Spontaneous Aryldiazonium Film Formation on 440C Stainless Steel in Nonaqueous Environments

The ability of three aryldiazonium salts to spontaneously assemble onto the surface of type 440C stainless steel is investigated in acetonitrile (ACN) and the model hydraulic fluids tributyl phosphate (TBP) and hexamethyldisiloxane (HMDS). Competition between native oxide formation and organic film growth at different diazonium salt concentrations is monitored by electrochemical impedance spectroscopy. At 1 mM diazonium salt, 70% of total assembly is complete within 10 min, though total surface coverage by organics is limited to ≈0.15 monolayers. Adding HCl to the electrolyte renders native oxide formation unfavorable, yet the diazonium molecules are still unable to the increase surface coverage over 1 M-10 μM HCl in solution. X-ray photoelectron spectroscopy confirms preferential bonding of organic molecules to iron over chromium, while secondary ion mass spectroscopy reveals the ability of these films to self-heal when mechanically removed or damaged. Aging the diazonium salts in these nonaqueous environments demonstrates that up to 90% of the original diazonium salt concentration remains after 21 days at room temperature, while increasing the temperature beyond 50 °C results in complete decomposition within 24 h, regardless of solvent-salt combination. It is concluded that the investigated diazonium molecules will not spontaneously form a continuous monolayer on 440C stainless steel immersed in ACN, TBP, or HMDS.