Many root cause analysis reports on steel bridge failures mention gusset plate corrosion as a likely cause for the failure. While gusset plates are a relatively small component, corrosion reduces the load carrying capacity of the structure. (To learn why, read Effect of Corrosion on a Material's Tensile Strength and Ductility.) In this article we will examine the role of gusset plates, why they are prone to corrosion, the corrosion types likely to be encountered, and best prevention practices.
What is a Gusset Plate?
A gusset plate is a thin piece of steel that is used to affix independent members of a structure to each other or to a beam to aid alignment. Gusset plates are used in joint, bend or otherwise disjointed structural locations that require additional support to withstand stresses. They typically are flat dish-like triangular or rectangular units that are fastened to beams with bolts or welds, and are critical load bearing members of metallic structure such as bridges and buildings.
The Role of Moisture and Pollutants in Gusset Plate Corrosion
Gusset plates and their associated fastener assemblies (e.g., washers, welds, bolt heads) can become corroded due to moisture and contaminants that accumulate on their intricate surfaces. Steel bridges in dry climates typically have a lifespan of more than five decades, while bridges in wet climates may only last 15 to 20 years.
Chlorides, dust, sulfur compounds and other airborne pollutants accelerate the corrosion process. The chlorides are carried by the wind from oceans and seas to interior regions. The sulfur compounds that are prevalent in urban and industrial areas are the result of the combustion of oil and coal, which emits sulfur dioxide gas (SO2) along with other pollutants. When water condenses on the steel structures, the SO2 reacts with the water to produce corrosive sulfuric acid (H2SO4).
In addition to naturally occurring rain, fog and snowfall, water splashed by vehicles onto the bridge is an important consideration because it can reach a height of around 6 meters (20 feet). Corrosive deicing substances used to treat roadways and bridges are another factor. The severity of the corrosion varies greatly, depending upon the average rainfall, humidity levels and environmental contaminants.
Ways to Prevent Gusset Plate Corrosion
There are several ways to prevent or reduce the likelihood of gusset plate corrosion. Usually most if not all of the methods must be employed to obtain the best results.
1. Use Corrosion Resistant Materials
Gusset plates manufactured from weathering steel (steel having silicon, copper, nickel, phosphorus and chromium) have limited corrosion resistance. They are unsuitable in locations with abutting vegetation, heavy persistent moisture or abundant rain or snowfall, especially when there is concentrated air pollution.
Conversely, corrosion resistant steels (ASTM A1010) with 10.5% to 12.5% chromium can slow the corrosion rate and reduce the cost of coating materials and their application.
Titanium alloy gusset plates have superior corrosion resistance. However, because titanium alloys are expensive, these gusset plates are often only used for critical applications in environments that are highly corrosive. It is important to separate titanium alloy components (e.g., gusset plates and bolts) from the steel structure with plastic washers or other nonconductive materials to prevent galvanic corrosion, also known as bimetallic corrosion.
2. Perform Surface Preparation for Subsequent Coatings
Gusset plate assemblies can be complicated because there can be hundreds of fasteners and welded joints within a large structure. Inspecting, cleaning and adequately preparing these surfaces can be more challenging than coating of the main structure itself. These activities are labor-intensive, expensive and time-consuming, but are critical for the structure’s longevity.
As far as welded joints are concerned, once the welding is finished, the weld and joint areas should be inspected for profile and cleanliness. Rough surfaces, poor quality weld starts and stops, weld spatter and other surface defects must be rectified through grinding and water blast cleaning or bristle brush cleaning at the site. (For more on this topic see Causes and Prevention of Corrosion on Welded Joints.)
Bolted connections must be prepared for coating. Lubricants that were applied to the threaded surfaces and loose rust must be removed. Hot dip galvanized surfaces damaged in transit may be prepared by power wire-brushing or water blast cleaning. Power brushing, if employed, should not excessively smooth the hot dip galvanized surfaces because the subsequent coating will not adhere to the surface. Liquid etching solutions are frequently used on galvanized gusset plate surfaces to increase the surface roughness.
Coating adhesion is also adversely affected by nut lubricants that spread to the surfaces of washers and bolts during tightening. Pressurized water cleaning or solvent cleaning methods may be used to remove the nut lubricating wax.
3. Use a Coating Method Appropriate for the Structure
Gusset plates with bolt assemblies present a complex configuration in very small areas for the coating technician, who has to apply a uniform film of different layers to ensure effective corrosion prevention. Surface tension impairs the flow of coatings into the crevices between steel surfaces and washer surfaces. Each bolt assembly has its own crevices. A stripe coat applied with a brush is the only commonsense solution to the challenge posed by surface tension. The coating technician forces the coating into crevices and around the edges with a brush and by thinning the coating if required.
Exercising diligence during the brush application is the key to providing an effective and durable coating. However, if there are large bolt assemblies then a combination of a spray coating immediately followed by a brush coating (to force the coating into crevices) is appropriate. Stripe coating of gusset plate bolt assemblies prevents corrosion of irregular surfaces with their innumerable crevices and edges. Caution is needed to prevent excessively thick coatings when spray coating, because excessive thickness can result in coating holidays, in the form of cracks, blisters and peeled coatings.
