Resistenza alla corrosione atmosferica di acciai inossidabili con diverse finiture superficiali (Atmospheric corrosion resistance of stainless steels with different surface finishing)

Thank to its mechanical and aesthetical characteristics, stainless steel is an interesting material suitable for large scale applications in the different building fields. Actually, there are many peculiarities of stainless steel, appreciated in construction and architectural sectors, starting from its extraordinary corrosion resistance, even in particularly aggressive environments, up to large availability of surface finishing obtained after submitting it to different technological cycles. In the present work, the effect produced by the surface finishing on the localized corrosion resistance of some stainless steels (AISI 304, AISI 316, AISI 430, AISI 443 and AISI 444) with 2B, BA and SB ("Scotch Bright") finishing was investigated. The study was performed by means of atmospheric exposure according to UNI EN ISO 8565 Italian regulation, using an urban-marine atmosphere as exposure environment, characterized by the presence of chlorides and traces of combustion gases. The atmospheric corrosion was expressed as percentage of "surface damage" due to red rust as a function of time and determined by using a suitable RGB filter on corroded sample pictures. Furthermore, a series of anodic potentiodynamic polarizations were performed (scan rate 0.166 mV/s) in a NaCl solution, 1% by weight; such polarizations have been started from -50 mV with respect to free corrosion potential up to the achievement of the current density threshold equal to 0.01 mA/ cm2, where the scan has been reversed up to the corrosion potential. In this way, in addition to the corrosion potential, the pitting and the protection potential were obtained, with which a ranking among stainless steels, together with their finishing, was made possible. Among the tested materials, the samples of ferritic steel AISI 430 showed the worst atmospheric corrosion resistance, independently from their surface finishing (SB or BA), indicating that they can not absolutely used for external urban-marine environments. With particular reference to BA finishing, better results were definitely obtained for AISI 443 ferritic steel (with the highest chromium content) and for AISI 444 ferritic steel (with percentages of chromium and molybdenum comparable to those of AISI 316 austenitic steel). The performances of these alloys, in terms of localized corrosion resistance, resulted completely comparable with those of austenitic stainless steel AISI 304 BA and AISI 316 BA and therefore these ferritic stainless steels can be considered for their use in place of these last ones (more expensive for the presence of nickel) in those applications where there is atmospheric exposure. Furthermore, for the tested ferritic stainless steels, the chemical composition (content of chromium and molybdenum) was more important than their surface finishing. Instead, the effect produced by surface finishing was particularly significant in the case of austenitic stainless steels, whose specimens with 2B finishing were more attacked with respect to specimens with BA finishing in the urban-marine atmosphere. The comparison between the two types of tests allows the conclusion that the electrochemical technique of anodic polarization is a fast and effective alternative to atmospheric exposure of the tested specimens, even if further studies are necessary. This is due to a certain disagreement between some results obtained with these experimental methods.