Corrosion protection for wind turbines– onshore and offshore

Content Page

1. Introduction 3

2. Relevant standards 4

3. Corrosion protection for wind turbines, installation on land (onshore) 5

4. Corrosion protection for wind turbines, installation in the sea (offshore) 7

5. Qualification of coating systems 9

6. Execution of the coating job 12

Corrosion protection for wind turbines– onshore and offshore

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Publication copyright of Hempel A/S. Publication of special feature in Wind Krafty Journal Jan 2003.

Authored by:

Karsten MühlbergTechnical managerHempel Germany GmbH Office WestNiederkasseler Straße 651147 Cologne (Germany)Phone: +49 (0) 2203 - 911 71-13Fax: +49 (0) 2203 - 911 71-9Mobile: +49 (0) 171-6712 200E-mail: mue@de.hempel.comwww.hempel-germany.de

AbstractA description is made of the basis on which standardizationof corrosion protection for wind turbines can be regulated andrealized. Requirements and criteria for approval of coatingsystems are described. The protection of rotor blades andother plastics will not be discussed.

1. IntroductionDuring the period 1999 to 2002 more than 7570 wind turbines wereerected in Germany. In 2002 alone, 2328 turbines were built,meaning that as far as steel towers are concerned, far more than2,000,000 square meters had to be protected. Additionally,mainframes, hubs, gearboxes, rotor shafts and generators, tomention but a few, also had to be protected.

Wind turbines are regarded as objects with a life time of 20-25 years.The corrosion protection also has to last at least for that periodwithout general maintenance. Due to their type of construction, i.e.steel towers with a height of the nacelle of up to more than 100meters, and due to their typical location, either at sea or on land,often in areas with low infrastructure, access for later maintenanceof the turbines is difficult and costly. Wind turbines also have to fulfila certain aesthetical function, and they are very often the subject ofpublic discussions.

Therefore is it of the utmost importance that corrosion protection isof very high quality. That means that relevant work in the factoriesin question has to be organised, carried out and supervised/controlled in a very professional way and on a very high level. Paintshave to be tailor-made to fit the needs of the different manufacturersof steel towers.

If these points are not taken into account, unacceptable quality ofthe corrosion protection will be the result. Defects related to this will,on the short or long term, influence the operational capability, causeloss of earnings, often become visible within a short period of time(on exterior areas) and may negatively influence the image.

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2. Relevant standardsHow is modern corrosion protection, especially for wind turbines, regulated nowadays? Out of numerous standards two essentialones should be emphasized:1. EN ISO 12944 „Paints and varnishes – Corrosion protection of steel structures by protective paint systems (1998)

2. NORSOK M 501, Surface Preparation and Protective Coating (4th Revision 1999).

EN ISO 12944 introduced in 1998 is applicable world-wide and is presently „the standard“. It regulates corrosion protection from lessaggressive environments like interior areas, to rural climate up to city and industrial atmosphere as well as steel constructions (bridges)at the coast and offshore. Corrosion protection for hydraulic steel works (immersed steel structures) and galvanized steel is alsodescribed in the EN ISO 12944. The standard furthermore contains indications about the lifetime of the various systems.

EN ISO 12944 (1998)

NORSOK M-501 (4th Revision 1999)

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Part 1: General introduction General definitions, health and safety and environment protection, protection periods.

Part 2: Classification of Corrosion categories for atmosphere, water and soil, loss of steel and zincenvironments content by corrosion in different environments.

Part 3: Design consideration Design criteria for steel structures to improve their resistance to corrosion.

Part 4: Types of surface and Methods of surface preparation for different surfaces, degrees of surface preparation cleanliness and surfaces roughness.

Part 5: Protective paint systems Different generic types of paints, paint systems for all corrosion categories fordifferent protection periods.

Part 6: Laboratory performance Preparation and evaluation of test panels, test methods for paint systems for all test methods corrosion categories and all kinds of protection.

Part 7: Execution and supervision Methods of application of paint materials, guidelines for application in workshopof paint work and on site, preparation of reference areas.

