Stainless steels 375
excellent condition. Of particular note in recent years have been the construction
of the Lloyd's building and Canary Wharf tower in London, where extensive use
has been made of stainless steel cladding. In addition, stainless steel is now being
used more frequently in doors, entrance halls, portals, stairways and roofing and
also in street furniture such as lighting columns and telephone boxes.
As illustrated in Figure 4.42, stainless steels are now available in a range of
textured finishes which increase the options open to designers and architects.
Fashion and preference have also turned away from the traditional, highly reflec-
tive finishes in stainless steel in favour of the No. 4 brushed finish. However,
this finish is produced by mechanical abrasion which can introduce fissures and
crevices into the surface which can act as initiation sites for pitting corrosion.
Therefore, in producing the No. 4 finish, great care has to be taken to produce
a cleanly cut surface and this is generally achieved with silicon carbide belts
rather than alumina abrasive belts. For architectural applications, major attention
must also be given to strip flatness and to the complete removal of oxide parti-
cles from hot-band material prior to cold rolling. Small particles of scale which
remain on the surface can introduce a galvanic corrosion effect which leads to
rusting during atmospheric exposure.
British Steel has operated atmospheric corrosion test sites in the UK since the
1960s in various rural, industrial and marine locations. A wide range of steels
and finishes are being investigated, but in the stainless steel range, attention has
been focused on the following grades:
Type 304 18% Cr 10% Ni
Type 315 18% Cr 10% Ni 1.5% Mo
Type 316 18% Cr 12% Ni 2.5% Mo
Type 317 18% Cr 12% Ni 3.5% Mo
The stainless steel samples were examined thoroughly for evidence of pitting
corrosion after 15 years exposure and the performance has been reported by
Figure 4.42 Textured finishes in stainless steel strip (Courtesy of British Steel Stainless)