If plain steel tying wires are used for reinforcement adjacent to Class F4 and F5 finishes, it poses the problem of rust staining which may impair the appearance of exposed concrete surfaces.
The rate of corrosion of plain steel tying wires is similar to normal steel reinforcement.
However, for tying wires with very small diameter, upon long exposure it stands a high chance of rusting completely and these rust will stain the formwork and significantly affect the concrete finish.
Therefore, stainless steel tying wires are specified for locations in the vicinity of high quality of finishes to avoid rust staining by corroded typing wires.
Note: Tying wires are wires used for fixing and connecting steel reinforcement bars.