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1 INTRODUCTION

Corrosion is defined as the chemical or

electrochemical reaction of a metal or an alloy with its

environment whose consequences have become a

major problem in offshore environments due to

extreme operating conditions and the presence of

aggressive corrosive elements. In the design of any

metal structure, it is extremely important to consider

the corrosion resistance that it offers since it

represents the difference between trouble-free long-

term operation and costly downtime. Steel structures

situated above the seawater in the so-called

atmospheric zone are in a high corrosivity category,

with a corrosion rate in the range of 80 - 200 µm per

year [6]. The most used method in the prevention of

external corrosion is to cover the exposed surfaces

with a high-efficiency coating. In the case of offshore

structures, the conventional approach consists of

using polymeric coatings, but this method requires

periodical maintenance that includes a number of

challenges associated with the physical environment

in which the work takes place Thus, i.e., the

application of coating systems for offshore steel

towers in modern facilities can cost up to 15 to 25€/m,

depending on the work conditions and on the coating

systems. Repairs of the corrosion protection can be

from 5 to 10 times more expensive and total cost can

easily rise to more than 1,000 €/m

[11, 13].

Offshore floating wind fams structures act as

artificial reefs, supporting an undesirable

accumulation of marine life by offering habitat for

microorganism, algae, fish and in-vertebrates.

Biofouling is defined as the adhesion and

accumulation of biotic deposits on submerged or

wetted surfaces, and the deposits consist of organic

components, such as microorganisms, plants, algae, or

animals, associated in a self-produced polymer matrix

called a biofilm, which can also include inorganic

Bioactive Ceramic Coating Solution for Offshore

Floating Wind Farms

D.S. Sanz, S. García, L. Trueba & A. Trueba

University of Cantabria, Santander, Spain

ABSTRACT: Biofouling is a natural phenomenon that consists of the accumulation of living organisms on an

artificial surface submerged or in contact with water like Offshore platforms. This study highlights the need for

offshore floating wind farms structures to consider the choice of material used in offshore applications to

minimize microbial-associated and corrosion problems. For this purpose, differences in the total of seawater

biofouling attached on two coated paints and three ceramic coatings in carbon steel for offshore structures were

evaluated and compared. All ceramic coatings were made of incorporating, by electrophoretic deposition, active

ceramic particles against biofouling as copper, silver, zinc and titanium. This experiment consisted of testing

ceramic coatings and conventional paints in a real environment with high biological activity and at the same

time in a shallow marine environment for a period of 1 year, which provided positive comparisons with the

standard system (ASTM-D3623) for using in protecting offshore marine structures.

http://www.transnav.eu

the International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 15

Number 2

June 2021

DOI: 10.12716/1001.15.02.19

408

components, such as salts and/or corrosion products

[4]. These biofilms cause microbial biofouling and can

also result in the accumulation of macro-organisms,

resulting in macro-fouling [10]. Biofouling consists

primarily of polysaccharides and water. The

components of polysaccharides vary depending on

species but typically include repeating

oligosaccharides such as glucose, mannose, galactose,

and xylose, among others [9]. The effects caused by

biofouling and corrosion are closely related to each

other since biofouling may induce corrosion, but

corrosion may also induce biofouling. Therefore, it is

necessary to study both parameters in conjunction.

Nowadays, chemically active antifouling paints with

biocides or non-stick fouling release coating is

practically the only method of fouling control. Today

there is still debate regarding the most optimal

coating systems for offshore structures [12, 15]. The

service life of anti-fouling paints is defined by the

coating thickness and has a considerable

environmental impact besides from continuously

maintenance operations due to limited life cycle and

durability [17]. While control of corrosion for

shipping is an enormous market, dynamic

performance coatings for ships are the subject of

intense development by paint companies and other

researchers and specific needs of this sector are

intensely cost sensitive [18]. Antifouling paints used

in floating offshore structures require periodic

maintenance. This maintenance is carried out in dry

dock, with its respective costs of inactivity and

transport, since an in-situ maintenance is difficult and

expensive. For this reason, it is required a long term

solution to corrosion and biofouling and coating

systems used for shipping are too short lived to be

entirely suitable. Current antifouling paints for

floating offshore structures use similar formulations

that are used in the shipping industry and rely either

on biocides that gradually leach out of the coating

(damaging surrounding ecosystems and only

providing a limited time period of effective bio-

activity) or on so called ‘self-polishing’ systems that

require a certain water velocity to remove

accumulated growth (unsuitable for static structures).

