30 www.paintsquare.com

Andreas W. Momber, Muehlhan AG,

Hamburg, Germany;

Peter Plagemann, Volkmar Stenzel;

Fraunhofer Institute for Manufacturing

and Applied Materials Research,

Bremen, Germany; and

Michael Schneider, Fraunhofer Institute

for Ceramic Technologies and Systems,

Dresden, Germany

he installation of offshore wind energy millsin both the North Sea and the Baltic Sea isone of the most recent approaches for alter-native production of energy. A number ofoffshore wind energy parks have alreadybeen established in these areas, and othersare under consideration. An offshore windenergy mill basically consists of a founda-tion, the actual tower, and the turbine-rotorconstruction (Fig. 1).

The tower is usually mounted on the foun-dation by a bolted flange. This article dealswith the corrosion protection of the towersand part of the foundation. These towers areambitious engineering constructions. Theycan be as high as 80 meters for offshoreinstallation; the diameter can be as large as 7meters; and the wall thickness can be in therange of several centimeters (Fig. 2).

Offshore wind energy towers are exposedto harsh and complex stresses, including thefollowing:• corrosive stress,• physical load, and• biological stress.

Part 1: Background and Test Program

Editor’s note: This article is based on an earlier version given at PACE 2008, January 27–30,in Los Angeles, CA. PACE is the joint conference of SSPC: The Society for ProtectiveCoatings and the Painting and Decorating Contractors of America (PDCA).

TInvestigating Corrosion Protection of Offshore Wind Towers

J P C L A p r i l 2 0 0 8

Burbo Bank Offshore Wind Farm (Liverpool Bay)Courtesy of Siemens Wind Power

Table 1: Corrosion Rates of Steelin Offshore Service4

Environmental zone

Buried in soil

Underwater zone (UZ)

Intermediate zone (IZ)

Splash zone (SZ)

Corrosion rate(mm/year)

0.1

0.2

0.25

0.4

tower, seems to require particular atten-tion for corrosion protection. However,using the values in Table 1 requires cau-tion, because they are based on the cor-rosion of unprotected steel, whereas thepresent investigation deals with the per-formance of protective coating systemsover steel.

Two corrosivity categories must beconsidered for offshore wind energymills:3

• C5-M: very high, marine; coastal andoffshore areas with high salinity, and• Im2: seawater or brackish water(including offshore structures).

Selection of Corrosion ProtectionSystems for Wind Energy TowersThe specification for a corrosion protec-tion system for wind energy towersshould address the following demands:• high corrosive stress due to elevatedsalt concentration in both water and air,• mechanical load due to ice drift orfloating objects,• biological stress, namely under water,• notable variations in temperature ofboth water and air,

This article deals mainly with the cor-rosive stress, although researchersfound that biological stress may alsoplay a role in the conditions offshore.The corrosive stress includes featuressuch as seawater exposure, wet-drycycles, temperature variations, con-struction details (joints, bolts, welds),and construction materials (materialcombinations).

The location of steel structures sever-al miles offshore is not a new situation.Oil and gas exploration and extractionplatforms have performed in such areasfor decades. The coating industry has,over the years, developed special coat-ing systems, to protect offshore struc-tures from corrosion. A simpleapproach for protecting offshore windenergy towers could be to adapt coat-ing systems for offshore platforms tothe wind towers. This approach wouldalso allow for the use of standardassessment schemes developed by theindustry and regulatory bodies.1,2

There are, however, critical differencesbetween platforms and towers, themost significant being that offshorewind energy towers are unmannedstructures with highly restrictedaccess. On oil and gas platforms, corro-sion protection systems are generallyunder permanent inspection, which isnot the case on offshore wind energytowers. Thus, whereas on oil and gasplatforms, areas of deteriorated coatingcan be recognised and repaired compar-atively easily, such repairs are not fea-sible on offshore wind energy towers.

