Colleagues and peers in field technical services may frequently encounter issues during construction where coatings fail to adhere properly to edges, sharp corners, or pointed areas of structural components, leading to substrate exposure or early rusting. I’ve faced this issue multiple times in my work. Let’s explore this problem in detail.
1. "Edge Retention Rate" or "Edge Coverage Rate"
Issue Overview:
When coating materials are applied to sharp edges of steel structures, a common issue is poor coverage at edges, corners, or tips (collectively referred to as edges). This is especially noticeable with light-colored coatings, often followed by early rusting.
Traditional Perspective:
In the electrocoating industry, a conventional belief is that reduced coating thickness at edges, caused by the coating film being pulled away during drying and curing, is the primary reason for localized corrosion at edges. However, this view may not hold true from either a mechanical or practical perspective.
2. Discussion on "Edge Retention Rate" or "Edge Coverage Rate"
The concept of "edge retention rate" or "edge coverage rate" is often discussed, but it may not fully explain why corrosion occurs at edges. Moreover, current testing methods may not provide reliable evidence to validate edge coating effectiveness.
Edges of steel structures are among the most challenging areas to protect against corrosion, typically where two planes intersect at a 90° angle. The traditional view attributes corrosion at these edges to reduced dry film thickness (DFT) at the edge, caused by the coating film being pulled away during curing or drying. The ability to maintain sufficient DFT at the edge is termed the edge retention rate, defined by standards such as MIL-PRF-23236D and NACE TM0304. These standards measure edge retention rate as the ratio of DFT at the edge to DFT on adjacent flat surfaces, expressed as:
Per MIL-PRF-23236D, a coating is deemed resistant to edge effects if the average edge retention rate across three test samples is 70%, with no single sample below 50%.
3. Does a 70% Edge Retention Rate Guarantee Corrosion Resistance?
3.1 Answer: Likely Not
The concept of edge retention for corrosion protection and the reproducibility or meaningfulness of edge retention rate tests may have several issues:
From the definition, successful edge retention depends on consistent DFT across both flat surfaces and the edge. A key challenge is ensuring that the wet film thickness (WFT) at the edge matches that on flat surfaces. For a valid comparison, the WFT on both areas should be uniform during application, so the DFT after drying/curing is comparable, assuming no external forces affect the coating. Neither MIL-PRF-23236D nor NACE TM0304 requires measuring WFT at the edge, making it impossible to confirm whether sufficient or excessive coating was applied at the edge.
Example: If a coating with 50% solid content is applied at 250 μm WFT on a flat surface, the DFT will be 125 μm. However, if only 150 μm WFT is applied at the edge due to application variations, the resulting DFT will be 75 μm, yielding an edge retention rate of 60%, failing to meet the 70% minimum requirement. Without knowing the WFT at the edge, it’s unclear whether the reduced DFT is due to external forces or uneven application. Additionally, DFT measurements on flat surfaces may vary, and the standards do not specify how many measurements are needed or where they should be taken. For instance, if one flat surface has a DFT of 150 μm and another 100 μm, the average DFT is 125 μm, but this average lacks relevance when comparing to the edge DFT. The surface contour of the edge also significantly impacts DFT measurements on flat surfaces.
3.2 Coating Application Methods:
The method of applying coatings can affect thickness. Spray application on edges can result in uneven coating:
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Direct spraying on flat surfaces may leave edges uncoated or undercoated.
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Angled spraying may overcoat edges while undercoating flat surfaces.
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Direct spraying on edges may reduce coating thickness at the edge.
MIL-PRF-23236D requires spraying flat surfaces first, followed by direct spraying on edges. This method may accumulate sufficient coating around the edge but reduce thickness directly at the edge.
4. Conclusion
The above indicates that, while edge retention rate may serve as an indicator of edge corrosion protection, flaws in the testing methods undermine the ability to measure it meaningfully. These limitations highlight the need for improved testing protocols to ensure reliable edge coating performance.