What wood to use for external steps on a metal box-section frame?
I assume that the scaffold boards are SPF (spruce, pine, or fir). If so, they'll gray out and start to rot within a year, mostly at the contact points with the supports. If you were to seal them well before installation they'd do a bit better, but only by a few years.
Some wood naturally resists rot, such as cedar, redwood, and teak. They should still be sealed, though, to preserve appearance.
Otherwise, treated lumber is generally the most cost-effective. Since you don't need it rated for ground contact, almost any will do. It'll last 10-20 years, depending on various factors relating to moisture.