On the electrografting of stainless steel from para-substituted aryldiazonium salts and the thermal stability of the grafted layer

In this study we explore the thermal stability of an organic layer electrografted onto stainless steel (ASTM 316) from four different aryldiazonium salts R-C6H4N2+ (R = NO2, F, H, or OCH3). The coverage of the surfaces was analysed electrochemically by employing redox probes and cyclic voltammetry. The results obtained clearly show that the steel surface after grafting is electrochemically passivated by the presence of a surface coating. Polarization modulated infrared reflection absorption spectroscopy (PM-IRRAS) was used to characterize the organic films on the surfaces and to monitor the thermal stability of these films from ambient temperature to 400 °C with 50 °C intervals. The PM-IRRAS spectra show a decrease in band intensities at 250 °C for nitrophenyl, independent of layer thickness and atmosphere (air or argon), 300 °C for methoxyphenyl, and 350 °C for phenyl and fluorophenyl films. All the characteristic IR bands were simultaneously and completely lost at 300, 350, 350, 400 and 400 °C for thin-layer nitrophenyl, thick-layer nitrophenyl, methoxyphenyl, fluorophenyl, and phenyl, respectively, which strongly indicates that the entire organic film is lost at these temperatures. The results show that it is mainly the substituent and the layer thickness that are responsible for the difference in thermal stability of the organic films and that all films withstood temperatures up to 200 °C.This study shows that electrochemical grafting from aryldiazonium salts is simple, fast, and has a low energy consumption which makes the procedure suitable for industrial applications.