royce]

OK, now I have to ask a newbie question. Howcome the galvanized steel end caps on the Argosy's don't destroy the aluminum they are attached to? Is it because of the sealant used between or some other protection, or do they destroy each other over time? Is my new to me Minuet going to self destruct?

[markdoane]

Royce,

You'll be ok if you keep the paint intact. Corrosion requires water (or some other electrolyte) to complete the galvanic connection.

[JDL]

Hmm,

I was thinking on this thread....

If I put an anode (zinc rod) on the frame of the trailer would that cause all electrolysis on the trailer to go to the anode?

[dwightdi]

Zinc anodes would protect it if the trailer was submerged in salt water. To be serious, it would only work if the entire surface of the trailer were exposed to a conductive layer of moisture. It might be moderately effective if you parked next to the ocean with it salt air for long periods. The galvanized steel fasteners serve this purpose untill the loss all their plating and start to rust. I personally still like stainless that do not require periodic replacement.

The pictures are from a trailer parked in the salt air for many years.

[A-Merry-Can]

check out how severe teh damage was to the tail light area on my globetrotter!



the aluminum under the rusted away light area crumbles in your hand. it's actually caused the aluminum skin to bubble on itself, so i'm replacing the entire rear quarters, and putting the tail lights inboard (where they should be, in my opinion!) ha!

jp

[Al's cousin]

This is an extremely interesting thread since I am considering an Argosy that was parked 10 miles from the ocean for less than 10 years. What else besides fresh water washings can I do to neutralize any residue salt air salt mist? Anything metal will be examined closely!

Al's cousin