At present, apart from the marine coating solutions,

the corrosion protection is accomplished by coupling

a less noble (i.e., more electronegative) metal in the

structure. Sacrificial anodes can provide added

protection in immersed regions, the problem is that

they are costly to install and re-place and provide no

protection in splash and tidal zones where corrosion

is most severe. Organic paint coatings have limited

lifetimes (<10 years) before needing substantial

maintenance and repair [19]. The main problem with a

conventional marine coating is the rapid corrosion of

the underlying steel in potential areas that have lost

protection due to scratching, etc. For this reason, the

offshore industry tries to detect new needs to

implement new solutions to overcome the challenges

of current painting systems in terms of durability

associated to corrosion protection, mechanical

resistance and fouling avoidance. A novel alternative

that is showing competitive results for similar and

harsh environments are related to ceramic enamel

coatings which have an excellent chemical and

abrasion resistance due to their sintered vitreous

structure.

Ceramic materials are attractive materials due to

their characteristics like high chemical resistance,

wear resistance, thermal resistance and corrosion

resistance [16]. Ceramic materials have this unique

performance. For example, ceramic tiles are coated

with ceramic glaze layers to provide an easy to clean,

wear resistant and high aesthetic surface. Ceramic

coatings are used for instance in the protection of

airplane turbines, where ZrO2 is applied by Thermal

spray; also chemical reactors are protected with a

ceramic enamel coating. The advantage of ceramic

enamel coatings is the capability to tailor the

properties of the vitreous matrix with other oxide and

functional particles [2].

Nowadays, there is two kinds of ceramic coatings:

enamels or glazes and both are made up of a mixture

of molten glass and diverse additives. The molten

glass can have diverse compositions, being made up

of oxides like SiO2, B2O3, Al2O3, Na2O, K2O, CeO2,

MgO, ZnO, CaO, ZrO2, TiO2 and adhesion promoters

for metal surface like, CoO2, NiO, Fe2O3, MnO; the

characteristic that a glass can be adjusted in

composition allows modelling their final properties,

like thermal expansion coefficient, melting point,

chemical resistance and biofouling adhesion. Ceramic

coatings can be modified by addition of raw materials

like quartz, titanium oxide, zircon silicate or ceramic

pigments to adjust to the metal type and final

application product [5]. At present the use of ceramic

enamel to protect metallic structures has been limited

by many factors. For example, conventional slurry

application techniques (e.g. dip coating, wet spraying)

are not suitable for large parts except for chemical

reactors and panels. Even more importantly, a

sintering thermal treatment is needed to consolidate

the coating, that could be incompatible with some

engineering materials (e.g. light alloys, quenched and

tempered steels, etc.), as they would bring about

unacceptable microstructural changes accompanied

by a loss of mechanical strength apart from warpage

and distortion of the coated items.

The electrophoretic deposition processes

encompass a family of coating deposition techniques

characterized by the use of a high-velocity and/or

high-temperature gas stream to project

softened/molten droplets of the coating material

towards the substrate. Whilst the droplets may attain

very high temperatures (hence, even refractory

coating materials can be processed), the substrate

remains relatively cold as it is rapidly scanned by the

gas+droplets stream along typical raster patterns [8].

A large variety of coating/substrate material

combinations is therefore pos-sible; in particular,

ceramic enamels can be sprayed [1] onto relatively

cold substrates, thus avoiding overheating,

microstructural alterations and distortions. Moreover,

thermal spraying techniques are applicable to large

structures and, with due adjustments, they are

portable for on-site work [14]. The main problem to

obtain tight, corrosion-resistant ceramic enamel

coatings by thermal spraying resides in the typical

voids and gaps between the flattened droplets

(lamellae), microcracks within the lamellae (due to

their rapid cooling after deposition), and entrained

gases. These limits have, up to now, hindered the

industrial uptake of thermal spray glass coatings

which have been developed and validated at lab-scale

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for a variety of applications on metallic and ceramic

substrates [3].

The experiment carried out for this study

evaluated the antifouling (AF) action of a ceramic

coated statically exposed to the seawater. The

conforming of the ceramic materials was carried out

by electrophoretic deposition. The aim was to

minimise the biofilm adhesion on the surface and

study the effect of new coated in composition and

structures of the biofilms produced. The scientific

relevance of this research in AF is very highlight

because it involves a new environment friendly

technology against biofouling to improve efficiency

and productivity in offshore floating wind farms

structures.

2 MATERIAL AND METHOD

2.1 Area of study

The study area chosen for the research was the

breakwater jetty of the Molnedo Dock (43º 27.713’ N,

03º 47.541’ W) after the authorization was granted by

the Santander Port Authority for experimental

activities to the Biofouling Research Group of the

University of Cantabria on September 23, 2020.

