Mild steel is perfectly substantial, oxidisation (rust) will only occur if water comes into contact with bare metal. Hence by coating the angle iron with hammerite, red oxide paint or by galvanising a protective coating is ensured and will last for years with no sign of metal fatigue.
Mild steel is used in many building applications such as Bridges, Commercial Buildings, Office Blocks etc. where strength and resistance to fatigue and corrossion are critical.
Stainless Steel in all the above applications just would not be financially viable and is best suited to marine applications where it will outperform lesser robust materials.
Im confident with the choice I have made.