Part 8: Development of Guidance for working out different specifications for corrosion protection andspecifications for new- maintenance.buildings and maintenance

1. Scope

2. Normative references

3. Definitions and abbreviations

4. General requirements Notes on application conditions, coating and steel materials, shopprimers,prequalification of products, personnel and procedures.

5. Health, safety and environment

6. Surface preparation Pre-blasting preparations (edges, welding seams etc), blast cleaning, finalsurface condition.

7. Paint application Notes on application equipment and application.

8. Thermally sprayed metallic coatings Material, application, repair.

9. Sprayed on passive fire protection Material, application, repair.

10. Qualification requirements Prequalification of products, qualification of personnel and procedures.

11. Inspection and testing Methods for inspection and testing and criteria for acceptance.

ANNEX A Paint systems for different areas on offshore facilities.

This standard was developed by „Norwegian Technology Standards Institution“ (Oscarsgt. 20, Postbox 7072 Majorstua N-0306 Oslo, NORWAY) with assistance from the Oil & Gas exploring and processing industry. It exclusively, but in great detail,describes corrosion protection of different offshore installations. The standard can be downloaded free of charge from thewebsite www.nts.no/norsok

A final note regarding the offshore standards:A draft for a new ISO standard for offshore purposes is being evaluated at the moment. It is called ISO 20340: Paints andvarnishes – Performance requirements for protective paint systems for offshore and related structures (11/2003). It isexpected early 2004. The standard covers laboratory test methods and evaluation criteria for coating systems for steelstructures (newbuilding) exposed to offshore environments. The environments in question are the C5-marine and the Im 2according to the ISO 12944, part 2.

3. Corrosion protection for wind turbines,onshore installation

Vital for the choice of good corrosion protection is the expected corrosion attack from the environment around the location ofthe wind turbine and its desired lifetime. The classification of our environment in this respect can be seen in EN ISO 12944part 2. Five corrosion categories are defined from not corrosive interior atmosphere (C1) up to industrial and sea-climate (C5 I+ C5 M). Decisive for the determination of the categories is the loss of unprotected steel and galvanized steel at outdoor storage(see table 1). Im 1 to Im 3 are describing the load of materials immersed in fresh and salt water as well as buried in soil.

Illustration 1: Statement of EN ISO 12944, part 2, corrosion categories C3, 4 and 5 as well as Im 1-3.

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Wind turbine farms in Germany are planned and built almost exclusively in areas with rural to city-like character. Erection inindustrial areas (e. g. in chemical plants) is rare. The present corrosion category complies with C3 (moderate load, seeillustration 1). In general a maintenance-free lifetime (protection time) of more than 15 years is expected from the coatingsystem. In part 5, the EN ISO 12944 recommends various multi-coat systems with dry film thicknesses from 160 – 240 µm.

Illustration 2: Statement from EN ISO 12944, part 5, paint system for corrosion category C3.

Definitions given in illustration 2 regarding expected time of protection (durability) short (2 – 5 years), middle (5 – 15 years) and

long (>15 years) are not guaranteed terms, but illustrate the period up to the first planned maintenance covering corrosionprotection. They therefore assist in choosing the right coating systems considering their lifetime.

A typical coating system for onshore wind turbines is nowadays a 3-coat system consisting of:2 comp. epoxy zinc rich primer 50 – 80 µm

2 comp. epoxy mid-coat 100 – 150 µm

2 comp. polyurethane topcoat 50 – 80 µm

Efficient 2-coat systems may also be used, such as:2 comp. epoxy zinc rich primer 80 µm

2 comp. polyurethane topcoat 120 µm

For the classical and most commonly used 3-coat system, most specifications in the market often show film thicknesses from240 to 280 µm, the latter almost matching the requirements of the highest corrosion category C5 (industry). For reasonsalready mentioned (long lifetime without maintenance, limited access, location at the coastline) this safety margin isacceptable. But 200 µm exterior and 160 µm interior (for steel towers) should be the lowest limit for onshore wind turbines.