2.2 Preparation of samples

The conforming of the three different ceramic

materials will be carried out by electrophoretic

deposition over carbon steel (A569/A569M6, 3 mm

thick by 200 mm x 300 mm) which will be visually

examined and tested once a month according to

ASTM D790. Table 1 shows a comparative table with

the elements of the three ceramic coatings used in the

investment. During the study, not only were the

antifouling properties against biofouling verified, but

their protection against corrosion was also verified, so

the two studies were carried out in parallel.

Table 1. Ceramic composition adjustment.

According to ISO 20340, steel structures for coastal

and offshore areas are classified as C5-M (due to the

high salinity) and should be coated where minimum

requirements for protective paint systems. In this way,

before coating application, the sample surface was

blast cleaned in order to get a final surface roughness

of Sa2.5 or Sa3 (ISO 8503) and cleanliness (ISO 8501).

After blast cleaning (not exceed rating 2 of ISO 8502-

3), dust and blast abrasives were removed from the

surface. For the application of the first coat, the metal

surface was completely dry, clean, free from

oil/grease, and had the specified roughness and

cleanliness. The paint coating applied had a total

thickness of 300 µm.

The atomistic deposition processes were the

method used for the application of ceramic coatings to

get lower deposition rates and thinner coatings.

Firstly, it was the deposition of dense metallic under-

layers between the functional ceramic topcoat and the

substrate by the high velocity air-fuel spray process

using compressed air instead of oxygen. The goal of

this is to protect the substrate from corrosion and

enhance the adhesion strength of the ceramic enamel

top layer. HVAF torches proved to be a viable means

of depositing watertight metallic coatings with low

flaw capacity generating even higher particle

velocities and lower particle temperatures. In

addition, following the manufacturer's instructions, a

coating with a biocide free silicone coating (Silicone

FR) were applied to carbon steel specimens. Finally,

before sample´s installation in seawater, they were

cleaned with FreeBact20 (AquaFix, Satsjbaden,

Sweden) and sterile water and airdried, and also

photographed.

According to the standard specifications of the

American Society of the International Association for

Testing and Materials (ASTM), the surface

topographies of the samples were denoted. Table 2, 3,

4, 5 and 6 show a comparison between the initial state

of the samples and the final chemical composition of

the experiment. Their surface roughness values were

measured using a surface roughmeter (Mitutoyo,

Surftest SJ-201 Series) in accordance with the

guidelines established in the standard ASME/ANSI

B46.1-2009.

Table 2. Sample 1 assayed.

Table 3: Sample 2 assayed.

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Table 4. Sample 3 assayed.

Table 5. Sample 4 assayed.

Table 6. Sample 5 assayed.

2.3 Biofouling assessment

The experiment tested and analyzed the behaviour of

three ceramic coatings compared to two conventional

paint coatings, all of them over carbon steel, against

marine biofouling. Samples, whit same geometry,

were submerged during 365 days in the shallow

marine environments, at a depth of 0.5 m.

At the end of the experiment, samples were

analyzed in the laboratory as follows: i) Measurement

of barnacle adhesion strength in shear as follow

ASTM D5618-94. This test method covers the

measurement of barnacle adhesion in shear to surfaces

exposed in the marine environment, ii) analysis

quantitative and qualitative of biofouling on sampled

surfaces and analysis optical by microscope of

biofouling as qualitative analysis.

The standard practice for evaluating biofouling

resistance and physical performance of marine coating

system ASTM D6990-05 establishes a practice for

evaluating degree of biofouling settlement on and

physical performance of marine coating systems when

panels coated with such coating systems are subjected

to immersion conditions in a marine environment.

2.4 Experimental setup

Sample’s installation in sea water was carried out on

06 February 2020 and were exposed until 06 February

2021 in realistic conditions of exposure of the

submerged zones in the breakwater jetty of the

Molnedo Dock. Samples were checked monthly by

visual inspection following the ASTM D 3623-78a and

roughness measurements were taken every two

months.

Temperature of seawater were measured once a

month during the experiment period. The chemical

parameters of seawater were measured once a month

during the experiment period.

3 RESULTS AND DISCUSION

AF technologies for marine applications are of large

interest mainly due to the economical and

environmental benefits. Table 2 shows the antifouling

performance of three ceramic coating in comparation

with two convectional paint coating after being

exposed for a period of 12 months in natural seawater.

The development of a biofilm is influenced by the

properties of the substratum, and it has been observed

that biofilms develop more quickly and attain a

greater biofilm thickness on rougher surfaces [9]. The

sample coating No. 1 was covered by 85% of hard

fouling organisms after 365 days and produced at 22%

losses of AF coating on the surface. Furthermore, it

was fouled by 13% of filamentous, 27% of barnacles,

40% of algae and 20% biofilm. The sample coating No.

