J. Coat. Technol. Res. DOI 10.1007/s11998-017-9979-5

REVIEW PAPER

Protective coatings for offshore wind energy devices (OWEAs): a review A. W. Momber, T. Marquardt

 American Coatings Association 2017 Abstract Coating specifications for offshore wind energy devices (OWEAs) are still based on specifications for the offshore oil and gas industry. Recently, two standards, developed for OWEA in the German offshore section (North Sea and Baltic Sea), were issued. This article reviews current offshore standards and commercial specifications used in the German OWEA industry. Stresses are defined for different zones of the structures. Specified coating systems for different stresses, including testing methods, are discussed. A review of 34 commercial coating specifications for OWEA in the North Sea and the Baltic Sea is provided. Evaluation parameters include number of layers, dry film thickness, and binder types for different coats. Special coating requirements are discussed, namely impact resistance, abrasion resistance, icing/ deicing performance, low friction, color and gloss stability, and low-temperature performance. Finally, trends for the utilization of thermal spray metals are reviewed. Keywords Offshore

Abrasion, Corrosion protection, Impact,

Introduction The production of electric power with offshore wind energy devices (OWEAs) plays an increasing role worldwide. Two basic structures of OWEA can be considered, namely tower constructions and transmission platforms. These structures are demanding engineering steel constructions. Figure 1 provides examples for both structural designs. OWEAs are exposed to a harsh environment, and they experience a A. W. Momber (&), T. Marquardt Muehlhan AG, Schlinckstraße 3, 21107 Hamburg, Germany e-mail: [email protected]

complex stress regime. Recent reviews about corrosion and corrosion protection of OWEAs are provided by Momber.1,2 Costs for the repair of protective coating systems of OWEA at sea (offshore) are assumed to be about 50 times higher than the costs considered for the initial application of the corrosion protection systems during the manufacture of the towers.3 For these reasons, protection systems for OWEA should be designed for a high reliability. The expected durability for corrosion protection systems for OWEA can exceed 25 years. Major design criteria for coating systems on steel include binder type, dry film thickness, and layer system.4–6 Because of a lack of particular OWEA standards, the industry used existing offshore coating standards,5–7 although the stress conditions are not precisely equal for OWEA and oil and gas platforms. For example, support structures of OWEA turbines are exposed to high dynamic stresses caused by wind, waves, and operation.8 The structures need to resist an extremely high number of stress cycles during their design life.9 The structures are unmanned, and systematic inspections and maintenance works are not possible. Moreover, the internal areas of foundations, namely monopiles, and of transition pieces were found to be vulnerable to corrosion.10 In 2016, two national standards, covering specifically the corrosion protection of OWEA structures in the German offshore section, were issued.11,12 One objective of this paper is to review existing standards for OWEA coatings and to provide a first review about 34 commercial protective coating specifications for OWEA structures in the North Sea and the Baltic Sea. Although the specifications are predominantly taken from German offshore wind parks, the results of this review are not restricted to the German offshore sector (except demands on gloss and color stability). Another objective is the discussion of special stresses acting on OWEA coatings in addition

J. Coat. Technol. Res.

Internal

Ameron (modified by author)

Atmospheric Atmospheric zone zone

Atmospheric zone Internal

Splash Splashzone zone Tidal Tidal zone zone

Splash zone

Submerged Submerged zone zone (under water)

Tidal zone Submerged zone

Buried zone

Buried (soil) zone (soil) Border wind

Buried zone

Fig. 1: Structure and corrosion zones of an offshore wind transmission platform and an offshore wind tower construction (the left graph is adapted from ‘‘4c-Offshore.com’’ and is modified by the authors)

to corrosive stresses. This contribution is restricted to steel structures. Rotor blades, although an important part of offshore wind mills, are not part of this review. Their loading system, which includes, among others, drop impact erosion, solid particle erosion, insects, icing, and lightning, is very different from the loading system for the steel structures.

Loading system General relationships In order to design protective systems, a loading system shall be defined. A recent review of coating damages to OWEA structures above sea level reveals that corrosive stresses and mechanical stresses are the dominating loading types.2 Additionally, biological stresses and special stresses shall be considered. Stresses being characteristic for the different zones of OWEA are summarized in Table 1.

Corrosive stresses The term corrosion is defined in ISO 8044.13 Corrosion can be described through a corrosion system, as illustrated in Fig. 2. It consists of three system phases, namely medium, material, and interphase. The domi-

nant media acting on OWEA are air (atmospheric with high salt content) and seawater. Additional media to be considered are condensation water in internal spaces and soil on foundations. In the regulative standards for the corrosion protection of steel, environments (media) are characterized in terms of corrosivity categories. Examples for OWEA constructions are provided in Fig. 1. The corrosive stress on OWEA is dependent on geographic and local environment. The composition of seawater, for example, differs for different geographical locations, and it is recommended to consider salt concentration and temperature in order to better assess the risk of corrosion.15 The local environment can be characterized through corrosion zones, as illustrated in Fig. 1. Corrosion zones are defined in ISO 12944-2.16 Results of corrosion measurements listed in Table 2 very well reflect the effects of corrosion zones on corrosion under offshore conditions. Under offshore exposure, corrosion speed is notably higher in the splash zone compared to the submerged zone and the atmospheric zone.5,18,20 Recent investigations have shown that the formation of electrochemically active species (e.g., cFeOOH) in the rust deposit, high concentrations of chloride in the inner rust layer, moisture regime in the outer rust layer, and pH changes during dry–wet cycles contribute to the high corrosion rates.21–23 Because the splash zone can also be subjected to mechanical stresses due to its location,24 particular attention shall be paid to its protection.

J. Coat. Technol. Res.

Table 1: Stress analysis for OWEA coatings Stress

Zone Atmospheric

Corrosion Condensation Impact Abrasion Color/gloss Skid Fouling Microbial Cathodic protection system Icing

Splash

Tidal

X

X

X

X

X X X

X X X

X

X

X X

X

Submerged

X

Buried

Internal

X

Special Decks

Boat landing

X

X

X X

X X X X

X

X X X

X X

X

X X X

X

X

Material selection

X

X X

Elimination of corrosive agents

Material Interphase

Medium (environment)

Un- and low-alloyed steel Un- and low-alloyed cast steel Cast iron Stainless steel Stainless cast steel Copper and copper alloys Aluminium alloys

Design

Air (atmospheric) Sea Water Condensed water Soil Oil, grease Fouling

Addition of inhibitors

Electrochemical protection

Linings, coatings

Fig. 2: Design of a corrosion system [modified from reference (14)] for offshore wind power constructions)

Table 2: Effects of corrosion zones on the corrosion rates of steel and hot-dip-galvanizing layers17–19 Corrosion zone

Buried in soil Underwater (submerged) zone Intermediate (tidal) zone Splash zone

Corrosion rate in lm per year Hot-dip galvanizing

Steel

16 Mn steel

A3 steel

– 8 10 10

100 200 250 400

50 140 70 230

60 180 110 270

Mechanical stresses Reviews about damages to coatings on OWEA reveal that about 45% of all damages are caused by mechanical stresses,2 mainly during the transport and the erection stage. Examples are provided in Fig. 3. Major

stress types are abrasion and impact. These stresses are part of the testing regime for coatings for offshore oil and gas applications,5 and they were recently introduced for the qualification of OWEA coatings.12 Mechanical stresses can also be caused by heavy waves.8

J. Coat. Technol. Res.

a

b Fig. 3: Mechanical damages to OWEA coatings.2 (a) Impact damage; (b) A brasive wear

Special stresses Special stresses include chemical stresses, stresses due to condensation, stresses due to low temperature, stress due to UV light, and stress combinations.16

Coating system structure and composition A coating system consists of a number of coating layers with different purposes.25 Basically, a corrosion protection coating system consists of the following layers4: – priming coat (first coat of a coating system); – intermediate coat (any coat between priming and topcoat); – topcoat (final coat of a coating system). Examples for coating systems designed for OWEA are provided in Fig. 4. Special coats, not considered in this review, include tie coat and stripe coat. Coating systems and individual coats are characterized through their dry film thickness (DFT), which is the thickness of a coat remaining on the substrate when the coating has cured. Regarding their protective mechanisms, corrosion protection coatings can be subdivided as follows25:

