Author Topic: Steel/Aluminium Transition Joint & repairs/patches to the superstructure  (Read 31041 times)

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Offline Hank Hargrove

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Re: Steel/Aluminium Transition Joint
« Reply #45 on: May 07, 2014, 03:49 AM »
It would be interesting to know how they handled that transition on the United States--despite her obvious woes she's nearly 60 years old and I've never heard of problems with the aluminum superstructure.

I think they used some sort of special tape between the steel and the aluminum. Then they welded it together. The ship has just been sitting around for the past 45 years, so it's hard to say. I've never heard anything either.
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Offline Adam Hodson

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Thanks for that information Hank.
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When Rob was showing slides of the building of QE2, at the QE2 Story Event on 4 May 2019, there was a question regarding why the aluminium structure had been left unpainted for some time whereas the hull had been painted "as soon as possible".

George C Griffiths was good enough to get this explanation from a friend, an Industrial Chemist and Metallurgist who took time to write this easy to understand write up:

Quote
Even in the atmosphere an aluminium SURFACE is not metal but consists of a micro layer of aluminium oxide.
It is extremely difficult to paint and would require the use of special etching primers.
 
When joining it to a ship's hull you have the added problem that aluminium metal is extremely anodic to steel and would corrode very quickly. 
Naturally it would require to be wet or damp to do this.
Not really difficult on a ship!

The easy way out is to use a non-conducting layer between the steel hull and the superstructure. 
The steel hull must then be painted but the aluminium itself can be left alone for a time, as the micro layer of aluminium oxide (ref. above) will act like a layer of paint thereby protecting the base metal.
Eventually the aluminium would require to be coated for long life - by first using the etching primer (applied any way at all) and then the paint.
If the aluminium was painted without the use of this etching primer the paint would just flake off.

So in short - it is the natural presence of the micro layer of oxide that protects it and negates the need for coating/painting for some considerable time.
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