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Measuring Coating Thickness Over Complex Geometries

Measuring Coating Thickness Over Complex Geometries

Corrosion of painted steel edges is an issue that affects a variety of industry sectors and results in significant costs relating to in-service repair, warranty claims and reduced resale value. Even small levels of rusting can lead to substantial staining of surfaces and adversely affect a customer’s perception of brand quality.

When polymer coatings are applied to sharp edges (either powder coating or liquid application), surface tension effects act to pull material away and result in low film thicknesses. Such regions can provide an insufficient barrier to moisture ingress and become points where corrosion can initiate.

Conventional techniques for determining paint thickness (such as magnetic and eddy current thickness gauges) are unable to operate accurately over surfaces with complex geometry, such as corners and edges. Often the only reliable method for determining coating thickness over a particular edge geometry is to cross-section the part and investigate with conventional quantitative microscopy techniques. While reliable and accurate, coating thickness data are only obtained within a single plane and preparation of metallographic sections is laborious and time-consuming.

TWI has used a non-contact metrology approach to determine the coating thickness over an entire profile, regardless of the geometry. By collecting 3D surface data using a focus variation microscope, coating thickness can be calculated over a variety of substrate geometries. This approach compares two sets of scan data; one taken of the initial surface and one taken after the coating is applied. One region of the test piece must remain uncoated to provide a reference surface that is the same in both scans. An area that is identical in both scans is required to ensure datasets correctly align and an accurate comparison can be made.

Figure 1 3D scan of an 8mm laser-cut carbon steel edge. The left hand side contains a through hole and a punch mark to aid scan alignment for later steps.

Figure 2 3D scan of the same sample having been powder coated. The left hand side was masked off to aid alignment with uncoated scan. [NB – powder coating has been modified to remove gloss, improving data collection from focus-variation microscope.]

Figure 3 False colour representation of nearest neighbour distance between scans of coated and uncoated edges, mapped onto scan data of the coated edge. This distance measures the minimum coating thickness at any given location.

Once aligned, datasets can be compared to determine the coating thickness over the entire geometry. Such an approach enables a step change in the information available when compared with a two dimensional cross-section. Mapping the coating thickness over the geometry allows for easy visualisation of potential regions with insufficient barrier properties. Quantitative data can then be extracted for further analysis in any desired location.

By readily visualising and quantifying coating thickness over features such as sharp edges, detailed studies can be performed into various approaches for mitigating problems associated with low film thickness. Engaging with both the supply chain and industrial end users, TWI has investigated the effectiveness of various approaches, including:

Altering cutting / surface preparation operations (potentially simple and cost effective to implement, but with limited opportunity for altering edge effects);
Additional edge preparation steps (straightforward concept that can be very effective, but not necessarily easy to implement cost effectively),
New polymer formulations designed specifically for high edge retention (potentially higher cost, but may provide improved long-term performance. Care is required to ensure other coating criteria are preserved, such as gloss levels).

Quantitative comparison of 3D datasets used to measure geometrical changes are not confined coating studies. Any application where material is either added (e.g. coating) or removed (e.g. machining, marking, etching, corrosion, etc.) can be suitable for this type of analysis. The only constraint is that one region of the part should remain unchanged to provide a reference surface for scan alignment. Such a region can be introduced artificially if required, such as the attachment of an inert material in corrosion studies.

Figure 4 (a) Detail from false-colour height map of coating thickness showing the reduced coating thickness over the sharp edge and the peaks of striations on laser cut face and (b) intersection of 2D plane, showing coating thickness variation over the edge.

Figure 5 Plot of coating thickness as a function of linear distance along edge, showing how surface tension effects during powder coating result in material being drawn away from the sharp edge.

Henry Begg Section Manager - Surface, Corrosion and Interface Engineering

Henry joined TWI as a Project Leader in 2013 having completed a PhD on the processing of advanced aluminium alloys. Since joining, he has managed a range of R&D activities relating to coatings and surface engineering across a diverse set of industry sectors. For the past three years, he has led the surface engineering team and has recently also taken management responsibility for the electrochemical corrosion and permeation testing activities at TWI.

Surface engineering activities at TWI span a broad range of process technologies and applications, from thin, functional coatings that create repellent surfaces, right up to thick weld overlays for heavy-duty industrial service. The surface engineering team has a particular focus on thermal and cold spray technology, providing support to industry across a wide range of TRL/MRL levels. They assist TWI members in developing new materials and processes, providing characterisation and performance testing in bespoke environments and supporting the automation and coating of large demonstrator components.

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