Imaging Corrosion at the Metal-Paint Interface Using Time-of-Flight Secondary Ion Mass Spectrometry

Summary

Time-of-flight secondary ion mass spectrometry is applied to demonstrate the chemical mapping and corrosion morphology at the metal-paint interface of an aluminum alloy after being exposed to a salt solution compared with a specimen exposed to air.

Abstract

Corrosion developed at the paint and aluminum (Al) metal-paint interface of an aluminum alloy is analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS), illustrating that SIMS is a suitable technique to study the chemical distribution at a metal-paint interface. The painted Al alloy coupons are immersed in a salt solution or exposed to air only. SIMS provides chemical mapping and 2D molecular imaging of the interface, allowing direct visualization of the morphology of the corrosion products formed at the metal-paint interface and mapping of the chemical after corrosion occurs. The experimental procedure of this method is presented to provide technical details to facilitate similar research and highlight pitfalls that may be encountered during such experiments.

Introduction

Al alloys have wide applications in engineering structures, such as in marine technology or military automotive, attributable to their high strength-to-weight ratio, excellent formability, and resistance to corrosion. However, localized corrosion of Al alloys is still a common phenomenon which affects their long-term reliability, durability, and integrity in various environmental conditions¹. Paint coating is the most common means to prevent corrosion. Illustration of the corrosion developed at the interface between metal and paint coating can provide insights in determining the appropriate remedy for corrosion prevention.

Protocol

1. Corrosion sample preparation

1. Al sample fixation in resin, and polishing
   1. Mount two Al alloy coupons (1 cm x 1 cm) using epoxy resin in 1.25 inch metallographic sample cups and place the coupons in the fume hood overnight or till the resin is completely cured.
   2. Take out the Al resin cylinders cups from the sample cups. Polish the Al resin cylinders using 240 grit paper with water at a 300 rpm platen/150 rpm in the holder for 1 min.
   3. Polish the Al resin cylinders using the polishing plate with 15 µm, 6 µm, 3 µm, and 1 µm water-based solution for 5 min (each step), sequentially.

Results

Figure 3 presents the comparison of mass spectra between the metal-paint interface treated with salt solution and the interface exposed to air. The mass spectra of the two samples were acquired using a 25 kV Bi₃⁺ ion beam scanning in 300 µm x 300 µm ROIs. The mass resolution (m/∆m) of the salt solution-treated sample was approximately 5,600 at the peak of m/z^- 26. The raw data of the mass spectra were exported after binning 10 channels. A graphical software w.......

Discussion

ToF-SIMS differentiates the ions according to their time of flight between two scintillators. The topography or sample roughness affects the flight time of the ions from different starting positions, which usually leads to a poor mass resolution with an increased width of peaks. Therefore, it is critical that the ROIs being analyzed are very flat, to ensure good signal collection⁸.

Another pitfall to avoid is charging. Since the Al-paint interface was fixed with the ins.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
0.05 µm Colloidal Silica polishing Solution	LECO	812-121-300	Final polishing solution
1 µm polishing solution	Pace Technologies	PC-1001-GLB	Water based polishing solution
15 µm polishing solution	Pace Technologies	PC-1015-GLBR	Water based polishing solution
3 µm polishing solution	Pace Technologies	PC-1003-GLG	Water based polishing solution
6 µm polishing solution	Pace Technologies	PC-1006-GLY	Water based polishing solution
Balance	Mettler Toledo	11106015	It is used for measuring the chemicals.
Epothin 2 epoxy hardener	Buehler	20-3442-064	Used for casting sample mounts
Epothin 2 epoxy resin	Buehler	20-3440-128	Used for casting sample mounts
Fast protein liquid chromatography (FPLC) conductivity sensor	Amersham	AKTA FPLC	Used to measure the conductivity of the salt solution.
Final B pad	Allied	90-150-235	Used for 1 µm and 0.05 µm  polishing steps
KCl	Sigma-Aldrich	P9333	Used to make the salt solution.
Low speed saw	Buehler Isomet	11-1280-160	Used to cut the Al coupons that are fixed in the epoxy resin.
MgCl2	Sigma-Aldrich	63042	Used to make the salt solution.
MgSO4	Sigma-Aldrich	M7506	It is used to make the salt solution.
NaCl	Sigma-Aldrich	S7653	It is used to make the salt solution.
NaOH	Sigma-Aldrich	306576	It is used for adjusting pH of the salt solution.
Paint	Rust-Oleum	245217	Universal General Purpose Gloss Black Hammered Spray Paint. It is used to spray on the Al coupons.
Pan-W polishing pad	LECO	809-505	Used for 15, 6, and 3 µm polishing steps
pH meter	Fisher Scientific	13-636-AP72	It is used for measuring the pH of the salt solution.
Pipette	Thermo Fisher	Scientific	Range: 10 to 1,000 µL
Pipette tip 1	Neptune	2112.96.BS	1,000 µL
Pipette tip 2	Rainin	17001865	20 µL
Silicon carbide paper	LECO	810-251-PRM	Grinding paper, 240 grit
Sputter coater	Cressington	108 sputter coater	It is used for coating the sample.
Tegramin-30 Semi-automatic polisher	Struers	6036127	Coarse/fine polishing/grinding
ToF-SIMS	IONTOF GmbH, Münster, Germany	ToF-SIMS V, equipped with Bi liquid metal ion gun and flood gun	It is used to acquire mass spectra and images of a specimen.
Vibromet 2 vibratory polisher	Buehler	67-1635-160	Final polishing step