The contract to corrosion protect the tower’s spire was awarded to Drytec Trans-Canada by the New York and New Jersey Port Authority, which owned the building at the time. From the outset it was clear to all involved that the sheer size of the workpiece posed an enormous challenge. The original plan to galvanize the high-strength steel was quickly dismissed, in view of the immense size of the spire components, the varying thicknesses and requirement for double dipping. Painting was also not an option, as the binders in the paint would have interfered with satellite communications. A pure zinc coating was chosen as this could provide the required conductivity, as well as achieve the required coating thickness of between 250 and 500 μm.
The search for a process
The next step was to find a process able to fulfil the coating thickness requirements and to accommodate the spire’s highly complex geometry. Ultimately the Drytec team presented flame spray technology to the building owners and then spent nearly a year testing the equipment.
Oerlikon Metco’s Key Account Manager was also contacted at this time, because Drytec was confronted with another challenge – the required coating bond strength of 4.8 MPa. Drytec, moreover, had to submit a process description to the New York City Inspector precisely documenting each step of the coating process, and describing the measures to prevent overspray, i.e. excess spray not applied to the workpiece.
“I don’t want to have to climb up there with a can of paint in a few years time to touch up the structure!”
The City Inspector to Oerlikon Metco‘s Key Account Manager.
Metco solution fulfilled all requirements
Shortly afterwards, Oerlikon Metco presented the capabilities of the Metco 16E combustion spray gun with zinc wire to the New York City Inspector at Drytec headquarters – and won him over completely. An adhesion strength of 8.3 MPa on all spire parts was actually achieved, thanks to the outstanding combination of spray equipment and spray material. The required coating specifications, SSPC SP10 and SSPC-CS23, the norm for thermal spraying (ISO 2063) and, of course, the customer’s required coating thickness, could all thereby be fulfilled.
Seventeen sections to objective
Having found the right process, the work could start. The lower six spire sections were too bulky and too heavy to be transported, so Drytec rented a hangar near the port to perform the final metallization. The individual sections were delivered in three parts and welded together on site. The application of the zinc corrosion coating required the simultaneous use of three wire combustion spray guns. Drytec kept six of these guns on hand at all times to enable any necessary maintenance to be performed without disruption to the coating process.
Seventeen sections, some comprising nested tubes, had to be zinc-coated. For some, hard-to-reach areas, an extension module – the Oerlikon Metco XT6-18T 45-degree-tilt angle spray gun – was needed.
Especially challenging was the narrow available time window between abrasive grit blasting and the application of the zinc coating, due to the rapid corrosion of highstrength steel: the coating had to be applied within four hours after grit blasting. Drytec used a total of 11,500 kg of zinc wire to coat the 139.6 metre high spire.
The Key Account Manager likes to recall the City Inspector’s reaction to his presentation: “He said to me: I don’t want to have to climb up there with a can of paint in a few years time to touch up the structure! If one of your customers should ever doubt the quality of this process, just tell him that the spire of the World Trade Center was coated using a Metco 16E and zinc wire.”
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