Corrosion Management of Elevated Lattice Galvanized Structures
Many countries, especially the western nations who had ramped up and developed immense industrial capacities during World War II, underwent huge expansions in their infrastructure after the end of the war, primarily in the 1950s, 1960s and into the 1970s. Not only did this include the building of roads, bridges, ports and railways; but expansive networks of high-voltage power distribution lines soon crisscrossed the countryside and soon after that, TV broadcasting and telecommunications networks were constructed and expanded. Large numbers of towers and masts were also erected to serve new radio distribution systems, radar facilities for airports, navigation aids and so on. Many of the structures for these networks were elevated lattice towers made from piece-small steel sections bolted together in the field. With very few exceptions these elevated assets were made from hot-dip galvanized carbon steel. Occasionally these were painted — and if so, this was typically done after erection — although most overpainting was performed for the purpose of civil aviation obstruction marking for structures proximate to airports or close to flight paths, rather than to augment the corrosion control aspects of the hot-dip galvanizing.
Given the protective system employed in these parts of the last century, it is not unreasonable to predict that a design life of around 50-to-60 years would have been a realistic expectation for untopcoated hot-dip galvanized steel in the more moderate urban and rural environments. However, with much of the population, industry and commerce of many of the larger countries located reasonably close to the sea and with considerable industrial developments since these assets were erected and commissioned, there are obviously many structures in environments that are now more severe than they were.
Galvanized steel in an atmospheric exposure has a finite life. The multiple layers (three in total, gamma, delta and zeta layers) of zinc/iron alloy, plus the mostly pure outer zinc layer (eta), that are produced when carbon steel is hot-dip galvanized, can provide durability terms ranging from a few years to multiple decades depending on the service exposure the steel is subjected to. Galvanizing has the ability to protect carbon steel due mainly to the principle of anodic protection (sometimes called galvanic protection). It also has some barrier properties that help keep ionic species, oxygen and the electrolyte from accessing the corrodible substrate. Over time, however, the zinc in the galvanizing is slowly consumed due to the concurrent mechanisms of galvanic action plus attack on the zinc itself by oxygen, water, CO2 and corrosives such as marine salt, acidic influences from urban atmospheres and other causes.
For the most part, these elevated lattice structures have survived particularly well, even though many are perhaps decades past the time when maintenance or individual member or structure replacement would have been expected if the initial 50-to-60 odd years of design life was realistic. However, even minor differences such as the following will affect the depletion or consumption rate of the galvanizing layers.
Location of the individual tower.
Corrosivity of the service environment.
Natural variation in the initial film build of the galvanizing.
Effects of orientation of the individual member.
For many networks, the multiple decades of a virtually maintenance-free life may give the asset owner a sense of security and apathy, which can mean that corrosion or section loss in bolts and members seems to appear quite suddenly and spread very quickly after initiation. Perhaps this is because after the initial dulling of the fresh galvanizing to a matte gray, nothing appears to visually change whether the galvanizing is one-quarter spent, at its half-life position or almost all consumed — there is no “gas-gage” needle that shows a progressively reducing residual life. Also, there is almost no chance that each member of a complex structure will reach its ultimate end of life at the same date.
Fig. 1: In corrosive environments, galvanized-only steel will inevitably deteriorate, but at different rates and consequences. Even in this photo, some steel (the main legs and compression members) would be best treated by site painting; other items could be changed for new; and other items (for example, fabricated ladders) might benefit best from refurbishment by re-galvanizing.
The process of hot-dip galvanizing does not result in the same thickness of zinc and zinc-iron alloy layers being deposited on all surfaces. The type of steel (i.e., its metallurgical formula); the weight of the steel; its section thickness; its surface roughness; the bath immersion time (with some steel grades); and the orientation of the member as it is withdrawn from the galvanizing bath can all affect the resulting zinc film build. Unlike some other forms of zinc-rich coating, for example, zinc silicate or zinc-rich epoxy, the lifespan of a galvanized item in a specific exposure condition is quite closely related to the original film build of the galvanizing layers.
