Innovative Stainless Steel Applications in transport ... - Euro Inox

Austenitic CrNi and CrNiMo stainless steels
Austenitic grades EN 1.4301 with a minimum of 17.5 % Cr and 8 % Ni and EN 1.4401
(minimum 16.5 % Cr, 10 % Ni and 2 % Mo) have good atmospheric corrosion
resistance and have been successfully used for decades in various architectural
applications. According to laboratory tests, field tests and practical experience, EN
1.4301 grade should provide adequate atmospheric corrosion resistance in most vehicle
applications (Figure 32, Figure 33 and Table 12). Apart from remaining intact in urban
atmospheres, it should also remain practically intact in marine atmospheres and even,
when properly treated, in urban atmospheres in the presence of de-icing salts (Figure
33(d)). Welding reduces corrosion resistance and can lead to pitting corrosion and
staining in the presence of chlorides, originating from sources such as seawater mist,
rainwater, de-icing salts or dust-control chemicals. Laser welding gives better corrosion
resistance than arc or spot welding. Excessive heat input is detrimental. Corrosion
resistance can be restored by post-weld surface treatments – again pickling provides the
best results.
Molybdenum and nitrogen alloying improve pitting-corrosion and crevice-corrosion
resistance. This was clearly seen both in the salt-spray and field tests, where grade
304SP, with 0.8 % Mo and 0.1 % N, was superior to standard grade EN 1.4301, both in
as-welded condition and after pickling (Figure 34 and Table 12).
Based on the nitrogen content, the corrosion resistance of grade EN 1.4318, with
minimum 17 % Cr, 6 % Ni and 0.1 % N, should be equal to or even slightly better than
that of standard grade EN 1.4301. This assumption is supported by salt-spray tests on
sandwich panels. Grade EN 1.4310 has also been successfully used in railway cars and
truck trailers. In building applications, grade EN 1.4310 is reported to suffer from rust
staining in the presence de-icing salts.
The risk of crevice corrosion can be minimised by careful design. Butt joints should be
preferred to lap joints and, if lap joints are used, weld bonding should be preferred to
spot welds. Although weld bonding itself can prevent crevice corrosion, pickling is still
required to restore the corrosion resistance of the outer surfaces, as shown in Figure
32(c) and (d). Pickling must be performed with care, to avoid deterioration of the
adhesive in the joint.
Surface finish has a major effect on localised-corrosion resistance in atmospheric
applications. Smoother surfaces retain less dirt and deposits and provide better
resistance against pitting corrosion, surface staining and tarnishing than do rough
surfaces. This was clearly seen in the salt-spray tests, where the colourisation of both
1D and brushed surfaces was more intense than that of the smoother 2B finish (BUS
p. 199).
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