Rather than describe a failure that has already occurred, this article addresses key steps to follow when painting bolted connections in order to prevent failures from occurring by focusing on surface preparation and painting practices that have been used successfully on bolted bridge connections such as splice plates and gusset connections for new construction projects or new steel additions to existing bridges. Larger connection surfaces are typically shop blast-cleaned and shop primed only with an inorganic or organic zinc-rich primer and then bolted together in the field using hot-dipped-galvanized (HDG) or mechanically galvanized bolt assemblies consisting of a nut, bolt and washer. Smaller shop-primed connections are sometimes bolted in the shop prior to shipping. Field finish painting of bolted connections is typically accomplished using an epoxy intermediate coat followed by a urethane finish coat. However, some bridge owners specify the application of an organic zinc-rich primer on mechanically galvanized bolt assemblies prior to the epoxy intermediate and urethane finish. Because galvanized bolt assemblies (as opposed to plain black bolts) are typically used in conjunction with steel connections primed with zinc-rich primer in the shop, that is our focus here.
As seen in December 2016 Issue of JPCL
While the paintable surface area of bolted bridge connections is small compared to the whole structure, on larger structures, individual connections can contain dozens or even hundreds of closely spaced bolt assemblies. Because the bolt assemblies have a more complex configuration and are spaced so closely, cleaning and painting these surfaces presents challenges that require more time, effort and expense to properly complete, versus other bridge surfaces. Proper painting of bridge connections is critical because these are the surfaces where premature coating problems often begin. Standard practices used by various bridge owners and recommended by paint manufacturers are addressed in this article.
SURFACE PREPARATION OF GALVANIZED BOLT ASSEMBLIES
Galvanized bolt assemblies are typically used in conjunction with new steel that has been shop-primed with a zinc-rich primer. The primary challenges encountered with the field surface preparation of galvanized bolt assemblies for painting includes preparing areas of rusting or damaged galvanizing and the removal of nut lubricants.
PREPARING RUST AND DAMAGED GALVANIZING ON BOLT ASSEMBLIES
Rusting and damaged galvanizing on bolt assemblies is most often a result of the field tensioning/tightening of the bolts by other crafts. The socket attached to the torque wrench typically damages (i.e. smears or burnishes) or removes the galvanizing on the outside corners (edges) of the bolt heads and nuts, and the bolt threads are sometimes damaged as the bolt assembly can also be damaged — usually smeared or burnished. As an example, Figure 1 shows damage and rusting along the two bottom rows of galvanized nuts that exceeds what is typically observed on bridge projects. In this case, the galvanizing was damaged by using too large of a socket when tightening was performed. Care must be taken to use the proper tools and techniques during installation.
SSPC-PA Guide 13/AASHTO NSBA Steel Bridge Collaboration S.8.1: “Guide Specification for Application of Coating Systems with Zinc-Rich Primers to Steel Bridges” provides guidance for cleaning and painting bolted bridge connections.
For removal of rusting and preparation of damaged galvanizing, SSPC-PA Guide 13 recommends cleaning in accordance with SSPC-SP 1, “Solvent Cleaning,” SSPC-SP 2, “Hand Tool Cleaning,” SSPC-SP 3, “Power Tool Cleaning,” or by pressurized water cleaning in accordance with SSPC-SP 12 (waterjetting), however, SSPC-SP 12 has been discontinued and replaced by four individual waterjetting standards, SSPC-SP/NACE WJ-1 through WJ-4. WJ-4, “Waterjet Cleaning of Metals – Light Cleaning” would apply in this case.
These cleaning methods only require the removal of loose rust, galvanizing or zinc-rich primer from the damaged or rusted area. Field experience has shown that more intensive cleaning to remove all spot-rust results in little, if any, improvement in paint system performance and the extra time, expense and effort required to accomplish removal is not warranted. Inquiries to bridge owners and coating manufacturers confirmed that their standard practices and recommendations for field cleaning of bolted connections are consistent with the methods discussed previously.
Also, when preparing galvanized bolt assemblies for painting, care must be taken that the galvanized surface is not overly smoothed or polished by the cleaning process — for example, by aggressive power wire-brushing. Such surfaces are not as easily “wetted-out” by coatings and do not provide sufficient surface roughness or a “tooth” to which coatings can adhere.
Because the surface of a mechanically galvanized bolt assembly is typically rougher and more porous than HDG, solvent cleaning and light hand-brushing with a wire brush or stiff composite bristle brush is typically effective for preparing the surface for painting.