In this article, the authors discuss anationally funded project on the perfor-mance testing of different corrosionprotection methods under site and labo-ratory conditions. This first part dealswith the rationale behind, and the set-ting-up of, the test program. A secondarticle will discuss the test results.

J P C L A p r i l 2 0 0 8 31www.paintsquare.com

Corrosive Stress and Corrosivity CategoryCorrosive stress depends largely on thelocation of a structure. An offshorewind energy tower, as a sea-based con-struction, has significant exposure inseveral zones, including the following:2,3

• underwater zone (UZ), the area per-manently exposed to water;• intermediate zone (IZ), the area wherethe water level changes due to naturalor artificial effects, and the combinedimpact of water and atmosphereincrease corrosion;• splash zone (SZ), the area wetted bywave and spray action, which can causeexceptionally high corrosion stresses,especially with seawater.

The environmental zones above canbe classified as per Fig. 1. Corrosionzones considered in this study aremarked. The corrosion rate of steel inthese environments can be greater than2.5 mm per year.4 It is already knownthat corrosion rates of steel are highestin the splash zone.4 Table 1 lists resultsreported in reference 4. In Table 1, thesplash zone, which features the flangeconnection between foundation and

5432

1

air

water

soil

Fig. 1 (Left): Corrosion zones on offshore windenergy towers. 1= buried in soil; 2 = underwaterzone (UZ); 3 = intermediate zone (IZ); 4 = splashzone (SZ); 5 = atmospheric zoneCourtesy of the authors

32 www.paintsquare.comJ P C L A p r i l 2 0 0 8

• long and irregular inspection inter-vals because of reduced accessibility,and• high maintenance and repair costs incase of coating failure.

The formal way to select a system isto consider the corrosivity categoriesaccording to reference 3. If the cate-gories are combined with a given dura-bility range, general coating systemschemes can be pre-selected.5 The gen-eral scheme covers the fol-lowing coating parameters:binder, primer type, num-ber of coats, and nominaldry film thickness. A typi-cal system that meets thecorrosivity categories C5-Mand Im2 would include azinc-rich, epoxy-basedpriming coat (60 µm); threesubsequent epoxy mid-coats, and onepolyurethane topcoat, witha total nominal dry filmthickness of 400 µm.However, this selectionprocess considers onlyorganic coating systems,not the detailed applicationof metal coatings, which arequite common on windenergy towers. Coating sys-tems, typically applied totraditional offshore struc-tures, are specified in refer-ence 2, where hot-dip gal-vanised and metallized steelsubstrates are included.

Reference 6 reviews coating systemsapplied to offshore wind energy tow-ers in the past. The systems basicallyconsisted of a Zn/Al-metallization,organic pore filler, several intermediateepoxy-based coats and a polyurethane-based topcoat. A typical total dry filmthickness was about 400 µm.Reference 7 describes a coating systemon offshore wind energy towers thatprovides high abrasion resistance.

Test ProceduresTesting and assessment methods for corro-sion protection systems may be subdivid-ed into laboratory tests under definedartificial stress conditions and site testsunder real stress conditions. Figure 3summarizes all tests for the present study.

The site tests included long-term expo-sure tests in a real corrosion environment.They were performed at the seawater testsite at the island of Helgoland, 70 km offthe German coast. The test site featuredthree galleries: one for the underwaterzone (UZ) environment; one for the inter-mediate zone (IZ) environment; and one

Table 2: Qualification Tests for Offshore Coating Systems (ISO 20340)

Test

Ageing resistance*

Cathodic disbondment

Seawater immersion

(ISO 2812-2)

Artificialscribe

Yes

ISO 15711

Yes

SZ

4,200 h

-

-

Testing duration for Im2IZ

4,200 h

6 months

4,200 h

UZ

-

6 months

4,200 h

Table 3: Parameters of the Cathodic Disbonding TestsParameter

Exposure time

Applied cathodic potential

Electrolyte

Test A, based on (9)

