A quaternary amorphous equiatomic FeCrSiNb HEA thin film was synthesised and characterised with the view for applications in demanding environments. The film was produced on silica and silicon substrates by ion beam sputter deposition. Scanning Electron Microscopy (SEM) was used to analyse the film morphology, and Energy-dispersive X-ray spectroscopy (EDX) was used to investigate the atomic composition. The crystal structure of the sample was investigated using X-Ray Diffraction (XRD) and Selected Area Electron Diffraction (SAED). The film is amorphous and uniform at the atomic level. An electrochemical cell with a three-electrode arrangement was utilised to assess the corrosion behavior of the sample in 0.6 M NaCl and H2SO4 solutions at room temperature under ambient pressure. In addition, the corrosion tests were conducted in a Libyan crude oil to simulate an oil field corrosion process. The corrosion results showed that the FeCrSiNb HEA thin film has an excellent corrosion resistance, and that the corrosion resistance is much better than that of 304 SS in 0.6 M NaCl, H2SO4 solutions, and in crude oil. The amorphous equiatomic FeCrSiNb HEA thin film has high hardness at around 15 ± 1.2 GPa, and reduced Young's modulus at around 229 ± 12 GPa.