Resistenza alla corrosione atmosferica di acciai inossidabili di largo impiego (ATMOSPHERIC CORROSION RESISTANCE OF STAINLESS STEELS SUITABLE FOR MANY APPLICATIONS)

The high corrosion resistance, good aesthetic aspect and good moldability has favoured the use of stainless steels in various market sectors. In the last few years, the use of stainless steels has become important also in the building trade, which has expanded their outdoor use. In this work, the atmospheric corrosion behaviour of two stainless steels (AISI 304 e AISI 316L) was studied. Several steel sheets, 10×15 cm in size and with two different standard surface finishings (2B e BA), were exposed to different natural atmospheres (marine, marine-urban and urban), by means of suitable exposure racks. Salt spray tests and anodic polarizations in NaCl 3,5 % were also carried out. A very slight corrosion attack was observed on the stainless steels with BA finishing in marine atmosphere, while the same steels remained substantially unattacked in the marine-urban and urban atmospheres. Among the samples with 2B finishing, those exposed close to the sea were the most damaged, followed by those exposed to the marine-urban atmosphere and then by those exposed to the urban atmosphere. At the end of the exposure time (133 days) in the marine atmosphere, AISI 316 L steel showed the same corrosion attack as AISI 304, which is less resistant at the beginning. On the contrary, AISI 304 steel with BA finishing showed good corrosion resistance, if it is not used very close to the sea. The results indicate that the stainless steels roughness and surface finishing strongly affect the corrosion resistance, in spite of their high chromium content. In particular, the use of the austenitic steels with 26 finishing is not indicated for outdoor applications, even far-away from the coast, because they are obtained by means of a technology intrinsically unsuitable for this purpose. In fact, the pickling treatment, necessary after solution annnealing treatment, makes their surface characteristic worse, because it forms some cracks at the grain boundary, where pitting corrosion can initiate. The advantage offered by the more expensive AISI 316 L steel with the same finishing is quite scarce. The anodic polarization data, in good agreement with the results of other authors, indicates that a low pitting potential and a higher probability of metastable pit initiation are correlated with a higher surface roughness. The salt spray tests did not let to an appreciable corrosive attack on the exposed samples; so this test does not appear suitable for simulating atmospheric exposure, because it not able to reproduce the wet-dry cycles. The morphological observation showed that atmospheric pitting corrosion proceeds by increasing the number of attacked areas, rather than by the growth of a damaged area. This phenomenon, which causes loss in brightness of the steels and the formation of corrosion spots, only affects the steel surface layers without worsening the functional characteristics.