30 days

-1,450 mVSCE

Potable water; added:

10 g/l sodium chloride

10 g/l sodium sulphate

10 g/l sodium carbonate

Test B, based on (8)

180 days

-1,050 mVSCE

Demineralized water;

added:

23.8 g/l sodium chloride

9.8 g/l magnesium chloride

8.9 g/l sodium sulfate

1.2 g/l calcium chloride

Fig. 2: Dimension of a typical wind tower constructionCourtesy of Muehlhan A/S, Vissenbjerg

Fig. 3 (Above): Summary of performance testsCourtesy of the authors

Performance tests

Site testsLong-term tests under practice

conditions

Site testsLong-term tests under practice

conditions

As per ISO 20340

Electro-chemicalimpedance

spectroscopy

No lifetime assessment possibleLifetime assessment probablypossible

Laboratory testsArtifical and controlled

conditions

Offshoreperformance

tests

Electro-chemicaltests

*See written text, pp. 34-35.

http://www.bayermaterialsciencenafta.com/2

for the splash zone (SZ) environment.Figure 4 shows the test site, where thespecimens for the SZ and the IZ can berecognized. The specimens for UZ, whichare submerged, can be seen as discol-oration of the water surface. All speci-mens were tested for three years. Part ofthe site test procedure was cathodic pro-tection, consisting of an impressed currentsystem. The applied potential was con-trolled to -880 mVAg/AgCl.

The laboratory tests were subdividedinto ageing tests according to ISO 20340,2

cathodic disbonding tests,8,9 and testsbased on electrochemical impedance spec-troscopy (EIS). All tests were performedin the laboratory at IFAM, Bremen. Theprocedure prescribed in reference 2includes a combination of UV/condensa-tion, salt spray, and low-temperatureexposure cycles. The exposure cycle inthe procedure lasts a week (168 hours)and includes the following stresses (seealso Table 2):• 72 hours (3 days) of exposure to UV(UV [B] lamps) and water,

34 www.paintsquare.comJ P C L A p r i l 2 0 0 8

Fig. 4 (Left): Outdoor test stand at Heigolandwith specimens. Courtesy of the authors

Fig. 5 (Left): Set-up forelectrochemical impedancespectroscopy measure-ments, and equivalentcircuitCourtesy of the authors

Fig. 6 (Below): Specimendesign for underwater zone(UZ); dimensions in mmCourtesy of the authors

150 (external)

for M 70

M 12

60

160

450

40

1:2

RCoat CCoat

40

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P

• 72 hours (3 days) of exposure tosalt spray, and• 24 hours (1 day) of exposure tolow temperature (-20 C ± 2 C).A total of 25 cycles (25 weeks)

were run. Part of the assessmentprocedure was cathodic disbondingaccording to references 8 and 9. Forthese tests, specimens with thedesign shown in Fig. 10 (p. 49) wereused. Holes were drilled through thecoating down to the substrate, andthe samples were exposed to syn-thetic seawater. Table 3 lists detailson the test parameters.

However, these tests are plainpass/fail tests. Although theyallow for a comparative evaluationof different paint systems, they donot provide information on thepaint degradation processes or oncorrosion progress. A promisingmethod for gathering degradationand corrosion information on coat-ings for wind energy towers isEIS.10,11 Therefore, additional EIStests were performed on a numberof laboratory samples. EIS was car-ried out on coated steel specimens,which had been stored in a 3%NaCl solution for up to 62 days.The measurements were performedaccording to the three-electrode-method with an onset cell of 8 cmdiameter. The testing device isshown in Fig. 5. The spectra weremeasured from 100 kHz to 0.01Hz; with a potential amplitude of20 mVAg/AgCl. From the obtainedspectra, barrier resistances weredetermined by fitting a simple RC(ohmic resistance/capacitor)equivalent circuit (Fig. 5).

