Stainless steel bolts are not always a good idea

[AL1]

At some time in the past a previous owner upgraded the collimation bolts on my Meade SN6.   For anyone else thinking of this take care in selecting compatible materials.   Aluminum in contact with stainless steel is vulnerable to galvanic corrosion.   As you can see there is lots of powdery white aluminum corrosion around the collimation screws.  My hypothesis is galvanic corrosion due to stainless bolt in an aluminum thread.   It should not be too hard to rectify as it has not gone too far.   Has anyone else encountered this?

 

[Clarkey]

I have seen this plenty of times - not only in telescopes but plenty of other places as a lot of mechanical engineers do not understand galvanic corrosion.

It is difficult to see in the picture, but are the machine screws definitely stainless or are they some sort of galvanised thread? There appears to be rust around the junction of one of the screws which made me wonder? Also, for a relatively dry environment, that appears to be a lot of corrosion for an alloy. I was wondering whether it was a zinc residue?

[AL1]

Hi Clarkey,

Thanks for you input..   Maybe that cast part is a zinc casting and not aluminum?  I will take a closer look at the bolts with my reading glasses  ( And yes I am a mechanical engineer). 

 

[Carbon Brush]

Is it possible the previous owner used a thread stiffening (or lubricating) compound but chose poorly?
I have seen corrosion on copper PCB tracks, ali & steel fixings, from various causes in my electronics work.
Anything from the wrong soldering flux, to mucky water, to rat urine, to corrosive food etching PCB tracks!
It must have been a strong curry🤣

[AstroKeith]

Stainless steel is generally OK in aluminium and aluminium alloys. I suspect the screws were zinc plated.

[Piero]

A few parts in my telescopes have stainless steel in contact with aluminium and they are completely fine.

I used A4 (mostly otherwise A2) stainless steel and 6082 alloy aluminium. All sold here in the UK.

I agree with Keith's comment above. 

[AL1]

I think maybe the bolts are bright zinc plated steel. 

[pipnina]

I stripped down and surveyed a bunch of middle sized electric motors recently, which were all cast steel for the stator housing and end caps... But they made the terminal boxes out of aluminium! Almost every single philips head screw holding those boxes to the steel was just corroded enough to be impossible to remove with a screwdriver (even when doing things health and safety wouldn't like to see). I had to drill the heads off of them all...

One thing that can work if you can get to the heads of the bolt however, is smacking it quite hard with a hammer. The percussive force causes the corrosion products that were acting as a glue to unstick even though the bolt itself doesn't really move... It works well on motors and 200 cube/hour pumps but maybe not on telescopes! You do have to swing rather hard...

If you want to never deal with corrosion in your parts however, make it out of NAB! It might cost a bit though haha.

Failing anything else in your case though, try drilling through the bolt head, with the tapping size drill for that hole or thread (if the thread is known, it's almost certainly M4, M5 or M6 looking at the image though.). Once you break through to the other side of the bolt, run the tap down itand it will clear the bolt out for you. if you do it right you successfully extract the bolt without damaging the threaded hole... If you don't do it right you have a new problem haha.

[Chriske]

Could it be zamak..?

ZAMAK (or Zamac, formerly trademarked as MAZAK^[1]) is a family of alloys with a base metal of zinc and alloying elements of aluminium, magnesium, and copper.

Zamak alloys are part of the zinc aluminium alloy family; they are distinguished from the other ZA alloys because of their constant 4% aluminium composition.^[2]

The name zamak is an acronym of the German names for the metals of which the alloys are composed: Zink (zinc), Aluminium, Magnesium and Kupfer (copper).^[2] The New Jersey Zinc Company developed zamak alloys in 1929.

The most common zamak alloy is zamak 3. Besides that, zamak 2, zamak 5 and zamak 7 are also commercially used.^[2] These alloys are most commonly die cast.^[2] Zamak alloys (particularly #3 and #5) are frequently used in the spin casting industry.

A large problem with early zinc die casting materials was zinc pest, owing to impurities in the alloys.^[3] Zamak avoided this by the use of 99.99% pure zinc metal, produced by New Jersey Zinc's use of a refluxer as part of the refining process.

[Elp]

Common bolts tend to have a zinc based passivate or equivalent post production coating to prevent early corrosion, typically zinc does suffer from "white rust" when in contact with moisture if not surface treated (why typically chrome automotive parts are made of zinc as they can be chrome plated quite easily). I was recently surprised when taking apart a greenhouse that the nuts and bolts were in fact aluminium.

[Peter Drew]

You can buy anti galling preparations to mitigate against corrosive seizure.     🙂