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12 3 COATING MATERIALS AND SELECTION 3.1 Alloy Selection TSMCs are used for the protection of the exposed surfaces of iron and steel components used in various corrosive environments. Long-term protection in excess of 20 years in both industrial and marine exposures has been documented. Zinc, aluminum, and zinc/aluminum alloy coatings provide sacrificial corrosion protection to a steel substrate, even when areas of the substrate are exposed to the corrosive environment. The relatively low corrosion rates of these coatings, in combination with the sacrificial corrosion protection that they offer, make them suitable for use in such harsh environments. Zinc has a higher electrochemical activity than aluminum has and thereby provides a higher level of cathodic protection to a steel substrate than does aluminum. Aluminum, with its lower electrochemical activity and adherent oxide film formation, provides a lower level of cathodic protection to a steel substrate. Electrical conductivity and pH contribute to the corrosivity in immersion environments. Due to the relatively high electrical conductivity of natural seawater, aluminum is the recommended thermal spray coating material for this environment. The higher galvanic interaction of zinc with the steel corresponds to a higher consumption rate in seawater immersion. In a freshwater environment, where the electrical conductivity is lower, zinc or 8515 weight percent (wt%) zinc/aluminum alloy coating will provide a better balance between cathodic protection and barrier protection and is the recommended thermal spray coating material. 3.1.1 Types of Exposure and Suitable Alloys Foreknowledge of the environmental stresses to which the protective coating system will be exposed is critical for the proper selection of the coating system. This is true of both paint and thermally sprayed coating systems. Exposure environments typically encompass one or more of the following environmental stresses: extremes of temperature, high levels of humidity, complete or partial or intermittent immersion, extremes of pH, solvent exposure, wet/dry cycling, thermal cycling, ultraviolet exposure, impact and abrasion, cavitation/erosion, and special exposures. The service environment is the single most important consideration in the selection of a coating system. Table 2 provides a summary of thermal spray metal coatings selection recommendations. Table 2 lists several environmental categories. It should be recognized that, particularly in atmospheric environments, a single category might not represent a particular environment. Moisture, wind direction, solar radiation, and local pollution effects (e.g., groundwater runoff discharge pipes and industrial drainage) can create microclimates that will affect the performance of materials and coatings. These conditions must be recognized in the selection of the proper coating. 3.1.1.1 Atmospheric high humidity. High humidity is often accompanied by condensation, which is considered to approximate the severity of freshwater immersion. An 8515 wt% zinc/ aluminum wire-arc spray coating to a thickness of 12 mils (305 m) is the recommended thermal spray system for high-humidity environments. Typically, high-performance paint

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13 TABLE 2 Thermally sprayed metal coating selection guide for 20- to 40-year life Environment Coating Thickness Sealer* mils [m] Atmospheric Rural Zinc or zinc-aluminum 68 [150200] No Industrial Zinc or zinc-aluminum 1215 [305380] Yes Marine Aluminum or zinc- 1215 [305380] No aluminum Immersion Freshwater Zinc-aluminum 1215 [305380] Yes Brackish Water Aluminum 1215 [305380] No Seawater Aluminum 1215 [305380] No Alternate Wet-Dry Freshwater Zinc-aluminum 1012 [250305] Yes Seawater Aluminum 1215 [305380] Yes Abrasion Zinc-aluminum 1416 [355405] Yes Condensation Zinc or zinc-aluminum 1012 [250305] Yes * See Section 6, "Sealer Selection and Application," for further information. systems, such as the epoxy and vinyl systems, are specified for high-humidity applications. Because paint systems are generally less costly to apply, they are more likely to be used for these types of exposures. However, the 8515 wt% zinc/aluminum thermal spray system should have a longer service life than paint coatings for this application. 3.1.1.2 Wet/dry cycling. A zone of alternating wet and dry is generally the most corrosive zone due to macrocell corrosion. This type of exposure is found in splash zones and tidal zones. Most TSMC systems will provide adequate protection under such conditions. Sealing and topcoating of the TSMC is generally recommended for such exposures. 3.1.1.3 Immersion. Immersion exposures range from immersion in deionized water to immersion in natural waters, including freshwater and seawater. Ionic content and pH contribute to the corrosivity of immersion environments. Typical sealers and topcoats are vinyl paints and coal tar epoxy coatings. Several epoxy and vinyl systems are appropriate for various immersion exposures depending on whether the water is fresh or salt and the degree of impact and abrasion. Epoxy systems are preferred for saltwater exposures, whereas the vinyl systems are generally preferred for freshwater exposures, especially where the level of impact and abrasion is significant. Seawater. Wire-arc sprayed aluminum to a coating thickness of 10 mils (250 m) is recommended for seawater immersion. Aluminum thermal spray has been used extensively by the offshore oil industry to protect immersed and splash zone platform components from corrosion. Aluminum thermal spray is thought to perform better in seawater immersion without an organic sealer and paint topcoat, and some specifications, such as U.S. Army COE CEGS-09971, recommend not using a sealer in seawater immersion. Wire-arc sprayed aluminum is the recommended thermal spray system for this application.