In addition, some expect that the paint adhesion on galvanized bolt assemblies should be equal to that system on the steel, which is often not the case. On one bridge project, concerns arose because field knife adhesion testing on HDG bolts caused forced separation of the system from the galvanizing, whereas the same paint on the surrounding inorganic zinc-primed steel could not be removed to the substrate. Further, laboratory adhesion testing on numerous painted test panels with galvanized bolts installed also showed that adhesion was weaker on the galvanized bolt assemblies even though the performance was excellent. Before field adhesion concerns are raised, the expectation should be known. Attempting to compare paint adhesion values on galvanized (particularly on smooth HDG) versus inorganic zinc-primed steel is likely not valid. If there is a desire to improve the adhesion of the coating to the HDG, cleaning/etching solutions are available that can be applied to the surface before painting. Some of these solutions require post-rinsing and neutralization (usually with clean potable water) but “no-rinse” formulations are also available.
Unless extensive paint damage or rusting is present on the bridge connection, complete abrasive blast-cleaning and repainting of bridge connections is typically not recommended. Blast-cleaning can damage or remove the superior corrosion protection properties provided by the galvanizing on the bolt assemblies and the shop-applied zinc-rich primer on the connection plate. Blast-cleaning often mandates the construction of a containment around individual bridge connections and the process also increases the potential for over-blast damage of adjacent painted bridge surfaces that must be repaired. All of this can add cost and prolong the project unnecessarily.
REMOVAL OF NUT LUBRICANTS FROM BOLT ASSEMBLIES
Nut lubricants are typically formulations of wax-based material pigmented with a blue or green dye (Fig. 2). The lubricant is shop-applied to nuts to reduce friction during field tensioning of bolts and is most commonly applied to the entire nut and internal threads. As the nut is tightened the lubricant often spreads onto the surface of the bolt, washer and connection plate. Because the lubricant is typically wax-based, if excessive amounts are present, it can be a deterrent to coating adhesion so some amount of removal is required before any paint is applied. Research into the cleaning methods recommended by paint manufacturers and specified by bridge owners indicates that solvent cleaning with methyl ethyl ketone (MEK), chemical cleaning with alkaline cleaners, or pressurized water cleaning can be effective for removing nut lubricants. SSPC-PA Guide 13 also addresses the removal of lubricants prior to painting and indicates that household cleaners such as ammonia are effective. However, issues with the removal of nut lubricants have occurred because the colored dye can stain the galvanizing on the bolt assembly. Further, complete removal of the dye from the many crevices present on the bolt assembly can be difficult. The visible presence of the dye after cleaning raises concerns as to whether wax is still present and as a result, some specifiers require that all dye be removed as a precaution. In the author’s experience, the dye is easier to remove from HDG bolt assemblies than from mechanically galvanized assemblies.
With reference to dye removal, SSPC-PA Guide 13, Section 5.4.4 and C5.4.4 (the commentary for that section) states the following.
“5.4.4 … The lubricant on exposed surfaces of installed nuts must be removed before painting. The identity of solvents and methods needed to remove the lubricant is obtained from the galvanizer and provided to the General Contractor, shop and field painters, the Owner and other interested parties, with a description of the cleanliness necessary for coating adhesion. Perform periodic evaluation to ensure that the lubricant and excess dye are adequately removed.”
“C5.4.4 … Any dye color remaining on the galvanized nuts after weathering or the required surface preparation is not believed to be detrimental to subsequent coating performance or appearance. A white cloth wipe test with no color transfer can be used to confirm that all lubricant and non-absorbed dye has been removed, leaving only the residual ‘stain’ on the surface.”
Because SSPC-PA Guide 13 does not address the amount of residual stain that is acceptable, a project-specific field mock-up demonstrating cleaning and painting of the bolt assembly is recommended. The mock-up should be performed before production work begins in the presence of all concerned parties. A mock-up allows for the expectations to be identified in advance and the means and methods for achieving the cleanliness to be demonstrated. Adhesion tests can also be conducted to verify the acceptability of the cleaning. Adhesion testing by probing with a razor knife similar to the methods described in ASTM D6677, “Standard Test Method for Evaluating Adhesion by Knife” can be used. An advantage of the field mock-up is that the amount of absorbed dye staining permitted to remain on the bolt assembly surfaces and in the crevices can be photographed and used as a job reference standard.
The author was involved with a project that required essentially every trace of dye to be removed. This process was extremely labor intensive and costly, which led to project disputes and delays. The ultimate resolution was to establish the cleanliness expectations through mock-ups involving different levels of dye removal. After progressive removal of the dye, the coatings were applied and adhesion testing was conducted.
Figure 3 shows photographs from the testing that depict both pre-cleaning and post-cleaning conditions of bolt assemblies. For this test, solvent cleaning with MEK was most effective for removing the blue-colored nut lubricant. On other projects, foaming glass cleaners containing ammonia and foaming alkaline-type household cleaners have also been effective in removing nut lubricants. The tests also indicated that supplementing the use of the wet cleaners with a non-scratch scouring pad or a composite bristle brush improved lubricant removal. The mock-up resolved the problem in this case, but only after considerable time and money had been spent. It should have been done in advance.
