Your first inclination might be to answer “of course,” but in fact, just the opposite is true. While iron has magnetic properties, it’s also electrically conductive. Eddy currents in the transformer core, induced by a magnetic field created by the unit’s primary windings, are actually undesirable. In this case, the core acts like a secondary wiring with a short circuit and that’s bad. Conversely, rust (or iron oxide) does not conduct an electrical current and indeed, the resistivity of rust reduces eddy current losses. The bottom line is clear: Rust may not be aesthetically pleasing on your transformer, but it’s almost certainly doing nothing bad to your amp.