Polarized Neutron Reflectometry of Nickel Corrosion Inhibitors

Abstract

Polarized neutron reflectometry has been used to investigate the detailed adsorption behavior and corrosion inhibition mechanism of two surfactants on a nickel surface under acidic conditions. Both the corrosion of the nickel surface and the structure of the adsorbed surfactant layer could be monitored in situ by the use of different solvent contrasts. Layer thicknesses and roughnesses were evaluated over a range of pH values, showing distinctly the superior corrosion inhibition of one negatively charged surfactant (sodium dodecyl sulfate) compared to a positively charged example (dodecyl trimethylammonium bromide) due to its stronger binding interaction with the surface. It was found that adequate corrosion inhibition occurs at significantly less than full surface coverage.

An X-ray and Neutron Reflectometry Study of Iron Corrosion in Seawater

“…The results for SDS are shown in Figure 15; it is evident that there is no improvement in corrosion inhibition over h, and indeed the surface seems to be significantly corroded after only 40 minutes. SDS is known to work as an effective corrosion inhibitor for metals in acidic environments, presumably due to the electrostatic attraction to the positively-charged surface at low pH 35 ; however, sulfate anions have been reported to have a catalytic effect on iron dissolution due to the formation of an intermediate Two samples were soaked in a saturated phosphate solution (bis(2-ethylhexyl)phosphate) in seawater over varying lengths of time, as shown in Figure 16. The phosphate does not completely halt corrosion; over 7 days, significant roughening of the surface is seen (the roughness of the iron layer increases from 12 to 32 Å), presumably due to patches of the iron corroding more quickly than other areas.…”

Neutron Reflectometry for Studying Corrosion and Corrosion Inhibition

“…By using two water contrasts-H 2 O and D 2 O-they were able to effectively contrast-match to the surfactant layer and nickel surface respectively and hence collect data pertaining to each specific part of the system in situ, as demonstrated in Figure 8. Importantly, they were able to demonstrate excellent corrosion inhibition at very low pH using sodium dodecyl sulphate (SDS) at a surfactant coverage of only 60%, corroborating the view that full coverage of inhibitor is unnecessary for protection of the surface [75]. This critical finding is believed to arise from the presence of 'active sites' at the metal surface that are particularly prone to corrosion and hence nucleate dissolution of the metal.…”

“…Their analysis allowed a detailed breakdown of the copper layer into oxides and hydroxides, which showed different behaviour under corrosive conditions. Similarly to the work by Wood et al [75], the NR technique was shown to be a powerful tool to characterise both the adsorbed inhibitor and metal surface by the use of contrast-matching. Their work also supported the view of corrosion inhibition as a mechanism that blocks specific active sites: whilst the amine layer coverage was only around 65%, compared to around 85% for the thiol layer, it proved to be a far better inhibitor.…”

“…This has been successfully applied to a range of materials, such as iron [65,73,74], copper [10], titanium [41], nickel [75], chromium [76], zirconium [77], ITO [78], stainless steel [76] and various other alloys [79][80][81][82]. Care should be taken during deposition and subsequent handling to ensure that the final surface represents the material desired: for example to what extent it oxidises upon exposure to air.…”

“…Despite the importance of iron and steel across a wide range of industrial applications, they remain relatively unstudied as a surface by neutron reflectometry, although a small but significant body of work concerning surfactant adsorption at iron/liquid interfaces does exist [65,73,75,[101][102][103]. The direct study of steel alloys is slightly more challenging due to the necessity of preparing thin (to ensure neutron transmission) and very flat films, although recent work has demonstrated that representative stainless steel films can be prepared using electron-beam deposition at low pressures [76].…”