ADDITONAL NUT LUBRICANT OPTIONS
Other options for nut lubricants that can minimize field cleaning requirements include purchasing nuts with the lubricant applied only to the nut threads and mating face rather than to the entire nut. This significantly decreases the amount of nut lubricant that must be cleaned prior to painting. Referring again to Figure 1, it is apparent that the nuts pictured were incorrectly installed. The lubricated face of the nut should be mating against the washer so that no lubricant is visible. However, for many of the nuts, the unlubricated face was installed against the washer and the lubricated face exposed. Workers installing the nuts must be properly instructed and trained. In this case, good decisions had been made to address the lubricant through product selection, but the project fell apart when the nuts were installed incorrectly.
While specialized lubricant is initially more costly than standard wax-type lubricants applied to the entire nut, those costs may be offset by the reduced cleaning effort that is necessary in the field.
PAINT APPLICATION ON BOLT ASSEMBLIES
The configuration of bolt assemblies is a combination of outside edges, inside and outside angles, bolt threads and crevices that can be challenging to paint. Edges, bolt threads and inside/outside angles are more difficult to coat with a uniform paint film because of their small surface area and complex configurations. Surface tension forces during the drying/curing process can actually pull many applied coatings away from edges and angles, resulting in a thinner protective layer on these surfaces. Surface tension also inhibits the flow-out of paint into the many crevices that are present on bolt assemblies such as crevices between the bolt head and the steel surface, the washer and the steel surface, the nut and the washer, and the nut and bolt threads.
The most common and practical solution to provide additional corrosion protection and coating coverage is to brush apply a stripe coat. The brushing action helps the paint to overcome surface tension by forcing the paint into the crevices and other irregularities. In addition, thinning of the paint often facilitates flow-out, wetting and ease of application of the stripe coat material. In many instances, particularly with large, concentrated bolt assembly patterns, it may be advantageous (more productive and less time-consuming) to spray-apply the paint to the connection followed by immediate back-brushing to work the paint into the bolt assembly. Some prefer conventional spray over airless spray for stripe coating because equipment adjustments can be made that give the applicator more control when painting the complex shapes and surfaces. The advantages of stripe coating are described in SSPC-PA Guide 11, “Protecting Edges, Crevices and Irregular Steel Surfaces by Stripe Coating.”
On several projects, inadequate coating coverage on certain faces of the nuts would have been prevented had brush stripe coating been specified. In these cases, the bolt assemblies were only spray painted. Because the spray application was not performed from all directions, some faces (typically the same nut faces on all connections) were not painted. For example, if the nuts are only sprayed from 2:00 through 6:00 and up to 10:00, the coverage is only good on those surfaces. The remaining surfaces from 10:00 through 12:00 to 2:00 are completely missed.
Thinner addition should always be in accordance with the manufacturer’s recommendations and the stripe coat thickness should be kept as thin as possible, applying just enough material to thoroughly coat all bolt assembly surfaces. It is the opinion of some that a bolted assembly is not properly coated until all crevices between the various assembly components have been visibly bridged and sealed with paint. However, if all exposed surfaces are painted, attempting to bridge crevices with paint is not necessary and can result in the application of an individual layer that is too thick. Excessively thick coatings are subject to increased internal curing stress (i.e. contractive shrinkage stress) that can lead to premature cracking and peeling, and/or solvent retention that can result in the formation of blisters, pinholes and voids in the paint film. In addition, if the bridge structure is subject to vibration and movement, excessive paint thickness around bolt assemblies may be more prone to cracking.
While some specifications require an organic (epoxy) zinc-rich primer to be used for the stripe coat on galvanized bolt assemblies, the use of epoxy intermediate coat material for this purpose is often preferred, particularly surface-tolerant, epoxy mastic-type products. The reason is that organic (epoxy) zinc-rich primers, which are highly filled with metallic zinc dust, may lack the ability to sufficiently wet-out the galvanized surface to develop adequate adhesion. On the other hand, epoxies are typically more user-friendly, more surface-tolerant, and flow-out, wet-out and adhere better to the galvanized fasteners than do epoxy zinc-rich primers. In some instances, multiple stripe coats are specified to improve coating performance on complex surfaces. Multiple stripe coats are advantageous if the thickness of an individual application is not excessive.
Field cleaning and painting of bridge connections can be more labor intensive, time consuming and costly to perform compared to painting any other bridge surfaces, but proper cleaning and painting of the connections is critical in order to prevent premature corrosion, poor adhesion and cracking or disbonding of the coating.
About the Author:
James D. Machen is a senior coatings consultant with KTATator, Inc., a coatings consulting engineering firm and distributor of inspection instruments, where he has been employed for over 20 years. Machen is an SSPC-certified Protective Coatings Specialist, a NACEcertified Coatings Inspector Level 3 (Peer Review) and a Level II Inspector in accordance with ASTM D4537. He performs coating failure analyses, coating system recommendations, specification preparation and major project management for a variety of clients in the transportation, water and waste, power generation, chemical processing and marine industries. He is a graduate of The Pennsylvania State University.