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Spectroscopic imaging of the buried substrate / polymer interface upon exposure to corrosive environments

https://doi.org/10.1016/j.vibspec.2021.103217Get rights and content

Abstract

Spectroscopic imaging of water at the substrate / polymer interface was demonstrated by a new experimental set-up based on FTIR-ATR focal plane array (FPA) imaging and simultaneous electrochemical impedance spectroscopy measurements. The local distribution of water at the substrate / polymer interfacial region could be imaged with a lateral resolution in the micrometer range. The kinetics of local water uptake vs exposure time on polymer coated Ge as well as on polymer coated and NaCl pre-contaminated Ge surfaces was obtained. The set-up can also be used to study interfacial processes at polymer coated thin film metallic or metal oxide surfaces.

Introduction

The stability of buried metal/polymer interfaces upon exposure to moist and corrosive environments is crucial for many applications such as painted metallic surfaces and adhesive structures. The analysis of chemical changes in the organic coating layer and at interface are essential to understand the interfacial stability during exposure to humid air or electrolyte solutions. Therefore, extensive research efforts have been made in order to study water uptake in coatings and understand the processes which leads to dis-bonding of the coating/ substrate as well as initiation of corrosion of metal substrate [[1], [2], [3], [4]].

Information about changes at the metal / polymer interface induced by exposure to electrolyte solutions has mostly been obtained by electrochemical impedance spectroscopy (EIS) which also provides information about the transport of water to the substrate / polymer interface [[5], [6], [7]]. Data on water transport in coatings are usually derived assuming homogeneous water uptake in the coating although more recent interpretations of EIS data can take into account inhomogenous water uptake [8].

However, although an increased understanding of the physicochemical processes which occur during water uptake, delamination and corrosion initiation can be obtained by EIS, little information about the chemical alterations which occur at the interface and in the coating layer are available with this technique. Unfortunately, few analytic methods are suitable for analysis of buried interfaces, in particular under in situ conditions, and powerful surface analytical methods, such as XPS or ToF-SIMS are not suitable for studies of buried polymer / substrate surfaces and are difficult to use in ambient conditions.

Optical spectroscopy such as infrared and Raman spectroscopy are more easily adapted for in situ measurements compared to techniques based on electrons or ions as probing particles, such as XPS and ToF-SIMS. FTIR attenuated total reflectance (ATR) spectroscopy has been frequently used for studies of water uptake in polymeric coatings deposited on internal reflection elements [[9], [10], [11], [12], [13]]. Simultaneous FTIR-ATR electrochemical investigations of model metal / polymer interfaces have been performed using internal reflection with a thin metal coating (in the order of 50 nm or less) and a polymeric coating on the metal [[14], [15], [16], [17], [18], [19], [20]]. Several studies of the effect of thin pretreatment film on the stability of the metal /polymer interface have been performed using this experimental approach [[21], [22], [23], [24], [25]]. However, these studies were performed at the macroscopic level and spatially resolved information cannot not be obtained below the millimeter/ centimeter range with the experimental approaches used previously.

Spectrochemical mapping and imaging of surfaces can be obtained by conventional FTIR-microscopy with single point detectors or with focal place array (FPA) detectors. FTIR-focal plane array (FPA) imaging enables visualization of the distribution of chemical species in a sample with a high lateral resolution with relatively short data acquisition times. The samples are illuminated in transmission/reflectance or with an ATR-objective and the focal plane of the objective is imaged onto the array detector which simultaneously record, for instance 64 × 64 or 128 × 128 spectra (depending on the detector size) over an defined area, for each image. The use of a FPA detectors enables spectral imaging of an area without the need to move the sample to record small areas defined by an aperture as in FTIR-mapping with a single point detector, which is very time consuming and less suitable for studies of dynamic processes. FTIR-ATR measurements can be performed with a lateral resolution down to 3–4 μm [26] with relatively short data acquisition times, which is considerably better than what is obtained practically with FTIR-mapping using a single-point detector and a thermal light source. The technique has been used to perform chemical imaging of cross sections of coatings [27,28] and obtain depth resolved information on degradation products in coil coatings [29]

More recently has infrared microspectroscopy with nanometris resolution, AFM-IR, been applied for ex situ studies of the substrate / coating interfacial region on cross sections of coated metal substrates [30,31]. However, AFM-IR would be difficult to use for in situ studies of the substrate / coating interface for coatings thicker than the order of one micrometer and in electrolyte solutions.

The aim of this work was to develop an experimental set-up for in situ spectrochemical imaging of the substrate / polymer interfacial region with a lateral resolution in the micrometer range using FTIR-ATR microscopy with a focal plane array detector. To our best knowledge this is the first report of in situ chemical imaging of the substrate / polymer in the micrometer resolution range during exposure to electrolyte solution or humid air.

Section snippets

Materials and exposures

The model coating used in the study consisted of a one component nitrile rubber and a vinylchloride-vinylacetate co-polymer casted on the flat surface of Ge hemispheres and then air dried for 2−3 h. The thickness of 55 +/- 5 μm as determined with gravimetry after removal of the coating from the substrate. For selected experiments NaCl particles were deposited on the flat surface of the Ge hemisphere from 5 μl NaCl saturated ethanol (99.5 %). After evaporation of ethanol the Ge surface was

Experimental set-up for in situ FTIR-FPA imaging of substrate/polymer interfaces

A description of the experimental set-up for in situ spectroscopic imaging of the buried substrate / polymer interface is shown in Fig. 1. The FTIR-microscope is equipped with a 20X objective which focus the IR-light on the flat surface of a Ge-hemisphere (r =25.4 mm) at which the incident light undergoes total reflection. Infrared spectra are obtained from the substrate / coating interfacial region by coupling of the exponentially decaying evanescent electrical field from the Ge-surface.

Summary and conclusions

A newly developed experimental set-up for FTIR imaging of water and molecules at the substrate / polymer interface based in FTIR-ATR focal plane array (FPA) imaging is presented. The local water up-take in the substrate / polymer interfacial region for the polymer coated Ge surface was studied and the images of the water band intensities could be obtained with a lateral resolution in the micrometer range while electrochemical impedance spectroscopy measurements can be performed simultaneously.

CRediT authorship contribution statement

Dan Persson: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization, Supervision. Dominque Thierry: Conceptualization, Writing - original draft, Supervision, Resources.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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