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14 Freshwater. Wire-arc sprayed 8515 wt% zinc/aluminum to a coating thickness of 12 mils (300 m) is recommended for freshwater immersion. These systems can be used either with or without sealers and topcoats. The 8515 wt% zinc/aluminum system combines the superior corrosion resistance of zinc and the improved impact and abrasion resistance of aluminum. Seal coats and paint topcoats may be used to add a further degree of protection to the TSMC systems used in freshwater immersion, but their use is not considered an absolute necessity. 3.1.1.4 Ultraviolet exposure. Resistance to ultraviolet (UV) radiationinduced degradation is an important aspect of coating performance. All thermally sprayed coatings are essentially unaffected by UV radiation. Organic sealers and topcoats used over TSMCs will be affected the same way as any other paint materials of the same type. Organic paint coatings are affected by UV radiation to varying degrees. Depending on the coating resin and pigmentation types, UV degradation may result in loss of gloss, color fading, film embrittlement, and chalking. Certain paints, including silicone and aliphatic polyurethane coatings, may exhibit superior UV resistance. Some coatings, including most epoxies and alkyds, have fairly poor UV resistance. The properties of a specific coating must be considered when selecting a coating that must have UV light resistance. 3.1.1.5 Impact and abrasion. Impact and abrasion are significant environmental stresses for any coating system. Abrasion is primarily a wear-induced failure caused by contact of a solid material with the coating. Examples include foot and vehicular traffic on floor coatings, ropes attached to mooring bitts, sand particles suspended in water, and floating ice. When objects of significant mass and velocity move in a direction normal to the surface as opposed to parallel, as in the case of abrasion, the stress is considered to be an impact. Abrasion damage occurs over a period of time, whereas impact damage is typically immediate and discrete. Many coating properties are important to the resistance of impact and abrasion including adhesion to the substrate, cohesion within the coating layers, toughness, ductility, and hardness. Thermally sprayed coatings of zinc, aluminum, and their alloys are very impact resistant. Zinc metallizing has only fair abrasion resistance in immersion applications because the coating forms a weakly adherent layer of zinc oxide. This layer is readily abraded, which exposes more zinc, which in turn oxidizes and is abraded; 8515 wt% zinc/aluminum is more impact/abrasion resistant than pure zinc or pure aluminum. 3.2 Concerns Related to Performance of the TSMC 3.2.1 Limits on Surface Preparation Coating selection may be limited by the degree or type of surface preparation that can be achieved on a particular structure or structural component. Because of physical configuration or proximity to other sensitive equipment or machinery, it may not always be possible to abrasive blast a steel substrate. In such cases, other types of surface preparation, such as hand tool or power tool cleaning, may be necessary, which, in turn, may place limits on the type of coatings that may be used. In some cases, it may be necessary to remove the old coating by means other than abrasive blasting, such as power tools, high-pressure water jetting, or chemical strippers. These surface preparation methods do not impart the surface profile that is needed for some types of coatings to perform well. In the case of thermally sprayed

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15 coatings, a high degree of surface preparation is essential. This kind of preparation can only be achieved by abrasive blasting using a good-quality, properly sized angular blast media. Thermal spraying should never be selected for applications in which it is not possible to provide the highest quality surface preparation. 3.2.2 Ease of Application Coating selection may also be limited by the ability of the applicator to access the surfaces to be coated. This is usually the result of the physical configuration or design of the structure. Items of limited access such as back-to-back angles, cavities, blind holes, and crevices may be difficult, if not impossible, to coat. Most items that can be coated by paint spray application may also be coated by thermal spray. Both methods require about the same amount of access area for hoses, maneuvering, and standoff distance. As a rule of thumb, if access to the surface allows proper blast cleaning, then thermal spray application is feasible. TSMCs perform best when sprayed in a direction normal (i.e., 90 degrees) to the surface and within a particular range of standoff distances from the substrate. Application at angles of less than 45 degrees to the vertical is not recommended. Maximum and minimum standoff distances depend on the material being applied, the manufacturer, and the type of thermal spray equipment used. If the standoff distance and spray angle cannot be maintained within the specified range, hand application of a paint coating may be necessary. 