Corr Protection for Windmills

J P C L / A p r i l 2 0 0 4 / P C E30

n 2001, 2,079 windmills were erected in Germany.

Considering the steel towers alone, this meant that more

than 2,000,000 sq m had to be protected. Components such as main frames,

hubs, gearboxes, rotor shafts, and generators, to name a few, also had to be

protected. On the basis of a predicted lifetime of more than 20 years after

erection of a windmill, and the difficult access expected in the future, it is

obvious how important a qualified corrosion protection system is. Defects

related to the system will sooner or later influence the operational availabili-

ty, are often quickly visible (on exterior areas), and may have a negative influ-

ence on the image.

This article will discuss the requirements for selecting protective coatings

for windmills, both on-shore and offshore, in terms of the available standards.

RELEVANT STANDARDSNowadays, how do we define and control the selection of a corrosion protec-

tion system, especially for windmills?

Out of numerous standards, two essential ones will be emphasized:

1. EN ISO 12944, Paints and varnishes— Corrosion protection of steel struc -

tures by protective paint systems (1998)

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

1999)

By Karsten Mühlberg,Hempel (Germany) GmbH, Pinneberg, Germany

C o r r o s i o nP r o t e c t i o nf o r Wi n d m i l l sO n - s h o r eand Off s h o r e

ITop: Windpark Utgruden

(Sweden) 2000Bottom: Windpark Tunoe Knob

(Denmark) 1995

It takes more than natureto keep windmills working.

Top right: Offshore windmills in operationCourtesy of Bonus Energy A/S

All other photos courtesy of the author

sion protection is the expected corro-

sion attack of the environment around

the location of the windmill and the

desired lifetime. The classification of

the environment in this respect can be

taken from EN ISO 12944 part 2. Five

corrosivity categories are defined from

(C1), not corrosive interior atmos-

phere, up to industrial and marine (C5-

I and C5-M). The determination of the

categories is the loss of a mass of

unprotected steel and galvanized steel

on outdoor storage. Im1 to Im3

describe exposure to water and soil.

Planning and erection of windmill

farms in Germany is done almost

exclusively in areas with rural to city-

like character. Erection in industrial

areas (e.g., in chemical plants) is rare.

Therefore corrosivity complies with

category C3 (moderate load). For a gen-

erally maintenance-free lifetime (pro-

tection time) of more than 15 years of

the coating, EN ISO 12944 recom-

mends in Part 5, various multicoat sys-

tems with dry film thicknesses from

160 to 240 µm.

Part 5 also gives definitions regard-

ing expected time of protection (dura-

bility) for the various systems, viz,

short (2 to 5 years), middle (5 to 15

years) and long (>15 years). These are

not guarantee terms but are rather the

period to the first planned mainte-

nance regarding corrosion protection.

They therefore help to select a coating

system with respect to its lifetime.

A typical coating system for on-

shore windmills nowadays is a three-

coat system consisting of:

• two-component epoxy-zinc rich

primer, 50–80 µm;

• two-component epoxy midcoat,

100–150 µm; and

• two-component polyurethane topcoat,

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In 1998, the EN ISO 12944 standard

(valid worldwide) was introduced and

currently is “the standard.” The various

parts define the corrosion protection

requirements, for minor aggressive

environments like interior areas, and

environments ranging from rural cli -

mates to city and industrial exposures

as well as for coastal and offshore envi-

ronments for steel constructions like

bridges. Corrosion protection for steel

structures in immersion service and for

galvanized steel is also described in EN

ISO 12944. The standard furthermore

contains indications about the lifetime

of the various systems.

The Norsok M-501 (revision 4, 1999)

standard was developed by “Norwegian

Technology Standards Institution” with

assistance by the oil and gas exploring

and processing industry. The various

parts describe solely and in detail the

corrosion protection of respective off-

shore-installations.

