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Specification for the Coating of Steel Cable Support Structures with a Galvanised/Polymeric Duplex Coating System

SCOPE

This specification covers the application, quality control an on-site installation requirements for the protection of steel cable support structures with a "duplex" coating consisting of a galvanised layer followed by a top-coat of an extrusion compounded thermoplastic such as Plascoat PPA 571 or PPA 571ES.

2) DESIGN OF CABLE SUPPORT STRUCTURES

Cable support structures should be designed in such a manner to take into account the ease of application of both the galvanised base layer and the polymeric coating and so that 100% coverage of the steel surface with the duplex coating can be achieved. The steel cable support structures should be free of sharp edges and burrs. All sharp edges and corners should be rounded off or ground to a minimum radius of 0.3 mm if possible. Weld splatter should be removed.

3) GALVANISING

Galvanising must be carried out to ASTM A525, coating weight designation G90 minimum.
The galvanised surface must be free of dross, white rust or contamination, when received by the applicator. A passivating chemical treatment should not be applied. Any protective film or layer on the galvanised surface must be removed prior to coating.
Under no circumstances should the galvaniser use touch-up paints to repair fettled edges. Ideally the galvanised structure should be coated as soon as possible and all efforts should be made to ensure that the time between galvanising and coating is kept to a minimum.

4) PRE-TREATMENT

Reference must be made to Plascoat's guide for the pre-treatment of galvanising steel prior to application of PPA 571 or PPA 571H. Ideally the galvanised metal should be degreased and chemically pre-treated with a calcium modified zinc phosphate. If this is not possible an effective sweep-blasting of the entire surface is suggested. 
After proper surface preparation, the cable support structures should be kept free from fingerprints, white rust or other contamination. The coating of prepared items should be accomplished within two hours.
If the galvanised layer has a tendency to "gas", then the metal should be "degassed" by heating to around 210ºC to 220ºC prior to coating.

5) APPLICATION

Plascoat PPA 571 is suitable for coating using the fluidised bed method. Plascoat PPA 571ES is suitable for application by electrostatic or flock deposition.

5.1 Fluidised Bed

Pre-heat temperatures should not exceed 280ºC and pre-heat times should be kept to a minimum to ensure that no damage is caused to the galvanic coating. A post-heat may be necessary to fully sinter the coating. Post-heat temperatures should not exceed 200ºC and post-heat times should be kept to a minimum.

5.2 Electrostatic Deposition

Plascoat PPA 571ES should be applied to the galvanised cable support structure using conditions conforming to the PPA 571ES data sheet - Voltage 30-80KV, 160ºC to 220ºC for 5 to 40 minutes, depending on metal thickness.

5.3 Flock Deposition

Plascoat PPA 571ES can be applied to a pre-heated item in order to achieve a film thickness greater than that possible with electrostatic deposition alone. Pre-heat temperature should not exceed 280ºC and pre-heat times should be kept to a minimum to ensure that no damage is caused to the galvanised coating.

6) QUALITY CONTROL PROCEDURES

6.1 Coating Thickness

A minimum of three non-destructive tests encompassing the whole length of each cable support structures should be carried out to ensure that the thickness of the polymeric coating conforms to the recommendations given in the relevant Technical Data sheets.
The thickness of the applied galvanised coating should be taken into account when using a magnetic thickness gauge.

6.1.1 Recommended Thickness

PPA 571 300-750 microns
PPA 571ES 150-500 microns

It is essential that the minimum thickness is 150 microns.

6.2 Holiday Detection

Detection for holidays and pinholes should be carried out along all edges and internal corners of each cable support structure. The recommended practice for Holiday Detection is outlined in ASTM D 5162. A high voltage spark tester (as defined in ASTM D 5162) and a test voltage of 2kV should be used for fluidised bed applied coatings (PPA 571). For coatings applied by electrostatic spraying (PPA571ES) should be tested with a Low Voltage Wet Sponge Tester (as defined in ASTM D5162).
The polymeric coating on each cable support should be free of holidays.

6.3 Adhesion

Small sections of typical cable support structure should be produced for adhesion testing. Plascoat would recommend one test piece per ten commercial cable support structures.
Adhesion should be performed in accordance with BS 1722 Part 16 (1992) {B.2.4.1}. Two tests should be conducted on each test piece a minimum distance of 250 mm apart. The coating shall not be capable of being lifted by more than 0.5mm at either test site.
In the event of an adhesion failure, Plascoat would recommend that adhesion checks are made on all commercial cable support structures produced between the last successful adhesion test and the failure, and the reasons for the adhesion failure identified.

It is recommended that, at the beginning of any production run, a minimum of three adhesion test pieces are coated and tested prior to full production of commercial cable tray structures.

7) ON-SITE INSTALLATION

When installing cable tray systems it is common to use a disc cutter to cut the tray to the desired lengths. Disc cutters are quick and do not require a vice or other clamping device. However, when using a disc cutter, the temperature of the metal can reach 700ºC. At this temperature, both the coating and the galvanised layer can seriously degrade. Such degradation will cause some peeling back of the coating and galvanising layer from the cut edge during service. 

1) The number of cut ends required is taken into account at the design stage and kept to a minimum.
2) The use of a band saw for on-site cutting (where less heat is produced) is considered.
3) Care is taken to minimise heat build-up from the disc cutter.
4) Cut ends are examined for breakdown of the galvanising layer, and, if necessary the coating is peeled back to where adhesion is adequate and removed. The exposed metal should then be repaired with a suitable fast-drying, two-pack polyurethane paint system. If no polyurethane systems are available, two-pack epoxy systems can also be used.
5) Repair all cut edges thoroughly. All cut ends should be repaired with a suitable fast-drying two-pack polyurethane paint system (or, if such a paint is not available, two-pack epoxy paint systems and/or butyl rubber self-amalgamating tape of 1mm thickness can be used).
Repairs to cut ends should be carried out so that no part of the metal surface is exposed and the cut/damaged area is effectively sealed from the atmosphere. It is the responsibility of the installer to ensure that all repairs are carried out satisfactorily.

Plascoat Systems can accept no responsibility for failures resulting from galvanised coatings of inadequate quality or from the application of our materials outside our recommended guidelines, or from failures resulting from damage to articles during transportation or installation.

Whilst the above guidelines are given in good faith, recommendations are made without warranty or guarantee. The successful chemical pre-treatment of galvanising is dependent on the quality of the galvanising and the length of time the galvanised surface is left unprotected. Poor adhesion between the zinc and/or zinc-iron alloy layers can result in failure of the whole protective system due to delamination of the galvanised coating. The formation of stubborn oxide layers and/or the inclusion of contaminants well adhered to the galvanised coating can affect chemical pre-treatment and subsequent adhesion to the protective coating.

It is the responsibility of the galvaniser to ensure that the galvanising as supplied to the coater conforms to the guidelines given in this specification. It is the responsibility of the applicator to ensure that our materials are applied to our guidelines and that the specified quality control procedures are employed to ensure the desired performance. Any problems discovered by the applicator due to the condition of the items being below the standard specified should be reported and discussed with the relevant suppliers and/or end-users before further processing takes place.

Case studies of Plascoat PPA 571 for the coating of cable trays