The reaction of carbon steel and sulphide was studied by electrochemical experiments. Sulphide forms a black film on a carbon steel surface. The rate of tile film formation was studied optically by measuring the change in the reflectance of light from the surface of a rotating steel disk electrode. The process was measured as a function of time at various potentials. A 2-dimensional and a 3-dimensional model were created for the film formation. The rate of film formation was studied as a function of sulphide concentration and potential.