– barrier effect; – inhibitive effect; – galvanic effect. Barrier coatings may be applied as priming coat and intermediate coat. These coats are characterized by an inert pigmentation, namely titanium oxide, micaceous iron oxide, or glass flakes. Inhibitive coats are widely used as priming coats. The anticorrosive mechanism relies on passivation of the substrate and the buildup of a protective layer. Inhibitive pigments include a number of inorganic salts.25 Galvanic effects can be realized with sacrificial coatings. They perform according to the principle of galvanic corrosion. The substrate is protected by a metal (or metal alloy), which is electrochemically more active than the substrate material; the protective metals include zinc and aluminum or their alloys. Zinc-rich organic coats are widely used. Other examples are metal-sprayed coatings and hot-dip galvanized coats. This type of coat is applied as priming coat only as the metal must stay in direct electrical contact with the substrate. The majority of coats for OWEA are chemically curing systems, consisting of two packs, namely base component (binder/resin) and curing agent component (hardener). Regarding their binders (or resins), coats can contain the following types4: alkyd (AK), chlori-

J. Coat. Technol. Res.

Substrate

Substrate

a

b

Substrate

c

Substrate

d

Fig. 4: Corrosion protection coating systems for OWEA (polished cross sections). References: a–c: Fraunhofer IFAM, Bremen, Germany; d: SIZ, Du¨sseldorf, Germany. (a) Organic multiple coat system: 1 3 EP (Zn) priming coat, 2 3 EP intermediate coats, 1 3 PU topcoat. (b) Spray metallization system: 1 3 Al/Mg, 1 3 organic pore filler (sealer). (c) Duplex system: 1 3 Zn/Al15, 1 3 EP intermediate coat, 1 3 EP topcoat. (d) Hot-dip-galvanized coat

nated rubber (CR), acrylic (AY), polyvinylchloride (PVC), epoxy (EP), ethyl silicate (ESI), and polyurethane (PU). Most commonly used hardeners include polyamines, polyamides, and polyisocyanates, whereby the former are more suitable for priming coats. Recently, high-solid coats are developed and applied to OWEA. These are coating materials that contain a volume of solids between 70 and 100%. This review investigates OWEA coating systems according to the following parameters: – number of layers; – total dry film thickness; – binder type for individual layers.

Coating systems for corrosion protection on OWEA Recommended coating systems for OWEA A review about offshore coatings and OWEA coatings recommended in international regulations is provided in Table 3 for external atmospheric exposure, in Table 4 for splash zone and tidal zone exposure, in Table 5 for soil and underwater exposure, and in Table 6 for internal atmospheric exposure. The tables cover total DFT, DFT of individual coats,

number of layers, priming coat, intermediate coat(s), and topcoat. Atmospheric exposure The governing corrosivity category for external sections of OWEA in an atmospheric environment is C5-Mhigh.16 [A more aggressive corrosivity category (CX) is currently under consideration.] Coating system thickness for external atmospheric exposure (Table 3) varies between 280 and 500 lm (except spray-metal), whereby the majority is in the range of 320 lm. Number of layers vary between 2 and 5, whereby the majority is between 3 and 4. The majority of the priming coats (62%; spraymetal excluded) contains zinc dust (zinc-rich). Three specifications recommend spray-metal. Intermediate coats are almost entirely EP coats. Topcoats are almost entirely PU coats, mainly because of the good UV resistance and color stability. The thickness of systems specified particularly for OWEA ranges between 320 and 440 lm (except spray-metal). DFTs lower than 280 lm and layer numbers less than three are not allowed for OWEA (except spray-metal). Splash zone exposure The governing corrosivity categories for external sections of OWEA in the water transition zone and the splash water zone are C5-M-high/Im2-high.16 Coating

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Table 3: Organic coating specifications according to standards and national regulations for external atmospheric exposure (C5-M-high) Standard

Priming coat Total DFT (lm) Layers total

Offshore general ISO 12944-54

ISO 203407 NACE5

320 500 320 400 280 350 275 325 280

Norsok6 Offshore wind (OWEA) BAW/VGB12 320 320 320 320 320 260b 11 GfKORR 400 440 320b 320b a

Binder

Intermediate coats

DFT (lm)

Binder

DFT (lm)

Topcoats Binder

DFT (lm)

2–4 2 4–5 3–4 3 3 3 3 3

EP/PU EP/PU EPa/PU/ESI EPa/PU/ESI Zn(R)a Others than Zn(R)c EPa EP EPa

80 250 60 60 40 60 75 125 60

EP – EP EPC – – EP EP –

– – – – – – 125 125 –

PU EP/PU PU EPC – – PU PU –

– 250 – – – – 75 75 –

3 4 3 3 4 5 3 3 4 3

EPa EPa EP PUa PUa Spray metald EPa EP ZnAl15 ZnAl15

50 50 50–100 50 50 80–100 60 180 100–150 100–150

EP EP EP PU PU EP EP EP EP EP

190 2 9 95 140–190 190 2 9 95 2 9 80 260 180 2 9 120 240

PU PU PU PU PU PU PU/PSX PU/PSX PU PU

80 80 80 80 80 80 80 80 80 80

Zinc-rich; b without spray metal layer; c recommended for repair or areas subjected to special stresses; d metal not specified

system thickness for splash zone exposure (Table 4) varies between 450 and 1000 lm, whereby the majority is in the range between 500 and 600 lm. Number of layers vary between 1 and 5, whereby the majority is between 3 and 4. The majority of the priming coats consists of EP; one-third (31%; spray-metal excluded) contains zinc dust (zinc-rich). The use of zinc-rich priming coats is, however, critically discussed in references (5,7). Two specifications recommend spraymetal. Intermediate coats consist entirely of EP; some contain glass flakes for better diffusion control and increased mechanical strength. Topcoats are almost entirely PU coats, mainly because of the good UV resistance. The thickness of systems specified particularly for OWEA ranges between 580 and 800 lm, whereby a higher number of layers are recommended compared to general offshore specifications. DFTs lower than 580 lm and layer numbers less than two are not allowed for OWEA. Underwater and soil exposure Coating system thickness for under water and soil zone exposure (Table 5) varies between 350 and 800 lm, whereby the majority is in the range between 350 and 500 lm. Number of layers vary between 1 and 5, whereby the majority is between 3 and 4. The majority

of the priming coats consists of EP; 30% (spray-metal excluded) contain zinc dust (zinc-rich). One specification recommends spray metal. Intermediate coats consist entirely of EP. Because UV resistance is not important, topcoats are almost entirely EP coats. Topcoats are much thicker (150–450 lm) than for the other exposure zones. The thickness of systems specified particularly for OWEA is (with one exception) 500 lm. Internal atmospheric exposure Corrosivity categories for internal sections in an atmospheric environment vary between C2-high and C4-high.16 Coating system thickness for internal atmospheric exposure (Table 6) varies in a narrow range between 85 and 280 lm. An exception is the system for dry internal sections in the splash zone of OWEA, which is notably thicker (400 lm). The number of layers varies between 1 and 3. The majority of the priming coats consist of EP, whereby about 54% contain zinc dust (spray-metals excluded). One specification recommends spray-metal. Intermediate coats consist predominantly of EP. Because UV resistance is not important, topcoats are almost entirely EP coats. There is no difference between general offshore specifications and OWEA specifications.