Galvanizing heavier-weight sections of steel usually results in a greater film build of zinc and zinc-iron alloy layers, partly because thicker sections take longer to come up to the temperature of the galvanizing bath — typically around 455 to 460 C (851 to 860 F). Steel that is withdrawn vertically from the bath will have a lower film build than that withdrawn at a flat level or at a low incline because the molten zinc will run off to a greater extent before freezing. The rate of withdrawal from the bath will also affect zinc builds with faster extraction rates, usually resulting in a higher thickness of the eta layer. Metallurgical influences such as the silicone, molybdenum and phosphorous content of the steel can also affect the build and form of the galvanizing layers.
One other influence — and this affects bolts and fasteners almost exclusively — is any post-galvanizing operation before quenching. Bolts, nuts and studs are spun centrifugally immediately after they emerge from the bath to spin off the excess zinc from the threads while it is still liquid so that the mechanical and physical properties of the matching threads do not result in a nut that will not turn on the bolt. Interestingly, nuts for tower and engineering bolts are tapped with a slightly oversized thread after galvanizing, so there is actually no zinc on the internal thread of a galvanized nut.
As can be seen, even if all of the steel sections for a lattice tower are galvanized on the same day through the same galvanizing plant, different members and items (including bolts) will have different film builds of zinc.
While the initial zinc film build is a guide to the potential lifespan of galvanized steel — taken to be the time until red rusting (from the carbon steel substrate) commences — there are other influences such as member orientation once erected; the degree of shelter or exposure to the prevailing weather, rain and corrosives; and the time-of-wetness.
With surfaces that are sheltered to some degree from the prevailing rainfall, such as the underside of angle steel sections, channel steel with downward pointing legs or surfaces in the lee of the prevailing wind and rain direction, the time-of-wetness can be less, however; there is a tendency for corrosives and wind-deposited materials to remain longer on these sheltered surfaces because they are denied the beneficial effects of fresh water washing during rainfall events so they often suffer from an increased corrosion rate. In contrast, some locations on surfaces that face the weather can suffer from a high rate of galvanizing loss as the corrosion/erosion process alternates, resulting in enhanced zinc depletion rates. These different exposure conditions (and hence durability) can exist only centimeters from surfaces with an altogether different orientation on the same piece of steel.
Once bolted into position on a structure such as a lattice tower, the consumption of zinc as this metal reacts with atmospheric oxygen, CO2, water, airborne acidic influences, chloride and other ionic materials commences. However, it proceeds at varying rates depending on orientation and the other exposure-related influences such as time-of-wetness and the concentration of any corrosives. Because various parts or elements of the structure start with different zinc film builds and they lose zinc at different rates, nothing gets to the end point at the same time, the end point being a condition whereby the zinc is depleted to a degree where it is unable to galvanically protect the carbon steel and its barrier properties are degraded or spent.
Usually, the first of many hundreds of individual galvanized components to show signs of red rusting are the bolts because they start with a much lower film build of zinc and possibly retain some edge damage during torquing. Next to lose the galvanic influence of the zinc galvanizing are usually the lighter secondary and tertiary braces such as 50- and 60-mm (2- and 2.3-inch) angle sections and the hoops and flat strap bars on caged ladders. Those with the longest lifespan are, fortunately, the heavier angles of the main legs and primary structural members.
The author’s research has indicated that many asset owners have approached the issue of corroding lattice structures by considering two main options for maintenance: either prepare and paint all of the tower steelwork with an organic coating system; or replace the structure with a new tower. Some elaborate decision-making protocols have been developed by these owners and their advisors to assess when and how to carry out surface preparation and painting; or when to replace the structure so as to provide the lowest long-term cost, to optimize the net present value and give the best metric of risk minimization.
