Cigre B2-313

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CORROSION PROTECTION OF STEEL TOWERS

AND CAMOUFLAGE OF LINES

USING THE DUPLEX-SYSTEM

Herbert LUGSCHITZ* Anton ERNST, Thomas GROS

Austrian Power Grid AG Hagentaler(Austria) (Austria)

Corrosion protection of steel towers using the DUPLEX-System is a relatively new method inthe transmission line business, whereas this principle has been used successfully for decadesfor other purposes. The idea is to coat galvanized tower members as soon as possible aftertheir production to use synergetic effects from zinc and the coating material. As a result, thelifetime of the protection against corrosion can be extended up to 40 - 50 years depending onthe environmental conditions.Another big advantage is that the towers can be camouflaged in the appropriate colour to

blend into the environment. Also the shining effect of new galvanized towers can be pre-vented.

Two methods shall be distinguished: i) the in-factory-DUPLEX-coating where members arecoated in the factory immediately after galvanizing before mounting and ii) the DUPLEX-coating on completely mounted towers where the structures are coated as soon as possible af-ter their erection. Both methods are proven and reliable principles for corrosion protection.Austrian Power Grid (APG) sees advantages in the DUPLEX-in-factory-coating and has beenusing this method successfully as protection for its steel structures since 1983. This contribu-tion therefore concentrates on the in-factory-coating.

Keywords: transmission towers corrosion protection DUPLEX coating in factory coating camouflage of lines

1. BACKGROUND

The lifetime of a transmission line may reach 70 to 100 years, proper maintenance provided.Within this time some elements need to be changed (maybe earth wires, conductors, clamps),others need refurbishment (foundations, earthing grid).In most cases steel towers need protection against corrosion.Hot dip galvanizing is a commonmethod for such protection. The galvanizing material does not last forever and after a certainperiod of time coating of the structures on site becomes necessary (maintenance coating).

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Session 2004B2-313

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A new way to extend the intervals and to reduce the costs of maintenance coating is to paintthe galvanized steel members as soon as possible after galvanizing. If this is done in the fac-tory, this is the so called in-factory-DUPLEX-method. [1]

The decision of APG in the year 1983 to use the in-factory-DUPLEX System was triggered

by three facts:

- former used coating materials were to be replaced by more environmental friendlymaterials

- maintenance costs were to be reduced- out of service times of lines for maintenance coating were to be reduced.

APG uses the wet coating method where liquid materials are applied. This is explained inthe present contribution.

The application of the DUPLEX system allows decades of smooth operation of such line

structures. The history of coating methods in APGs grid over the years is given in table 1 andis representative for many other utilities.

Table 1: History of protection methods in APGs grid over the decades

till 1955 Unprotected towers delivered. Material containing oil-minium applicated on erectedtowers on site immediately after erection (two coatings)

since 1955 Galvanized towers delivered, no additional coating after erection. First maintenancecoating 15-20 years after installation, periodical treatment

1983 First attempt of in-factory DUPLEX-coating (10 towers coated) using solvent-containing materials

1984 First large-size test-application of in-factory DUPLEX-coating

1985 - 1993 several lines in-factory DUPLEX coated with solvent-containing material

1989-1993 tests with water-soluble materials

1993-2003 several lines in-factory DUPLEX coated with water-soluble material

2. THE PRINCIPLE OF DUPLEX-COATING

DUPLEX-coating means that newly hot-dip-galvanized steel parts are coated with an appro-

priate material immediately after galvanizing. This leads to a synergetic effect between thezinc and the coating material which results in a protection factor of 1.5. The lifetime of the

protection of DUPLEX coated materials therefore increases by 1.5 compared with the stan-dard measures.

As a result of environmental protection measures in Central Europe the quality of air im-proved remarkably during the last decades. Due to this the reduction factor for the coatingmaterial (and for the zinc coating as well) decreased. The coating nowadays lasts longer thanin former decades and conclusions from the 1960s may not be directly applied to decisions to

be taken in 2004. To reach the desired lifetime of the protection of 40-50 years a dry layerthickness of 70m shall be achieved if a reduction of 5 m per year is assumed. This leads to

a wet material thickness of app. 150 m.

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3. IN-FACTORY-DUPLEX-COATING

3.1Experience of APG with in-factory-DUPLEX-coatingAPG decided in 1983 to test the coating on some 380kV-towers. The results were very en-

couraging and this method was adopted as standard protection method for newly erected lines.

Table 2: Overview on coating of APGs transmission line towers following the in-factoryDUPLEX-System

Year number of in-factory

DUPLEX-coated towers

m2

coating coating material

1983 10 6.470 m2 solvent-containing



1985 370 208.000 m2. solvent-containing




1995 228 173.000 m2 water-soluble

1997 - 2003 344 266.440 m2 water-solubletotal

1984-20031275 1.041.500 m

2

All parts of the towers are coated except the joint plates, bolts and the parts of the tower legswhich are in the concrete foundations. The reasons behind are economic ones (tower legs) andtoo complicated processes for small parts (joint plates). Test were carried out to find out if thecoating on tower legs reduces the adhesion in the foundations but no reduction was found.Uncoated joint plates can be mounted easier. They are coated on site after mounting with littleeffort during the securing works for the bolts and nuts.

To guarantee an undisturbed electric connec-tion with the ground potential it is importantnot to exceed a certain given resistance at theconnecting plates and members. This is pro-vided with a suitable coating material in the

desired thickness. The roughness of the galva-nized surface is sufficient to penetrate thecoating material and to provide the electriccontact.Aircraft warning coating in red and white col-our needs to be stable against ultra-violetlight. These materials are special mixtures andneed additives to achieve the appropriateelectric conductivity.