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Table 4: Coating specifications according to standards and national regulations for splash zone exposure (C5-M/ Im2-high) Standard

Total DFT (lm)

Layers total

Offshore general ISO 12944-54 540 500 800 450 800 600 ISO 203407 450 450 600 NACE5 1000 Norsok6 600 Offshore wind (OWEA) BAW/VGB12 580 580 580 580 580 580 580 580 GfKORR11 700 800 600b 720b a e

Priming coat Binder

Intermediate coats

Topcoat

DFT (lm)

Binder

DFT (lm)

Binder

DFT (lm)

3–5 3 1 3 3 1–3 3 3 2 2 2

EPa EP EP ESIa EP EP Zn(R)a Others than Zn(R)c Others than Zn(R)c EPd EP

60 80 800 60 80 – 40 60 200 500 –

EP EP – EPd EPd EP – – – – EP/PE

– – – – – – – – – – –

PURC PU – EP EPd PU – – – EPd –

– – – – – – – – – 500 –

3 4 5 5 2 3 4 5 4 3 4 3

EPa EPa EPa EP EP EP EP PUa EP EP ZnAl15 ZnAl15

50 50 50 50–100 500 250 160–170 50 200 350 150 150

EP EP EP EP – EP EP PU EP EP EP EP

450 2 9 225 3 9 150 400–450 – 250 330–340 3 9 150 2 9 200 350 3 9 160 2 9 300

PU PU PU PU PU PU PU PU PU PU PUe PUe

80 80 80 80 80 80 80 80 100 100 120 120

Zinc-rich; b without spray metal layer; c recommended for repair or areas subjected to special stresses; d with glass flakes; polysiloxane or polyaspartic also possible

Commercial coating specifications for OWEA in the North Sea and the Baltic Sea Examples for commercially specified coating systems for OWEA for a North Sea offshore environment are provided in Fig. 5. The upper graph contains coating systems for a transition piece, whereas the lower graph features coating systems for a foundation structure (tripod). External atmospheric exposure Analysis results for commercial OWEA coating systems for external atmospheric exposure are summarized in Table 7. Regarding the dry film thickness, about 47% of all systems feature a total thickness >300–600 lm. This range is consistent with recommendations in Table 3, where nominal dry film thicknesses between 320 and 500 lm are recommended. Very thick systems (>900 lm) are specified in 4% of all specifications, whereby the reason for the specification of these unusually thick systems, particularly for sec-

ondary parts, remains unclear. Regarding the number of layers, two-thirds (66%) of all specifications specify three layers, which agrees with Table 3. Binders for the priming coats of primary parts are EPs (solid contents between 67 and 100%), partly with zinc (mainly zinc dust, but also zinc sulfate). This is in agreement with Table 3. For secondary parts, about 46% of the priming coats are spray metals (steel is considered here as substrate), which agrees with Table 3. Intermediate coats are entirely EPs with solid contents between 72 and 100%. The majority of all materials contained polyamine-adducted hardener. Two products contained glass flakes, which are not prescribed (and not required) for this environmental category. Topcoats are dominated through PU (100% for primary parts, 73% for secondary parts) with solid contents between 56 und 70%. Splash zone exposure Examples for commercial OWEA coating systems specified for splash zone exposure are provided in

J. Coat. Technol. Res.

Table 5: Coating specifications according to standards and national regulations for soil exposure (Im3 high) and under water exposure (Im2 high) Standard

Total DFT (lm)

Offshore general ISO 12944-54

ISO 203407 NACE5

Layers total

540 500 800 450 800 600 350 800 350 250–375b 350

Norsok6 Offshore wind (OWEA) BAW/VGB12 500 500 500 500 500 500 500 500 GfKORR11 350 500d a

Zinc-rich;

b

without spray metal layer;

c

Priming coat

Intermediate coats

Binder

DFT (lm)

3–5 3 1 3 3 1–3 2 1 2 1 2

EPa EP EP ESIa EP EP – – EPe Al EP

60 80 800 60 80 – 150 800 175 250–375 –

EP – EP – – – EPc – EPc – EP – – – – – – – EP (sealer, thinned) EP –

PURC PU – EP EPc PU – – EPe EP (sealer) EP

– – – – – – 200 – 175

2 3 4 4 1 2 3 4 2 2

EPa EPa EPa EP EP EP EP PUa EP EP

50 50 50 50–100 500 250 160–170 50 180 250

– EP EP EP – – EP PU – –

EP EP PU – – EP – – EP EP

450 225 150 – – 250 – – 180 250

with glass flakes;

d

Binder

Topcoat

DFT (lm)

Binder

– 225 2 9 150 400–450 – – 330–340 3 9 150 – –

in compartments with water exchange;

e

DFT (lm)

–

high solids

Table 6: Coating specifications according to standards and national regulations for internal atmospheric exposure (C3–C4 high) Standard

Total DFT (lm) Layers total Priming coat Intermediate coats Topcoat Binder DFT (lm) Binder DFT (lm) Binder DFT (lm)

Offshore general ISO 12944-54

280 280 240 240 Norsok6 150e 85e Offshore wind (OWEA) BAW/VGB12 240 240 240 200b GfKORR11 240 240 280 400d a

Zinc-rich;

b

2–3 2–3 3–4 3–4 1 2

EP EP EPa/PUa/ESI EPa/PU/ESI EP EPa

160 80 60 60 150 60

AY/CR/PVC EP AY/CR/PVC EP

– – – –

EP (tie coat)

25

2 3 2 3b 2 3 2 2

EPa EPa EP TS EPa EPa EP EP

50 50 50–100 80–100 60 60 200 200

– EP – EP – EP – –

– 190 – 20c – 120 – –

without spray metal layer;

c

sealer;

d

dry internal section in splash zone;

e

– PU – PU

– – – –

EP EP EP EP EP PU PU EP

190 80 140–190 180 180 60 80 200

fully dry and ventilated areas

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Transition piece internal (C3/C4 H)

Transition piece external (C5-M H)

2 × 200 μm EP 1 × 70 μm PU Σ 470 μm

1 × 200 μm EP 2 × 200 μm EP 1 × 70 μm PU

Σ 670 μm

Monopile external (C5-M H, lm2) 1 × 200 μm EP 1 × 200 μm EP Σ 400 μm

Accessories (internal ladders, platforms, catwalks) (1 × 125 μm FVZ) 2 × 90 μm EP 1 × 60 μm PU Σ 240 μm Additions external (landings, ladders) (C5-M H, lm2H) (1 × 85 μm SM) 2 × 250 μm EP 1 × 70 μm PU Σ 570 μm

atmospheric, external (C5-M H)

Additions, external (landings, ladders) (C5-M H)

1 × 60 μm EP(Zn) 1 × 200 μm EP 1 × 60 μm PU Σ 320 μm

1 × 60 μm EP(Zn) 1 × 200 μm EP 1 × 60 μm PU Σ 320 μm

Tripod structure,

Tripod, external (C5-M H + lm2 H) 3 × 200 μm EP 1 × 70 μm EP Σ 670 μm

Accessories, external (platforms, catwalks) (C5-M H + lm2 H) 3 × 200 μm EP 1 × 70 μm PU Σ 670 μm

Fig. 5: Selected coating system specifications for OWEA structures. Upper graph: Transition piece; lower graph: Foundation (tripod). SM = Spray-metal, FVZ = Hot-dip galvanized

Fig. 5. Analysis results for coating systems for this scenario are summarized in Table 7. Regarding the dry film thickness, 30% of the systems feature a total thickness >300–600 lm, and 57% of all systems feature a total thickness >600–900 lm, which is basically in the range of Table 4. Coating systems thinner than 450 lm, however, do not meet the requirements for splash zone exposure (compare Table 4). Very thick systems (>900 lm) are specified in 13% of all specifications, although this high number is recommended in one of 23 cases only (compare Table 4). Regarding the number of layers, two layers and three layers cover about 56% and 32% respectively. These values correspond to general offshore requirements, but they are on the lower limit of the OWEA recommendations

(Table 4). Twelve percentage of the systems cover one layer only, which is allowed according to ISO 12944-5, but not in the offshore/OWEA recommendations. About one-third of the priming coats (38%) contain zinc-rich epoxy. Intermediate coats are dominated through EP (88%), whereas topcoats are dominated through PU (79%). In contrast to atmospheric exposure, however, polysiloxane (3%) is specified for topcoats, which is allowed in the GfKORR guideline.11 Coatings for splash zone applications sometimes contain glass flakes for better diffusion control and to improve mechanical strength; this agrees with Table 4. A typical glass-flake requirement reads as follows: minimum of 25 wt% of the total pigment content; maximum flake length: 300 lm.

