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From page 260... ... 258 6.1 introduction This chapter is a best-practices guide and discussion for preventing corrosion of exposed structural steel for bridges and includes factors to be considered for design through installation, inspection, and maintenance. Various types of coatings, including painting, galvanizing, and metalizing, are discussed along with other methods of corrosion prevention that include the use of steels with higher resistance to corrosion, such as weathering steel. Read the entire page →
From page 261... ... 259 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES 6.2 deScriPtion oF methodS For corroSion Prevention This section discusses the corrosion process and describes the three main methods used to prevent corrosion of steel bridges. Read the entire page →
From page 262... ... 260 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 6.2.2 Description of Coating: Painting In a cost-effective, multicoat paint system, the primary purpose of the coating layer closest to the steel surface is to provide corrosion protection for the steel surface. Any special aesthetic considerations are accommodated in the subsequent coating layers, principally the topcoat. Read the entire page →
From page 263... ... 261 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES of SHRP 2 Project R19A indicated that all states responding use a system consisting of a zinc-rich primer. Read the entire page →
From page 264... ... 262 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE component is the solvent system used in the formulation that is a component of the wet film, but not the dry film, of the coating. 6.2.2.1.1 Vehicle Resin The vehicle resin (or binder) Read the entire page →
From page 265... ... 263 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES Industrial Coatings Pigment Vehicle Inhibitors (NV) Read the entire page →
From page 266... ... 264 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE The pigment in a coating may also provide corrosion protection. If used for this purpose, the pigmentation must be formulated into the primer layer (the layer adjacent to the steel substrate) Read the entire page →
From page 267... ... 265 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES Environmental Protection Agency (EPA) Read the entire page →
From page 268... ... 266 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE particles to form a solid film. The coalescing solvent then evaporates from the coating film. Read the entire page →
From page 269... ... 267 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES 6.2.2.3 Coating Systems Defined In many cases, coatings can be combined to create a coating system, which includes both the surface preparation and the application of one or more coating layers. Read the entire page →
From page 270... ... 268 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 6.2.2.3.4 Topcoat Function The topcoat, or finish coat, is the first line of defense in a corrosion-protection system. It must also be aesthetically compatible with the specifier's priorities and is often required to maintain color and gloss levels for long periods of time. Read the entire page →
From page 271... ... 269 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES 6.2.2.4.2 Airless Spray Airless spray has long been the most common method used for applying bridge coatings. Read the entire page →
From page 272... ... 270 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE "attracted" to the component or part having the opposing electrical charge, significantly reducing overspray and material usage. The coating, however, must be able to accept an electrical charge, and the addition of a polar solvent is sometimes required to inhibit the coating's natural resistivity. Read the entire page →
From page 273... ... 271 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES The Specifier's Guide continues: Below the name of each layer in [Figure 6.5] Read the entire page →
From page 274... ... 272 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Pickling. Dilute solutions of either hydrochloric or sulfuric acid remove surface rust and mill scale to provide a chemically clean metallic surface. Read the entire page →
From page 275... ... 273 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES influencing the thickness and appearance of the galvanized coating include chemical composition of the steel, steel surface condition, cold working of steel before galvanizing, bath temperature, bath immersion time, bath withdrawal rate, and steel cooling rate. Read the entire page →
From page 276... ... 274 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE steels that provide good galvanized coatings: Levels of carbon less than 0.25%, phosphorus less than 0.04%, or manganese less than 1.35% are beneficial; Silicon levels less than 0.04% or between 0.15% and 0.22% are desirable. Silicon may be present in many steels commonly galvanized even though it is not part of the controlled composition of the steel. Read the entire page →
From page 277... ... 275 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES ASTM, CSA, and AASHTO specifications and inspection standards for galvanizing recognize that variations occur in both coating thickness and compositions. Read the entire page →
