Surface preparation of structural steel
March 14, 2002
This article highlights two of the accepted industry specifications, which are provided by the American Institute of Steel Construction (AISC) and the Society for Protective Coatings (SSPC). It covers many of the more salient topics, point by point, and explains them in detail.
Editor's Note: This article is the first part of a two-part series about structural steel corrosion prevention. Part I discusses surface preparation. Part II, which will appear in the April issue, concerns paint application.
How difficult can it be? It's just paint!
That sums up a common view of corrosion protection systems. However, a corrosion protection system—the process of identifying the service environment, selecting a coating system, preparing the surface, determining the most appropriate application method, and applying the coating according to the governing specification—isn't as simple as it seems. A corrosion protection system can be difficult and, furthermore, costly.
In fact, a recent survey of steel bridge fabricators conducted by the American Iron and Steel Institute (AISI)—National Steel Bridge Alliance (NSBA) committee revealed that 100 percent of the shops surveyed consider paint to be the most costly item for them. Comments included, "Paint systems are complicated and high-priced; cure times create bottlenecks; handling is expensive; blast cleaning to SSPC-SP10 is difficult; total shop painting is the worst; and edge preparation is time-consuming and expensive."
Based on this survey, it is evident that many fabrication shops perceive protective coating systems as problematic. In fact, the financial success or failure of a project often depends on whether the coating system is applied correctly the first time or whether it must be removed and reapplied multiple times.
Gaining a better understanding of specification requirements that govern corrosion protection systems can help fabrication shops prepare and paint structural items more successfully.
To help ensure success with paint, fabricators should:
Two shop painting certifications are widely recognized for structural and architectural applications. One is available through the American Institute of Steel Construction (AISC); the other is available through the Society for Protective Coatings (SSPC).
The AISC Sophisticated Paint Endorsement (SPE) is the more common of the two certifications. While it has undergone revisions over time, the intent of the endorsement is to demonstrate, through an independent audit, that a fabrication shop is proficient in surface preparation and painting operations and has the necessary quality control programs.
Many facility owners stipulate the SPE as a bid requisite; some even specify the type of shop that can bid the work (totally enclosed [P1], covered [P2], or outside [P3]). Almost 200 shops in the U.S. are SPE-certified.
Similarly, SSPC maintains a fabrication shop certification program known as Qualification Procedure 3 (QP3), which became available in 1995. Reciprocity between the two certification programs has enabled certified shops to bid on projects requiring AISC SPE or SSPC QP3 while holding only one of the two certifications.
There is no standard specification for surface preparation and coating work. Requirements vary depending on customer demands, the level of surface preparation required or desired, the coating system to be applied, and the service environment of the finished product.
For example, specification requirements for painting structural steel for a new football stadium are somewhat different from requirements for a steel bridge that will be exposed routinely to deicing salts and impact damage from gravel and other road debris. The specification for the football stadium may have strict requirements for aesthetics, while the specification for the bridge may allow a percentage of runs or sags to remain.
Following are some of the most common surface preparation and coating application specification requirements. However, this article does not review every aspect of the specifications. Reading and comprehending the governing specification for a particular project, and identifying any requirements that are unfamiliar or potentially problematic, are necessary before beginning a project.
Remove all visible grease and oil contamination in accordance with SSPC-SP1.
It is generally accepted that mechanical methods of surface preparation, such as abrasive blast cleaning, do not remove oil and grease contamination and may spread the contamination or even embed it into the substrate. Also, if a recyclable abrasive media is used (steel shot or steel grit), residual oil or grease on the surface can contaminate the abrasive media and spread the contamination to other surfaces.Therefore, SSPC-SP1, Solvent Cleaning, is a prerequisite to all methods of surface preparation identified by SSPC.
If a specification requires a near-white blast (SSPC-SP10/NACE 2) but does not explicitly require solvent cleaning, solvent cleaning is required nonetheless because the specification requires removal of visible grease and oil contamination before blast cleaning the surface. Using clean solvent, clean rags, and proper technique is important. Otherwise, a small, isolated patch of contamination can become widespread.
