Paper SS+2D-WeM2
Quantitative Chemical State base on XPS Energy Scan Image Applied to Ni Fe corroded Samples

Wednesday, November 9, 2016, 8:20 am, Room 104D

1. Introduction

Corrosion is a natural phenomenon where metals have a tendency to revert back to their natural, lower energy state. During many years, the most effective corrosion protection systems were based on the use of chromate-rich surface treatments. However, recently, the legislation imposed by the european commission, REACH (Registration, Evaluation, Authorization and Restrictions of Chemicals) prohibited the use of hexavalent chromium. Many alternatives have been explored so far, including the protection with monolayer and/or multilayers of thin films (Cr, CrN, Ti, TiN, NbN) deposited by magnetron sputtering process. Corrosion is a physical-chemical interaction between the metal and its environment. Most hard coatings (TiN, CrN, NbN) possess an higher corrosion resistance than less noble materials like steel, Al or Mg alloys. When the coatings are deposited on such less noble substrates and exposed to a corrosive atmosphere (ex : NaCl), the coated parts suffer from serious corrosive attack (pitting corrosion) due to inherent coating defects or inhomogeneities . This kind of corrosion is localized to the defect area and is characterized by the anodic dissolution of the substrate material with a high anodic current density at the defect site. It is generally called galvanic corrosion. In this study, Nickel thin film (2 micrometers) was deposited on steel substrate (XC38) using Direct Current Magnetron Sputtering from a Ni target. In order to simulate the corrosive attack, the coated part was exposed to a corrosive medium, i.e salt spray test. The test was conducted under continuous spray (24 hours) conditions (5wt.% NaCl) at a temperature of 35 °C according to the ASTM B117 standard procedure.

The morphology and the chemical environment of the corrosion products were analyzed respectively by optical microscopy and XPS investigations.

The samples have conducting and isolating regions to avoid spatial differential potential the samples were measured in a floating condition. The Parallel XPS images of 128 pixels by 128 pixels where done with FOV of 900 x 900 microns at an energy resolution of about 1eV and an energy step of 0.2 eV for O 1s, Na KLL, Ca 2p, C 1s, Ni 3p and Fe 3p with charge compensation. Ni and Fe were measure in one region. To treat the 735 images corresponding of more than 1.2 10⁷ intensity points, images were smoothed and projected on PCA images. All images were projected on 6 abstract factors. Then images were converting in 81920 spectra. After spectra quantification, energies regions were converting back in quantitative images. Then Fe map was classified in 5 false colors part by intensity.