There are obviously some maintenance strategies or practices used that fit in-between these two binary choices. For many assets owners, this typically involves selected member or bolt replacement on a like-for-like basis, mostly performed in a more ad-hoc manner.
For some large facility owners in Australia — spanning power distributors, large switchyard operators (for example, at aluminum refineries) and telecommunications and broadcasting services — a more pragmatic model for elevated asset durability has been developed where whole tower painting or complete structure replacement are not the adopted maintenance actions on aged towers except in isolated cases.
Instead, the principal method of ensuring durability of the asset and prudent risk control involves an engineered combination of partial in-situ painting and some selected member replacement, but with different action drivers for these two methods. There is also a similar strategy for new structures, especially those in corrosive environments such as tropical marine conditions on offshore islands, where the conventional method of erecting regular hot-dip galvanized steel structures has proven time and again to deliver an unsatisfactory life span.
CARING FOR THE AGED
Fig. 2: Bolts are normally the first to show signs of corrosion. Painting bolts in this condition will prove mostly futile. Bolts can be much more corroded than those pictured here before they must be changed out.
If the choice is made to either fully repaint the tower/mast or replace it with a new structure, an on-site corrosion and condition assessment must be made by someone with suitable skills. There are tools and standards, such as SSPC-VIS 21, that can be used, although this pictorial guide depicts degrees or the extent of rusting as a scale or percentage, rather than the rate at which an item or member is losing steel section. These two factors are not the same and the author considers the latter metric to be more important. Nonetheless, the assessor must make a judgement about the average extent of corrosion, breakdown or the extent of section loss for the whole structure in order to categorize the asset.
A major facility owner in New Zealand2 has developed a system of “condition assessment” scores, ranging from CA100 down to CA20 (CA100 representing as-new condition, with scores cascading down with more deterioration and CA20 denoting replacement). In this system, this asset owner has determined that the optimum CA score to repaint is around CA40 or CA30, depending on whether the environment is very severe, severe, moderate or low. CA scores below this level suggest replacement of the structure.
Assigning a single CA score is not always easy to do because the structure being assessed is subject to a high order of sensitivity, especially when members or zones are close in condition (i.e., the degree of corrosion) to the crossover point between the candidate actions.
Given that all of the lattice members on the tower are bolted, and as it is usually the lighter sections such as secondary and tertiary braces, ladder cages and cleat plates that corrode first, these often only require the removal of a few bolts to facilitate their extraction and replacement; and if carefully performed, changing lighter members will not threaten the integrity of the structure during the time that the bolts are removed and replaced.
The strategy is to identify the steelwork members on a tower that are vulnerable to a loss of structural integrity from corrosion and separate these into the following hierarchy of members with slightly different approaches needed for maintenance.
Primary members such as the main legs and the deeply embedded steelwork, for example, items that almost cannot be replaced or removed and must be maintained in situ. This would include items in compression, principal members of and around portal frames, major platform support steelwork, and so on.
Next would be secondary members that could possibly be removed and replaced if required, but which are still critical to the integrity of the tower. These would include main braces and diagonals, cable runway supports and main antenna mounting frames.
Tertiary members would include the lighter braces, handrail angles, platform mesh or checker plate, ladder sections, ladder cages, lighter antenna mounting bars and braces.
Lastly would be bolts and fasteners including tower bolts, step bolts, feeder and wave guide cable clamps, and so on.
Primary members must be protected and maintained at a high level, not only because they are pivotal to the structural integrity of the tower but because there is little opportunity to extract and replace these if damaged by corrosion. This would include items that are very difficult or impossible to replace, such as the main leg angles; compression members; heavily embedded platform or bracing members; and the cable feeder runway stiles (on broadcasting/communication towers), as opposed to those that can more easily be removed by the extraction of a small number of bolts or fasteners.