Remove all weld spatter and steel laminations prior to surface preparation. Laminations that are raised during surface preparation shall be ground smooth and the surface reroughened in these areas.
Weld spatter must be removed before surface preparation and painting, because it might become dislodged after painting, leaving a holiday (pinhole) in the coating film. Also, the coating will draw thin on the spatter, leading to corrosion in this area first. Laminations that protrude above the coating film can cause premature rusting and even under-film corrosion if left unchecked.
When laminations are ground smooth, some specifications require that the burnished area be reprofiled to ensure adhesion of the primer. This is particularly important with inorganic zinc-rich primers and metallizing (discussed later), which rely heavily on mechanical adhesion to the underlying steel.
All corners and edges of flanges shall be ground.
This specification language varies, ranging from qualitative requirements such as break the edge to more quantitative requirements (for example, 1/16-inch chamfer, 1/16-in. radius, 1/8-in. chamfer, or 1/8-in. radius). Coatings that shrink during the drying/ curing process tend to pull away from the edge, leaving a thin film of coating. It is not unusual for coated edges to show early failure because of this phenomenon. One solution is to reduce the sharpness of the edge by grinding, followed by application of a stripe coat on the edges, then a full coat on all surfaces.
A recent study conducted by the NSBA demonstrated that many inorganic zinc-rich coatings do not draw thin on edges because they contain a high zinc loading and shrink very little during the curing process.1Therefore, when using inorganic zinc primers, specifiers may choose to limit grinding requirements to simply breaking the corner, thereby reducing the number of labor-hours required for this operation.Abrasive blast cleaning shall be performed using a mixture of steel grit and steel shot.
Inorganic zinc-rich primer, a common shop primer, often is specified to be applied to a surface that has angular peaks, resulting in a more concentrated peak count per unit area (increased surface area). Steel shot, which produces rounded or peened surface roughness, does not meet this requirement.
Therefore, specifications often require both steel grit and steel shot to achieve the desired level of surface cleanliness and profile shape. Because the abrasive changes shape and size during repeated use, the challenge is to maintain a consistent mix throughout the project.
Abrasive blast cleaning shall be performed using centrifugal or open-nozzle systems.
Centrifugal blast machines literally throw or hurl abrasive media at the steel surfaces using centrifugal wheels located within a large housing. Steel members are conveyed or traversed through the machine, and the abrasive media impacts the surfaces and simultaneously removes mill scale and rust, generating a surface profile or anchor pattern. Many shops use these systems for preparing steel surfaces for painting.
Because these systems use recyclable abrasive media (steel shot, steel grit, or a mix of the two), the abrasive media must be checked routinely to ensure it is free of oil contamination. A vial test in which a mixture of abrasive and water is observed for an oil film on the water surface can confirm or refute the presence of oil.
Optimizing an open-nozzle abrasive blast cleaning system leads to higher productivity. The compressor or plant air source must be able to supply the proper volume of compressed air to the blast system to maintain ample pressure at the nozzle (100 pounds per square inch [PSI] for most expendable abrasives and higher for steel abrasives). According to Clemco Industries Corp., a supplier of blast cleaning equipment, to maintain 100-PSI blast nozzle air pressure using a No. 7 blast nozzle (7/16 in. diameter), a minimum of 254 cubic feet per minute (CFM) is required.2
Reducing the blast nozzle size or increasing the air volume are two methods for achieving the proper pressure at the nozzle. A hypodermic needle pressure gauge can measure blast nozzle pressure. Reductions in air pressure are caused by hose or pipe size restrictions and bends, blast hose length, coupling leaks, and so forth.
Venturi and straight-bore nozzles are two types of blast nozzle. Venturi nozzles are more productive than straight-bore nozzles, and the longer the nozzle, the faster the abrasive exits it. Generally, the larger the nozzle orifice, the more productive it is, provided the compressor or plant source provides a sufficient volume of air.