In addition, probable contact cor-rosion between dissimilar metalswas investigated. These investiga-tions were applied to the contactbetween the bolt material in theflange area and construction steels.The contact areas were assessedvisually.

J P C L A p r i l 2 0 0 8 37www.paintsquare.com

Table 4: Coating Systems

Systemnumber

1

2

3

4

5

6

SEM cross sectionPrimer

Zn-EP

(80 µm)

Zn-EP

(80 µm)

Zn/Al

(85/15)2)

(100 µm)

Zn/Al

(85/15)2)

(100 µm)

EP5)

(1,000

µm)

Al/Mg

(95/05)2)

(350 µm)

System composition (DFT)2. Layer

EP

(300 µm)

EP

(450 µm)

EP3)

(20 µm)

EP3)

(20 µm)

-

EP6)

(40 µm)

3. Layer

EP

(300 µm)

EP

(450 µm)

EP

(450 µm)

EP4)

(450 µm)

-

-

4. Layer

PUR1)

(70 µm)

-

EP

(450 µm)

EP4)

(450 µm)

-

-

Total DFTin µm

750

980

1,020

1,020

1,000

390

1) topcoat; 2) metallization; 3) primer + pore filler; 4) particle reinforced;5) applied in one layer; 6) (pore filler); SEM - scanning electron microscopy

No image available

Test SamplesOutdoor Test SamplesDesign rules for offshore wind energytower follow the function of the tower.Low-corrosion design is a secondaryissue. Therefore, the towers are complexstructures with construction detailssuch as bore holes, bolted connections,flanges, weld seams, bracings, steel sec-tions, and coating overlap. Fewapproaches have been made in the pastto consider these details. Bailey et al.12

were probably the first to simulate con-struction details of offshore structures.Their specimen featured a plate withbore holes, I-beams, a pipe section, edges,weld seams, and bolts. Wilds13 manufac-tured specimens containing a weldedpipe section, angled parts, weld seamsand I-beam, and he investigated the per-formance of organic repair coating sys-tems. The author found a notable effect

at the weld seams.For the present study, researchers

designed and manufactured three typesof special specimens: for the UZ, for theIZ, and for the SZ (Figs. 6 to 8). The useof the specimens embodying on a smallscale the typical structural features of areal offshore wind energy tower is con-sidered a new approach in testing. Alloutdoor samples were made from high-strength, weldable construction steel, S-355.

The samples for the UZ were steelpipes. The pipes were filled with seawa-ter and then sealed. These specimenscontained weld seams at the uncoatedand coated sections. They also featured aconnection for cathodic protection (seeFigs. 6 and 8). About 60% of the surfacewas coated. The specimens for the IZzone were simple steel plates (Fig. 8).After an exposure for 13 months,researchers scribed the IZ-samples topromote corrosion.

The specimens for the SZ consisted oftwo parts bolted together (Figs. 7 and 8).The lower part embodied the end of thefoundation structure where the actualtower construction, embodied by theupper part of the specimen, rests. Bothparts of the specimen featured a flangeend, which was welded to the main body.The flange sections were metallized butnot coated. Made from high-alloyed steel(AISI 304), the bolts represented the con-tact between dissimilar metals. The SZspecimens also contained an angled steelpanel welded to the lower part. This con-struction detail may characterize designthat promotes corrosion (Fig. 8).