The recommended values are primarily related to components exposed to steady weathering, such as the exterior of steeltowers or hubs. Other parts of the wind turbine (generator, gearbox, rotor shaft, main frame etc.) and the tower interior maybe protected in the same way, but lower film thicknesses may be used, as during operation of the wind turbine the dew pointis hardly every reached, and some other parts will never be in contact with the surrounding atmosphere.

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4. Corrosion protection for wind turbines,installation in the sea (offshore)

For corrosion category C3 (moderate load acc. to EN ISO 12944, part 2) the requirements for corrosion protection of onshorewind turbines have been defined. C5-M (heavy corrosion, sea, see table 1) characterizes the conditions to which offshore windturbines are exposed. Im 2 (also part 2, EN ISO 12944) describes permanent exposure to water (seawater and brackish water).As coating systems the EN ISO 12944 recommends multi-coat applications from 320 up to 500 µm (atmospheric exposureC5-M) and 480 up to 1000 µm (immersed, Im 2). These statements apply to almost maintenance-free periods of corrosionprotection exceeding 15 years.

Illustration 3: Statement from EN ISO 12944, part 5, paint system for corrosion category C5.

The standard NORSOK M 501 specifies similar systems. For atmospheric exposure 335 µm are dictated, and for permanentimmersion in water 450 µm (both are minimum thicknesses).

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Illustration 4: Statement from NORSOK M 501, coating system 1, atmospheric exposure.

Illustration 5: Statement from NORSOK M 501, coating system 7, permanent immersion.

A typical coating system looks as follows (above waterline):2 comp. epoxy zinc rich primer 50 – 80 µm

2 comp. Epoxy Mid-coat, in case 2 x 100 – 150 µm

2 comp. Polyurethane Topcoat 50 – 80 µm

min. film thickness totally 320 / 335 µm

Submerged areas (e. g. monopiles):2 – 3 x epoxy-coating 225 µm each

min. film thickness totally 450 / preferably 600 µm

According to NORSOK cathodic protection has to be added to the coating system for submerged areas. Possibilities areinstallation of impressed cathodic corrosion protection or welding of sacrificial anodes.

In short, for corrosion protection at sea generally the same coating systems and coating materials can be used as for theprotection of onshore wind turbines. They differ merely in film thickness and number of coats. From the very beginning onlytop quality products based on epoxy and polyurethane resins have been used for the protection of wind turbines.

In this connection we wish to draw your attention already here to an important point regarding „offshore corrosion protection“in comparison to „onshore corrosion protection“.

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On basis of the described differences the simple but important conclusion is that qualified execution of the coating job is the

decisive criterion for successful „offshore corrosion protection“. Some negligence and variations in quality, which are more orless normal in the daily business, can often be „tolerated“ and they are normally without direct consequences for onshorewind turbines, but they cannot be tolerated in offshore service, as this would have dramatic consequences. To highlight thequality assurance, here is a short review of a damage analysis.

Damage analysis, 120 cases, Australia (Journal of Protective Coatings and Linings, January 2000)

5. Qualification of coating systemsIn the field of „heavy corrosion protection“, coating materials on basis of epoxy and polyurethane resins still dominate, as theyhave proven their quality and they have impressive long-term references. Nevertheless, due to their extensive variety, in particularfor special applications, to which offshore definitely belongs, it is necessary to make a selection of coating materials by testingthem in a careful way. Such tests comprise short term stress on the coating systems with a pre-set definition of their conditionbefore and after the tests. The judgement of damages of the coating systems due to their exposure to the different tests is donevisually as well as with instruments. Also here please refer to EN ISO 12944 and NORSOK M 501.

