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Analysis of Coating Composition used on the Humber Bridge

The Humber Bridge Board had been trying to determine both the long and short term maintenance strategy for the Humber Bridge. Part of that strategy included understanding the layers of coating that were applied over the years. Under normal circumstances there would be detailed maintenance records, however, the last time the bridge was painted was when paper records were the norm and these had since been lost. TWI was approached to help establish what had been used, particularly for the highly successful top layer.

In the absence of records, the only way to determine the paint system used was through analysis of samples extracted from the bridge coating. TWI undertook analysis using a number of physical and chemical techniques to build up a picture of the system used and identify the components for an effective system for the combination of substrate and operating conditions.

Initially, optical and scanning electron microscopy (SEM) were used to assess a cross-section of the retrieved specimens. This gave a detailed view of the multiple layers present in the sample, as identified by the different colours in the optical image. It also provided an estimation of the thicknesses of the individual layers and the overall structure. Energy dispersive x-ray spectroscopy (EDX) was used in conjunction with SEM, which is a technique that can be used to determine the elemental composition of a sample and estimate the abundance. It does this by firing a high energy beam of charged particles to cause the material in question to eject an electron from its inner shell and as an electron from the outer shell fills the electron hole, it can release x-rays. The number and energy of these emitted x-rays allow the elemental composition to then be measured.

Backscattered SEM image of cross-sectioned specimen
Backscattered SEM image of cross-sectioned specimen

The EDX analysis showed that the base yellow coat was filled with zinc phosphate, consistent with what would be expected of an anticorrosive primer. The grey middle layer was found to have large flakes of iron-based metal particulates as the filler and the top coat was heavily loaded with a silicate-based filler. On the very top surface, a thin layer with sulphurous compounds was found. This was believed to be deposits most probably arising from traffic exhaust fumes.

The specific chemistries of the coating binders were investigated using two main techniques, pyrolysis gas chromatography with mass spectrometry (Py-GCMS) and Fourier transfer infrared (FTIR) microscopic analysis. The Py-GCMS technique uses pyrolysis to break down complex molecules, such as polymers, to smaller molecules that can then be separated using gas chromatography. This enables them to be more readily analysed by mass spectrometry, allowing identification of their specific chemical components. During FTIR, infrared radiation is passed through a sample, some radiation is absorbed by the sample and some is transmitted. The resulting signal at the detector is a spectrum representing a molecular fingerprint of the sample, which is useful because different chemical structures produce different spectral fingerprints. The FTIR and Py-GCMS analyses identified the differences in the chemistry of the resins used for the different layers.

Drawing on TWI’s expertise, the compositional information concerning the fillers and binders was used to enable currently available equivalent commercial products to be identified. The detailed analysis of the existing paint scheme used on the Humber Bridge thus allowed the Board to make informed decisions regarding the maintenance strategy and the planned repainting scheme.

For more information, please email contactus@twi.co.uk

Cross sectional image of a paint flake using optical microscopy
Cross sectional image of a paint flake using optical microscopy
Avatar Chris Graham Project Leader – Tribology and Coatings Engineering

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