Laboratory Test SamplesThree types of laboratory sampleswere manufactured. The first type cov-ered the specimens for the degradationtests according to ISO 20340.2 Thedimensions were slightly modified (Fig.9). The coated specimens were provid-ed with two artificial scribes to simu-late localized mechanical damage.Position and dimensions of the scribes

38 www.paintsquare.comJ P C L A p r i l 2 0 0 8

Fig. 7: Specimen design for splash zone (SZ);dimensions in mm

Courtesy of the authors

Fig. 8: Coated specimens for outdoor testing. Upper: Splash zone (SZ) specimens. Center: Intermediatezone (IZ) (originally without scribe, no effect after 13 months; scribe added after 13 months to produce

mechanical damage to the systems.) Bottom: Underwater (UZ) specimensCourtesy of the authors

5 - 8

M 12

1:2

front view (1:5)

side view

225

450

450

50

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can be read from Fig. 9. The second typecovered the specimens for the cathodicdisbonding tests (Fig. 10). The specimenconsisted of a lower primary section andan upper, smaller secondary section,whereas the top part of the upper sec-tion remained uncoated. A hole with acontrolled cross section (Ø 10 mm) wasdrilled through the coating down to theplain steel in the center of the specimen.The third specimen, used for EIS-mea-surements, was a simple, coated plate 30mm x 30 mm.

Coating Systems and Coating MaterialsCorrosion protection scenarios can besubdivided into three categories: activemethods, passive methods, and tempo-rary methods. Active methods includethe selection of corrosion-resistant mate-rials, designs reducing the risk of corro-sion, and cathodic protection. Passivemethods include applying coatings or lin-ings to protect the steel. The methodsinvestigated in this project included thefollowing:

• cathodic corrosion protection ofunpainted steel;• thick, single-layer organic coating,• multi-layer organic coating system,• duplex system: metal-sprayed coatingwith organic top coat, and• metal-sprayed coating with organicsealer.

Details of these protection methodscan be found in Table 4. Systems 1 and 2are basic inexpensive versions, whereasthe systems 3 and 4 represent moreadvanced versions. Systems 5 and 6were applied to the samples for the UZonly.

The systems differed not only in termsof composition and thickness, but also interms of primer coat and type of inter-mediate coat. The organic coating materi-als with particle reinforcement werenon-commercial products. Duplex sys-tems are routinely used for onshorewind energy towers, and they have beenapplied to offshore wind energy towersat places6. Duplex systems are high-levelsystems because the protection of steelagainst corrosion can be ensured even ifthe organic coating fails. Multi-layerorganic coating systems are standardsolutions, but their performance dependson the details of the systems. Therefore,multi-layer systems with different inter-mediate layers have been tested. Single-layer organic coating systems are notcommon in the offshore industry, butthey could offer advantages in terms ofapplication. Their performance underoffshore conditions has not yet beeninvestigated systematically. The system“Al/Mg-metallization + pore filler” is anuncommon variant for offshore con-structions, but it would allow for a com-parison between different metallizationsystems, Zn/Al and Al/Mg.14

All samples were blast cleaned accord-ing to ISO 8504-2.15 Abrasive materialwas steel grit with a particle sizebetween 0.2 and 2 mm. Fine cleaningwas performed. The surface profile wasmeasured with a stylus instrumentaccording to ISO 8503-4.16 The average

Fig. 9: Specimen design for the tests according to ISO 20340 (2); dimensions in mm“1” = 2-mm-wide scribe; “2” = 0.05-mm-wide scribe

Courtesy of the authors

Fig. 10: Specimen design for the cathodicdisbonding tests; dimensions in mm

Courtesy of the authors

15

coating limit

welded150

150

20

70

10

2

20 2030

70

100

2020

3030

42 www.paintsquare.comJ P C L A p r i l 2 0 0 8

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maximum roughness had a value ofRy5=69 µm, with a standard deviationof 6 µm. The surface preparation gradewas Sa 21⁄2 (for the organic systems) andSa 21⁄2 to Sa 3 (for the metallized sys-tems). The weld seams were ground andcleaned to a P3-quality according to ISO8501-3.17 All coatings were applied inaccordance with the manufacturers’

specifications. The organic systems wereapplied with airless spray systems. Themetallized coatings were applied with aspecial metallizing technique18 with thesample preparation and test programthis described.

The second part of this article, to bepublished in an upcoming issue, will dis-cuss the results of this testing.