J. Coat. Technol. Res.

Table 7: Parameters of the coating systems from the 34 reviewed commercial OWEA coating specification (all numbers in percent) Assessment parameter

Readings in % Atmospheric internal C2–C4

Number of layers 1 Layer 2 Layers 3 Layers Total dry film thickness in lm <300 301–600 601–900 >900 Binder-type priming coat EP (Zn)a Spray metal Others Binder-type intermediate coats EP Polyester Others Binder-type topcoat PU PS EP Spray metalb a

Sometimes with Zn;

b

Atmospheric external C5-M

Splash and tidal zone C5-M + Im2

Submerged zone Im2

Total

Primary

Secondary

12 44 44

0 34 66

0 23 77

0 48 52

12 56 32

100 0 0

81 17 2 0

27 47 22 4

8 50 38 4

47 43 5 5

0 30 57 13

0 83 0 17

65 35 0

72 24 0

100 0 0

54 46 0

38 0 62

0 0 0

88 0 12

100 0 0

100 0 0

100 0 0

88 12 0

0 0 0

70 0 18 12

96 0 4 0

100 0 0 0

73 0 27 0

79 3 18 0

0 0 100 0

with sealer

Underwater zone and soil zone Permanently exposed sections (under water zone and soil zone) are found to be uncoated in almost all commercial OWEA specifications. These areas are predominantly protected with cathodic protection systems. In three cases only, submerged sections were coated, whereby the coating was a one-layer EP coating.

metal was also specified to some extent (35%). Intermediate coats mainly contained EP (88%). Topcoats are dominated through PU (70%), whereby the reason for this high number remains unclear. Spraymetal topcoats cover 12% of all specifications. [Although spray metals range under top (final) coat, it is in some cases associated with a tie coat, or sealer.]

Coating systems for mechanical stresses Internal atmospheric exposure

Impact resistance

Analysis results for commercial OWEA coating systems for internal sections in an atmospheric environment are summarized in Table 7. Regarding the dry film thickness, 81% of all systems feature a total thickness <300 lm. This is in agreement with Table 6. Thicker systems (>300 lm) are specified in 19% of all specifications, although this thickness is not required for dry internal atmospheric sections. Regarding the number of layers, 12% of all specifications specify onelayer systems, which would be sufficient for category C2,16 but not for higher categories. Priming coats were dominated through zinc-rich EP (65%), but spray

Mechanical stresses are a design criterion for offshore coatings in the oil and gas industry5 for many years, and they are introduced into recent OWEA standards.12 Table 8 lists respective testing scenarios and criteria. Impact resistance is particularly important for splash zone coatings, boat landings, escape routes, and lay down areas,6,24 but it is also critical for transport and erection processes.2 Information on impact testing is provided in Table 8. Impact tests on organic offshore coatings are performed in references (27–30). Figure 6 illustrates effects of testing temperature. The impact damage is more severe at the lower testing tempera-

J. Coat. Technol. Res.

Table 8: Recommended mechanical tests and acceptance criteria for offshore coating systems5,12 NACE5

Test Test method Impact strength

Abrasion test

a

VGB/BAW12

Acceptance criterion

Test method

Acceptance criterion

>3.4 J for non-deck areas Falling weight (ISO 6272-1) Height ‡60 cm (1.0 kg) ‡5.9 J No cracking (holiday testing) >5.6 J for decks and boat-landing splash zone area Taber abrasion Coating thickness reduction BAW Standarda Abrasion numberb aW £ (ASTM D4060) <50 lm per 1000 cycles Rotating drum with water–grit mix- 40 lm per 10,000 rev ture (2:1) Grit: 4 kg basalt, 3–12 mm Falling weight (ASTM G14)

See reference (26) for a detailed description;

b

abrasion number aW = DDFT 9 10,000/200,000 (DFT = dry film thickness)

Fig. 6: Effects of temperature on impact resistance31; equal impact height: 1.0 m. Left: 40°C; right: 0°C

Table 9: Results of impact tests on offshore coatings29 Coating

EP + PU EP PU (Zn) EP + PU EP EP Rubber a

Total DFT in lm

Layers total

1200 525 500 615 375 1500 4000

Accelerated aging according to ISO 203407;

3 3 4 3 3 3 1 b

Failure impact energy in J 20C

60C

Ageda

12.2 11.6 32b 7.6 10.5 10.5 32b

12.2 7.4 29.5 5.2 5.3 5.4 –

– 32b 32b 10.5 16.8 – 32b

maximum available impact energy

ture. Table 9 summarizes results of impact tests on offshore coatings, namely effects of testing temperature and accelerated aging. Impact resistance decreases for lower temperatures, and it increases after aging. This is in agreement with results in Bjoergum et al.27 for impact tests on offshore coatings with impact energies up to 10 J. Three typical types of damage can be identified on impinged coating surfaces. Figure 7 categorizes these damage types as follows: (i) plastic deformation,

associated with material pileup around the impact site (Fig. 7a); (ii) elastic–plastic deformation and the formation of radial cracks (Fig. 7b); (iii) elastic–plastic response with severe coating detachment (Fig. 7c). Figures 7d to 7f illustrate deformation and fracture features in brittle thin glass coatings. The similarity between the damage morphologies of the organic coating system and the glass coating is obvious. Depending on the impact energy (EI), the damage types can be categorized as follows:

J. Coat. Technol. Res.

a

d

R R R R R R

R

R R

e

b R

L R

L

R R R L

c

L

R

R

R

L L

L

R

R

L

f

Fig. 7: Contact damage morphologies for organic coatings and for brittle coatings (glass).30 Left column (a–c): Organic coating.30 Right column (d, e): Soda-lime glass coating (180 lm)32; (f): Glass coating (4 mm).33 (a, d) Plastic response with permanent depression (EI > EY). (b, e) Elastic–plastic response with radial crack (‘‘R’’) formation (EI > ER). (c, f) Elastic– plastic response with radial (‘‘R’’) and lateral (‘‘L’’) crack formation and coating detachment (EI  ER)

EI < EY fi no damage EI > EY fi plastic response (Figs. 7a and 7d) EI > ER fi elastic–plastic response with radial cracking (Figs. 7b and 7e) EI  ER fi elastic–plastic response with radial and lateral cracking (Figs. 7c and 7f) Here, EY is the impact energy required for plastic deformation, and ER is the impact energy required to form radial/lateral cracks. Based on the identical damage morphologies for polymeric coatings and glass coatings in Fig. 7, threshold criteria for deformation and fracture of brittle coating materials may be applied to discuss the results in Table 9. Ball indentation tests

on thin brittle coatings reveals the following criterion for the initiation of radial cracks34: PR / HC1 PY

ð1Þ

Here, PY is the indentation force required for plastic deformation, PR is the threshold indentation force for radial crack initiation, and HC is the coating hardness. and E = ½ 9 mB 9 v2B, equation (1) With P  v6/535 B can be rewritten as follows: ER 5=3 / HC EY

ð2Þ

J. Coat. Technol. Res.

temperature promotes abrasion. Figure 8 depicts a linear increase in coating abrasion over exposure time. The rotating drum abrasion test12,26 delivers an abrasion parameter, which is estimated as follows:

Here, ER is the critical impact energy for radial crack formation, EY is the critical impact energy for plastic deformation, mB is the ball weight, and vB is the velocity of the impinging ball. Hardness of organic offshore coatings increases with a decrease in temperature.36 Kotnarowska and Wojtyniak37 found hardness to decrease after UV radiation and exposure in NaCl solution. Therefore, equation (2) depicts at least qualitative trends correctly. The indentation approach was recently proven for effects of artificial aging38 and for effects of temperature30 on the impact resistance of organic offshore coatings.

aW ¼

Abrasion resistance is particularly important for coatings in the tidal zone, subjected to floating ice and fouling removal tools,24 but also during transport and erection processes.2 Information on abrasion testing is provided in Table 8. The Taber abrasion test5 delivers a wear index, which is estimated as follows: 1000ðA  BÞ : C