From page 278... ... 276 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Thermally sprayed metal coating (TSMC) , referred to as metalizing, is the process of applying metallic zinc in wire form to clean steel by feeding it into a heated gun, where it is heated, melted, and spray applied by using combustion gases or auxiliary compressed air to provide ample velocity. Read the entire page →
From page 279... ... 277 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES 6.2.4.1.2 Wire-Arc Process Due to its high deposition rates, excellent adhesion, and cost-effectiveness, wire-arc spray is the preferred process for applying TSMCs to steel [bridges] Read the entire page →
From page 280... ... 278 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Again quoting from Ellor et al., TSMCs should always be applied to "white" metal (SSPC-SP 5/NACE # 1, [White Metal Blast Cleaning] Read the entire page →
From page 281... ... 279 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES cleanliness degrades or a change on performance (as per bend or tensile test) Read the entire page →
From page 282... ... 280 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE The materials and application costs of a TSMC system are higher than the cost of conventional liquid-applied coatings; however, the dominant factor may not be either material or application labor cost. Instead, the cost of taking the facility out of service, contractor mobilization, environmental constraints, and monitoring costs are often the major considerations of the total cost. Read the entire page →
From page 283... ... 281 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES high-pressure water jetting, or chemical strippers. Read the entire page →
From page 284... ... 282 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Weathering steels have been successfully used on coal hopper cars, buildings, and electric transmission towers and began appearing in bridges on a large scale in the mid 1960s (McEleney 2005) Read the entire page →
From page 285... ... 283 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES • Locations of continual or persistent rainfall (or wetting) Read the entire page →
From page 286... ... 284 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE McEleney further summarizes research performed by the TxDOT that recommends the following periodic measures: • Flush debris, dirt, and bird and bat droppings from the bridge structure. • Clear vegetation from pier and abutment areas to enhance air circulation. Read the entire page →
From page 287... ... 285 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES Detail exposure. Read the entire page →
From page 288... ... 286 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE As a result of this type of exposure, a surface that is normally expected to be dry is in effect an area of severe exposure. These areas are categorized by the SSPC as SSPC Category 2A (frequently wet by fresh water) Read the entire page →
From page 289... ... 287 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES 6.3.2.4 Splash-Zone Exposure When traffic travels beneath a steel overpass or through a steel truss or similar structure, an area of the steel above and often beside the traffic is bathed in water from the roadway. Read the entire page →
From page 290... ... 288 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Figure 6.14. Bearings below roadway joints are difficult to protect. Read the entire page →
From page 291... ... 289 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES During storms, salt water from nearby brackish water or from ice- or snow-melt can be blown onto steel surfaces. Read the entire page →
From page 292... ... 290 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE An FHWA-sponsored study (Appleman et al. Read the entire page →
From page 293... ... 291 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES 6.3.3.2 Prevention or Passivation of White Storage Stain The white deposit described in Section 6.3.3.1 is called wet storage stain. Read the entire page →
From page 294... ... 292 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE considered. Such systems are said to provide 1.5 to 2.5 times the service life of the sum of both galvanizing and paint if each is considered separately, and an aesthetically pleasing palette of colors is available. Read the entire page →
From page 295... ... 293 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES Phosphating is not recommended if a zinc-rich primer is going to be applied. Read the entire page →
From page 296... ... 294 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE The primary benefit of metalizing over other coating technologies is its durability and corrosion resistance, especially in salt-rich environments. For this reason, metalizing should be considered as an option for bridge structures in salt-rich environments or for areas or components of bridge structures that receive considerable exposure to salt and moisture from drainage and runoff. Read the entire page →
From page 297... ... 295 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES 6.3.5 factors Affecting Service Life of Steel Bridge Elements Specific to Weathering and noncorrosive Steels 6.3.5.1 Corrosion-Resistant Weathering Steel With the introduction of steel as a material of construction for bridges in the late 1800s, the industry has sought to find a form of steel that can overcome its most basic limitation: corrosion. Read the entire page →
