Khanna A.S. (Ed.) High-Performance Organic Coatings: Selection, application and evaluation

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Organic coatings for marine and shipping applications 347

O=O

0.5 nm

transport of

ions to the

coating/metal

interface

•formation of an

electrochemical double

layer at the metal surface

•potential drop 10

V/cm

•extremely extended diffuse

double layer at the

metal/polymer interface

•potential drop only 10

V/cm

→ slow electrode kinetics

metal electrolyte

metal coating

O=O

0.5 nm

transport of

ions to the

coating/metal

interface

→ fast electrode kinetics

16.1 Schematics of the characteristics of the double layer for electro-

lyte covered metal (top) and polymer-coated metal (bottom) [15].

Transport of corrosive species to the metal surface

Following the pathway formation, the next step in the degradation of coated

steel is the transport (Fig. 16.2: step 2) of water oxygen and ions from the

environment to the metal surface. Although both water and electrolyte can

tion of conductive pathways. The regions of direct electrolyte penetration

have indeed been shown to be small, localized and randomly distributed.

Furthermore, these regions have been demonstrated to correspond to initial

sites of corrosion. The molecular origin of the direct electrolyte penetration

regions is not entirely clear, but in some cases, they have been associated

with inhomogeneous crosslinking.

water and oxygen to such a degree that the transport of the materials is

not rate controlling. In case of the presence of soluble materials under the

coating, water uptake may be accelerated due to osmosis (Fig. 16.3a, b).

its, not removed prior to the paint application. At this stage, all ingredients

for corrosion are present in the blister: anodic species (substrate material),

electrochemical double layer is formed. At the initiation of corrosion, the

anodic and cathodic sites on the metal will be randomly distributed and

constantly changing over the metal surface (Fig. 16.3b). The build-up of

osmotic pressure may eventually lead to collapse of the blister (Fig. 16.3c)

leading to a similar situation as depicted in Fig. 16.2d.

penetrate films, electrolyte tends to penetrate films only locally by forma-

As stated earlier in this chapter, typical organic films are permeable to

One source of underfilm electrolytes could be salts embedded in rust depos-

cathodic species (water and oxygen) and an aqueous film in which the

348 High-performance organic coatings

O, O

, NaCl

O, O

coating

steel

hydrophylic

region

O, O

NaCl

cathodeanode

conductive pathway

cathode

anode

corrosion products

, OH

5 6

cathode

anode

, OH

corrosion products

cathode

coating

steel

hydrophylic

region

coating

steel

hydrophilic

region

cathodeanode

conductive pathway

cathodeanode

conductive pathway

cathode

anode

corrosion products

, OH

5 6

cathode

anode

corrosion products

, OH

5 6

cathode

anode

, OH

corrosion products

cathode

NaCl

16.2 Model for the degradation of an initially intact organic coating on

steel in a neutral NaCl solution [17].

Regardless of the rate-controlling step, the transport of ions from the

environment to the metal surface is through discrete, least resistant, con-

ductive pathways in the coating. This view is supported by the fact that the

solubility of ions in a polymer matrix is extremely low, approximately

−8

mol/l in equilibrium with one molar electrolyte. Ion transport through

Organic coatings for marine and shipping applications 349

cathode

anode

, OH

NaCl

, OH

corrosion products

cathode

suloble material, e.g. salts

coating

steel

dissolved

material

osmosis

osmotic pressure

O, O

, NaCl

4 5

cathode

anode

, OH

corrosion products

cathode

soluble material, e.g. salts

coating

steel

dissolved

material

osmosis

osmotic pressure

4 5

16.3 Model for the degradation of an organic coating on steel in a

neutral NaCl solution initiated by osmosis.

conductive pathways is in agreement with the impedance equivalent circuit

models, which generally consists of the coating capacitance shorted by a

resistive element, representing the presence of ionically conductive path-

ways through coatings.

Based on this conceptual model, the steps (circled numbers in Fig. 16.2)

involved in the degradation of an organic-coated steel panel containing no

apparent defects exposed to a neutral NaCl electrolyte may be summarized

as follows:

1. Conductive pathways develop by water attack in the hydrophilic regions

scopic defects (in the coatings) accelerate this process.

2. Ions migrate through conductive pathways to the metal surface.

3. Anodes develop on the metal surface at the base of the pathways.

of the film, followed by the interconnection of these regions; macro-

350 High-performance organic coatings

4. Cathodic contacts develop under the coating at the periphery of the

pathways.

