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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,
J P C L / A p r i l 2 0 0 4 / P C E 31www.paintsquare.com
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
J P C L / A p r i l 2 0 0 4 / P C E32 www.paintsquare.com
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
J P C L / A p r i l 2 0 0 4 / P C E 33www.paintsquare.com
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).
J P C L / A p r i l 2 0 0 4 / P C E34 www.paintsquare.com
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.
J P C L / A p r i l 2 0 0 4 / P C E 35www.paintsquare.com
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