If spraying is done from a single direction then parts of the surfaces will remain uncoated. Brush application can make up for the deficiencies.
4. Choose an Effective Coating Formulation
Stripe coats of mastic type epoxy coatings, applied in multiple coats, are advantageous for galvanized gusset bolt assemblies because epoxies have excellent adhesion on these surfaces.
Top coats of polyurethane are also often used because of their improved mechanical properties and chemical and water resistance. Polyurethane top coats are also preferred in situations where abrasion from windblown sand and severe impact loading from heavy trucks and strong cyclonic winds is expected. Acrylic urethane top coats are appropriate when gloss and color retention are necessary. Top coat epoxies suffer from the disadvantage that the top film may be susceptible to deterioration from sunlight.
5. Prevent Uniform Corrosion
Uniform corrosion affects the surfaces of gusset plate assemblies in a uniform manner. This can be prevented or minimized by following proven surface preparation practices and diligent coating practices.
Modern structures such as steel bridges are built with prefabricated parts that have had an initial coating applied in a controlled environment with advanced coating systems. If galvanized truss gusset plates are used, then any coatings on welded pieces and fastener connections that were damaged during transit will require special attention at the work site. In corrosion prone areas such as wet regions, the galvanized surfaces of the entire structure will require carefully planned surface preparation and an additional organic coating such as an epoxy or polyurethane. The corrosion protection efforts for gusset plate assemblies should be at least at the same level of diligence, if not better, as the corrosion prevention efforts for other structural parts.
6. Prevent Pitting Corrosion
Pitting corrosion is clearly identified by corrosion damage to a pinhole point or a very small area, which creates numerous cavities in the gusset plates and fasteners. (To learn more about pitting corrosion read All About Pitting Corrosion.) Chlorides and other aggressive chemicals in the environment can cause pitting corrosion, as can any inclusions in the substrate material.
The solution lies in the selection of substrate materials that are relatively free from inclusions and that have very good resistance to pitting corrosion in the expected service environment. ASTM G48 alloys can be considered as well as chemical resistant coatings such as polyurethane to help prevent pitting corrosion.
7. Prevent Crevice Corrosion
Crevice corrosion of fasteners and gusset plate assemblies occurs within small gaps and cracks present on their metallic surfaces. Crevices collect and maintain stagnant moisture because of capillary action. The crevice initially acts as a moisture repository that gradually causes the electrochemical reactions that result in corrosion. Crevices may be present in the confined space between washers and bolt heads, washers and gusset plates, nuts and washers, and threaded joints. Entrapped construction debris can contribute to the onset of crevice corrosion. Damaged coatings can also lead to deterioration.
Minimizing the use of washers reduces the risk of crevice corrosion by decreasing the number of joint faces in the fasteners. Ensuring smooth surfaces free from cracks and crevices is another method to minimize this type of damage. The recommended method to coat crevices was discussed in the section on uniform corrosion.
At the design stage, the risk of crevice corrosion can be minimized by eliminating the bolted joints, which are replaced by butt welds between the gusset plate and connecting structural members such as beams and trusses, although this may not be feasible in every situation. Stagnant moisture conditions can be lessened by designing in proper drainage. The use of molybdenum as an alloying element in steel and polytetrafluoroethylene (PTFE) gaskets can also minimize crevice corrosion in the bolted connections.
8. Prevent Galvanic Corrosion
Galvanic corrosion of gusset plates with fastener connections may occur due to the use of two dissimilar metals in the galvanic series. The use of plastic washers may be feasible to overcome this problem because they effectively isolate the gusset plate from the main structural elements such as beams and trusses. Organic coatings such as epoxies and polyurethanes can provide protection against galvanic corrosion. Care must be taken at the material selection stage for rivets, nuts, bolts, gusset plates and washers to avoid connecting dissimilar metals.
9. Prevent Stress Corrosion Cracking
Stress corrosion cracking (SCC) of fasteners and gusset plates is caused by a combination of corrosive agents (e.g. chlorides, hydrogen sulfide) in the environment, stress concentration at weak corroded sections, notches and edges, and the use of metal susceptible to cracking due to stress corrosion.
The solution lies in minimizing any stress concentration at the design stage and choosing materials that will not crack in the specific environment.
10. Inspect and Maintain to Preserve the Coating’s Integrity
The gusset plate coating should be routinely inspected (e.g., biannually) to assess its integrity. First, the gusset plate assemblies and their coatings are visually inspected, and the thickness of the coating and the steel sections are measured. Steel thickness is measured by using ultrasonic test equipment to identify any deterioration. If remediation is required, the surface should be prepared for recoating by removing peeled coatings and accumulated rust and dirt. Then, a new coating is applied by brush or a combination of spray and brush where the area to be recoated is large.
Prevention of gusset plate corrosion must begin at the design and material selection stage. Costly titanium alloy gusset plates may become a cost-effective choice for highly corrosive environments. Galvanized steel gusset plates may be coated with epoxies and polyurethane top coatings to improve the longevity of the structures. Steps should be taken to address the types of corrosion that are inherent to gusset plates and the structures they support. Biannual inspection and restoration of coatings can ensure the continued integrity of the structure.