5. Sodium ions migrate along the coating/metal interface from the defect

to the cathodic sites to neutralize the hydroxyl ions; this transport

process is the rate-determining step.

6. Alkalinity of NaOH solution at cathodic sites causes disbondment

(cathodic delamination).

7. Hygroscopic NaOH materials at cathodic sites produce a water activity

difference between the environment and interface, setting up an osmotic

pressure gradient; water is driven to cathodic sites through the coatings

by an osmotic pressure.

8. For some coatings, blisters develop, enlarge (probably facilitated by

mechanical stresses) and eventually coalesce, resulting in total

delamination.

It has been shown that pore-free polymers initially provide a rather good

corrosion protection, due to low solubility of ions within the polymer. If,

however, part of the polymer is destroyed (formation of pores, or conduc-

tive pathways) or mechanically removed, then the electrolyte has direct

access to the metallic substrate. At the borderline of the defect an interest-

ing electrochemical situation occurs. At the defect a rapid metal dissolution

is possible (active electrode), whereas below the adherent coating the dis-

solution reaction is strongly inhibited (passive electrode) [18].

16.3.2 Mechanisms and techniques of corrosion protection

Additional to the corrosion protection provided by organic coatings, pro-

tection to steel marine and ship structures can be improved by cathodic

protection. The conventional basis for cathodic protection relies on induc-

ing a negative steel potential shift which reduces the tendency for iron to

dissolve as positive ions. Key factors that determine what is required for

fully effective cathodic protection in seawater are:

•

the required current density at the protected surface and the current

• Dissolved oxygen – with the corrosion of steel under cathodic control,

the required current density is directly proportional to the product of

seawater.

• Temperature – this affects electrochemical charge transfer and diffusion

kinetics, the solubility of the calcareous deposits and the electrochemi-

• The formation of calcareous deposits – these reduce the current demand

in the long term and improve the spread of protection.

available from the specific anode geometry and anode distribution.

the oxygen concentration and the oxygen diffusion coefficient in

cal capacity of sacrificial anode materials.

Seawater salinity and resistivity – these have a strong influence upon

Organic coatings for marine and shipping applications 351

The principle of cathodic protection is in connecting an external anode to

the metal to be protected. The external anode may be a galvanic anode,

where the current is a result of the potential difference between the two

metals, or it may be an impressed current anode, where the current is

impressed from an external DC power source.

Galvanic anode systems use reactive metals that are directly electrically

connected to the steel. Commonly used metals are aluminium, zinc and

magnesium. Impressed current systems employ inert (zero or low dissolu-

tion) anodes and use an external source of d.c. power to impress a current

from an external anode to the cathodic surface. Cathodic protection is com-

monly applied to a coated structure to provide corrosion control to areas

where the coating may be damaged. It may be applied to existing structures

to prolong their life. In practice its main use is to protect steel structures

buried in soil or immersed in water. Among other purposes cathodic pro-

tection is generally used for ship hulls and tanks, but is functional only when

immersed.

The provision of an organic coating to the structure will greatly reduce

the current demanded for cathodic protection of the metallic surface

(approximately 110 mA/m

for unprotected structure and 5 mA/m

for pro-

tected structure, a factor of 22). The use of a well-applied and suitable

coating increases the effective spread of cathodic protection current. A

combination of both a coating and cathodic protection will normally result

in the most practical and economic overall protection system. Ideally, the

coating has a high electrical resistance, is continuous and will adhere

strongly to the surface to be protected. Compatibility with the alkaline

environment, such as epoxy binders provide, created or enhanced by

cathodic protection is desirable.

In the case of critical areas with the danger of damage or failure of the

coating, additional cathodic protection is applied. This is always the case

for the underwater part of the hull of a ship and for offshore constructions,

pipelines and seawater ballast tanks. Due to the cathodic protection, at the

coating–metal interface, in the vicinity of damage, conditions of high pH

will be created, which will cause detachment of the coating. Modern protec-

tive coatings for these kinds of applications are designed to tolerate normal

required protection for unalloyed steel: 200 to 300 mV lower than the open

corrosion potential.