Picture 1: joint plates are coated on site after

complete erection of the towers

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3.2 The in-factory-coating process

To guarantee a good adhesion of the coating, the hot-dip-galvanizing process for in-factory-coating may not use after-treatment with chrome, phosphates or oil. This means that such gal-vanized parts need to be coated within few days in order to prevent oxidation of the surfaces.

All coating works are carried out inside the factory which allows complete independencefrom the weather. This guarantees defined conditions to reach high quality, good adhesionand reduced losses of coating material. Risks as they often occur when outdoor objects arecoated (e.g. too high humidity or too low object temperature) can be completely eliminated.Before coating the parts need sweeping which is a kind of very soft sandblasting usingkorund or granulate from refined steel. The abrasion is app.10 m and guarantees that the sur-face is free from any pollution from transport or storage.

The coating material is applied by means of AIRLESS coating. It is done by hand. The ex-perience with coating machines was poor as they did not work sufficiently reliable and less

economically. The application by hand brought much better results and higher reliability. Af-ter coating the material needs drying in the factory over app. 24 hours. During this time thetemperature may not fall below +10 degree centigrade and the humidity may not exceed 75%.

APG sees the following advantages for the in-factory-method:- independent from the weather conditions (temperature, humidity, etc.)- independent from the season (can be done all over the year)- constant thickness of applied material- constant quality (can be checked and controlled easily)- if necessary 24 hours production per day- no pollution of soil and air- no extra out of service time for the lines

3.3 Shipping and mounting of in-factory-coated membersAfter the drying process the coated parts are stapled. They are separated by angles of alumini-um-coated cardboard to prevent damages. The weight of the bundles may not be too high inorder not to squeeze the coating material.

The handling on site needs fewadditional considerations inorder not to damage the coat-ing. Of course some minordamages cannot be preventedand must be repaired by thecontractor. This can be donewithout much effort at thesame time with the securingworks of the bolts.

Picture 2: staple of coatedtower members ready to be

transported on site

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3.4Costs for in-factory-DUPLEX-coatingBased on prices including all necessary extra works (sorting, sweeping, coating, storage fordrying as well as preparation for transport to the construction site and the necessary coatingmaterial) the additional costs for the in-factory-DUPLEX-coating amounts to approx. 2.5 %

of total costs of a line(including towers, foundations, conductors, hardware, compensation,erection works, etc.). This must be balanced against the savings from reduced maintenancecoatings.

The defined and constant conditions during the coating process allow the elimination ofmany uncertainties compared with the coating on site (e.g. weather, time needed for coating,quality) and therefore allows a precise estimation of costs. The savings depend highly on theenvisaged lifetime of a line and on the grade of aggressive atmosphere. As a very general andrough estimation the reduction of costs for corrosion protection can be calculated as given intable 3.

Table 3: reduction of costs for corrosion protection due to DUPLEX-System (based on pricesof date of erection)

estimated lifetime of towers app. cost reduction for protectiondue to DUPLEX

50 years 50 %

100 years 17 %

120 years 13 %

3.5 Environmental advantages from in-factory-coating

According to the experience of APG so far the period of time till the first maintenance coatingmay be estimated with 40 to 50 years. Under the same environmental conditions this is muchlonger than for traditional protection methods.

The hot-dip-galvanizing process for in-factory-coating shall not use aftertreatment withchrome, phosphate or oil, and therefore reduces the pollution with those materials. The pollu-tion during the in-factory-process is considerably reduced as it is a closed process. If watersoluble materials are used, no solvents pollute the environment.

4. DUPLEX-COATING ON SITE

As mentioned above another method for DUPLEX coating is to coat the mounted and erectedstructures on site. The measures to be taken during the works are well known from mainte-nance coating and range from protection of land around the towers, compensation paymentsto landowners, cleaning work before coating, etc. The coating works are normally done byhand and highly depend on the weather conditions and the season as well as on the possibili-ties to take lines out of service. The variable length of day may influence the progress of theworks.

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5. CAMOUFLAGE OF LINES

Overhead lines are technical objects in the landscape and are under discussion in many cases.Many methods and approaches have been developed worldwide to get more acceptance fromthe public. One of this methods is to camouflage lines to blend into the environment and is

explained in [2]. It was found that the visibility of overhead lines and the impression on thegeneral public can be reduced if they are coated in an appropriate colour [3]. In some regionsit has become a precondition to get the permission for building a line to coat the towers in thedesired colour. Often the shining effects from newly erected galvanized towers disturbs land-owners and it takes years till this effect is naturally reduced.

If towers are coated using the DUPLEX-method they are camouflaged from the first momentof their erection. If not, the next possibility to coat the towers will be at the first maintenancecoating which can be decades after installation.

6. REFERENCES

[1] A. Ernst, Corrosion protection of lattice steel towers by the in-factory-DUPLEX-system Langlebiger Korrosionsschutz von Stahlgittermasten durch stabweiseAnwendung des DUPLEX-Systems [sterreichische Zeitschrift fr Elektrotechnik,Jg. 42, December 1989, Vienna]

[2] H. Egger, H. Lugschitz, Experience with a 380 kV Camouflage Line [Cigr 1998,22/33/36-04 ]

[3] E. Gehmacher, A. Piereder-Hagen, NANU- Natures Non-Economic Utilization Measuring the psychological and ecological impact of technical constructions withnatural landscapes: The case of electrical overhead lines [Forschung im Verbund, Is-sue 34, 1998, Vienna]

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