Examples of coating system tests

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ONSHORE

General cyclic condensation without or with minor salt effect

Moderate corrosion on holidays and weak areas of the coating

OFFSHORE

Extended exposure to condensation combined with strong salt effect and UV-light

Heavy corrosion on holidays and weak areas of the coating

1983-1990 1990-2000without ISO 9000 with ISO 9000

faulty coating material 2 % 2 %

wrong specification 19 % 41 %

changed environmental conditions 11 % 11 %

faulty processing / wrong application 68 % 46 %

EN ISO 12944, part 6 NORSOK M 501

onshore exposure offshore exposure offshore exposurecorrosion category C3 corrosion category C5M/Im2 atmospheric/immersion

480h neutral salt spray 1440h neutral salt spray 1 cycle:(ISO 7253) (ISO 7253) 72h salt spray test (ISO 7253)

16h drying time240h water condensation 720h water condensation 80h QUV- test (ASTM G53)

(ISO 6270) (ISO 6270) 25 cycles in total (4200h)

3000h water immersion like atmospheric exposure,(ISO 2812-2), but plus ASTM G8 (cathodic

1440h salt spray test (ISO 7253) disbonding, 30d, -1500mV)

Typical evaluation criteria are mainly:• degree of blistering and cracking (ISO 4628)

• degree of rusting and flaking (ISO 4628)

• degree of chalking (ISO 4628)

• adhesion (ISO 4624)

• underlying corrosion caused by artificial scribe

For clarification, below are some examples on test equipment and evaluation criteria:

Salt spray test (ISO 7253)

Corrosion creep from scribe

Equipment for testing of the resistance to UV-light exposure (QUV- test)

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Statements from ISO 4628: Paints and varnishes – Evaluation of degradation of paint coatings – Designationof intensity, quantity and size of common type of defects.

Cracking Cracking Flaking

Blistering Blistering Rusting

Chalking

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Adhesion acc. to ISO 4624 (Paints and varnishes – Pull-off test for adhesion), DIN EN ISO 2409 (Paints and varnishes – Cross-cut test) und ASTM 3359 A.

6. Execution of the coating jobSuccessfully passed tests are necessary but not by themselves an adequate criterion for successful corrosion protection and notat all for the requirements for „Offshore Service“. Already during planning, suitable corrosion protection of the steelworks must be

considered (EN ISO 12944, part 3). All significant points have to be clearly documented in a specification (EN ISO 12944, part 8).This specification must be distributed to all parties involved. This begins with the predefined existing rust grade of the steel to beused (ISO 8501/part 1, not below B). Furthermore treatment of welding seams and sharp edges has to be specified, consideringlater coating treatment (ISO 8501/3). Clear directions regarding cleanliness of the surface and roughness as well as execution andcontrol of all steps of the coating job must be part of the specification (EN ISO 12944, part 7).

The lifetime of a coating system depends in general on five factors:• Quality of surface preparation• Actual coating thickness obtained• Quality of workmanship• Quality of the coatings• Conditions at location

The points marked in red are in the hands of the application company and are always the crucial ones. Experience shows that execution of the whole job according to existing technical standards, well-organized quality assurance and external qualitycontrol are essential prerequisites for successful implementation of all aspects of corrosion protection.

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Some pictures, which will make this clear:

Modern blast cleaning equipment (highly automated)

Modern application and drying cabins (can be heated) with very good lighting and access

Modern two-component spray equipment (mixing unit, right: paint supply from containers)

A last hint. Besides qualified coating systems and execution of all coating jobs according to the best technical standards, especiallyin the offshore business, you should refer to experience and references. The above examples of modern working conditions are notstandard everywhere. The field of corrosion protection for wind turbines, especially for steel towers is today, more than ever before,influenced by strong competition. Coating application around the clock, 24 hours a day, under pressure of time, carried out by foreignlabourers with whom communication is often very difficult, are typical constellations for this business today. Therefore, there is anincreasing demand for faster drying of the coating, lower thicknesses and fewer layers. Does this make sense in every case?