ð4Þ

Here, aW is the abrasion parameter in lm per 10,000 revolutions, DDFT is the total reduction in film thickness in lm, and the numbers correspond to the number of revolutions of the drum. The acceptance criterion is aW £ 40. Typical aW-numbers for highsolid EP and PU corrosion protection coatings range between 26 and 72.31 This test, originally developed for coatings on weirs and river barriers,26 is specified for OWEA coatings in the VGB guidelines.12 It remains unclear how this test could replicate abrasion stresses to OWEA coatings. It is shown by Binder42 that the test overestimates real abrasion rates for organic coatings. The ratio nature/laboratory is 12 for PU and 159 for EP. The abrasion ratio EP/PU is about 24 for the laboratory test, whereas it is about 2 under real abrasion conditions.42 In order to investigate the wear of coatings on boat landings of OWEA, Bjoergum et al.43 utilized a rubber sliding test. A rubber sample was slit against the coatings with a force of 200 N and a frequency of 0.1 Hz. The force value was thought to correspond to the impact force of a service boat (10,000 N). Results are provided in Table 11. Powdercoated hot-dip galvanized surfaces exhibited an excellent wear resistance under dry conditions, but under

Abrasion

WI ¼

DDFT  10;000 : 200;000

ð3Þ

Here, WI is the wear index in lm per 1000 cycles, A is the average DFT at 1000 or less cycles, B is the average DFT after 3000 cycles, and C is the number of abrasion cycles. The acceptance criterion is WI £ 50. Results obtained on offshore coatings are provided in Table 10 and in Fig. 8. Table 10 reveals that systems with special inclusions (Al2O3, MoS2) provided an extremely high abrasion resistance and that a low

Table 10: Results of abrasion tests on offshore coatings with Taber abrader Coating

Total DFT in lm

Layers total

Abrasion mg/1000 cycles39

Zinc silicate/EP polyamide Inorganic zinc/EP polyamide Zinc silicate/PU coal tar Zinc silicate/EP EP coal tar PE + glass flakes EP/PU EP (Al2O3) PU (Zn) EP/PU EP EP PAI (MoS2) a

395 295 400 375 380 1000 1200 525 500 615 375 1500 30

Acceptance criterion: 50 lm/1000 cycles (Table 8)

3 3 3 3 4 2 3 3 4 3 3 3 2

64 51 48 29 82 54 – – – – – – –

lm/1000 cyclesa,40,41 20C

0C

– – – – – – 33 15 41 47 26 22 9

48 10 53 41 39 37 12

J. Coat. Technol. Res. 120

90

Coating 5

Coating 6

Temperature 20 0 Output

Temperature 20 0 Output

Coating abrasion in µm

Coating abrasion in µm

120

60

30

90

60

30

0

0 0

500

1,000

1,500

2,000

0

500

Number of revolutions

1,000

1,500

2,000

Number of revolutions

Fig. 8: Results of abrasion tests for two offshore coating systems at two temperature levels40

Table 11: Results of rubber sliding tests on offshore coatings43 Weight loss in ga

Coating system Priming coat EP (Zn) EP (Zn) EP (Zn) EP (Al) EP (modified) EP Hot-dip-galvanized HDG PE (glass flake)

Intermediate coat

Topcoat

DFT in lm

Air (700 s)

Artificial seawater (1800 s)

EP PSI EP (modified) EP (surface tolerant) EP (surface tolerant) EP (glass flake) (HDG) Powder coating

PU

310 350 280 450 500 500 200 300 1500

0.004 0.003 0.009 0.005 0.012 0.009 0.006 0.001 0.019

0.004 0.006 0.0015 0.0025 0.01 0.013 0.0065 0.008 0.032

PSI PSI PSI

A rubber sample was slit against the coatings with a force of 200 N ( fi 10,000 N from service boat force) and a frequency of 0.1 Hz in two different media (air and artificial seawater) a Approximated from the graph published in reference (43)

wet conditions other coatings performed better. Epoxy with glass flakes performed worst, which conflicts with the recommendations in NACE5 and DNVGL.24 Friction coefficient measurement on organic offshore coatings (PU, PSO, EP) revealed friction coefficients between 0.41 and 0.50 in air and between 0.12 and 0.19 for water.27 Detailed investigations of organic offshore coatings40 have shown that Taber abrasion resistance depended on filler content and filler material hardness at moderate temperatures. Based on SEM inspections of worn surfaces, the authors identified a number of different material removal modes, including plowing, microcrack formation and extension, filler fragmentation, filler particle pullout. Two examples are provided in Fig. 9. The left column illustrates the separation and detachment of filler/pigment particles, whereas the

right column illustrates a plowing process. A model equation for the abrasion of organic offshore coatings reads like follows40: WA ¼ ½ð1  nF Þ  WM þ ðn1 þ n2 þ n3 Þ  WF   kF |fflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflffl} |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} Matrix



Filler

kA ð1=TR Þ |{z}

Abrasive

ð5Þ kF  1; kA  1

kF ¼ f ðn2 Þ

In the equation, WA is the volume of the totally abraded coating, and WM is the volume of the abraded

J. Coat. Technol. Res.

Fig. 9: Images of two worn offshore coatings (left and right columns, respectively) after Taber abrasion.40 Upper row: 3D white light interferometry; middle row: 3D laser confocal microscope (note different depth scales); lower row: SEM photograph (white light interferometry and laser confocal microcopy were performed at different locations). Left column: filler pullout; right column: plowing/scratching

J. Coat. Technol. Res. 0

10 min

50 min

80 min

90 min

Fig. 10: Development of hoar frost accretion on an offshore coating44

800

800

New Aged

Hoar frost thickness in µm

Hoar frost thickness in µm

New Aged 600

400

200

600

400

200

0

0 0

30

a

60

90

120

Static contact angle in °

0

b

10

20

30

40

50

60

Specific surface energy in mN/m

Fig. 11: Relationships between coating surface parameters and hoar frost thickness.44 (a) Static contact angle; (b) Specific surface energy

Table 12: Results of icing/deicing tests on offshore coatings27 Ice adhesion in Nma

Coating system Type

Layers total

DFT in lm

30C

60C

EP (Zn)/EP/PU EP (Zn)/EP/PSI Glass flake EP (Al)/EP Rubber (vulcanized)

3 3 2 2 1

310 280 1500 450 4500

14 15 32 10 45

45 53 80 33 63

a

Frozen artificial seawater

matrix. The model ranks the contributions of generic matrix material (subscript ‘‘M’’), filler material (subscript ‘‘F’’), and abrasive material (subscript ‘‘A’’). It also considers temperature effects (1/TR). The parameter nF is the relative filler content (ratio of filler volume to total coating volume). For nF = 0, the abrasion is that of an unfilled, plain matrix material, and the second term of equation (5) can be omitted. It is expected that abrasion models developed for unfilled

polymers can be applied to that situation. The parameter n1 characterizes the amount of soft filler materials (ratio between volume of fillers with a hardness lower than the abrasive material hardness to total filler volume). The quantity n1 9 WF is the filler volume removed due to plastic deformation/plowing (see Fig. 9, right column). The parameter n2 measures the amount of filler particles deficiently bonded to the surrounding matrix related to the total filler volume.