A compressed-air cleanliness test (blotter test) as described in ASTM D4285, Test Method for Indicating Oil or Water in Compressed Air, checks for oil or water contamination that may be generated by the compressor.
It may be advantageous for fabrication shops to combine centrifugal blast with an open-nozzle system to optimize productivity. For example, rather than sending a member through the blast machine to clean a defective area, an open-nozzle system can be maneuvered by a laborer for spot cleaning, especially in limited-access areas.
After abrasive blast cleaning, the surface cleanliness shall meet or exceed SSPC-SPxx.
Surface cleanliness is key to the success of a protective coating system. Active mill scale or rust beneath the coating film can reduce the life of the coating system.
Three abrasive blast cleaning cleanliness standards govern fabrication shop projects. They are SSPC-SP10/NACE 2 (Near-White Blast), SSPC-SP6/NACE 3 (Commercial Blast), and the least common, SSPC-SP5/NACE 1 (White Metal Blast). For all three standards, all mill scale and rust must be removed from the surfaces.
An SP6 level allows up to 33 percent staining from rust and mill scale, while SP10 allows up to 5 percent staining. An SP5 blast allows no staining to be present.
The amount of staining is assessed over 9-sq.-in. increments of the surface. This is based on the approximate size of the photographic standards, SSPC Visual Standard No. 1 (VIS 1-89), that are used to supplement the written definitions. VIS 1-89 provides a pictorial representation of these levels of cleanliness over four initial conditions—intact mill scale, flaking mill scale and rust, 100 percent rusted, and rusted and pitted surfaces.
Such photographs typically are used to determine the actual level of cleanliness achieved. However, these photographs rarely match the visual appearance of the initial condition of the steel or the level of cleanliness achieved. They are a useful tool for determining the difference between the various degrees of cleaning, but they cannot be held directly on the steel with an expectation of finding a precise match.
Therefore, it is advantageous to develop a project-specific visual standard (in conjunction with the VIS-1 standard) based on concurrence from quality assurance, quality control, and other parties, as appropriate. This project-specific cleanliness standard should be preserved (wrapped in plastic with a desiccant, or sealed in a clear urethane) in the event of disputes later in the project.
After abrasive blast cleaning, the surface profile shall be angular and 1.5 to 3 mils deep.
The surface profile anchors the coating system to the underlying steel. While an insufficient profile depth may result in adhesion problems, excessive surface profile can result in pinpoint rusting. Not only is the depth critical, but for some coating systems, the shape of the surface profile also can be important. Some coatings require an angular roughness or sharp peaks rather than a peened surface.
Three methods for measuring the depth of the profile are described in ASTM D4417, Field Measurement of Surface Profile of Blast Cleaned Steel. Method A is a visual comparator; Method B is a depth micrometer; and Method C is a replica tape. While all three methods are reliable, Method C is not only the most common, but also the most repeatable and reproducible, based on a study conducted by ASTM.
The number of surface profile measurements that should be made varies from project to project. If the governing specification does not require a specific number of readings, then common sense must prevail, because no guidance for the number of readings is provided in the standard.
It may be prudent to prepare a small section of the surface and confirm that the profile depth is adequate before initiating production blast cleaning runs. This way, modifications to the abrasive mix can be made if necessary to achieve the specified profile depth.All visible dust shall be removed by double blow down with clean, dry compressed air, or by vacuuming.
Application of a coating system over dust and debris can result in adhesion problems. A fine layer of dust often is held to the surface by static charge. This dust typically can be removed by blowing the surface off with compressed air or by vacuuming (blow-down is more common). Before blow-down, the compressed-air source should be tested for cleanliness from oil and water contamination according to the procedure described in ASTM D4285.
1. KTA-Tator Inc., "Edge/Corner Preparation of Steel Members and Its Effect on Zinc Rich Primer Performance," NSBA Item No. H053.
2. Blast Off 2, Your Guide to Safe and Efficient Abrasive Blasting (Washington, Mo.: Clemco Industries Corp., 1994).