Finally, regarding offshore standards,

a draft of a new ISO standard has been

issued for voting. It is called ISO

20340: Paints and varnishes—

Performance requirements for protective

paint systems for offshore and related

structures (11/2003), and is expected

to be finalized this year.

The objectives of this standard are

laboratory test methods and evaluation

criteria for coating systems for steel

structures (new building) exposed to

offshore environments. The environ-

ments in question are the C5-marine

and the Im2, according to the ISO

12944, Part 2.

CORROSION PROTECTION

FOR WINDMILLS

Installation on Land (On-Shore)

Decisive for the choice of good corro-

50–80 µm.

Also possible are high-performance,

two-coat systems such as

• two-comp. epoxy-zinc rich primer, 80

µm and

• two-comp. polyurethane topcoat,

120 µm.

The classic and most commonly used

three-coat systems in the market often

have specifications with total dry film

thicknesses from 240 to 280 µm,

whereby the last mentioned almost

matches the requirements of the high-

est corrosivity category C5 (industry).

Due to the reasons already mentioned

(long lifetime without maintenance,

limited access, location at the coastline)

this safety margin is acceptable. But

200 µm outside and 160 µm inside (for

steel towers) should be the lowest limit

for on-shore installed wind turbines.

The recommended values are pri -

marily related to components exposed

to steady weathering, like the exterior

of the steel towers or hubs. Other

parts of the windmill (generator, gear-

box, rotor shaft, main frame, etc.). The

tower inside may be protected similar-

ly, but lower film thicknesses are possi-

ble because during operation of the

windmill, the dew point is seldom met

and some other parts will never have

atmospheric contact.

Installation in the Sea (Offshore)

The requirements for corrosion protec-

tion of windmills installed on-shore

have been defined with the corrosivity

category C3 (moderate load according

to EN ISO 12944, part 2). C5-M (high

corrosivity, sea) characterizes the con-

ditions to which offshore windmills are

exposed. Im2 (also part 2, EN ISO

12944) describes permanent exposure

to water (seawater and brackish

tems and coating materials can be used

as for the protection of on-shore wind -

mills. They differ merely in film thick-

ness and, in some cases, in the number

of coats. This statement is valid so long

as only top-quality products based on

epoxy and polyurethane resins have

been used for the protection of wind-

mills.

In this connection, I want to draw the

r e a d e r’s attention here to an important

point regarding “offshore-corrosion-pro-

tection” in comparison to “on-shore-cor-

rosion-protection” (Table 1).

On the basis of the described differ-

ences, the simple but important conclu-

sion is that qualified execution of the

coating job is the decisive criterion fo r

successful “offshore corrosion protec-

tion.” Some slackness and variations in

q u a l i t y, which are more or less normal in

the daily business, can often be “tolerat-

ed” and they are normally without direct

consequences for on-shore windmills.

They cannot be tolerated in offshore ser-

vice, for they would result in dramatic

failures. A short review of a damage

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water). EN ISO 12944 recommends

multicoat systems with dfts from 320

to 500 µm (atmospheric exposure C5-

M) and 480 up to 1,000 µm (immersion

Im 2). These statements are valid for

almost maintenance-free periods of cor-

rosion protection exceeding 15 years.

The NORSOK standard M 501 speci-

fies similar systems. For atmospheric

exposure, a dry film thickness of 335

µm is quoted while for permanent

water exposure, 450 µm (both are min-

imum thicknesses) is given.

A typical (above waterline) coating

system looks like:

• two-component epoxy-zinc rich

primer, 50–80 µm;

• two-component epoxy midcoat, 2

coats at 100–150 µm; and

• two-component polyurethane topcoat,

50–80 µm.

• The minimum film thickness total is

320–335 µm.

A system for areas under water (e.g.,

monopiles) might be

• 2–3 x epoxy coating, 225 µm each.

• The minimum film thickness total is at

least 450 and is better at 600 µm.