From page 298... ... 296 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 6.4.2 general Strategies for Producing an Effective Corrosion-Protection System Regardless of the option selected from the list provided in Section 6.4.1, there are five major strategies that can result in an effective corrosion-protection system. These strategies are described in the following sections and listed here: 1. Read the entire page →
From page 299... ... 297 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES corrosion. Read the entire page →
From page 300... ... 298 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE A composite approach is often adopted on weathering steel bridges when girder ends are cleaned and coated. Figure 6.17 shows several structural details that are suitable for composite protection. Read the entire page →
From page 301... ... 299 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES M represents metalize, and P represents a paint layer. Read the entire page →
From page 302... ... 300 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Many states use Technical Advisory FHWA T5140.22, Uncoated Weathering Steel in Structures (FHWA 1989) , as a guidance document. Read the entire page →
From page 303... ... 301 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES This "corrosion-proof" steel is just beginning to be used on bridges. Read the entire page →
From page 304... ... 302 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 6.4.2.5.1 Surface Preparation The initial condition of the surface to be cleaned will determine the amount of work, time, and money required to achieve any particular degree of surface cleanliness. It is more difficult to remove contaminants from rusty steel and to remove mill scale from new steel than it is to wash surface film off steel in good condition. Read the entire page →
From page 305... ... 303 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES The existence and treatment of rust and particularly pack rust can make bridge repair so expensive that bridge demolition may appear to be a feasible option. Read the entire page →
From page 306... ... 304 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE The sun's ultraviolet light causes exposed organic coating to chalk to some extent. Chalk is the residue left after deterioration of the coating's organic binder on exposed surfaces. Read the entire page →
From page 307... ... 305 Chapter 6. Read the entire page →
From page 308... ... 306 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE From a cost perspective, the difference in cost between spot or zone cleaning and painting versus full removal is dramatic. According to industry sources, a typical leadpaint removal project in the northeastern part of the United States (in 2011) Read the entire page →
From page 309... ... 307 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES manufacturer according to product. Read the entire page →
From page 310... ... 308 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Maintenance overcoating is a process in which new coating is applied over existing coating. Based on industry knowledge and DOT survey information obtained for this project, the coating systems currently in use for this purpose include acrylic, calcium sulfonate, epoxy sealer–epoxy–urethane, epoxy sealer–urethane, polyester, and polyaspartic. Read the entire page →
From page 311... ... 309 Chapter 6. Read the entire page →
From page 312... ... 310 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE moisture. Most acrylic coatings are not suitable for immersion service or strong chemical environments. Read the entire page →
From page 313... ... 311 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES Acrylic urethanes are perhaps the most widely used corrosion-protection urethanes for atmospheric service on bridges. Read the entire page →
From page 314... ... 312 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 6.5.1.3.1 Types of Zinc-Rich Coatings Zinc-rich coatings can be subcategorized into two types: those with organic or with inorganic binders. The organic types are similar in many ways to the epoxy and urethane coating systems previously discussed, except that sufficient zinc dust pigment is added to provide galvanic protection. Read the entire page →
From page 315... ... 313 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES During the condensation phase of the reaction, the partially polymerized silicate combines with atmospheric moisture to eliminate alcohol, which vaporizes. Read the entire page →
From page 316... ... 314 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Metalizing spray application generates a smaller spray pattern, and application is normally slower than spray painting; accordingly, zinc thermal spray is generally more costly. FHWA funded two studies relative to zinc thermal spray that are reported in Publications FHWA-RD-91-060 (Kogler and Mott 1992) Read the entire page →
From page 317... ... 315 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES proven field history. Read the entire page →

From page 260...

... 258 6.1 introduction This chapter is a best-practices guide and discussion for preventing corrosion of exposed structural steel for bridges and includes factors to be considered for design through installation, inspection, and maintenance. Various types of coatings, including painting, galvanizing, and metalizing, are discussed along with other methods of corrosion prevention that include the use of steels with higher resistance to corrosion, such as weathering steel.