J. Coat. Technol. Res.

The quantity n2 9 WF is the volume of filler pullout (see Fig. 9, left column). The parameter n3 characterizes the amount of filler material vulnerable to fracture and fragmentation (ratio of brittle filler materials to total filler volume). The quantity n3 9 WF is the volume of filler material fractured during the abrasion process. The dimensionless parameter kF describes a reinforcement effect due to excavated hard filler particles, which contribute to the abrasion process. It is related to the parameter n2 and to the hardness of the fillers; for n2 = 0, or for filler materials with a hardness lower than that of the abrasive material, the following condition applies: kF = 1. The dimensionless parameter kA finally describes a supporting effect of free abrasive particles torn off the rotating disk during the abrasion process. If no free, unbonded abrasive debris is involved in the abrasion process, kA = 1. If abrasive debris contributes to the abrasion process, kA > 1. The process then turns into a three-body

Fig. 12: Low-skid coating on an OWEA deck with safety painting (Muehlhan AG, Hamburg)

abrasion process (coating; abrasive disk; abrasive debris). The dimensionless temperature parameter is TA = 1 for 20C, and it increases with a decrease in temperature.40

Coating systems for special stresses Icing/antiicing The icing/antiicing performance of coatings is a major issue for OWEA rotor blades, and a number of investigations address this issue. However, icing can be a serious safety issue on offshore structures. The icing/antiicing performance of OWEA corrosion protection coatings has not been investigated in detail yet. Hoar frost accretion on new and artificially aged offshore coatings was investigated in Momber et al.44; see Fig. 10. The authors utilized statistical methods in order to judge relationships between hoar frost thickness and topcoat wettability parameters, namely static contact angle and specific surface energy. Two examples are provided in Fig. 11. None of the investigated parameters showed statistically significant relationships to the accretion of hoar frost. Bjoergum et al.27 investigated the adhesion of ice to offshore coatings. Ice was formed from distilled water and from seawater. Results of the sea-ice tests are provided in Table 12. Ice adhesion increased at lower temperatures. This trend was also noted for ice made from distilled water. Rubber and glass-flake-containing materials revealed a bad icing performance. Ice adhesion was high on these surfaces, and ice will become difficult to remove. Wettability parameters for typical offshore topcoats are investigated by Momber et al.45

Table 13: Recommendations for low-friction coating systems5,6 NACE5

Parameter System 1 Total DFT (lm) Layers total Priming coat Binder DFT (lm) Intermediate coat Binder DFT (lm) Antiskid coat Binder DFT (lm) Topcoat Binder DFT (lm) a

Norsok6

System 2

System 3

650 4

700 4

300 2

3000 1

EP (Zn) 75

EP 125

– –

Nonskid EP screed

EPa 250

EPa 250

– –

EPb 250

EPb 250

Metal spray (Al)d 300

PUc 75

PUc 75

Sealer (no additional DFT)

High solid content; b mixed with fine abrasive grit; c safety marking; d metal-spray process adjusted in a way that finished Al has an antiskid profile set at a desired coarseness specification

J. Coat. Technol. Res.

Low-friction coatings Low-friction (nonskid) coatings are specified on OWEA transformation platforms for decks, runways, and escape routes. An example is provided in Fig. 12. Typical coating systems recommended for decks, floors, and helidecks of offshore constructions are provided in Table 13. Low-friction coatings can be selected according to friction classes. Friction classes are specified according to an inclination angle in a defined testing arrangement.46 Friction classes (‘‘R’’ classes) specified for OWEA include the following (numbers in brackets designate the corresponding inclination angles46:

a 0.6

CIE Y

1 – Colour range for new coating 47 2 – Colour range for coating in service 47 3 – Assessment results for Fig. 13a

0.5

– external decks: R10 (>10–19); – external decks, runways: R11 (>19–27); – internal decks: R9 (6–10). In order to realize the higher friction classes, it might be required to blend fine quartz (or other particulates) into the coating layer. For walkways, escape routes, and lay down areas, Norsok6 recommends the use of light colored nonskid aggregates with particle sizes between 1 and 5 mm. Examples from the investigated commercial OWEA specifications, however, feature finer grit: 0.1–0.3 mm for R9 and 0.7– 1.2 mm for R11. Further demands on nonskid coatings include impact resistance, hardness, and flexibility.6

1 2 3

0.4 0.4

0.5

b

0.6

Color and gloss

CIE X

Fig. 13: Normal color table, illustrating yellow color change due to aging. (a) Sections marked (16 measurement marks each) for color measurements; (b) Color ranges and measurement results. Denotations ‘‘1’’ and ‘‘2’’ are color ranges according to reference (47); denotation ‘‘3’’ depicts measurement results from the sections marked in photograph (a); data from reference (2)

Requirements on color and gloss originate from UV exposure in the external atmospheric zone and in the splash zone. For the North Sea and the Baltic Sea, requirements are specified in the VGB guidelines12 and in the WSV guidelines.47 Colors for outside sections of OWEA specified in the investigated commercial specifications include RAL 1023/yellow (72%) and RAL

Table 14: Effects of low temperatures on the performance of organic offshore coatings28,29 Coating Type

EP/PU EP PU (Zn) EP EP EP a

Performance parameter Layers total

3 3 4 3 3 3

DFT in lm

1200 525 500 615 375 1500

AE

a

Scribe creep in mm

Adhesion in MPab

20C

60C

20C

60C

20C

60C

94 76 87 77 83 81

87 25 71 47 32 77

1.0 19 4 14.4 7.7 10.4

4.2 >25 2.8 13.6 >25 14.3

21.8 22.1 3.3 9.3 8.6 22.3

23.3 22.2 5.2 17.5 16.6 21.0

Anticorrosive effect, for new coatings: AE = 100;

b

Pull-off test

J. Coat. Technol. Res.

a

b

–20 °C

–60°C

–20°C

–60°C

Fig. 14: Corrosion protection performance of two offshore coating systems at low temperatures.52 (a) Temperatureinsensitive system (2 3 EP + PU, total DFT = 1200 lm). (b) Temperature-sensitive system (3 3 EP, total DFT = 525 lm)

Table 15: Species found on towers of different OWEA locations53–59 Species Algaes Anemones Barnacles Cnidaria Crabs Green laver Mussels Polyp Sea urchin Seaweed Small fish Sponges Starfish Tang Worms Molluscs a

Barrow

Coast of England

Helgolanda

North Sea

Baltic Sea

Nysted

Egmond

X X X – X

X X X – X X X X – X X X – X X –

X X X X X – X – – – X X – – X –

X X – – X – X X – – – – X – – –

X – – – X X X – – – – – – – X X

X – X – X – X – – – – – – – X X

X X X X X X X – X – – X X – X X

X X X – X – X – – –

OWEA test samples (see Fig. 15)

7035/gray (25%). In the investigated specifications, 67% of all specified gloss numbers require gloss 40–60 (60), and 33% require gloss 50–70 (60).48 Topcoat color stability is an important safety issue of offshore wind farms. Particularly, the luminous yellow color in the immediate transmission zone must meet specific criteria in terms of color stability.47 For this reason, UV-resistant topcoatings are required. The VGB guidelines12 prescribe the testing of the yellow color (RAL 1023) in the atmospheric zone, splash water zone, and tidal zone.49 Two criteria are defined regarding the color difference: (1) DEab  3 new against RAL; (2) DEab  3:5 after UV exposure against new. Requirements are illustrated in Fig. 13. The threshold criteria for a newly applied yellow topcoat (designated ‘‘1’’) and a topcoat in service

(designated ‘‘2’’) in a color space are provided in Fig. 13b. The graphs illustrate an alternative assessment technology, based on photographic images.2 The reviewed sections are marked in Fig. 13a. Each section is subdivided into 16 measurement fields. As can be seen, the in-service color data (designated ‘‘3’’) are located outside the specified acceptance ranges. There is a trend toward the ‘‘red’’ section of the color space for the situation ‘‘3.’’ High color stability can be achieved with polysiloxane topcoats; their color change is less than DE = 1 after long-time atmospheric exposure.50 Polysiloxane is recommended for OWEA topcoats under splash zone conditions (see Table 4). Gloss can be held with polyurethane, polysiloxane (silane), and silicone amine.51 Epoxy performs bad, even if UV absorbers are added to the resin.51

J. Coat. Technol. Res. Zn/Al + EP

Zn/Al + EP

EP (Zn) + PU

EP (Zn) + EP

Front side with fouling

Front side after algae removal

Front side after final cleaning

Rear side

Fig. 15: Conditions of coated OWEA samples after 36 months in the intermediate zone of an offshore location53

Table 16: Effects of OWEA coating systems on fouling in the submerged zone under North Sea conditions after 12 months60 Coating system Type PU EP Foil EP Textile EP EP