3.2.3 Regulatory Requirements The use of paint coatings is regulated in terms of the types and amounts of solvents or volatile organic compounds (VOCs) they contain. Certain types of solvents, such as water and acetone, are exempt from these regulations because they do not contribute to the formation of photochemical pollution or smog in the lower atmosphere. Regulations vary by geographic location and by industry. Different rules apply for architectural and industrial maintenance painting, marine painting, and miscellaneous metal parts painting. The specifier should consult with local and state officials to determine which rules, if any, affect the proposed coating work. There are no VOC emissions associated with the use of TSMCs, and their use is not regulated by any such rule. TSMCs offer an excellent VOC-compliant alternative to paint coatings for many applications. However, the sealers and topcoats recommended for thermal spray systems are not exempt from VOC-type regulations. The thermal spray coatings will often perform just as well without the sealers and topcoats, which can therefore be omitted for reasons of compliance with air pollution regulations. It should also be noted that there are typically low-VOC paint coating alternatives for most applications. The relative merits of these products should be weighed against those of the zero-VOC TSMC systems. Thermal spraying of metals produces airborne metal dusts and fumes. Finely divided solids or other particulate accumulations are an explosion hazard. Fine metal particles might damage some types of equipment, such as electronics and bearings. Metal fumes can pose a health hazard (e.g., "metal fume fever"). Proper containment and ventilation may be required in order to reduce these risks. See Section 2 for a discussion of appropriate safety and environmental concerns.

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16 3.2.4 Field Conditions The conditions under which the coating work will be performed are another important consideration in coating selection. Certain atmospheric conditions, including high levels of humidity and condensation, precipitation, high winds, and extreme cold or heat, place severe limitations on any type of coating work. 3.2.4.1 Moisture. Moisture on the surface should always be avoided to the greatest extent possible. Certain types of paint are more tolerant of small amounts of water on the surface than others and should be specified for work where such conditions cannot be avoided. Thermally sprayed coatings should never be applied if moisture is present on the surface. 3.2.4.2 High winds. High winds may affect the types of surface preparation and coating application methods that are practical for a given job. High winds will tend to carry surface preparation debris and paint overspray over longer distances. This problem can be avoided by using methods other than open abrasive blasting and spray application of paints. 3.2.4.3 High atmospheric temperatures. For sealers and finish coats, the pot life of multi-component catalyzed coatings such as epoxies can be greatly reduced by high atmospheric temperatures. High ambient air and surface temperatures can also adversely affect paint application and the subsequent performance of the coating; for example, vinyl paints are prone to dry spray at high temperatures. Most paints should not be applied below a certain minimum temperature because they will not cure or dry. Most epoxy paints should not be applied when the ambient and substrate temperatures are below 50F (10C); however, there are some specialized epoxy coatings that can be applied at temperatures as low as 20F (-7C). Latex coatings should never be applied when temperatures are expected to fall below 50F (10C) during application and drying. Vinyl paints can be applied at quite low temperatures compared with most paints. Vinyl application at 32F (0C) can be performed with relative ease. There are generally no upper or lower ambient or surface temperature limits on the application of TSMCs, although there are practical limits at which personnel can properly perform their tasks. 3.2.5 Maintainability The future maintainability of a coating system should be considered by the specifier. Some protective coatings are easier to maintain than others. The specifier should also be cognizant of how maintenance painting is normally achieved, whether by contractor or with in-house labor. In-house labor is usually sufficient for low-technology processes requiring minimal training and equipment. For example, "touch-up" painting with brushes or rollers of paints exposed to the atmosphere is readily accomplished with in-house labor. More sophisticated, dedicated, in-house paint crews can carry out more complicated work including abrasive blasting and spray application of paints for immersion service. TSMC and maintenance, because of their specialized nature and relatively high equipment costs, are ordinarily best accomplished by outside contractors. TSMCs are more difficult to repair than are most paint coatings. The ease of spot repair of TSMCs approximates that of the vinyl paint systems. As with the vinyls, special care must be taken to properly feather the edges of the blast-repaired areas without causing the adjacent coating to disbond or lift from the surface. Because of the difficulty of effecting appropriate repairs, TSMC systems, like the vinyls, are generally kept in service until total recoating is needed.

