Definately the latter. Alu is only good to about mach 2.3. Steel all the way.
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Grrrrreat! 
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In aviation we use a sacrificial material, so a cheap aluminium washer or alike that will start corroding at first before anything else. Works as well. Or some really nasty protective materials that you really donât want to touch.
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Oh Gods of corrosion, take from us thine humble washer in holy sacrifice so that our metallurgical sins be forgiven amen.
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Why does being alloys change anything?
It changes the susceptibility to corrosion a lot, certain aluminium alloys hate water for example or are extremely sensitive to scratching. It pretty much determines how you handle and apply the materials in an aircraft.
Normal, unalloyed aluminium will pretty much not corrode with metal given itâs almost instantly creation of a small layer of corrosion that isolates the rest of the material. In aviation we use ALCLAD like techniqueâs, using 2024-T4 with a protective layer for example. ALCLAD means Aluminium Clad. In my example itâs a copper based alloy of aluminium where they waltz on a thin layer of pure Al to protect it.
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WellâŠ.it does but other pure metals (e.g. iron) can actually be worse on their own. The alloys have some potential for an electron âexchangeâ depending on the materials they have been alloyed with. True, pure iron, for example, is much more susceptible to corrosion than steel alloys (i.e. iron + carbon + chromium or nickel, etc. all the way up to stainless steel), but the alloys do have various levels of electron donor / electron acceptor.
The purpose of making alloys is that they are stronger in some way than a pure metal. The other elements set up a crystalline matrix, depending an the amount introduced, that is stronger than the original, pure metal. That said, when you start messing around making alloys, the electrochemical properties of what you make is different from the original metal and the elements added.
Technically âŠ
An electrode at which a reduction reaction occurs is called a cathode. Reduction involves a gain in electrons, as occurs in reactions ⊠The reduction of hydrogen ions and oxygen are thus the cathodic reactions of corrosion and the area of a corroding metal where these reactions occur is a cathode. THE ELECTROCHEMISTRY OF CORROSION
So in this case the aluminum is an anode (gives up electrons) and the steel is a cathode (takes electrons). Put them in hard contact with each other (bolt together) and add an electrolyte like humid air (especially sea air containing salt) and corrosion will start. The place it will start is likely the bolt holes where the compression of the two metals is the strongest.
A better example is baking lasagna:
In this example, the salty food (lasagna) is the electrolyte, the aluminum foil is the anode, and the steel pan is the cathode. If the aluminum foil only touches the electrolyte in small areas, the galvanic corrosion is concentrated, and corrosion can occur fairly rapidly.
Galvanic corrosion - Wikipedia
âŠand now Iâm hungry. 
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That put itâs really nicely! Well written there Hangar!
135cm (4â 5") I could do with less, but I wanted the sensors to see a bit âaroundâ the structure.
Thanks for your praise! 
It is SimBox no.2. It helped having built the old one, and tinkered with that for a few years.
Makes it easier to see what you want and needâŠ
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Zinc is often used in marine applications. Would brass work as well, for washers?
Not sure, doubt it though. Besides, aluminium is about as cheap as it can get, but yes itâs much like zinc in marine applications!
I am glad to see you purchased the cup with the lid. No spillage in the SimBox NG when going inverted / pulling negative Gs.
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Most immpressive. But what about more âbottle-lyâ beverages?
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I will have the engineering department work all night, on a response to your question.
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Getting ready for that «4G negative dive» 
The ConTrollR engineering department pulled an all-nighter and came up with this solution.
We at ConTrollR would like to take this opportunity to thank you for your valuable input on the matter. We hope you will be satisfied with our efforts.
I got a stainless steel cup holder insert at the boat interior department of Biltema.
Yes, @Hangar200. Boaty stuff, in the SimBox NG!
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Shareholder value has just trippled!
You are awesome, Sir!
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Nice!
I found something similar, but it was made from plastic. Canât have that!
Your cup holder wouldâve passed QC though! 
I normally use the cup holder that is part of the computerâŠyou know, its near the top, you press an âejectâ button and it slides out. The one thing I donât like about my iMacâŠno cup holder! âŠwhat?
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