Corrosion inhibition in acidic environments: key interfacial insights with photoelectron spectroscopy.

In many engineering scenarios, surface-active organic species are added to acidic solutions to inhibit the corrosion of metallic components. Given suitable selection, such corrosion inhibitors are highly effective, preventing significant degradation even in highly aggressive environments. Nevertheless, there are still considerable gaps in fundamental knowledge of corrosion inhibitor functionality, severely restricting rational development. Here, we demonstrate the capability of X-ray photoelectron spectroscopy (XPS), supported by ab initio modelling, for revealing key details of inhibited substrates. Attention is focussed on the corrosion inhibition of carbon steel through the addition of an exemplar imidazoline-based corrosion inhibitor (OMID) to aqueous solutions of both HCl and H2SO4. Most notably, it is demonstrated that interfacial chemistry varies with the identity of the acid. High resolution Fe 2p, O 1s, N 1s, and Cl 2p XPS spectra, acquired from well-inhibited carbon steel in 1 M HCl, show that there are two different singly protonated OMID species bound directly to the metallic carbon steel substrate. In sharp contrast, in 0.01 M H2SO4, OMID adsorbs onto an ultra-thin surface film, composed primarily of a ferric sulfate (Fe2(SO4)3)-like phase. Such insight is essential to efforts to develop a mechanistic description of corrosion inhibitor functionality, as well as knowledge-based identification of next generation corrosion inhibitors.

Determining the Chemical Composition of Corrosion Inhibitor/Metal Interfaces with XPS: Minimizing Post Immersion Oxidation.

An approach for acquiring more reliable X-ray photoelectron spectroscopy data from corrosion inhibitor/metal interfaces is described. More specifically, the focus is on metallic substrates immersed in acidic solutions containing organic corrosion inhibitors, as these systems can be particularly sensitive to oxidation following removal from solution. To minimize the likelihood of such degradation, samples are removed from solution within a glove box purged with inert gas, either N2 or Ar. The glove box is directly attached to the load-lock of the ultra-high vacuum X-ray photoelectron spectroscopy instrument, avoiding any exposure to the ambient laboratory atmosphere, and thus reducing the possibility of post immersion substrate oxidation. On this basis, one can be more certain that the X-ray photoelectron spectroscopy features observed are likely to be representative of the in situ submerged scenario, e.g. the oxidation state of the metal is not modified.