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17 3.3 Cost 3.3.1 Cost Considerations Coating systems are cost-effective only to the extent that they provide the requisite corrosion protection. Cost should be considered only after the identification of coating materials that will perform in the exposure environment. Given that a number of alternative coating systems may perform in a given application, the next consideration is the cost of the coating job. Ideally, protective coating systems will be selected based on life-cycle cost rather than simple installed cost. However, given the realities of budgets, this approach is not always practical. Therefore, coating systems are sometimes selected on the basis of first or installed cost. Because TSMC systems are almost always more expensive to install than paint systems for a given application, they are often passed over, when, in fact, they can have significantly lower life-cycle costs than paint systems. Additional information on the cost of TSMCs and how to perform cost calculations is provided below. 3.3.2 Cost Analysis 3.3.2.1 Cost of materials and application. The cost of a TSMC system in terms of materials and application is higher than the cost of conventional liquid-applied coatings; however, the major cost of a coatings project is not the materials and application. The dominant factor in coating rework is not material and application cost, but rather it is the cost of taking the facility out of service, contractor mobilization, environmental constraints (e.g., containment and disposal), and monitoring. In most complex coating rework, the actual cost of materials and application is less than 20 percent of the total process. Thus, if one is able to gain a three- fold life extension by using TSMC, the process can pay for itself. 3.3.2.2 Life-cycle cost. Whenever possible, coating selection should be based on life-cycle cost. In reality, the engineer must balance competing needs and may not always be able to specify the least expensive coating on a life-cycle cost basis. Because of their somewhat higher first cost, TSMCs are often overlooked. To calculate life-cycle costs, the installed cost of the coating system and its expected service life must be known. Life-cycle costs for coating systems are readily compared by calculating the average equivalent annual cost (AEAC) or present worth (PW) for each system under consideration. The present worth can be calculated using the following relationship: M (1 + r1 ) M (1 + r1 ) M (1 + r1 ) P1 P2 Pn PW = F + + +L+ (1 + r ) (1 + r ) (1 + r ) P1 P2 Pn where F = cost of initial coating system, M = cost of maintenance in year Pn, r = interest rate, r1 = inflation rate, n = number of maintenance actions required to achieve life of structure, P1 = number of years to first maintenance,

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18 P2 = number of years to second maintenance, and Pn = number of years to last maintenance. (For more information, refer to E. L. Grant, W. G. Irerson, and R. S. Leavenworth, Principles of Engineering Economy, 7th ed., John Wiley & Sons, New York, 1982 and ASM Handbook, Volume 13--Corrosion, 9th ed., ASM International, Materials Park, OH, 1987, pp. 369374). 3.3.2.3 Installed cost. The basic installed cost of a TSMC system is calculated by adding the costs of surface preparation, materials, consumables, and thermal spray application. The cost of surface preparation is well known. The cost of time, materials, and consumables may be calculated using the elements: (1) Surface area to be coated (SA). SA = length width (2) Volume (V) of coating material needed to coat the area. V = SA coating thickness (3) Weight of the material to be deposited (Wd). The density (D) of the applied coating will be less than that of the feedstock material. A good assumption is that the applied coating will be about 90 percent of the density of the feedstock material. Densities are as follows: aluminum--0.10 lb/in.3 (2.70 g/cm3), zinc--26 lb/in.3 (7.13 g/cm3), and 8515 wt% zinc/aluminum wire--0.207 lb/in.3 (5.87 g/cm3). Wd = V 0.9D (4) Weight (W) of material used. Deposition efficiencies (DE) of zinc, aluminum, and 8515 wt% zinc/aluminum, applied by wire-arc spray, are estimated to be 60 to 65 percent, 70 to 75 percent, and 70 to 75 percent, respectively. W = Wd /DE (5) Spray time (T). Spray rates (SRs) for wire-arc sprayed materials vary depending on wire diameter and current settings. Table 5 (in Section 5) provides typical spray rates for materials and wire sizes. T = W/SR (6) Electricity or oxygen and fuel gas consumption (C). Typical consumption rates (CRs) for electricity, fuel gas, and oxygen are available from equipment manufacturers. C = CR x T (7) Cost of materials (CM). CM = W cost per unit weight