Fig. 3: The effort and cost to clean, prepare and multi-coat corroded items such as these angle braces would not be a good financial investment. They should be left until their structural capacity is nearly spent and then changed out like-for-like.
Members that fall into the first category should be prepared and repainted in situ. This is because the effort to extract these members is likely to be too great. Often, if a member is in compression rather than in tension in its loaded state, extraction is more difficult or perhaps impossible. For example, it is very hard to change out a main leg angle on many towers or masts as this would require a complex leg splint to be installed, jacks be fitted to unload the bolts and the stability and integrity issues of the whole structure while the individual member is extracted and replaced must be considered. The site painting will usually entail carrying out cleaning using freshwater jet-washing, surface preparation most probably using wet abrasive blasting and then applying a high-performance (field-applied) coating system. Often, the sooner these items are addressed when zinc depletion is pending, the lower the intensity of surface preparation that will be required, specifically because after cleaning, the remnants of the galvanizing can remain to be part of the ongoing protective system.
Secondary members could potentially be extracted and replaced like-for-like, even if this happens to be quite an exercise. The feasibility of extracting a particular member would depend on how much other steelwork needs to be removed to get the affected item out. Therefore, some secondary members might require treating as-fitted while others could be removed. Simple, straight and un-fabricated members, such as some primary and most secondary angle braces, cleat plates, feeder clamp angles and handrail members, usually fall into this category. In some instances it may prove more feasible to consider assemblies or sub-assemblies as candidates for replacement as a whole. The cross arms on power towers that support the electrical conductors are an example of a group of members that could be considered for replacement as an assembly.
One important difference between these options is the degree of degradation that can be tolerated if the next action is one of the following.
To repaint the member over the existing galvanizing.
To abrasive blast and then paint it.
To remove and replace it.
If the next action is to simply paint any member or part of a structure, it follows that the galvanizing must still be functional prior to painting, because if not and/or there is any corrosion present, more extensive abrasive blasting or some other mechanical surface preparation will be required first, which puts this item into the next category and thus adds to the cost and complexity. This also means that no corrosion should be present on the member and no metal loss can have occurred.
For the other members of the tower where physically removing and replacing them with new, like-manufactured items is possible, some delay and a much worse state of degradation can be tolerated. As the next action for such members is replacement with a newly galvanized and freshly shop-painted item, these can be left until their structural functionality is threatened due to corrosion, or until their corroded appearance is no longer tolerable, whichever comes first.
The reasoning is that if the next maintenance activity is to blast and paint any member(s) on a structure, it follows that there is a small limit to how much structural strength each member can afford to lose by corrosion, because it is intended to stay in place after it’s painted. This implies that the galvanizing could be well-consumed and even red rust present, yet the item could potentially still be suitable to properly blast and paint. Whether it is economical to pursue this option is another matter. However, if a change-out is the next action, its replacement can be delayed for much longer because that member is to be scrapped and replaced with a structurally new and uncompromised item. In the latter case, the member could be very badly corroded, even to the point of perforation, but if deemed to be still structurally adequate, it doesn’t yet need replacement. In this instance, it is the remaining structural capability of the member that will determine when the change-out is needed and not the condition of the galvanizing, the paintability of the item or its appearance.
Selected items that can be removed, specifically some of the more complex or fabricated sections, can also be considered for extraction, refurbishment and re-installation. This could include ladder assemblies which can be abrasive blasted and re-galvanized prior to being reinstalled. For such fabricated items, this should be less expensive than refabricating new assemblies, even if it means making a temporary arrangement to allow for continued access, for example, with scaffold ladders. Likewise, some of the regular and repeated items that exist on the tower in multiple locations could have some of their number removed and replaced with new, and some refurbished by blasting and re-galvanizing before being swapped out for like items. Some of the brackets that restrain the feeder cables to the feeder runway, repeats of face diagonal angles and various handrails could fall into this category.