For areas under water (immersion

service) according to NORSOK cathod-

ic protection has to be used with the

coating system. The possibilities are

installing impressed cathodic corrosion

protection or welding sacrificial

anodes.

In summary, for corrosion protection

at sea, generally the same coating sys-

OFFSHORE

Extended exposure to condensationcombined with strong salinity and UV-light

Heavy cor rosion on holidays and weakareas of the coating

ON-SHORE

Generally cyclic dew/condensationwithout or with minor salinity

Moderate cor rosion on holidays and weakareas of the coating

Table 1: Comparison of On-Shore and Offshore Protection

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 %

Table 2: Damage Analysis of 120 Cases*

*From Mark Weston, “Who Pays When the Paint Fails?” JPCL (January 2000), pp. 56–59.

EN ISO 12944, part 6 NORSOK M 501On-shore exposure Offshore exposure offshore exposur e

corrosivity category C3 cor rosivity category C5M/ Im2 atmospheric/ immersion480h neutral salt spray 1,440h neutral salt spray 1 cycle:

(ISO 7253) (ISO 7253) 72h salt spray test (ISO 7253)16h drying time

240h 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 (cathodic1440h 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), and- corrosion creep from scribe.

Table 3: Examples of Tests for Coating Systems


a n a l y s i s of 120 cases, in Australia is

given in Weston, Journal of Protective

Coatings & Linings, January 2000, pp.

56–59 (Table 2).

QUALIFICATIONOF COATING SYSTEMS

In the field of “heavy-duty corrosion

protection,” coating materials based on

epoxy and polyurethane resins still

dominate as they have proven their

quality and they have impressive long-

term references. Nevertheless, due to

their extensive variety, especially for

special applications like offshore, it is

necessary to select coating materials by

testing them carefully. Such tests com-

prise temporal stress for coating sys-

tems (short-term tests) with a prede-

fined definition of their condition

before and after the tests. The assess-

ment of damage to the coating systems

from exposure to the different tests is

done visually and with instruments

(also refer to EN ISO 12944 and NOR-

SOK M 501). Table 3 gives examples.

REALISATION OF

THE COATING JOB

Successfully passed tests are necessary

but alone are not adequate criteria for

successful corrosion protection and not

at all for the requirements for

“Offshore Service.” When corrosion

protection of steelworks is being

planned, a suitable design must be con-

sidered (EN ISO 12944, part 3). All sig-

nificant points have to be put into a

specification, clearly and without ambi-

guity (EN ISO 12944, part 8). This

specification must be distributed to all

parties involved.

The process starts with the prede-

fined existing rust grade of the steel to

be used (ISO 8501/part 1, not worse

than B). Furthermore, it must be speci-

fied how welding seams and sharp

edges have to be prepared considering

Tunoe Knob outside: 80µm Metallization100 µm 2-pack epoxy sealer 100 µm 2-pack epoxy intermediate50 µm Acrylic polyurethane topcoat

inside: 40µm Epoxy zinc dust primer 2x140 µm 2-pack high-build epoxy

Vindeby outside: 120 µm Metallization100 µm 2-pack epoxy sealer 100 µm 2-pack epoxy intermediate50 µm Acrylic polyurethane topcoat

inside: 75 µm Epoxy primer/intermediate150 µm High-build epoxy

Utgrunden outside: 75 µm Zinc-rich epoxy primer 2x110 µm High-build epoxy intermediate 50 µm Acrylic polyurethane topcoat

inside: 70 µm Zinc-rich epoxy primer 150 µm High-build epoxy

Middel- outside: 100 µm MetallizationGrunden 100 µm 2-pack epoxy sealer

120 µm 2-pack epoxy intermediate50 µm Acrylic polyurethane topcoat

inside: 80 µm Metallization100 µm 2-pack epoxy sealer 100 µm 2-pack high-build epoxy

