Steels_ Metallurgy and Applications, Third Edition

376 Steels: Metallurgy and Applications

Stone et al. 43 and also by Needham. 44 As indicated in Figure 4.43, the marine
location is the most aggressive because of the high chloride content in the atmo-
sphere. However, as anticipated, the corrosion behaviour of these steels correlates
strongly with the molybdenum content. This is illustrated by the following pitting
data in Tables 4.21 and 4.22 which were obtained after 15 years exposure in a
marine environment.
Thus both the maximum pit depth and the pit density decrease progressively
with molybdenum content. It is also apparent that washing, either by the natural
action of the rain in unsheltered conditions or by manual washing in sheltered
areas, exerts a beneficial effect in reducing pitting attack. Thus the design of
structures is important and, ideally, the material should be unsheltered such that
rain water will remove airborne salts and other damaging debris.
The information derived from this work has confirmed that Type 316 is the
most appropriate grade of stainless steel for architectural applications, particularly
where the atmosphere is relatively high in chloride ions. The long-term data
obtained in marine locations were also instrumental in convincing operators of
offshore oil and gas platforms of the benefits of using stainless steels for topside

100 -

80-

E -I

:E; 60-

40

20-

0

• Semi-
  Rural industrial
  site site

Heavy Marine

industrial site

site

Figure 4.43 Atmospheric corrosion in Type 304 steel after 15 years exposure (unwashed
samples) (After Needham 44)

Table 4.21

Type Pit density (pits/cm 2)

Unsheltered Sheltered

Washed Unwashed Washed Unwashed

304 3060 3172 3798 4214

315 647 728 789 890

316 364 377 419 450

317 37 46 52 67

After Stone et al. 43