Fouling mass increase in % Layers 1 3 1 1 1 2 1

21 22 17 27 28 50 37

Low temperature Low temperatures can deteriorate the protection performance of offshore coatings. Results of detailed investigations performed in Momber et al.28,29 are provided in Table 14. The results reveal a decrease in corrosion protection performance and an increase in

scribe creep, but an increase in adhesion at the low temperature. In terms of corrosion protection, the authors noted two types of coatings: temperaturesensitive coatings and insensitive coatings.52 This guides a way for the selection of coating systems for low temperatures. Examples for either coating type are shown in Fig. 14. Low temperatures have effects on abrasion resistance (Table 10), impact resistance (Table 9), and icing (Table 12). Antifouling Fouling and settlement of structures are phenomena that can affect corrosion as well as the performance of corrosion protection coatings, mainly in the underwater zone. Settlement can occur due to bacteria, plants, or animals. Investigations covering these circumstances have been performed on OWEA, but not in terms of corrosion protection, but rather from the point of view of ecological effects. A selection of reported results can be found in Table 15. Figure 15 shows examples for the fouling of coated steel samples, which were subjected to offshore conditions in the intermediate (tidal) zone

J. Coat. Technol. Res.

Table 17: Spray metal systems and duplex systems for offshore and OWEA Zone

External atmospheric

Decks and floors Splash zone

Submerged zone

Internal atmospheric

Total DFT (lm) Layers total

420–470a 420–470a 250b 340–360c 300d 240i,j 450i,j 320i,j 300–400b 750a 870a 250b 450i,j 320i,j 250–375b 450i,j 320i,j 280–300c 160i,j 240i,j 450i,j 320i,j

4 3 3 5 4 3i 3i 3i 2 4 3 3 3i 3i 3 3i 3i 3 2i 3i 3i 3i

Spray metal

Intermediate coats

Metal

DFT (lm)

ZnAl15 ZnAl15 Al Spray metale Al, ZnAl Al, Zn, ZnAl Al, Zn, ZnAl Al, Zn, ZnAl Al ZnAl15 ZNAl15 Al Al, Zn, ZnAl Al, Zn, ZnAl Al Al, Zn, ZnAl Al, Zn, ZnAl Spray metale Al, Zn, ZnAl Al, Zn, ZnAl Al, Zn, ZnAl Al, Zn, ZnAl

100–150 100–150 250 80–100 100 – – – 300–400 150 150 250 – – 250–375 – – 80–100 – – – –

Binder

DFT (lm)

EP 2 9 120 EP 240 EP (thinned sealer)f,g EP (+sealer) 160 (+20) +(sealer) 125 EP, PU (sealer)g EP (sealer)g EP, PU (sealer)g EP (sealer)g EP 3 9 160 EP 2 9 300 EP (thinned sealer)f,g EP (thinner)g EP, PU (thinner)g EP (thinned sealer)f,g EP (sealer)g EP, PU (sealer)g EP 180 EP, PU (sealer)g EP, PU (sealer)g EP (sealer)g EP, PU (sealer)g

Topcoat Binder

DFT (lm)

PUh PUh EP (sealer)g PU – EP, PU EP, EPC EP, EPC

80 80

PUh PUh EP (sealer)g EP, EPC EP, EPC EP (sealer)g EP, EPC EP, EPC EP (sealer) EP, PU EP, PU EP, EPC EP, EPC

80 75 240 450 320 120 120 450 320 450 320 20 160 240 450 320

a GfKORR11 for OWEA; b NACE5 for offshore; c VGB12 for OWEA; d Norsok6 for offshore; e not specified; f seals the porosity of sprayed Al; g no additional DFT; h Polysiloxane or polyaspartic also possible; i without spray metal layer; j ISO 12944-54

Table 18: Corrosion rates of spray metals in lm/year after 11 years North Sea exposure65 Zone

Atmospheric Splash water Tidal Submerged

Spray metal (on S235JR steel substrate) Zn

ZnAl2

ZnAl4

ZnAl15

ZnAl22

ZnAl

ZnAlMg5

2 10 >50 >50

<1 9 18 18

<1 9 9–11 9–11

<1 <1 <1 <1

<1 <1 <1 <1

<1 <1 2 1.8

<1 <1 <1 <1

for a period of three years. Values for the biomasses of more than 40 species, which had settled on OWEA in the Swedish part of the Baltic Sea, are summarized and categorized in Wilhelmsson and Malm.56 A detailed determination of antifouling effects of typical OWEA coatings has not been provided yet. Bessell59 provided some data on the fouling on OWEA coatings in the North Sea; results are provided in Table 16. Clear trends cannot be derived from these numbers, particularly because of missing information about the coatings used, but it seems that a PU-based system performed better than EP-based systems. An antifouling foil seems to provide a very good antifouling

performance. This is a topic for further systematic investigations. Cathodic protection Cathodic protection (CP) can lead to delamination of protective coatings if the coatings are not CP compatible. The testing scenario for cathodic disbonding tests for offshore coatings is outlined in ISO 20340.7 For OWEA coating systems, the reader may refer to Momber et al.,51 who reported about compatibility testing of OWEA coating systems (organic, duplex)

J. Coat. Technol. Res.

Atmospheric zone

Splash water zone

Tidal zone

Red rust due to external removal

Fig. 16: Metallurgical cross sections of ZnAl spray metal after application (upper row) and after 11 years North Sea exposure in different exposure zones (lower row)65; substrate: S235JR steel

Table 19: Evaluation results for long-term OWEA coating systems66 Coating system

Assessment

Spray Thickness in metal lm

Organic layers

Layers total

Total DFT in lm

ISO 20340a

Barrier performancea,b

Field exposurea,c

Corrosion creep in mm 20340 Fieldc

ZnAl – ZnAl – – – – a d

100 – 100 – – – –

1 = best, 6 = worst; with glass flakes

EP EP PSI PEd EP EP + PU EP (Zn) b

3 3 1 2 1 2 3

560 560 350 1500 550 700 800

6 2 6 1 4 4 3

4 4 7 6 1 1 1

based on electrochemical impedance spectroscopy (EIS);

according to ISO 15711-262 and ASTM G8.63 Mahdavi et al.64 provide a review about recent development in testing for the cathodic delamination of coatings.

Spray-metal coatings and duplex coatings Spray metal is recommended in particular for mechanically stressed sections of OWEA structures.

c

3 6 1 1 6 5 4

>20 5 >20 1 7 7 6

3 7 0 0 9 5 4

after 16 months North Sea exposure;

Spray-metals are usually applied to flange connections, but also to frames and platform railings. However, they are frequently applied to entire tower sections as duplex systems (in combination with an organic coating system). A review about recommended spray-metal coatings and duplex systems for general offshore and OWEA applications is provided in Table 17. The recommendations include Al, Zn, and ZnAl, whereas reference (5) recommends pure Al. Spray-metal layers range between 80 and 150 lm. Exceptions are spray-

J. Coat. Technol. Res. Organic EP (DFT = 560 µm)

After site exposure (2.5 years, splash zone, North Sea)

After ISO 20340 testing

Duplex ZnAl15 + EP (DFT = 660 µm)

Fig. 17: Two offshore coating systems (left: duplex; right: organic) after different testing scenarios68

0

4 weeks

metal layers for the submerged zone (375 lm) and for decks and floors (400 lm). Detailed investigation into the protection performance of spray-metals in different corrosivity categories after long-term offshore exposure (11 years) revealed that ZnAl15 and ZnAl20 alloys provide very good protection (in terms of corrosion rate) in the splash zone, the tidal zone, and the submerged zone. Plain Zn layers performed worst in all zones.65 Results are provided in Table 18 and in Fig. 16. The samples in Fig. 16 do not show any red rust originating from the substrate. The two spots on the splash water zone samples originate from external material removal. The steel substrate (S235JR) is not affected. The alloy ZnAl15 is recommended in the GfKORR guidelines11 in combination with an organic system for OWEA in the atmospheric zone and the splash zone (Tables 3 and 4). A number of authors performed comparative investigations on duplex systems and on organic systems.53,66,67 Some results are provided in Table 19. It can be seen that duplex systems perform excellently in the field, but performed worst in the ISO 20340 aging test. The barrier properties estimated with EIS (electrochemical impedance spectroscopy) were also very low for the duplex systems. This discrepancy between field exposure, EIS investigation, and accelerated aging was earlier reported in Momber et al.,61 where this effect is contributed to ‘‘microfogging.’’ Bjoergum68 achieved similar results. Examples are provided in Fig. 17. Whereas the duplex system performed better under site exposure conditions, it failed after the accelerated aging test. She noted that correlations between laboratory tests and site exposure tests exist for organic systems only, but not for Zn duplex systems. The

