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Rusty screws !!


AstroJOE

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I've noticed the heads on some screws which attach the metal housing either end of my C11 have started to rust a little. Same with the nylon tipped metal screws which hold the finder in place. Only slightly at the moment but its inevitably going to get worse.

The scope has been permanently set up in my obsy for about a year. The obsy is water tight, I always tend to wipe the dew off after each session and also cover it when its not in use.

Has anyone else experienced this and if so what can be done about it?

James

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Lesson to the wise as my dad always said ,paint with clear women's nail polish I,did this 2 years ago none of mine have rusted yet ,remove if possible ,use the back of a sponge the green pad rub with water and dry and then paint with clear nail polish or black if you have it I brought a bottle after my mrs moaned I,used hers once

Pat

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Moisture + oxygen = rust. Presume the screws are zinc plated which offers some protection but eventually will break down as is a very thin coating. As Todd says you can coat the heads with something but that will not protect the threads. I would suggest removing if you can and spraying with WD40 as this will prevent rust and act as a moisture repellent. Any standard screws can be replaced with stainless steel, BUT NOT is they are in contact with aluminium!!!

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You might want to consider a de-humidifier in your obsy to remove the water in there. I have one running set to maintain 40% humidity continuously. It must be damp in there and apart from the screw heads other things (less obvious like elctronics) will be suffering from the dampness. Corrosion on contacts can cause problems, lens and mirror coatings will degrade, mold and mildew can form in hidden dark places.

ChrisH

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For some reason, probably electrolytic, stainless steel and aluminium don't get on well together causing corrosion of the aluminium. A coating of grease on the threads might provide a barrier to this effect . :smiley:

Mmm, I ask because although I don't own it now , I changed some screws on my old dob tube for stainless steel screws, I think the body is aluminium ? Hope the new owner hasn't got a tarnished ota .
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Stainless steel and aluminium have different electrode potential and in contact with each other and an electrolyte (in this case water) the aluminium can corrode more quickly than it would normally. It is called galvanic corrosion and is similar to the effect in a battery, the aluminium (anode) dissolves and deposits itself on the stainless steel (cathode). The first time I came across this was with aluminium double glazing where the sections were held together with stainless steel screws. The aluminium section began to corrode and we had to replace with aluminium screws. I am not saying it is going to be a major problem but it is better to avoid putting the two together. (If they are not in direct contact it should not a problem)

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No panic. The effect is pretty minimal and long term, the result is usually a white powdery deposit on the threads.  :smiley:

It depends on the environment. The powdery deposit you mention is the normal oxidisation of aluminium in air. Aluminium contrary to popular belief oxidises very quickly but the powdery substance produced will normally go on to protect the aluminium from further corrosion. Add water, salt water, acid + stainless steel and you have a different scenario. The corrosion of the aluminium can be significant over a period of time.

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Our local hospital was built in the early 80's, with stainless steel cladding - I believe the design architect failed to specify appropriate fittings and eventually paid out, but the entire build had to be re-clad in zinc titanium at a cost of around £ 25M.    Whoops!

Here's a chart that shows the susceptibility of dissimilar metals to galvanic corrosion - the further apart the metals are, the greater the electrolytic potential:

Corros1.gif

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  • 3 weeks later...

Why can't stainless Steel screws be used with aluminium ? Just for future reference .

I am now concerned as I am using stainless steel socket cap screws to attach the aluminum backplate to my 'modded' ETX105. Should I be worried as the backplate is about6mm thick snd I did use thread-lock? Only the heads are in contact with the backplate!

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I make a habit of only using stainless steel screws on things, in plain old steel a bit of copper grease stops the screws galling, in aluminium a couple of winds of teflon tape keeps things separate.

Im not saying its the best solution but it seems to work for me so far.

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I make a habit of only using stainless steel screws on things, in plain old steel a bit of copper grease stops the screws galling, in aluminium a couple of winds of teflon tape keeps things separate.

Im not saying its the best solution but it seems to work for me so far.

You need some form of insulating material between the stainless steel and the aluminium. Providing there is no electrical contact between the two metals there should be no problem.

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http://www.snelsons.co.uk/aluminium_corrosion.html

Let's put some facts on this. You can get galvanic corerosion but without constant salt and water penetration, only a little around the head of your screws. Also al oxide is hard and transparent and used to protect alloy surfaces by galvanizing.

Unless you are a set of motorcycle front forks or a building cladding, you'll be fine for at least 20 years.

Mike

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http://www.snelsons.co.uk/aluminium_corrosion.html

Let's put some facts on this. You can get galvanic corerosion but without constant salt and water penetration, only a little around the head of your screws. Also al oxide is hard and transparent and used to protect alloy surfaces by galvanizing.

Unless you are a set of motorcycle front forks or a building cladding, you'll be fine for at least 20 years.

Mike

Agree, no one should panic, not a major problem but IMO prevention is always better than cure. Aluminium Oxide which is the WHITE powdery coating you see on aluminium that has been exposed to air, water etc. does act as a barrier against further atmospheric corrosion, I am not sure whether the same applies to galvanic corrosion. http://www.aluminiumdesign.net/design-support/aluminiums-corrosion-resistance/

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To put the lid on this we might need to get really pedantic about which al oxide is produced by corrosion and which produced by oxidizing in air.One can't be white and transparent at the same time unless it's the difference between bulk and thin layer. But white rubs off easily :(

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To put the lid on this we might need to get really pedantic about which al oxide is produced by corrosion and which produced by oxidizing in air.One can't be white and transparent at the same time unless it's the difference between bulk and thin layer. But white rubs off easily :(

Not quite sure I understand your comment - corrosion is the result of oxidisation by air and other means, isn't it? As far as I am aware there is no transparent layer of oxide on aluminium, but I stand to be corrected if anyone knows better.

I repeat: The white or if you prefer dull grey powdery coating on the aluminium is aluminium oxide. Aluminium, against popular belief, oxidises very quickly and it is the formation of this layer that slows down further oxidisation. If you remove the layer then you reveal the aluminium beneath and corrosion will increase again until a fresh layer of oxide is formed.  

Anyway we are getting way off the original reason for my first reply that was to bring to members attention that the use of stainless steel fasteners with aluminium may be something to avoid if there is an alternative.  Hopefully members can make their own decision as to whether or not it is a risk to their equipment based on their own environment. On a personal note I really do not care whether aluminium oxide is white, grey, transparent or pink with blue spots.

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