Will Galvanized Steel React with this Ductwork

Unfortunately, you're correct: the moisture in the air can and will affect both.

In reality, both of the scenarious you describe happen pretty often. Sometimes it's terrible, sometimes not.

The ultimate choice for you is a balancing act based on practicality: How easy/cheap to install vs. how meaningful/terrible are the consequences (and after how much time).

When it comes to flashing, I'd do it right, first time, no exceptions. The consequences are just too dire for a poorly done job.

As for using an aluminum patch on a gavanized duct: yes it'll corrode after a while. But no big deal IMHO.

Normally gym equipment is powder-coated. in your picture, they have used an electrostatic powder coating process and finished with a sort of hammertone finish..

IMO- your options are as follows:

• Stainless steel is the best metal in terms of durability. it will never rust, however, its more expensive and is slightly harder to drill/bend. I would recommend pricing the bench with Stainless, then you can assess the viability. In terms of stainless, the most common stainless is 304, which is industry grade.. This grade is perfect... but will turn slightly yellow over time, if you are concerned with tarnished stainless you can go for the more expensive 316 stainless.

• Mild Steel is your other option, and obviously this will rust, so your rust prevention options are

  • Metal Oxide primer with enamel coating (paint)... which will not look as clean and will rust after a short time
  • Powder coating - you can imagine this as a sort of plastic coating which is baked onto the metal. in terms of finishes. this looks really neat, however the impact of the power bar on the bench will chip away the coating and it will show signs of rust after a while. not to mention that the coating is only on the outside of the tubing and the tubing will rust on the inside.

  • Chrome is another option, in which the metal is coated in chrome, this is also on the exterior of the metal, but I am not too certain if you will find a person who can chrome such a large item

  • Your last option (and most cost effective) is to galvanize the metal. this results in a zinc layer being coated on all surfaces of the metal (inside and outside) your second photo looks like it has been hot-dipped galvanized. the galvanizing will look really shiny at first, but will form a dulled coating and will last for many years.. Galvanizing comes in two types

    • Hot dipped , where your bench will be dropped into molten zinc and will come out with a thick layer of zinc covering all surfaces. with this your bench will last for 20+ years without rusting.
    • electrogalvanized, where the bench is dipped into a solution and a current creates a charge which attracts the zinc coating (this coating is a lot thinner so will not last as long), however the finish is a lot prettier. I would not recommend this in your case.

So to summarize, I would say your options are stainless steel, or hot-dipped galvanized steel. Both will be extremely durable but I like the look of stainless more

If you decide on Galvanized, I would also place some rubber strips in the areas where the power-bar lands on the bench, since the zinc coating will wear away there over time.