Preparation of Edges for Coating Performance

Introduction

Over 2,000 years ago, the Greek philosopher Aristotle discovered (or uncovered) the fact that our world wasn’t flat. While the earth itself is round, the topography of what is on this earth is neither flat nor round, but rather highly variable, with hills, valleys, mountains and plains. Similarly, our industrial infrastructure is far from uniform, and rarely completely flat (see left photo). Many industrial structures contain edges, corners, weldments, drain/weep holes, crevices and connections that are assembled using various types of fasteners. For decades we have acknowledged that these geometries pose real challenges for coatings and are frequently the first to show signs of rust staining and eventual corrosion. While there has been some progress on removing these types of configurations at the design stage (think lacing bars on a bridge, versus box beams) many will remain. Rather than engineering the problem away, we’ll need to determine how best to manage the corrosion on these challenging configurations. This brief article describes the problems commonly encountered when trying to protect edges, explores how specifications typically address edge protection, reviews some research done by the National Steel Bridge Alliance (NSBA) in 1999/2000 and describes SSPC Paint Application Guide 11, Protecting Edges, Crevices, and Irregular Steel Geometries by Stripe Coating, which is under revision during the time this article was prepared.

The Problem with Edges

Sharp edges such as those generated by torch cutting or shearing operations are difficult to coat, as many coatings that tend to shrink significantly during the drying/curing process tend to pull away (draw thin) on the edge as they cure due to surface tension that is created on the sharp edge (see image, left).  Even rolled edges of I-beams can be difficult to protect depending on the coating system used and the prevailing service environment. Some coatings have “edge retentive” properties, while most do not. It can be difficult for a specifier to determine whether edge preparation is necessary prior to coating application, since it is so dependent on the configuration of the structure (presence of edges, corners, weldments, drain/weep holes, crevices, fasteners), properties of the coating, the number of coating layers, the service environment and the degree to which future maintenance is feasible. While alkyd coatings are generally regarded as having poor edge retentive properties, inorganic zinc-rich primers (commonly used for new steel), high-build epoxies and 100% solids coatings often possess a degree of edge-build. A method for measuring the edge retention properties of a coating is published in MIL-PRF-23236[1].This test method examines the film thickness of three specimens cut from a non-chamfered 90-degree angle containing the

candidate coating. The ratio of the film thickness at the apex vs. the film thickness on the flat area is calculated and expressed as the percentage of edge retention of the coating, as shown in the image to the left. An average value of 70% edge retention for triplicate test specimens generally indicates that the coating has edge-retentive properties.

Addressing Edge Preparation

There are generally four ways to address edges: eliminate them (usually not feasible), grind, stripe coat, or grind and stripe coat. Grinding and stripe coating are described.

Grinding: There are several options associated with grinding a sharp edge to enable coatings to flow and protect better, including 1/16” chamfer or radius, 1/8” chamber or radius, or simply “break” the edge. While incorporating a radius into the edge is optimum it comes with considerable cost. Creating a chamfer followed by abrasive blast cleaning generates a similar radius at a comparatively lower cost (since abrasive blast cleaning will be performed as part of the surface preparation process). Grinding in a steel fabrication shop is commonplace, while field grinding can be difficult due to access, etc.

Section 8.2, Edge Profile of SSPC-PA 18/S8.2-2017, Specification for Application of Thermal Spray Coating Systems to Steel Bridges states, Before the abrasive blasting process, remove the hardened surface along thermal-cut edges by grinding or machining this surface to a depth that will allow for conventional grit blasting to produce the required surface profile for application of the thermal spray coating (TSC). Flame-cut or sheared edges must also have the corners “broken” to remove sharp or irregular edges. A radius is not required. The Applicator is to break corners by providing at least a 1/16″ chamfer or radius. Rolled edges do not need to be broken or ground unless they are sharp or irregular.

Stripe Coating: The SSPC Protective Coatings Glossary defines a “Stripe Coat” as: “A coat of paint applied only to edges or to welds on steel structures before or after a full coat is applied. SSPC Paint Guide No. 11 defines “Stripe Coat” or “Striping” as: “A coat of paint applied only to edges, welds, outside corners, bolt heads and threads, nuts, and crevices, either before or after a full coat is applied to the entire surface, for the purpose of imparting additional barrier properties to the applied coating system. The practice of applying a stripe coat is frequently referred to as “striping.”

A May 2017 KTA University article authored by James D, Machen titled, Importance of Stripe Coating for Extending Coating Service Life” states that brush application is typically the preferred method for applying stripe coats to weldments, fasteners, and crevices, while spray is preferred for edges to prevent pulling the coating of the edge during application.