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19 (8) Cost of application (CA). CA = T unit labor cost (9) Cost of consumables (CC). CC = T unit cost of consumable (10) Total cost (TC) of the TSMC. TC = CM + CA + CC. 3.3.2.4 Factors that increase cost. Factors that increase the cost of thermal spray and other coating jobs include the cost of containment, inspection, rigging, mobilization, waste storage and disposal, and worker health and safety. These can have a significant effect on coating cost and might be independent of the type of coating system being considered. They should be considered when comparing annual cost or PW of different systems. 3.3.2.5 Cost-effectiveness of TSMCs. In 1997, the Federal Highway Administration (FHWA) compared the performance of a number of coating systems, including paints and thermal spray. Coating life expectancies were estimated based on their performance in an aggressive marine atmospheric exposure and a mildly corrosive environment. Installed and life-cycle costs were calculated for each coating system for each exposure. Average equivalent annual costs were calculated based on a 60-year structure life. For the more severe marine atmospheric exposure, TSMCs of aluminum, zinc, and 8515 wt% zinc/ aluminum alloy were the most cost-effective coatings. For the less severe mildly corrosive atmospheric exposure, thermal spray was no more or less cost-effective than other coating options. Report FHWA-RD-96-058 provides the details of the study. For example, the costs used were the following: Costs per square foot Item Epoxy Annual Zinc/Aluminum Mastic/ Escalation @ 6 mils Polyurethane Rate, % Surface preparation (labor + material), $ 1.25 $ 0.60 3.94 SSPC-SP 10 Coating application (labor) $ 2.50 $ 0.30 4.00 Coating material $ 1.50 $ 0.42 1.91 Containment and air filtration system $ 2.00 $ 2.00 3.00 Rigging $ 0.50 $ 0.50 3.00 Mobilization $ 0.50 $ 0.50 3.00 Hazardous waste storage and disposal $ 2.50 $ 2.50 6.00 Worker health and safety $ 2.00 $ 2.00 4.00

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20 While the surface preparation, coating material, and application costs were initially higher for the zinc/aluminum TSMC, the savings came in the number of times the coating must be applied. The TSMC requires two applications within the 60-year life of the structure (one initial coat and one maintenance recoat), whereas the epoxy/polyurethane requires eight coats within the 60-year life of the structure (one initial coat and seven maintenance recoats). This worked out to a total present value for the epoxy mastic/polyurethane system of $21.37/ft2 ($1.99/m2) compared with $18.38/ft2 ($1.71/m2) for the zinc/aluminum TSMC. 3.4 Design Proper design can improve the performance of coatings by removing some features that tax the coatings' ability to protect the structure. NACE International Recommended Practice RP0178, "Fabrication Details, Surface Finish Requirements, and Proper Design Considerations for Tanks and Vessels To Be Lined for Immersion Service," while it addresses tanks, has some pertinent recommendations that are applicable to piles. These include the following: Avoid dissimilar metals in direct contact with each other. Examples to be avoided are aluminum or stainless steel fasteners connected directly to steel without dielectric bushings and washers and aluminum conduit and straps connected directly to steel without dielectric insulation. Where welding is used, use a continuous weld bead. Remove weld spatter and weld metal irregularities that could interfere with obtaining an adequate coating film thickness. Edges should be beveled or rounded to break up sharp corners. Optimally, edges should have a minimum radius of 1/8 in. (3 mm) and preferably 1/4 in. (6 mm). Allow drainage in the case of horizontal members by installing drain holes or orienting the member such that it does not hold water. Make the drain hole large enough to reduce the likelihood of becoming clogged. Avoid lap joints (faying surfaces) where possible because these do not permit coating of the surfaces within the joint. If unavoidable, seal weld lap joints. Avoid pockets where the abrasive blast equipment cannot effectively clean and thermal spray cannot effectively coat the surface. Avoid back-to-back angles because the interior facing surfaces cannot be cleaned and protected. Alternately use a T-section or other shape that allows open access to all surfaces. Seal weld back-to-back angles if they must be used. Ensure that all corrosion-prone surfaces are accessible for applying TSMCs both during initial fabrication and during the lifetime of the structure. 