Bolts and fasteners should never be painted in an attempt to preserve their lives. New bolts are cheap when compared to the cost of providing access, manpower and materials to clean and repaint them aloft. The only exception would be where primary members or deeply embedded steelwork is being blasted and coated in situ, then it makes some sense to coat the bolts at the same time. Tower bolts and similar high-strength fasteners can lose a lot of metal from both the head and the nut due to corrosion, long before the bolt loses tension in the shank, which is, after all, where and how bolts work.
This makes the trigger point for action different for primary members, secondary members, tertiary members or other items and surfaces. For many painters and coating contractors familiar with maintenance painting on structures, this concept can be hard to grasp especially if the members are adjacent to each other and the painters are accustomed to painting all areas showing distress. Maintenance painters are usually conditioned to address all breakdowns as a default, so their inclination to tackle all corroded members must be restrained so that the work on the structure doesn’t expand by scope-creep. Contractors should be educated specifically on what items to address and how, as well as what members and surfaces they should ignore.
The potential for measurable savings using this approach comes from choosing the most appropriate treatment option — which in some instances means doing nothing.
Therefore, selective replacement of badly corroded lattice tower members with newly galvanized and shop-painted items, on a like-for-like basis, is a strategy that should be considered for use with site painting in order to achieve the longest lifespan at the lowest possible cost with the best risk profile for the structure.
Fig. 4: Fabricated items such as corroded ladder runs are candidates to be removed, reblasted and re-galvanized.
It should be understood that a straight piece of new angle section steel, with a few bolt holes and galvanized (then painted) after fabrication, is actually quite cheap when compared to the complex and expensive exercise of erecting scaffolding and containment and manually blasting and painting parts of an elevated structure. Using rope access methods and having the pre-fabricated replacement members and new bolts already on-site allows change-out to take place in a very short time.
The usual procedure is to survey the structure and identify members that are corroding through or below the galvanizing, indicating that they are losing section thickness and threatening structural integrity. This requires an experienced inspector to climb the tower or conduct an inspection from an alternative vantage point using a camera-equipped, remotely piloted vehicle (RPV) or a quadcopter (sometimes incorrectly referred to as a drone). The members are identified preferably on the original drawings or if this is not possible, the item is physically measured. A list of all of the steel required for the member change-out program is then assembled.
The process of cutting simple angle lengths from stock steel, punching the holes correctly, hard stamping the mark number, abrasive blasting and then hot-dip galvanizing each piece, is a very straightforward exercise that can be handled by a competent steel workshop or a steel merchant. Likewise, profile-cutting steel cleats or attachment plates can be performed by any number of steel workshops or profile-cutting operators working from an electronic drawing or CAD file. If a reasonable quantity is ordered, the cost per item should be quite economical.
One very attractive feature of adopting a selective member change-out program as described earlier is the flexibility and compatibility that it has alongside the other maintenance options.
Adopting a member change-out strategy can reduce the work scope and/or delay the expenditure of painting, particularly on bolted lattice structures. Adding to its attractiveness is that it allows a corroding member to deteriorate to a greater degree by remaining exposed until more of its galvanizing is spent. Because the trigger for its replacement is now purely its structural strength and not its appearance or the effort to blast and paint it, the member could feasibly last years longer as part of the structure.
TREATING NEW SECTIONS OR ASSEMBLIES
Fig. 5: Neither the angle section nor the bolts pictured here are candidates for site painting. It would be better to let these corrode until more of the residual life is spent and then replace them with new parts.
If new steelwork members or assemblies are to be fitted to a structure on a like-for-like basis it makes good sense to maximize their potential durability. The default procedure would be to fabricate and hot-dip galvanize these in the same manner as they were made originally. However, there are some initiatives that can be adopted that can significantly increase the life expectation of these members.
The following steps should be followed for all new steel items.
