EcoVntr,
First, I am not a materials expert or metallurgist. However, I am a long term owner of an ocean going sailboat as well as an FWC Camper. I also have considerable experience from my 45+ year career in aerospace painting and bonding parts and assemblies for use in Aircraft and in Space.
The following text was edited on 3/23/2024 to add additional information.
In order for a galvanic action to occur there needs to be two dissimilar metals in contact with each other as well as a dielectric material present, typically water or water vapor of some kind (rain, fog, condensation, etc).
Here is a link with a discussion of galvanic corrosion that may be useful. Note the diagram showing how to use dissimilar metals and minimize corrosion risk.
https://galvatech200...rosion/?lang=en
In general, Stainless Steel and Aluminum have a potential difference large enough to be concerned about galvanic corrosion of the Aluminum, particularly in the presence of salt water (or salt water vapor) but even with rain or fog to lesser extent, corrosion just takes longer to happen. After all Stainless steel is much closer to Steel on the galvanic chart than it is to to Aluminum
Important note: As with other metals there are a variety of alloys or types of Stainless Steels. I believe only two alloys of Stainless have very low amounts of carbon and are considered non-ferrous and applicable to making screws and other parts useful on our campers. Those are type 304 and type 316. Either will work on our campers. Type 316 is preferred by boaters due to its excellent resistance to salt water induced corrosion. Type 304 is a slightly cheaper alternative. Now the mystery of why Stainless BBQ grills and other items made out of Stainless show signs of rusting when left outside in the rain is solved, they are not made of 304 or 316 Stainless.
If you look to buy something and it says Stainless Steel but it does not say it is 304 or 316 then it most likely isn't. Always be sure to ask.
Now it is fair to ask if other types of Stainless will work instead of 304 or 316. If you employ the strategy to minimize galvanic corrosion by employing the techniques in the link above then probably it is OK - but be sure you also understand the strength needed for the application as the strength varies among alloys. I think it would be appropriate to ask a metals expert at one to the metal suppliers online if a particular type of Stainless would work in the specific application location and conditions (outside, near the coast, rainy whatever) that you intend to use it.
In the end you may just want to use less expensive and likely stronger carbon steel fasteners while employing the corrosion mitigation approaches outlined at the link (a coating of paint or a coating of Zinc works too if you can avoid scratching the coating off of the treds and the fastener head when installing the fastener). ;-)
https://www.ryerson....-the-difference
https://www.ryerson....stainless-steel
All of this winds up protecting the Aluminum from degrading. But if the main issue is just the fasteners rusting then the protective coating on the fasteners is either getting removed somehow or is not adhering properly to the surface of the fastener resulting in the fastener rusting. If the coating has pinholes or is too thin water can get between the coating and the steel body of the fastener and then corrosion happens. Similarly if the surfaces of the fasteners are not properly prepared and cleaned then the paint or protective coating may not stick adequately or fail prematurely allowing water to react with the steel. Stainless fasteners may solve the rusting problem but if they are not painted or used in a way to mitigate galvanic reaction then you may wind up with the Aluminum parts corroding.
Relative to sealants and painting:
The single best tip I can give for for sealants and paints - always prepare the surface in accordance with the manufacturers instructions and clean with acetone and/ or alcohol then prime as recommended for the paint or sealant you are using. Polyurethanes stick well to properly prepared surfaces.
In the space business we bond a lot of things together and we clean the surfaces to bright metal then often clean with acetone, deionized water (a strong solvent) or distilled water followed by alcohol since good high purity alcohol does not leave a residue when it evaporates whereas Acetone often does.
How do we know the surface is clean enough? It is referred to as a water break free surface. If you drop a drop of water on the cleaned surface the drop will spread out (literally instantly flash over) and wet the entire clean part of the surface and will not bead up. If the water beads up anywhere then that part of the surface is not clean enough and needs more cleaning. This level of clean does not last long, maybe 30 min to an hour at best, because the surface will soon oxidize (rate depends on the material cleaned).
Paint and adhesive bonds on properly prepared surfaces are very strong and it is not unusual when a good bod is tested to the limit of failure to see some of the substrate material the adhesive is applied to is pulled out with the adhesive.
Of course we do not need this level of clean to do bonding on our campers (although it would do no harm) but the understanding and knowledge that a good clean properly prepared surface maximizes bond strength and longevity is important.
Most paints and adhesive bonds fail prematurely because the substrate was not properly prepared and or cleaned.
I hope this is helpful.
Craig
Edited by ckent323, 23 March 2024 - 07:52 AM.