Machen goes on to state that the cost of the effort to stripe coat is not always warranted. Structures in mild or interior exposure environments may not need the added corrosion protection of a stripe and therefore, the extra cost and effort of stripe coating may not be warranted.  However, the cost and extra effort is typically worthwhile when the structure is in immersion service (water or chemicals); is in highly corrosive environments; has a history of pitting corrosion, crevice corrosion, pack rust formation, and rust bleed problems; and/or includes complex configurations such as built-up box or lattice box-type members, complex gusset connections, rivets, nut/bolt assemblies, etc.

When applying stripe coats to blast cleaned steel surfaces the stripe coat is more commonly applied after the full primer coat has been applied.  Attempting to apply the stripe coat to blast cleaned steel may result in rust back of the steel surface before the full coat can be applied.  However, applying the stripe coat prior to any subsequent full coat typically results in improved appearance of the overall coating system versus applying the stripe coat after the full coat.  Applying the stripe coat before the full coat makes it easier to visualize the surfaces being stripe coated, improves localized protection, and is easier to inspect (as noted in photo).  Applying the stripe coat after full coats typically necessitates the use of contrasting colors to make the stripe coat visible to applicators and inspectors. 

Industry experience has shown that both wet-on-wet and wet-on-dry methods of stripe coat application are used.  Wet-on-dry stripe coatsmust typically dry for a certain time-period before the full coat is applied and can involve any coat. The dry times typically conform to the manufacturer’s “Dry-to-Recoat” interval listed on the product data sheet. Wet-on-dry stripe coating is typically a separate and distinct operation that requires more time to apply but allows better visibility for inspection.  Wet-on-wet stripe coating typically requires special approval from the manufacturer because the full coat is applied as soon as the stripe coat has released most of its solvent and is still “tacky” to the touch. While wet-on-wet stripe coating is less time-consuming,  it requires that inspection be performed during application if the installation of the stripe coat is to be visually verified.  Both methods achieve the intended goal, which is the application of extra thickness and assurance of complete coverage on the surfaces being stripe coated.

Industry Guidelines

A landmark, three-phase study commissioned by the National Steel Bridge Alliance (NSBA)[2] in 1999 concluded that grinding of the corners (edges) in the shop, for the purpose of improving the surfaces for coating coverage and ultimately corrosion protection, is unnecessary when employing ethyl silicate inorganic zinc-rich primer systems with a minimum zinc loading of 83%. Limited testing of organic zinc-rich coatings (two epoxy zinc-rich and one urethane zinc-rich) with minimal zinc loading of 84% used in Phase 3 indicated that minimal corner (edge) preparation (breaking the corner) generates a surface that provides sufficient coating performance. However, the study did not consider all manufacturers and edge retention properties can be formulation-driven. Therefore, coating manufacturer’s instructions and specification requirements should always be followed.

A revised version of SSPC Painting Application Guide 11, Protecting Edges, Crevices, and Irregular Steel Geometries by Stripe Coating (in ballot at the time this article was published) discusses the technique called “stripe coating” or “striping” as a way of providing extra corrosion protection measures on edges, outside corners, crevices, bolt heads, welds, and other irregular steel geometries, including optional pre-surface preparation techniques for sharp edges to improve coating performance. Appendix A contains recommendations and sample language to assist the specification writer in addressing corrosion protection of Edges, Crevices, and Irregular Steel Geometries, and Appendix B contains representative photographs of configurations/geometries that may benefit from stripe coating.

Summary

Most industrial structures contain edges (and other complex geometries) that are difficult to protect and are often the first to reveal signs of deterioration. While design of new structures may consider minimizing edges to the extent feasible, older structures frequently contain many edges that must be addressed during maintenance painting work, and one simply cannot eliminate edges. Proper treatment of edges can include grinding to relieve the corner, followed by abrasive blast cleaning that further treats the edge; stripe coating to achieve a greater barrier between the prepared steel and the service environment; or a combination of methods. SSPC Paint Application Guide No. 11 is an excellent resource for specification writers to determine the best route to address edge treatment for both new construction and rehabilitation work.


[1] MIL-PRF-23236 (latest edition); Coating System for Ship Structures. Washington Navy Yard, DC: Naval Sea  Systems Command, available online at http://assist.daps.dla.mil/quicksearch

[2] Edge/Corner Preparation of Steel Members and its Effect on Zinc Rich Primer Performance available from https://www.aisc.org/globalassets/nsba/technical-documents/edge-corner-preparation-of-steel-members-and-its-effect-on-zinc-rich-primer-performance.pdf

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