After fabrication, countersink all drilled or punched holes (just to break the edge), remove all weld spatter and deburr all guillotined or cut edges and corners to provide a minimum radius of around 2 mm. Achieving a P2 grade of surface finishing as per ISO 8501.33 is a good achievement.
Abrasive blast to at least Sa 2½ (AS 1627.44) or SSPC-SP 10/NACE No. 25 to produce a sharp, angular surface profile.
Hot-dip galvanize to a locally-recognized standard such as AS 46806; preferably quench in fresh water (with no chromic acid or similar white rust suppressing agents) or air cool.
Lightly whip abrasive blast the galvanized steel using a non-metallic abrasive to produce a fine angular surface profile.
Apply a high-performance tower paint vinyl coating system to a minimum DFT of about 180 microns (7 mils).
Suitably preparing the steel after fabrication as described can make quite a difference in the durability of a duplex (galvanizing-plus-paint) system. While forming a radius on edges and countersinking holes is not technically required for hot-dip galvanizing, if an organic paint system is to follow the galvanizing, these preparatory steps can prevent the thinning of paint build on sharp edges and corners, which is where the breakdown ultimately will commence.
The second step — that of abrasive blasting prior to galvanizing — is equally important. This is the simplest method of increasing the collective film build, and thus the life span, of hot-dip galvanizing. By increasing the effective surface area of the steel versus the planar area, the reactivity of the steel substrate with the molten zinc can be enhanced, allowing for the formation of higher builds of the zinc-iron alloy layers, specifically the delta and zeta layers, during the standard hot-dip operation. Testing has shown that blasting the steel to a near-white condition results in galvanizing film builds of around 200 to 230 microns (8 to 9 mils) versus about 120 to 140 microns (4.7 to 5.5 mils) for unblasted steel galvanized on the same dipping jig at the same time. An increase in galvanizing film build equates to a longer life potential, not always in a lineal manner, but significantly so.
After galvanizing, the steel is again abrasive blasted, this time at a reduced blasting pressure and using a sharp, angular, non-metallic abrasive (to avoid embedment and the chance of bi-metallic corrosion) to roughen the outer surface for paint adhesion. The aim is not to remove any zinc thickness, but simply to profile the eta layer. An angular profile with amplitude of around 25 microns (1 mil) is more than adequate, but as always, it is the shape of the profile that matters more than the height.
While vinyls are decades-old products, their performance, reliability and durability is legendary. Tower paint vinyls were mostly developed for field application to erected structures but they are equally useable as shop-applied materials. Well-formulated vinyls have a moisture vapor transmission (MVT) rate way below almost every other protective coating available, which means a much lower film build is required to provide a very low rate of permeability of oxygen and moisture through to the reactive substrate (the outer surface of the galvanizing, not the carbon steel beneath). Typically, three coats to a collective film build of about 180 microns is adequate. Being single-pack with a drying mechanism that is purely by solvent evaporation means that maximum recoat windows are measured in decades not hours. While multi-coat epoxy layers with a polyurethane topcoat have been used on towers as well, the best long-term performance, in the author’s opinion, has been delivered by vinyl products. Epoxies tend to ultimately crack along the section edges of angle steel members as they age and embrittle, which does not tend to occur with vinyls due to their lower film build, better long-term flexibility and higher tensile strength. Modern tower paint vinyls have volume solids of approximately 50 percent and often have some MIO (micaceous iron oxide) pigment incorporated which aids slip resistance when painters are working aloft, but also likely further lowers permeability. The vinyl primer usually needs to be formulated differently from the subsequent coating layers so it will tolerate the zinc and bare steel substrate; some acid modification is one way of ensuring high levels of adhesion and compatibility.
Implementing this blast/galvanize/blast/paint treatment on all replacement steel is a sound way to ensure that the life of new steel members is well beyond what would have been delivered had the same treatment been employed as was used when the towers were originally built. Performing the correct site touch-up of the shop-applied paint system after erection safeguards against compromises due to shipping, handling and erection damage.
