Bicycle Mechanics - Interesting fact for steel frame owners...

Hi Guys,

Following my previous thread about stainless bolts, I started thinking of aluminium bolts. I was worried about galvanic corrosion and seizing but I looked it up on the net and apparently aluminium can be used to sacrificially protect steel.

This means that if you have aluminium bolts, they will corrode in preference to your frame (assuming the anodizing gets scratched off a bit as you tighten them up). Of course if the bolts are well anodized, there is no effect at all.

Just thought I'd share this strange fact... :)

I've run into trouble when the aluminum and steel forge together.

My worst nightmare is when an aluminum seatpost froze to a steel frame.

I think it would be screaming hell trying to get frozen aluminum out of a steel frame. You would probably have to drill and most probably re-tap too.

By the way, where are aluminum fasteners used on bikes? I can't remember seeing them. Aren't aluminum fasteners too weak for anything with bike-type loads?

I use an Aluminium bolt to hold a dynamo lamp onto its mount. I was given the thing, and its works fine, but I wouldnt like to use Al in any weightbearing role.
If you want to prevent Al and steel corroding together, you need to skip up the electrochemical series to zinc. Put a small zinc tab on your bike, and this will corrode preferentially to Al.
You can use Al as a breakaway bolt for the rear deraileeur on Al frames. On impact, the bolt will break, instead of bending the dropout.

Originally posted by MichaelW
If you want to prevent Al and steel corroding together, you need to skip up the electrochemical series to zinc. Put a small zinc tab on your bike, and this will corrode preferentially to Al.


This sounds fascinating, MichaelW. Tell us more. What kind of zinc tab do you use? Where do you get it?

Where on your bike to you place it?

Are there any disadvantages?

If 2 metals are in contact, with one higher up the electrochemical series :

http://www.efunda.com/materials/corrosion/electrochem_list.cfm?sort=com

they will act as a battery cell, and generate a voltage. The one higher up will act as an anode and corrode/oxidise.
Sometime the oxide will form a protective layer (and firmly glue the 2 surfaces , like a seatpost).

Zinc is used as a sacrificial metal in boats to protect the bronze propellor, and in galvanized steel roofing sheets.

http://www.ocean.udel.edu/mas/masnotes/corrosion.html

Im not sure how well it will work in a non marine environment.
Zinc plating is a messy process, and zinc is hazardous when brazing, so there are good reasons not to use galvanised steel, but a zinc coated steel washer may help in places.

Whenever two dissimilar metals are put into contact a galvanic action occurs. This is the sacrifical anode effect you are referring to. For a bike there would be no benefit of doing this at all, with either zinc or aluminum. If you use aluminum fasteners and do not protect the threads with teflon tape or grease, you may never get them out with out. Because the galvanic action transfers electrons from the aluminum to the steel it also bonds to the steel. As for using a zinc anode I don't see what benefit that would be. First off you would have to strip the paint off in the area of the zinc and attach the zinc with some sort of a steel bolt. If you glued it there would be no contact and no galvanic action. also unless you ride in a saltwater enviroment such as a coastal area, a steel frame corrodes so slowly (if properly maintained) that you would never see the benefit. Zinc anodes are used on ships and offshore oil rigs and they work great for that application.
While I'm on the subject of wieght savings and aluminim, has anyone else ever noticed that during the Tour de France the riders often wear quite a bit of gold jewerly, that gold is heavy you know, at the same time they will use a titainum crank bolt to save weight. Just an observation.

I was thinking of using them for no load app's such as bottle holder, crudcatcher, brakes etc. I know about the zinc thing as well but I found this strange website all about aluminium and it said that apart from zinc, cadmium and magnesium, aluminium is the least noble of all metals. Personally I wouldn't have thought aluminium had this property...

I know all about the frame thing too. At uni a few years ago, a mate of mine got his aluminium seatpost post stuck in a £700 custom Dave Yates frame ! We tried a lump hammer (and he's a big guy), Plusgas freeing agent - everything. In the end he had to use a hot air gun to really heat the frame and then whack with a lump hammer for about half an hour. He ended up having to get the frame re-sprayed. Next time, he remembered to grease the post properly :p .

How does the shear strength of a typical alumin(i)um alloy bolt compare to that of an equivalent stainless steel fitting?

The biggest risk in buying an older bike is that a previous owner failed to grease the seat post and handlebar stem.

I've used Loctite 222 for years on my archery equipment. There are a lot steel to aluminum connections there. Haven't had any problems. It dries to about the consistency of bubble gum. You can still remove the bolt easily. I was told that it was originally designed for race cars. Don't know if that's true or not, but I know it works.

The strength of aluminum is approximately 1/3 that of steel in nearly all respects.

Originally posted by D*Alex
The strength of aluminum is approximately 1/3 that of steel in nearly all respects.

... which would not seem to make it an ideal material for a small-diameter threaded fastener.

Yeah, you're right. I certainly wouldn't even dream of using aluminium bolts for stuff like seat post clamp, stem clamp, crank bolts etc but how strong does a bolt have to be to hold a water bottle holder etc

Originally posted by mike
I've run into trouble when the aluminum and steel forge together.

You could probably deal with the galvanic problem by coating Al screws w/anti-sieze. I suppose an Al screw in an steel frame is no weaker than a steel screw in an Al frame.
:confused:

Yeah, in a way, aluminium screws in an aluminium frame are probably a good idea - less chance of stripping the aluminium threads than with a steel screw or bolt. Mind you, using anti-seize justs adds copper to the galvanic equation - lol :) .