3.5 Areas Requiring Special Treatment 3.5.1 Portions Below the Mudline The TSMCs discussed in this guide are generally applicable to areas above the water, in tidal and splash zones, and below the waterline. Figure 1 shows typical corrosion losses with low- alloy and carbon steel. Significant corrosion occurs in the splash, tidal, and immersion zones and in the zone a few feet below the mudline, with relatively little corrosion deeper into the mud. This is because there is little oxygen below the mudline to support aggressive

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21 Figure 1. Corrosion of sheeting piling at various locations (Source: AASHTO Highway Structures Design Book, Vol. 1, 1986, p. 10.49). corrosion. The application of a TSMC below the mudline is not necessary; however, coating a few feet below the mudline will allow for changes in bottom elevation with time and situations in which the piles are not driven to their intended depth. However, if this is considered, it should be kept in mind that macrocell galvanic corrosion reactions occur below the waterline and mudline. These might result in reduced TSMC life below the water line. Consideration should be given to applying TSMC on the whole pile to eliminate this galvanic corrosion. 3.5.2 Faying Surfaces Faying surfaces are surfaces that are in contact with each other and joined by bolting or other means. Faying surfaces should be seal welded, or the TSMC should be applied to the faying surfaces before joining. TSMCs can be applied to slip critical surfaces. 3.5.3 Sheet Pile Interlock Joints Figure 2 illustrates a typical sheet pile interlock joint. The surfaces within the joint are of relatively close tolerance and if coated with the full thickness of the TSMC, the combined thickness of both joints might prevent the piles from being joined together. The coating must be applied to a lesser thickness within the joint. The coating thickness should be no more than 3 to 5 mils (76 to127 m) unless experience indicates that a thicker coating will work. In some cases, sections of sheet pile might be already joined, and it will not be possible to put any coating on the inside of the joint. In this case, the coating should be as evenly applied as possible across the joint interface.

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22 Figure 2. Schematic of an interlock joint. 3.6 Effect of Steel Composition Small variations in steel hardness caused by alloy content will have little effect on the adhesion or performance of TSMCs; however, it is important to ensure that the appropriate surface profile is achieved prior to applying the TSMC. Hardened areas, such as the heat- affected zones of welds, might require special care to ensure an adequate surface profile. Flame-cut areas will require special care to ensure that an adequate profile is achieved. 3.7 Effect of Holidays on Protective Ability of Coating Coating holidays will affect the ability of the metallic coating to protect the steel substrate. Small holidays, such as pinholes and narrow scratches, will be protected by the galvanic action of the coating. Large holidays, those exceeding 1/2 in. (1.3 cm), in immersed areas should be protected by galvanic interaction with the coating. Large holidays in the atmospheric, splash, and tidal zones will not be fully protected by the coating. All holidays will eventually result in deterioration of the TSMC, leading to corrosion of the steel. The consequence of the corrosion (e.g., small hole leading to seepage through the piling, a large corroded area resulting in structural weakening of the pile, or aesthetic requirements) should be taken into consideration when determining whether to repair the coating on existing structures. All visible holidays on new structures should be repaired using appropriate materials prior to the pile being placed in service. 3.8 Thermally Sprayed Metal Wire Storage Temperature, humidity, and dew point cause problems if thermal spray feedstock is not properly stored. All of the active metal wires will oxidize if exposed to moisture. The oxide film can cause feed problems in both flame and arc equipment. Extreme temperature changes may also cause zinc and zinc/aluminum alloy wire to recrystallize and become brittle. Thermal spray wires should be securely sealed and protected from moisture intrusion to prevent oxidation of the material until they are to be used.