Horns Rev outside: 100 µm Metallization100 µm 2-pack epoxy sealer 120 µm 2-pack epoxy intermediate50 µm Acrylic polyurethane topcoat

inside: 80 µm Metallization100 µm 2-pack epoxy sealer 100 µm 2-pack high-build epoxy

Samso outside: 80 µm Metallization120 µm High-build epoxy 100 µm High-build epoxy50 µm Acrylic polyurethane topcoat

inside: first 10m 60 µm Metallization200 µm Epoxy sealer

then 50 µm Zinc-rich epoxy primer 100 µm High-build epoxy

monopiles: 3x300 µm High-solids epoxy (direct to metal)(outside/icecone)

West Alli- above Zinc-rich epoxy primerance 2002 waterline: High-build epoxy intermediate

Acrylic polyurethane topcoatbelowwaterline: High-solids epoxy (direct to metal)

Stena Don above Zinc-rich epoxy primer waterline: High-build epoxy intermediate

Acrylic polyurethane topcoat below 2-pack high-solids epoxy (direct to metal)waterline:

Table 4: Specifications for Steel Towers for Windmills

Note: Metallization was generally zinc/aluminum (85/15).

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later coating treatment (ISO 8501/3).

Clear directions regarding cleanliness

of the surface and roughness as well as

execution and control of all steps of the

coating job must be part of the specifi-

cation (EN ISO 12944, part 7).

The lifetime of a coating system

depends in general on five fa c t o r s .

• Quality of surface preparation

• Actual obtained coating thickness

• Quality of workmanship

• Quality of the coatings

• Conditions at location

The first three points are the responsi-

bility of the subcontractor and are

always the decisive ones. Execution of

the whole work according to existing

technical standards, well-organized qual-

ity assurance, and external quality con-

trol are, as experience shows, essential

prerequisites for successful implementa-

tion of all points given regarding corro-

sion protection.

Besides using qualified coating sys-

tems and executing all coating jobs

according to the best technical stan-

dards, you should refer to experiences

and references. The above examples of

modern working conditions are not stan-

dard everywhere. The field of corrosion

protection for windmills, especially fo r

steel towers, is today, more than ever

b e fore, marked by strong competition.

Coating application around the clock, 24

hours every day, under pressure of time,

carried out by foreign labourers with

whom communication is often very dif-

ficult, are typical components of a day of

coating work. Therefore, the demand

increases for faster drying of the coating,

lower thicknesses, and fewer coats. Does

this make sense in every case?

To d a y, it is possible to qualify one- and

two-coat systems with partly reduced

film thicknesses than mentioned befo r e ,

for the highest requirements. It is one

thing to apply the very best coating

under laboratory conditions onto per-

fectly prepared test panels and expose

these to a laboratory test. But it is a

completely different thing to reproduce

the same on site on thousands of square

meters around the clock and sometimes

under less than optimal conditions. In

r e a l i t y, it is all much more complicated

and can, therefore, not always be real-

i z e d .

The more demanding a coating mate-

Modern application and curing cabins with very good lighting and access

Table 4 shows a series of systems

suitable for the inside and outside of

steel windmills. Offshore windmills

and an offshore oil rig protected with

some of these systems are shown here

and on p. 30.

After studying Chemistry at the University of

Leipzig, Karsten Mühlberg worked for

Lacufa, a former East German paint manu-

facturer. In 1992, he joined Germany-based

rial is, the higher are the requirements

during application. Therefore, much

more responsibility is transferred to

the applicator. This should be absolute-

ly clear. Coating materials should

therefore not only be customer-orient-

ed, but also be versatile and able, to

some extent, to tolerate variations that

occur in daily practice. Nothing else

tells more about these remarkable

points than practical references,

preferably from the offshore field.


Hempel. He is currently responsible for

Hempel’s technical service department.

Stena Don, offshore rig, 2001(Kvaerner Warnow Shipyard)

Modern blast cleaning equipment, highly automated

Karsten Mühlberg

Windpark Middelgrunden (Denmark) 2001

Corr Protection for Windmills

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