Metals that react

You may all have seen a comment posted by a mate Adam Johnson against my last blog on the Rear axle universals.

I am looking at making aluminium caps to cover up the ugly circlips on the universals and Adam was concerned that the metals might react and give cause to ugly corrosion. I have seen this myself where Aluminium forms a nasty white powdery substance when it is in contact with other metals. I have to admit Adams comment did get me thinking, so I made a point of finding out a bit more about it.

After reading a fair bit of material I think it goes something like this……the higher the activity of the metal (faster oxidising) the more the metal will take on an increased rate of corrosion when in contact with a lower activity one. So in other words a faster rusting metal will rust even quicker if it’s put with a slower rusting material.

So what is the order of activity…..faster rusting to slower rusting?

·         Francium

·         Caesium

·         Rubidium

·         Potassium

·         Sodium

·         Lithium

·         Strontium

·         Calcium

·         Magnesium

·         Aluminium

·         Manganese

·         Mild Steel

·         Zinc

·         Chromium

·         Iron

·         Cadmium

·         Cobalt

·         Nickel

·         Tin

·         Lead

·         Copper

·         Silver

·         Mercury

·         Gold

·         Platinum

But I hear you say Aluminium is good in the elements and it doesn’t rust, so why is it toward the top of the list? Actually Aluminium corrodes very quickly, in fact as soon as it makes contact with air, but the main reason it isn’t ordinarily destroyed very quickly is a quirk of its oxidation product, aluminium oxide (aluminium rust) adheres with extraordinary tenacity to pure aluminium, so if nothing else is present, an aluminium object will last nearly forever, protected by its own oxide.

In contact with other metals though, aluminium can be destroyed quickly as Adam has witnessed in the past. This will be sped up if there is an acid anywhere nearby like sea water or maybe even rain water that has contaminants.

Apparently in the old days steel ships used to sink at sea as the sea water used to speed up the reaction between the iron rivets and the steel hulls and would literally dissolve the rivets. To overcome this they strapped a block of zinc on the hull which becomes a sacrificial anode. A sacrificial anode is a block of metal that is more reactive than the metal it is protecting. The more reactive a metal is the easier it gives away electrons. This reactive block of metal on ships acts as a source of electrons for the iron. When oxygen takes electrons from the iron during the process of rusting, iron atoms simply take electrons from the reactive metal in this case zinc, so the block of zinc corrodes and the ship and its rivets don’t.

Similarly you might think Chromium is not far down the list either so why doesn't it rust or corrode? Look again at the list though, mild steel which chrome is applied to is toward the more active end of the list, so what happens is the mild steel actually donates its electrons to the chrome. When ever you see a rusty bumper on a car its not the chrome that has corroded its the mild steel underneath.

So yes this does mean the aluminium caps I propose making for my axles could in fact go white and powdery, as they will corrode more quickly than the chrome and the mild steel they are attached to. But....is this a bad thing? Actually no and for two reasons…..its a hot rod and polishing will be regular so should protect things to a great extent but also this faster corroding aluminium will act as a sacrificial anode to the chrome and mild steel axles, so in effect, as I see it, it will stop the chrome rusting.

Now I can afford to replace a few aluminium caps if they start to deteriorate (or maybe even just repolish them), one things for sure though, that’s a whole lot cheaper than re-chroming the rear end.

Thanks Adam for raising the point it made me look into it and I know more now that I did this time yesterday! I guess only time will tell now as to whether or not I am correct in my assumptions. It must be true though I read it on the internet!