Fig. 6: Some corrosion and loss of galvanizing is not critical if the next action is to remove and replace members. However, if repainting in situ is the chosen strategy, deterioration beyond about this degree will start to become more costly.
TREATMENT FOR SITE-PAINTED STEELWORK
The procedures and practices that have been developed over a number of decades to field-paint aged towers are well-proven. These apply whether entire structures are to be cleaned and painted or only selected pieces and items. While high-pressure fresh water jet-washing was once the preferred treatment method, with some added mineral abrasive to aid the removal of corrosion, staining and zinc corrosion products; many contractors now employ wet abrasive blasting. This treatment has the flexibility to dilute or remove soluble materials and other corrosives with the flexibility of adding abrasive to enhance the physical removal of all types of corrosion products and to profile the surface for paint application and adhesion. Catching the members that are to be field-prepared and painted in situ before the onset of steel corrosion, can significantly reduce the intensity of surface preparation and the cost, and also assist with the logistics.
Usually, if the structure has a previous coating system or if it is in an urban area or proximate to sensitive receptors, erecting a containment system around the members to be prepared and coated is required. Many of the older red and white civil aviation obstruction marking paints were lead- or chromium-based, or had these pigments as colorants. The cost, poor productivity, and logistics of erecting containments around elevated structures is one reason why replacing those members that can be extracted should be maximized.
Asset owners who have been maintaining aged elevated structures by field painting for many years have mostly gravitated to user-friendly, high-performance paint systems that have demonstrated their capability on a long list of structures in many exposure conditions over decades. Single-pack coating materials are preferred by many field painting contractors, especially for intermediate and topcoats. In addition to tower paint vinyls, some moisture-cured polyurethanes have a long and positive history, and although both single-pack and catalyzed zinc-rich coatings are used as primers, epoxy zinc-rich is favored.
The use of single-pack zinc-rich organics as multi-coat, single-product systems is not ideal. A good zinc-rich is certainly the best primer that can be applied to aged, atmospherically-exposed steel after an appropriate surface preparation, but the best durability and performance comes from overcoating the zinc-rich with a low-permeability, well-adhered polymeric topcoat system. A good zinc-rich epoxy over correctly prepared galvanizing or areas of spot bare steel, followed by solution vinyl topcoats surpasses all other contending systems for cost, logistics, performance, durability, recoatability and risk management.
When the nominated tower member items are being site cleaned, prepared and painted, the question arises as to where to terminate the treatment on affixed members. Ideally, splitting all connections would minimize unsealed joints, for example, diagonal braces that are bolt-fixed to leg members, potentially allowing all surfaces of a nominated item to be treated, but this is not really practical. A very good result can still be achieved without opening up these connections and running the repaint treatment for a distance of about 150 to 200 mm (6 to 8 inches) past the boundary of the nominated member along the affixed item. Careful attention to painting around the joint is needed, which may involve some selected spot gap filing with a compatible and paintable sealant. Moisture-cured polyurethane sealants have been found to be about the best in this situation, although a full paint treatment into and around the joint must be applied before a sealant is introduced because there is no corrosion protection in sealants. The cured sealant should then be stripe-coated with the topcoat.
LOOKING AFTER THE NEW-BORN
Australia’s largest telco owns or looks after some 27,000 elevated structures including towers, masts and poles. Some are new-build structures where services are being extended; others are new-builds where the existing structure has become unserviceable due to corrosion, fatigue or damage due to location, environment or extreme weather events. On one of the islands in the Torres Strait (between Australia and Papua New Guinea) a hot-dip galvanized mast that was erected about 12 years ago had suffered severe section loss due to corrosion in the harsh tropical marine environment and a cyclone to the extent that it required replacement. Erecting a new structure was the only feasible action; however, with such a short life span afforded by galvanizing alone in this environment, the need for a more appropriate solution drove a dedicated design effort.
