Observatory Project: Galvanic Corrosion and a Change in Track Selection!
Date: June 16, 2024
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NOTICE!
This post shares my preliminary design thoughts for a Roll-Off-Roof Observatory.
I make NO representations regarding the fitness or soundness of the designs and design decisions discussed.
Use this information at YOUR OWN RISK!
If you decide to build your own version of this project, you ASSUME ALL LIABILITY for your efforts and their results.
Earlier Posts in this Series
I have been thinking about building an observatory for a while now - below are previous posts leading up to where I am today.
Observatory Update: Designing the Roll-Off-Roof Track System! May 2024
The Move is Complete, Now Pivoting to the Observatory Project! April 2024
An Observatory Project Update: Success! We Just Bought A Property and Will Be Moving! Nov 2023
An Observatory Project Update - One Year In Feb 2023
Goals for my Observatory Project March 2022
Here is the Video Companion to this post
In The Last Post…
I reviewed my selections for the V-groove and Flat wheels and the track that would go with them.
The wheels were made of iron, and the track was made of aluminum.
I chose the iron wheels for strength and their ability to handle loads.
I chose the aluminum track for its corrosion resistance - as they would be exposed to the weather on the outriggers and for their attractive cost.
In doing this, I became aware that when different types of metal come into contact, a weird form of corrosion called Galvanic Corrosion can occur.
So, What IS Galvanic Corrosion?
Galvanic corrosion is also known as Bimetallic or Dissimilar Metal Corrosion. It is an electrochemical process in which one metal corrodes when it is in electrical contact with another in the presence of an electrolyte.
A similar galvanic reaction is exploited in batteries to generate a useful electrical voltage to power portable devices. This phenomenon is named after Italian physician Luigi Galvani (1737–1798). This process will cause corrosion of one of the metals.
For this process to occur, two different metals with different galvanic potentials must come in contact, and an electrolyte—which can often be water or moisture—must be present. Sea Water, with its dissolved salts, is an ideal electrolyte, but often, all you need is moisture to enable this reaction.
So I chose iron wheels and aluminum tracks.
In the case of aluminum and steel, aluminum is more anodic (less noble) than steel in the galvanic series, which means it is more likely to corrode when in contact with steel. When these two metals are in contact with an electrolyte, such as when used together in structures or equipment, the aluminum will corrode preferentially.
Other metals can cause an issue—here is a chart of metals that play in this little drama, showing the possibilities.
My Initial Thoughts
In my initial assessment, I did not think this form of corrosion would be much of a factor.
Here was my thought process:
My tracks or rails are 40' long
The roof is 20' long but only contacts the tracks at the location of the five wheels on each side of the roof.
The roof does not spend much time in the open position, so the contact points there are not likely to be an issue.
The Roof spends a lot of time in the closed position. In this case, it can rest for long periods of time with its wheels in contact with one set of locations on the rails.
For Galvanic corrosion to occur, you need moisture, and in the closed position, the points of contact are protected from weather by the roof itself. The only source of Moisture would be in the air itself, potentially condensing on cooler metal parts of the track system.
With this thought process, I sort of dismissed the concern.
Rethinking Things
When I posted the last blog, I received a comment asking about galvanic corrosion. Since I had not discussed it in either the blog or the video, I responded by sharing my initial thoughts about it.
As I did this, another thought came to me!
While the wheels have narrow areas of contact that are protected from gross moisture by the roof, what about the screws attaching the rails to the top plate of the observatory? These can be found all along the 40’ track, and in the outrigger area, they will be exposed to the weather!
I would not use aluminum screws on the aluminum track, so I would likely use steel.
So here I could have a case where I have steel screws, aluminum track, and moisture.
So, the more I thought about it, the more I thought I should be conservative and change the tracks to steel. Specifically, Galvanized Steel to avoid corrosion.
I would keep the same wheels.
But I would go steel for the rails. This might cost more, but I decided I wanted peace of mind.
Going Steel for the Tracks
I found a galvanized V-track on Amazon. It costs about $100 for 6-foot lengths. You can see it HERE.
From Amazon.
This seemed to be a good solution, and it was about the same price as the Aluminum track. I have seen Other Galvanized track—very similar to this one—listed for 50% more, so this was a good deal.
I had some problems finding a 3" wide flat track. Either I could not find what I wanted, or what I found was too expensive or too short to be useful. I talked to my friend, who is helping me fabricate the piers (a subject of a future video), to see if he could source 3" wide flat rails.
He looked at it and found he could get galvanized stock for about $200 and stainless steel for about double that.
These are really good prices, so I will likely take the stainless steel route here.
So, given my change in thought process and selections, I wanted to share this update with you.
A Final Surprise
I will end this with what, to me, was another surprise.
I will use pressure-treated lumber to support the rails for at least the exposed tracks on the outrigger.
I was going to lay the Steel track on the top plate of this pressure-treated section.
So that is no problem - right?
Well - not so fast.
Pressure-treated lumber is used for wet environments because it is treated to prevent decay and breakdown. This treatment involves copper compounds that bind to the actual cells of the wood.
So I added steel tracks and fasteners into this equation, and guess what?
You have copper metal in the wood
You have steel
You have moisture
And Galvanic Corrosion raises its ugly head again!
It turns out that a plastic sheet between the wood and the rail would not be a bad idea.
For the best results, use building-code-approved, corrosion-resistant screws/fasteners.
Hot-dipped galvanized steel is recommended and should conform to ASTM A153 for fasteners and ASTM A653, G185 for connectors.
For coastal installations, use code-approved stainless steel.
Stainless steel offers the best protection. Type 304 or higher stainless steel is recommended for wet environments, such as poolside decks. If you live on the coast and are exposed to salt mists, type 316 or higher is recommended.
Conclusion
I’d like to say that this wraps up my hardware selection. But the truth is that I continue to research this topic - looking for better solutions or more economical solutions - so these are likely to change. However, I will share any changes that are made in the interim!
Stay Tuned!
Clear Skies,
Pat
