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

The overall goal of this experimental procedure is to prevent the oxidation of metallic substrates during sample transfer from an inhibited acidic solution to an x-ray photoelectron spectrometer. This procedure allows one to more reliably determine the chemical nature of corrosion inhibitor matter interfaces in acidic solutions, which is a key component of mechanistic understanding and performance prediction. A particular advantage of this approach is that it is relatively easy to apply.

And so, can be adopted by other researchers undertaking similar measurements. The protocol is relatively straightforward, but one should practice manipulating samples in the glove box as this is the most exacting segment of the method. Beyond corrosion inhibition, this approach should also be considered in other areas where XPS measurements are undertaken from air sensitive interfaces formed in a fluid environment.

We first decided to try this approach as we were concerned the previous work was compromised by oxidation of the inhibited interface upon exposure to air. To begin this procedure, prepare the carbon-steel substrate and the hydrochloric acid solution with added CI as outlined in the text protocol. Next, pour 25 milliliters of the prepared HCL plus CI solution into a small glass beaker.

Using acid resistant tweezers, pick up the disk shaped carbon-steel sample, then lower it into the HCL plus CI solution. Orient the sample so that the cylindrical faces are in the vertical plane. First, connect the glove box to a source of inert gas such as nitrogen or argon.

Then, adhere a small square of double sided conductive carbon tape onto the XPS sample bar. Insert all of the hardware required to transfer the sample to the XPS instrument into the glove box through an open port. After this, place the glass beaker containing the submerged substrate into the glove box.

Seal every port. Then, purge the glove box with inert gas. Allow the sample to remain submerged for the desired immersion period.

Monitor the relative humidity value within the glove box. Once the relative humidity is below eight percent, reach into the glove box gloves. Then, cover these with nitrile gloves.

Using tweezers, remove the carbon-steel sample from the solution. Immediately blow the sample dry using an empty wash bottle. Next, cover the beaker containing the HCL plus CI solution with a plastic paraffin film.

Affix the dried sample to the small square of tape on the XPS sample bar. After this, vent the XPS load lock chamber to the inert gas. Open the load lock flange, transfer the sample bar into the load lock chamber and slide it onto the sample holding prong.

Then, close the flange. Switch on the turbo rotary pump combination to pump down the chamber. Once the pressure reaches approximately five times 10 to the minus seven millibar, manually transfer the sample into the intermediate chamber using the transfer arm.

Using the keypad, orient the sample to the desired photoelectron emission angle. Next, open the XPS data acquisition software. Navigate to the instrument manual control window.

Input 10 milliamps and 15 killivolts as the values for the anode emission and anode HT parameters respectively. Then, click the On button in the Xray gun section to power up the monochromated aluminum K alpha x-ray source. After this, click the On button in the Neutralizer section to turn on the charge neutralizer.

In the Analyzer section, select the spectrum Hybrid measurement mode from the mode and lens drop down menus. Next, input the desired parameters into the acquisition scan control section. Using the keypad, optimize the sample position to maximize the signal from the selected core level.

Then, acquire XPS data as outlined in the text protocol. In this study, an alternative approach for introducing samples into an ultra high vacuum XPS instrument is used to minimize oxidation after immersion in an acidic solution containing a corrosion inhibitor. The XPS data for a polished sample exhibits three prominent features:the iron 2p signal arises from the carbon-steel composing the sample.

The oxygen 1s signal derives from both a surface oxidic film and absorbates while the carbon 1s signal is due to an adventitious carbon. Submersion in either of the inhibited one molar HCL solutions results in significant changes to the corresponding overview spectrum. A nitrogen signal arises due to surface absorption of the inhibitor while the oxygen signal is nearly eliminated.

The higher resolution oxygen 1s and iron 2p spectral profiles indicate that surface oxidation of the polished sample has occurred resulting in iron oxides and hydroxides. These species are absent from the immersed samples indicating that little, if any, surface oxide or hydroxide remains. The effect of the ambient laboratory atmosphere on an inhibited interface is determined by transferring a sample inside a partially purged glove box.

This sample exposed to a higher concentration of oxygen exhibits signs of surface oxidation. Thus, surface oxidation is minimized only when sample transfer occurs inside a well purged glove box. After watching this video, you should have a good understanding of how to avoid oxidation of corrosion inhibitor matter interfaces prior to XPS measurements.

The procedure is relatively simple, and once mastered, the sample transfer step can be done in less than 30 minutes if it is performed properly. However, while undertaking the procedure, it is important not to touch the surface to be probed by XPS at any time during sample immersion or transfer. Finally, don't forget that working with acids can be hazardous and appropriate care should be taken.