The solution adopted was the design of a structure that would potentially allow each and every member to be extracted and replaced — including the main mast leg and primary compression members — if and when required. This necessitated changing the method of fixings at member-to-member connections so that an engineered splint strongback could be affixed to a main leg member, even while the tensile loads from guys are maintained, so that a safe extraction and reinstallation could be conducted. As well as these and related steps, the durability potential of the hot-dip galvanized steel was enhanced. This involved almost exactly the same steps as are detailed above for the replacement members on older structures including careful metal finishing after fabrication, abrasive blasting, hot-dip galvanizing, whip blasting, and application of a high-performance three-coat tower paint vinyl system.
Interestingly, the design team insisted that if a high-performance coating system was to be applied to the piece-small sections, a method would have to be conceived to ensure full electrical continuity. Minimal electrical resistance must exist across every member-to-member bolted joint. The solution adopted was to use circular, self-adhesive masking stickers that were applied to every fixing hole in every member right across the structure. The diameter of the stickers was slightly larger than the outside diameter (OD) of the flat washer to be used with the galvanized tower bolt so that both the bolt head and the nut/washer would be free of paint and in full electrical contact across the joint after torquing. The stickers were applied after whip blasting and prior to priming with the tower paint vinyl and were then removed after painting was completed. Very careful attention was paid to proper packing and stacking of painted steel sections to avoid or minimize damage from transit, shipping, handling and erection. The extra effort in designing, planning and execution of this project should result in achieving a life span between 200 and 250 percent of that of the original structure.
Other initiatives worth noting for highly corrosive sites are careful examination of tower/mast design and use of steel sections that are much more amenable to corrosion protection and durability, have fewer pockets and unsealed joints, and include engineered connections allowing for easier member extraction/replacement. Adopting tubular or solid round bar sections for the main tower legs will bypass the multiple sharp edges associated with angle members. Creating bespoke knuckle joints that connect in-line and multiple angled connectors will ease extraction and minimize clamped, ponding and unsealed joints. A suggestion has been proposed to make towers or masts with six legs (thus as a hexagon in cross section) as opposed to three or four legs, on the basis that an individual leg and other members could be more easily extracted; the cross bracing of a hexagon would be more effective; lighter individual sections could be used due to load sharing; and torsional resistance of the structure with eccentric antenna loads in high winds would be much improved.
Collectively, these initiatives could make future elevated structures much more durable and considerably longer lasting, as well as easing maintenance practices in the decades ahead.
As Seen in JPCL, June 2016.
ABOUT THE AUTHOR
Mark Dromgool is the managing director of KTA-Tator Australia Pty Ltd, based in Melbourne, Australia. He has been active in the protective coatings industry for 38 years. Dromgool’s experience includes 10 years as a coating application contractor and about seven working for two of the largest protective coating suppliers in Australia and New Zealand. In 1994, he formed KTA-Tator Australia as a protective coating engineering, inspection and consulting company.
A long-standing member of SSPC and NACE, Dromgool is former president of the Blast Cleaning and Coating Association (BCCA) of NSW. He has written and published many papers on coatings and linings and has lectured widely at local and international conferences. In 1996 and again in 2007, he was the recipient of the JPCL Editor’s Award for papers entitled “Maximizing the Life of Tank Linings,” and “Epoxy Linings – Solvent-Free But Not Problem-Free,” respectively. In 2006, Dromgool was awarded the John Hartley Award for Excellence by the BCCA of NSW.
Dromgool has qualifications as a mechanical engineer; is an ACA-certified Coatings Inspector; a NACE-accredited Protective Coating Specialist; an SSPC-accredited Protective Coatings Specialist and a NACE-certified Coating Inspector – Level 3.
SSPC-VIS 2, ”Standard Method of Evaluating Degree of Rusting on Painted Steel Surfaces“; SSPC 00-08; Pittsburgh, PA.