The film formation of film forming amines (FFAs), commonly used as corrosion inhibitors in water/steam cycles, has been subject of various studies. However, less is known of the impact of alkalising amines (AA) on the film formation. AAs have a solubilising effect on the FFA and as a consequence might indirectly influence the properties of the film.
Therefore, the film formation of oleyldiamine, a commonly used FFA, on carbon steel has been studied with electrochemical impedance spectroscopy (EIS) in defined test water, containing 200 mg L-1 of NaCl, with and without the presence of different AAs. The studies have been carried out at two different pH values over an exposure time up to 5 h.
All EIS spectra of the blank show one semi-circle, whereas two distinct semi-circles have been observed in the spectra of the test solutions with FFA. Therefore, the steel solution interface has been described by two different equivalent circuits using resistances and constant phase elements. All EIS spectra could be fitted with good agreement to the experimental data. The spectra showed significant and systematic changes throughout the exposure time. All polarisation resistances measured with FFA at pH 9.0 are much higher compared to the data obtained at pH 8.2. The polarisation resistance of the blank solution is strongly decreasing over time following roughly an exponential law and almost comes to a steady state after 5 hours. In contrast to this the solutions containing FFA show a linear increase of the polarisation resistance over time. Obviously the film formation has not yet completely been finished within the experimental period.
The FFA solutions containing the two different AA show a similar behaviour over time and almost the same high polarisation resistance, reflecting the excellent corrosion protection. Overall, the data indicate that the film obtained with oleylamine in the solutions containing DEAE provides a slightly better protection than FFA solution with CHA. Thus, the more toxic CHA can be replaced by DEAE.