Transformer rust. How to refurb?

[bobschwenkler]

I came across this on the internet just now and it just so happens I was talking to another tech about a power amp in the studio that's got some rust going on on one of the transformers:

Surface rust on those old transformers won't hurt a thing. AAMOF, don't ever remove the rust! Sanding it off to bare metal will allow conduction between the laminations, which will cause larger eddy currents and a loss of performance. When the laminations were made, they were annealed in a furnace to form a non-conductive oxide coating, after that they'd been zapped with a strong electromagnet while at red heat to orient the grain in one direction. If you can't stand the way the rust looks and you just must do something about it, repeatedly wipe it down with a soft cloth dampened with denatured alcohol until it no longer stains a clean section of the cloth, then paint it.

This tech was telling me the opposite: sand it and reseal it.

So what's the deal?

 

[PRR]

If you POWER-sand it, you schmear the thin iron and short the lams.

And rust on the lams absolutely does no harm. (However it suggests green crud inside on the copper, which is bad, and not repairable short of re-winding, which is usually absurd.)

If it offends your buyer, rub lightly with plastic scrubee, thin some RustOleum and apply sparingly.

 

[PRR]

> When the laminations were made, they were annealed in a furnace to form a non-conductive oxide coating

They are annealed for magnetic property.

They are then insulated in another step.

Shellac is classic, but thick and awkward (try hanging 200 lams to dry).

The insulation does not have to be good. A few minutes in a hot damp oven works, quick, cheap.

This insulation is between lams so the iron does not touch. Rust on the outside does no good, and no harm.

(Yes, if you find a tranny at the bottom of the sea rust-pitted like Columbus' anchor, the core properties will be less by the depth of the pits; the copper will be in far worse shape.)

 

[edanderson]

the rust won't hurt the lams, in fact most modern steel lams have a very thin oxide layer (nonreactive rust) that is specifically to electrically insulate each lam from its neighbors.  but if you want to touch it up cosmetically, i would get some rust stain spray (CLR or the like) and use a bit sparingly on a plastic scrubber or sponge as prr suggested.  then dry thoroughly and seal with a non-metallic paint or lacquer.

ed