[ARC5] restoring rusty metal tubes

[Brian Clarke]

Hello Leslie,

 

The external envelope of the metal tube can reach 270°C. Ordinary gloss enamel will blister and peel off. Any rust removal product that uses phosphoric acid will form a coating of iron phosphate – good for preventing further corrosion. But, it does reduce the thickness of the metal tubing. No pot of phosphoric acid I have used has mentioned survival at 270°C.

 

My suggestion, echoing but also eschewing advice from some of our erstwhile US cousins is:

1.       Test the tubes for heater continuity, emission, gain and gas. Manufacturers usually understate the specifications so that published figures are often less than you achieve with a NIOB tube; a new tube should be at least 25% better than spec. So, any tube that is not close to spec becomes a good candidate for small-arms target practice.

2.       Mark (scratch, engrave) the phenolic base indelibly with the type nomenclature. Scratching the nomenclature into the metal tube is a sure way to start later corrosion. Probably not a good idea to engrave the end of the octal key, because that part often gets broken off.

3.       Gently remove the rust by abrading with green Nylon mesh pot cleaner. Leave the old paint in place wherever you can. A wire buff will remove some of the metal – where the metal is thinner it will get hotter.

4.       Apply a metal primer that will withstand 270°C, eg, a paint designed for baking.

5.       Spray on an auto engine exhaust black paint formulation that will finish matt – most auto parts shops carry it. A gloss finish will not radiate heat as well as a ‘black body’.

6.       If you are a dab hand at calligraphy, rewrite the nomenclature over the black matt with a high temperature white paint. Artist’s acrylic probably won’t cut it. I got some high temperature white matt from an auto parts store for redoing a toaster. Spray the paint into a small pot and scribe with a fine artist’s brush or a tooth pick – prefer the brush because it holds more paint; the tooth pick may give a blobby result.

 

Cheers, Brian, VK2GCE

 

From: arc5-bounces at mailman.qth.net [mailto:arc5-bounces at mailman.qth.net] On Behalf Of Leslie Smith
Sent: Wednesday, 15 September 2021 9:14 PM
To: arc5 at mailman.qth.net
Subject: [ARC5] restoring rusty metal tubes

 

Hello all!

I bought a box full of metal tubes recently.  12A6, 12K8, 12SK7 and so on.  Many have an unattractive amount of rust on the surface, some have a small amount.  None belong to a class I would call 'collectable'.

Can anyone advise what I can do to preserve these?  My thoughts are along the lines of clean the metal case with a buffing wheel; after that re-paint with gloss spray.  All observations welcome.  I would like to save a box of common, not too flash, but probably quite functional metal octal tubes.

73s from Australia.

Leslie

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20210916/a75ee77b/attachment-0001.html>