2 was covered by 78% of biofouling organisms and

produced at 25% losses of AF coating on the surface.

Furthermore, it was fouled by 18% of filamentous,

20% of barnacles, 32% of algae and 30% biofilm. The

sample coating No. 3 was covered by 90% of

biofouling organisms and produced at 19% losses of

AF coating on the surface. Furthermore, it was fouled

by 15% of filamentous, 4% of barnacles, 29% of algae

and 52% biofilm. The sample coating No. 4 was

covered by 100% of biofouling organisms and

produced at 76% losses of AF coating on the surface.

Furthermore, it was fouled by 17% of filamentous, 5%

of barnacles, 25% of algae and 53% biofilm. The

sample coating No. 5 was covered by 95% of

biofouling organisms and produced at 74% losses of

AF coating on the surface. Furthermore, it was fouled

by 20% of filamentous, 3% of barnacles, 27% of algae

and 50% biofilm. Analyzing these parameters, coating

No. 2 had the best antifouling release performance

under static conditions.

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The samples coating No. 4 and No. 5, silicon-

based, produced the depletion and leaching of these

silicon-based biofouling as the surface wears out,

leading to changes in the surface chemical

composition (eventually also topography) and

lowering of the AF performance. This explains why

the coatings No. 4 and No. 5 did not have long

durability and high AF performance levels all through

the coating life-cycle.

4 LIFE CYCLE ASSESSMENT

In a life cycle assessment (LCA), biofouling adhesion

on ceramic coatings is compared to the equivalent

adhesion on conventional paints. The LCA study

consists of four stages under the ISO 14040 guidelines:

Stage 1: This experiment consisted of testing

ceramic coatings and conventional paints in a real

environment with high biological activity and at the

same time in a shallow marine environment for a

period of 1 year, which provided positive

comparisons with the standard system (ASTM-D3623)

for using in protecting offshore marine structures. To

compare the different ceramic coatings with

conventional paints, the samples were extracted once

a month to check their weight, the variation in

roughness of the fouling layer, and photograph them.

Stage 2: In this step, inventory analysis gives a

description of materials used in the ceramic coating

elaboration, which appear in table 1.

Stage 3: Ceramic coating systems may provide a

long life-time due to their high biocorrosion-erosion

resistance and excellent coating adhesion to steel

surface, together with non-degradation (UV

resistance) and non-lixiviation of materials during

their whole life-time, so they may be a more durable

and environmentally friendly solution than the

currently used biofouling protection systems.

Stage 4: The results of the study shows that the

antifouling performance of the ceramic coating No. 2,

had the best antifouling release performance under

static conditions. In comparison with two

convectional paint coating after being exposed for a

period of 12 months in natural seawater. The results

are explained in detail in the section 2 “Results and

discussion”.

5 ECONOMIC ASPECTS

A 30% of failures in ships and other marine

equipment are consequence of marine corrosion, with

an annual cost of over $1.8 trillion [7]. The result of

these studies has shown that ceramic coatings offer

distinct advantages for long-term corrosion protection

over conventional coatings for marine service. This

factor makes it possible to substantially reduce the

maintenance of the structure and avoid dry-docking

in the case of floating structures. The cost of one dry-

docking can be as high as $0.2M to $0.7M. Dry-

docking can also adversely affect the flexibility of

operational schedules by taking offshore structure out

of service.

Ceramic coating is applied by Thermal spray. One

method to estimate the cost of thermal spray

application method is per square inch. It can range

from under $1 to spray some lower cost materials to

more than $50 for higher value components with

expensive coatings. Ceramic particles are a relatively

inexpensive material, which makes this coating

economically viable.

6 CONCLUSIONS

The results of this study indicate that biofouling is

extensive and formed by a diverse group of

microorganisms in coatings with different

compositions. Therefore, the addition of different

compositions into glass of coating affects the number

and species of microorganisms attached to the surface.

One of the factors that directly affect

microorganism’s development on coating surface is its

roughness, thus maximizing the negative

consequences of biofilm accumulation. The low

roughness offered by ceramic coatings hinders

biological adhesion, as has been demonstrated in the

marine field tests carried out for a year, even under

static conditions. The functionality of ceramic coatings

is based on antifouling efficiency relies on low

adhesion strength and diverse AF additives to reject

biofouling adhesion. The ceramic coatings with

suitable compositions are recommended to the

offshore anti-biofouling applications.

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