9 weeks

16 weeks

25 weeks

Fig. 18: Cold-gas-sprayed duplex systems after ISO 20340 testing.70 Scale bar: 20 mm; Coating system: Zn/Al (N2) 50 lm + 2 3 100 lm EP + 50 lm PU

J. Coat. Technol. Res.

Table 20: Repair/maintenance systems for offshore applications5 Zone

Total DFT Layers (lm) total

Priming coat

Intermediate coats

Binder

DFT (lm)

External atmospheric

300–425 225–325 225–375

3 3 3

EP Organicb Urethanec

Splash zone

300–2000 325–675 450–550 500–1000

1 2 1 1

EPd EP 125–175 EPe EP (under water curable)

Submerged zone a

High solids;

b

zinc-rich;

c

moisture-cured;

d

surface tolerant;

author concluded that the ISO 20340 test is not suitable for ZnAl15 duplex systems.68 The utilization of active gas to the thermal metalspray process was investigated in Bobzin et al.69 and Kro¨mmer and Momber.70 The latter reference investigated the effects of different carrier gases on the performance of duplex spray metals in the artificial aging test according to ISO 20340. They utilized Al and Al/Zn alloys with varying thicknesses. All spray metals were covered with organic layers (sealer; 2 9 100 lm EP; 50 lm PU). The authors found that the use of an N2/active carrier gas had the capability to increase the corrosion protection performance of duplex Al/Zn coatings. Compared with air as a carrier gas, scribe corrosion and scribe delamination were notably lower if N2/active gas was used. The cathodic protection period of the spray-metal coatings could be extended if N2/active gas was used, and pull-off strength increased. The best performing system was a Zn/Al alloy with a thickness of 75 lm, covered with a three-layer organic coating. Bobzin et al.69 investigated effects of N2 and N2–H2 active gases on arc wire spraying of ZnAl15 in comparison with compressed air. Investigations of layer composition and oxygen content did not reveal effects of the carrier gas. They contributed this to the formation of a thin passive alumina film on the outside of the molten particles which prevent chemical reactions with the surrounding atmosphere. Therefore, the better corrosion protection performance of duplex systems with metals sprayed with N2/active gas must be caused by other effects than oxygen reduction in the sprayed metal layers. Kro¨mmer and Momber70 performed preliminary investigations into cold-gassprayed Al/Zn duplex systems for OWEA applications. Some results are displayed in Fig. 18. Compared with thermal-sprayed ZnAl duplex systems of the same thickness, cold-gas-sprayed duplex systems provided a satisfactory corrosion protection after 25 weeks. This result was partly attributed to the denser structure of the cold-gas-sprayed coatings.

125–175 50–75 75–125

e

Binder

DFT (lm)

EPa 125–175 EP 125–175 Urethanec 75–125

EPe

Topcoat Binder

DFT (lm)

PU 50–75 PU 50–75 Urethanec 75– 125

200–500

with glass flakes

Repair coatings Repair coatings for offshore structures require a number of special properties. These include a one-coat system, compatibility with damp surfaces,5 or fewer demands on substrate surface preparation. Table 20 lists recommended repair/maintenance systems for offshore applications. From the application point of view, repair/maintenance coatings shall either be 1pack systems or premixed/predosed 2-pack systems. Pot life shall be long and drying time shall be short. Regarding the special requirements for repair/maintenance systems, the majority of the coatings in Table 20 are not real repair/maintenance coatings. Although there are some investigations available regarding the interactions between surface preparation method, repair coating type, and coating layer number,71 they do not deliver suitable information about the development and performance of particular repair/maintenance coatings. The authors of this review and their coworkers performed a number of systematic ISO 20340 aging tests with one-layer offshore repair coatings,72 which will be reported about in the near future.

Testing standards A summary of recommended testing scenarios for different corrosivity categories and loading zones is provided in Table 21. The scenarios include tests for corrosion protection (salt spray, immersion, condensation, cyclic aging), for mechanical resistance (impact, abrasion), for color stability, for CP compatibility, and for site exposure tests. The latter test, however, is recommended in one standard12 only. Considering the results for the testing of duplex systems in ‘‘Spraymetal coatings and duplex coatings’’ section, this qualification test shall be given more attention. The VGB guidelines12 require the most thorough testing for coating qualification, particularly in the tidal zone

a

In hours;

b

Buried

Submerged

Tidal

Zone Splash

Atmospheric

– – – –

– – –

– 1440 – –

– – –

in years;

–

3000

c

– – – –

– – – –

d

– – –

– – – –

–

– – –

– – –

– 1440 – –

3000

– – – –

– – –

– – – 4200

4200

X – – –

decks and boat landing

– 5c –

– – – 15b

–

X X – 4200a

– – –

– – – X

–

– – – –

– 5c –

– – – 15

–

X X – 4200

– – –

– 1440 – –

3000

– – – –

– – –

– – 4200 4200

4200

– – – –

– – –

– – – X

–

– – – –

– 1440 – –

3000

– – – –

2000 – 5c – – –

1440 2160 4200 15b

–

X X – 4200

– – –

– – 4200 4200

4200

– – – –

X X – 4200

–

– – – –

– – –

2000 – 5c – – –

– 1440 720 1440 2160 1440 4200 – X 15b –

X

– Xd – –

– – –

– – 4200 –

4200

– – – –

– – –

– – – –

–

– Xd – –

2000 – –

720 1440 – –

–

– – – –

12944 20340 NACE VGB 12944 20340 NACE VGB 12944 20340 NACE VGB 12944 20340 NACE VGB 12944 20340 NACE VGB

in months;

Adhesion Impact Abrasion Immersion (NaCl) Immersion (water) Condensation Salt spray Cyclic aging Cathodic protection compatibility Color Site exposure Chemical resistance

Test

Table 21: Testing scenarios for offshore coatings

J. Coat. Technol. Res.

J. Coat. Technol. Res.

and the splash zone. The core qualification test for OWEA coatings is the aging procedure as per ISO 20340.7 It consists of the 25 exposure cycles (4200 h). One exposure cycle lasts a full week (186 h); it includes: – 72 h of UV exposure and condensation; – 72 h of salt spray; – 24 h of low temperature (20C). The test panels (150 9 75 mm) feature a scribe which simulates a mechanical damage. Examples are shown in Figs. 14, 17, and 18. The assessment criteria include blistering [0 (S0)], rusting (Ri0), cracking [0(S0)], flaking [0(S0)], chalking (if agreed between interested parties), corrosion from scribe [3.0 mm for Zn(R) primer; 8.0 mm for other priming coats], cathodic disbanding (˘ = 20 mm), and pull-off strength (2–8 MPa).

Summary Coating specifications for offshore wind energy devices (OWEAs) are still based on specifications for the offshore oil and gas industry. Recently, two standards, exclusively developed for OWEA in the German offshore section (North Sea and Baltic Sea), were issued. This paper provides a thorough review about current offshore standards and 34 commercial specifications used in the German OWEA industry. Stresses are defined for different zones of the structures. Specified coating systems for different stresses, including testing methods, are discussed. Assessment parameters include number of layers, dry film thickness, and binder types for different coats. Special coating requirements are discussed, namely impact resistance, abrasion resistance, icing/deicing performance, low friction, color and gloss stability, and low-temperature performance. Finally, trends for the utilization of thermal spray metals are reviewed. Acknowledgments This investigation was funded through the German Federal Ministry of Education and Research (BMBF) under OWS-MV 03WKCR3E.

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