Citric acid aerospace stainless steel passivation: a green approach
The aim of the present work is to propose a completely green sustainable stainless steel passivation procedure, using baths based on citrus fruit waste derived citric acid, as a promising alternative route to the currently employed nitric passivation process. Stainless steels are widely used in the aerospace sector, because of their high mechanical properties and corrosion resistance. A clean and polished stainless steel part spontaneously forms a chromium oxide film that completely covers all its surface and prevents further surface corrosion by blocking oxygen diffusion to the steel surface, and blocks corrosion from spreading into the metal's internal structure .However, during machining process a microscopic amount of exogenous surface contamination may be transferred to the surface of the stainless steel work piece . If not removed, these foreign particles can reduce effectiveness of the original protective film. In order to maximize the natural orrosion resistance and to remove contaminants from the surface, an acid cleaning of stainless steels, called passivation, is commonly performed in industry. Passivation process is controlled by industrial standards  and it is almost exclusively based on the use of a nitric-hydrofluoric acid mixture (HNO3) . The process is undoubtedly economic and leads to excellent results, but the use of nitric acid involves extremely serious ecological problems. Actually, highly polluting nitrogenoxide (NOx) vapours are released to the atmosphere, and high nitrate concentrations are dissolved in wash water and spent baths, both types of pollutants requiring treatment prior to disposal. The removal of NOx from air and of nitrates from baths involves huge plant operation problems and high operating costs, with no certainty about the obtainment of targets complying with the regulations in force .In the recent past years, passivation treatment in citric acid (C6H8O7) baths has been found to be a successful greener passivation process. Citric acid is a biobased acid which exists in greater than trace amounts in a variety of fruits and vegetables, particularly in citrus fruits such as lemons and limes, comprising as much as 8% of the dry fruit weight . It can be performed on a large number of stainless steel families, and it is considered environmentally friendly in every respect. However, it has not been clearly demonstrated if citric acid passivation is an adequate alternative to nitric acid passivation .On the basis of these considerations, selected stainless steels were submitted to the passivation in both citric and nitric acid baths, in order to compare these two procedures in terms of effectiveness (potentiality of the citric acid based passivation) and processing time. As a preliminary step, a green sustainable method for citric acid extraction, from citrus fruit waste, is attempted. Corrosion tests were performed using traditional (potentiodynamic anodic-cathodic polarizationtest) and advanced (EIS, bulk electrochemical impedance spectroscopy and LEIS, localised electrochemical impedance spectroscopy) electrochemical techniques. The samples were investigated before and after corrosion tests with optical microscopy (OM), scanning electronmicroscopy (SEM-EDS), x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS).