What is the difference between mild steel and red oxide painted universal beams or columns?

Red oxide is mainly an anti-corrosion primer. It:

• slows rust formation,
• protects steel during transport/storage,
• provides a base coat for later painting.

It is not usually considered a final decorative or long-term protective finish unless specified.

Strength difference?

Normally none.
The beam strength depends on:

• steel grade (for example S275 or S355),
• beam size,
• section properties,

—not whether it has red oxide primer.

When to choose each

• Choose bare mild steel UB if:
  • you will galvanize it,
  • fabricate/weld extensively,
  • or apply your own coating system.
• Choose red oxide painted UB if:
  • you want quick rust protection,
  • the beam may sit outdoors temporarily,
  • or you want it pre-primed for painting.

Common steel grades for UBs in the UK include S275 structural steel and S355 structural steel.