Today it is possible to have one or two-coat systems with lower film thicknesses than mentioned earlier, but still meeting thehighest requirements. One thing is to apply the very best coating under laboratory conditions onto perfectly prepared test panelsand then expose these to the test. A completely different matter is to reproduce the same on site on thousands of square meters,around the clock, and sometimes under adverse conditions. Reality is often much more complicated and all conditions cantherefore not be realized. The more demanding a coating material is, the higher are the requirements during application, andconsequently much more responsibility is transferred to the applicator. This should be absolutely clear. Coating materials shouldtherefore not only be customer oriented, they have to be versatile and to a certain extent able to tolerate variations which occurin daily practice. Practical references, preferably from the offshore field, verify these points.

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Finally some examples for illustration: All objects are protected by products which are still in use for wind turbines.

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Windpark Utgrunden (Sweden), 2000

Windpark Middelgrunden (Denmark), 2001

Windpark Vindeby (Denmark), 1991

Windpark Samso (Denmark), 2003

Windpark Tune Knob (Denmark), 1995 Windpark Horns Rev (Denmark), 2003

West Alliance 2002, offshore rig (Keppel Shipyard Singapore) Stena Don, offshore rig, 2001 (Kvaerner Warnow Shipyard)

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Tunoe Knob exterior: 80 µm Metallization100 µm Epoxy paint (Hempadur 45230)100 µm Epoxy paint (Hempadur 45200)50 µm Polyurethane paint (Hempathane 55210)

interior: 40 µm Epoxy zinc dust paint (Hempadur Zinc 1536)2 x 140 µm Epoxy paint (Hempadur 45200)

Vindeby exterior: 120 µm Metallization100 µm Epoxy paint (Hempadur 45230)100 µm Epoxy paint (Hempadur 45200)50 µm Polyurethane paint (Hempathane 55210)

interior: 75 µm Epoxy paint (Hempadur 15570)150 µm Epoxy paint (Hempadur 45140)

Utgrunden exterior: 75 µm Epoxy zinc dust paint (Hempadur Zinc 1736)2x110 µm Epoxy paint (Hempadur 45880)

50 µm Polyurethane paint (Hempathane 55210)interior: 70 µm Epoxy zinc dust paint (Hempadur Zinc 1736)

150 µm Epoxy paint (Hempadur 45880)

Middelgrunden exterior: 100 µm Metallization100 µm Epoxy paint (Hempadur 45230)120 µm Epoxy paint (Hempadur 45140)50 µm Polyurethane paint (Hempathane 55210)

interior: 80 µm Metallization100 µm Epoxy paint (Hempadur 45230)100 µm Epoxy paint (Hempadur 45140)

Horns Rev exterior: 100 µm Metallization100 µm Epoxy paint (Hempadur 45230)120 µm Epoxy paint (Hempadur 45140)50 µm Polyurethane paint (Hempathane 55210)

interior: 80 µm Metallization100 µm Epoxy paint (Hempadur 45230)100 µm Epoxy paint (Hempadur 45140)

Samso exterior: 80 µm Metallization120 µm Epoxy paint (Hempadur 47140)100 µm Epoxy paint (Hempadur 47140)50 µm Polyurethane paint (Hempathane 55210)

interior: first 10 m60 µm Metallization

200 µm Epoxy paint (Hempadur 45140)than

50 µm Epoxy zinc dust paint (Hempadur Zinc 1736)100 µm Epoxy paint (Hempadur 47140)

monopiles (exterior/icecone): 3 x 300 µm Epoxy paint (Hempadur 45540)

West Alliance 2002 above waterline: Epoxy zinc dust paint (Hempadur Zinc 1736)Epoxy paint (Hempadur 45880)Polyurethane paint (Hempathane 55210)

below waterline: Epoxy paint (Hempadur 45750)

Stena Don above waterline: Epoxy zinc dust paint (Hempadur Zinc 1736)Epoxy paint (Hempadur 45880)Polyurethane paint (Hempathane 55210)

below waterline: Epoxy paint (Hempadur 45750)

Specifications for steel towers for wind turbines

The Hempel Group’s Head Office:

Hempel A/SLundtoftevej 150DK-2800 Kgs. LyngbyDenmark

Tel: +45 45 93 38 00Fax: +45 45 88 55 18E-mail: protective@dk.hempel.com http://www.hempel.com