References1. M. Mitchell, “An update and review:

offshore systems and current testscenarios,” Protect. CoatingsEurope, Vol. 11, No. 4, pp. 36-41(2006).

2. ISO 20340, “Performancerequirements for protective paintsystems for offshore and relatedstructures”, InternationalOrganization for Standardisation,Geneve (2005).

3. ISO 12944-2, “Paints and varnishes—Corrosion protection of steelstructures by protective paintsystems—Part 2: Classification ofenvironments,” InternationalOrganisation for Standardisation,Geneve (1989).

4. P. Ault, “The use of coatings forcorrosion control on offshore oilstructures,” Protect. CoatingsEurope, Vol. 11, No. 4, pp. 42-46(2006).

5. ISO 12944-5, “Paints and varnishes- Corrosion protection of steelstructures by protective paintsystems—Part 5: Protective paintsystems,” InternationalOrganisation for Standardisation,Geneve (2007).

6. K. Mühlberg, “Corrosion protectionfor windmills on-shore and offshore,” Protect. Coatings Europe,Vol. 9, No. 4, pp. 30-35 (2004).

7. D. Levie, M. Roehl, “Anforderungenund Erfahrungen bei derBeschichtung von Fundamenten beiOffshore-Windenergieanlagen,”Korrosionsschutz in der maritimenTechnik, Tagungsband zur 2. Tagung,Hamburg, pp 87-94 (2003).

8. ISO 15711, “Paints and varnishes—Determination of resistance tocathodic disbonding of coatingsexposed to seawater,” InternationalOrganisation for Standardisation,Geneve (2003).

9. ASTM G8, “Standard testingmethods for cathodic disbonding ofpipeline coatings,” ASTM

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10. Y. Korobov, P.D. Moore,“Performance testing methods foroffshore coatings: cyclic, EIS andstress,” Corrosion 2004, NewOrleans, March/April 2004, Paper04005.

11.M. O’Donoghue, R. Garrett, V.J.Datta, S. Osborne, and P. Roberts,“Windmills—fast productionschedules with novel zinc primersand polyaspartic ester topcoats,”Proc. PACE 2005, Las Vegas,Conference-CD (2005).

12. G. Bailey, D.H. Deacon, and W.R.King, “Results of offshore tests ofselected coating systems,” Trans.Inst. Mar. Eng. (C), Vol. 91, Conf.No. 1, pp. 43-50 (1979).

13. N. Wilds, “Surface tolerant coatingsfor offshore maintenance,” PACEConference and Exhibition 2006,Tampa, February, Conference-CD(2006).

14. Y. Li, J. Liu, J. Duan, and B. Hou,“Thermally sprayed aluminium andzinc coatings for tidal zone cathodicprotection of offshore platformlegs,” Materials Performance, No. 12,pp. 16-19 (2006).

15. ISO 8504-2, “Preparation of steelsubstrates before application ofpaints and relatedproducts–Surface preparationmethods–Part 2: Abrasive blast-cleaning,” InternationalOrganisation for Standardisation,Geneve (2002).

16. ISO 8503-4, “Preparation of steelsubstrates before application ofpaints and related products—Surface roughness characteristics ofblast-cleaned steel substrates–Part 4: Method for the calibration ofISO surface profile comparatorsand for the determination of surfaceprofile—Stylus instrumentprocedure,” InternationalOrganisation for Standardisation,Geneve (1995).

17. ISO 8501-3, “Preparation of steelsubstrates before application ofpaints and related products –Visual assessment of surfacecleanliness–Part 3: Preparationgrades of welds, edges and otherareas with surface imperfections,”International Organisation forStandardisation, Geneve (2007).

18. ISO 2063, “Thermal spraying—Metallic and other inorganiccoatings—Zinc, aluminium andtheir alloys,” InternationalOrganisation for Standardisation,Geneve (2005).

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The authors can be contacted throughDr. Andreas Momber,momber@muehlhan.com