Steels_ Metallurgy and Applications, Third Edition

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)