An electrochemical and spectroscopic investigation of the corrosion inhibitor nonylphenylethoxy phosphate ester and of the films formed on 316L stainless steel in acidic solutions.

摘要

In-situ surface-enhanced Raman spectroscopy (SERS) and electrochemical polarization were applied to study nonylphenylethoxy phosphate ester, an effective corrosion inhibitor for pipeline steels in carbon-dioxide-saturated formation brine.; Comparison of results among the phosphate ester, a nonylphenyl ethoxylated alcohol (Nonoxynol-9), and the sulfate ester---molecules that are essentially identical except for the headgroup---indicates that the effectiveness of the first as a corrosion inhibitor is crucially dependent upon the phosphate headgroup. Its effectiveness is attributed predominantly to the ability to complex with the surface, and not to its electrostatic interaction with the surface. Polarization curves obtained in brine containing the phosphate monoester or diester, separated from the as-received mixture, indicate that the monoester is superior. While SERS indicates that the ethoxylates of the phosphate ester interact with the steel surface, they, by themselves, are not sufficient to anchor the molecule. Spectral features assigned to the phenyl ring suggest that it does not interact significantly with the steel surface. SERS demonstrates that Cl- adsorption correlates with a decrease in phosphate ester adsorption and this occurs in the range of potentials where pitting would be expected according to the polarization curve.; SERS has also been used to investigate the films that form on the surface of 316L stainless steel in a number of acidic solutions at room temperature---0.75 M HCl, 0.75 M HBr, 0.867 M HClO4, 0.75 M H2SO4 , and 0.75 M NaCl + 10-3 M HCl (pH 3). Two surface species characterized by SER peaks at 395 and 460 cm-1 form in all solutions; the relative intensity of the peaks depends on pH.; The SER spectra of the films do not correlate with any cited reference spectrum of the oxide, hydroxide, or oxyhydroxide of iron, chromium, nickel, or molybdenum. The presence of a component giving rise to a peak at ≈214 cm-1 correlates with the absence of pitting at the higher potentials. This component appears to be destabilized by the presence of either Cl- or Br-, and by low pH. Collectively, the results indicate that the identity of the film is primarily determined by the pH of the solution and is relatively insensitive to the identity of the anion present, except at high potentials where pitting occurs.