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5 CHAPTER 1 INTRODUCTION AND RESEARCH APPROACH Zinc and aluminum have been used as steel coatings since tions (3). The excellent barrier properties and low rate of self- the early 1900s, with early application of thermally sprayed corrosion for the aluminum coating make it attractive for sea- metal coatings (TSMCs) to bridge structures in the 1930s (1). water exposure. Because TSMCs are porous, sealers are often TSMCs have been used widely in the European bridge indus- specified to reduce the porosity and improve the service life try, in the U.S. Navy, and on offshore oil exploration drill plat- of the coating. Common sealers include epoxies and vinyl forms for quite some time. TSMCs of zinc, aluminum, and coatings. Because of the advances in coating technology, part their alloys can offer substantial advantages when compared of this study was to investigate other sealer materials. with other coatings typically used to protect steel pilings. Organic coatings can fail prematurely as a result of corrosion progression from coating defects. Transportation, handling, Quality Assurance Requirements installation, or simple long-term material deterioration may cause these defects. TSMCs offer advantages in generally TSMC materials are sensitive to surface preparation and higher mechanical damage resistance, low self-corrosion rates, application conditions (4). Most specifications require sur- and the ability to provide steel corrosion control via cathodic face preparation and application conditions that meet the protection at coatings defects. The objective of this research Society for Protective Coatings Surface Preparation Specifi- was to develop a guide for highway agency personnel on the cation 5 (SSPC-SP-5), "White Metal Blast Cleaning," for selection and use of TSMCs for highway pilings that would application of a TSMC (5). This can be difficult to achieve in be suitable as an AASHTO reference. The Thermally Sprayed all conditions, especially if field coating is being considered Metal Coating Guide, containing all of the information gath- rather than shop coating. Other parameters, such as abrasive ered from the literature, industry research, and laboratory test- type for surface preparation, required profile range, and accep- ing, is the primary product of this research. tance environmental conditions, can affect porosity, adhesion, The extensive body of information on TSMCs, including and corrosion performance of the coating. existing guides, was researched so as not to repeat basic research and development work or conflict with industry standards where those standards are applicable. Applicable Damage Tolerance of the Coating portions of existing materials were used in the preparation of A key aspect of the coatings is their resistance to damage the Thermally Sprayed Metal Coating Guide. This study also in transportation, handling, and installation. Regardless of sought to resolve any issues that were unclear concerning the whether shop or field coatings are used, there is a tendency use of TSMCs on steel pilings. for there to be impact and flexure damage to the coatings. The performance of the alternative alloys and application conditions must be qualified in these regards. ISSUES OF CONCERN Alloy Selection RESEARCH PLAN There are several TSMC alloys available. Most commonly The research plan consisted of eight tasks, briefly used metals for the protection of steel are anodic to steel. This described below. eliminates the need for a completely pinhole-free barrier because the TSMC provides sacrificial protection to the steel substrate (2). Zinc, aluminum, and alloys of the two metals Task 1--Collect and Review Domestic and are thus favored for the protection of steel. Ideally, the alloy Foreign Literature and Information should have a very low self-corrosion rate and be an efficient and effective sacrificial anode. Aluminum TSMCs have been The review of existing literature and other information found to protect steel well under seawater immersion condi- included the following:
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6 · Existing specifications and guides on coatings for steel the information available to date and delineating areas requir- pilings and for metallizing in general, ing further research. They also asked that a second interim · Published research studies of metallized coating perfor- report be prepared. mance in immersion environments, · Interviews with suppliers of metallizing materials and application equipment and review of their literature, Task 5--Work Plan Execution · A visit by researchers to a coating fabrication shop where steel piles were being coated, The work plan execution consisted of conducting laboratory · Laboratory studies to perform certain basic testing on tests to provide additional information about the effectiveness TSMC, and of sealers, different sealer materials, surface preparation mate- · Interviews with coating applicators. rials, and the application variables on performance. The tests consisted of both standard laboratory tests and seawater expo- sure tests. Researchers also intended to perform field inspec- Task 2--Evaluate and Summarize the Literature tions of several TSMC structures, including some existing structures operated by the U.S. Army Corps of Engineers. The This consisted of the evaluation of information gathered in inability to access these structures without extensive dewa- Task 1 to determine the completeness and relevance of exist- tering procedures prevented this from occurring. ing information. From this evaluation, a detailed work plan was developed that would provide the information necessary for the guide to TSMCs. Task 6--Final Guide to TSMCs This task consisted of preparing the final guide on the Task 3--Develop a Detailed Experimental basis of the combined results of the literature search and Work Plan laboratory tests. The purpose of Task 3 was to develop an experimental work plan built around key issues related to TSMC perfor- Task 7--Long-Term Validation Plan mance. It was anticipated that the work plan would consist of laboratory tests designed to provide information to supple- The objective of this task was to develop a long-term ment the literature in the areas of surface preparation, seal- implementation plan for the use of TSMCs for pilings. The ers, and application parameters. plan was to have the guide to TSMCs accepted as an AASHTO guide by demonstrating the usefulness and appli- cability of the guide and TSMCs to state DOT officials. Task 4--Interim Report The interim report presented the results of Tasks 1 through Task 8--Final Report 3. As a result of the interim report, the panel determined that an expanded literature search was needed. The panel directed This report contains a discussion of all of the work that a draft guide to TSMCs be prepared incorporating all of described in Tasks 1 through 7.