16.4 Coatings for antifouling protection

16.4.1 Introduction

Since ships are used for transport, the use of all kinds of ‘antifouling’ prod-

ucts is also well established. Depending on availability, technical possibil-

ities and knowledge of the effects on the environment, many different

352 High-performance organic coatings

products have been applied: wax, tar, asphalt, pitch, shielding with copper

or lead. Coatings used early in the last century were based on linseed oil,

shellac or rosin, containing all kinds of toxic elements: copper, arsenic or

mercury oxide mixes.

In the latter half of the last century a new technology was originated:

organometallic compounds. There were developed and applied as antifoul-

ing compounds in paints: organo-arsenic, organo-lead and in the 1970s

organotin. This last group, in particular tributyltin (TBT), was very success-

ful, because of its:

• effectiveness (in combination with copper oxide it covers the whole

range of the marine growths concerned)

• combination with the acrylic binder which gives self-polishing proper-

ties, making a new effective surface continuously available, moreover

decreasing drag resistance by smoothing the surface

• compatibility with anticorrosive systems

•

• acceptable price.

However, because of the high toxicity of TBT to oysters, molluscs and

crustaceans and their bioaccumulation, the application has been banned

globally by IMO since January 2003. Moreover, from 1 January 2008 no

coating on ship hulls containing organotin may be in contact with

seawater.

much research for alternatives has been started, though because of the

‘excellent technical suitability’ of organotin-based antifoulings it is an

almost impossible challenge to develop similar products. Furthermore,

another practical barrier is raised in the development and application of

new antifouling agents: for their admission, production and use in the US

and EU, approval of the respective authorities is needed (the US

Environmental Protection Agency, and the various directives issued in the

EU resulting in the actual REACH – Registration Evaluation and

Authorisation of Chemicals). The procedure for approval is costly and

risky; for this reason, despite the various initiatives, including those looking

for new compounds ‘learning from nature’, few novel products have reached

the market during the last decade. For example, in Europe much research

was conducted to copy active compounds (secondary metabolites) from

sponges retaining antifouling properties and to produce them synthetically

[19, 20]. These projects were successful from a research point of view: three

different compounds have been developed. From the practical or commer-

cial point of view they have not been successful: they have not been taken

into consideration for development because of the registration costs and

risks.

relative long lifetime, up to five years

Since the first signs of the environmental problems caused by organotin

Organic coatings for marine and shipping applications 353

16.4.2 Current and future developments in antifouling

Antifouling techniques can be subdivided into the following principles:

• Chemical. Chemical compounds acting as active, mainly toxic agents.

Coatings containing these compounds form the largest group in prac-

tice. Metal cladding and release of toxic metal ions also belongs to this

principle.

•

ings: in this case organisms can attach, but they are easily removed

during sailing. Application of gels can also be attributed to this group.

• Biological. Interfering with the metabolites of marine organisms, for

example with enzymes. Predation by other marine organisms can also

be considered as a biological technique.

• Electrical. Using direct current, electrical pulses or electrolysis of the

seawater.

• Mechanical. Mechanical cleaning of the surface (Fig. 16.4).

In practice, for the protection of the hulls of ships against fouling, mainly

coatings based on the chemical principle are used (estimated at 90%) and

the remainder are mainly low energy surface coatings (Fouling Release

Coatings, FRCs).

Due to the restrictions for environmental reasons, paint manufacturers

have been forced to commercialize environmentally less harmful products.

In antifouling coatings based on the chemical protection principle, alterna-

tives to TBT-based antifoulings are nowadays available. Almeida et al. [1],

Table 16.2, gives a complete overview of the products actually available

that replace the organotin antifouling [1, 21–31]. The principal antifouling

16.4 Mechanical cleaning of fouling.

Physical. Low energy surface (silicone or fluoropolymer based) coat-

Table 16.2 Overview of the main alternatives for TBT based antifouling available on the market [1, 21–31]

Type of paint Company Product designation Basic known

components

Proposed

mechanisms

Reference

Controlled depletion

paints (CDPs) (increase

of soluble matrix

technology with use of

new resins)

Chugoku Marine

Paints

Kansai Paint

TFA-10/30

Sea Tender

10/12/15

New Crest

CDP. Hydration

International

Marine

Coatings

Interspeed 340

Interclean 245

CDP

Contact leaching

Transocean

M.P.A.

Optima 2.30–2.36 CDP

Tin-free self-polishing

systems (TF-SPCs)

(identical mechanism to

self-polishing technol-

ogy but without tin)

Ameron ABC-1a4 SP. Hydrolysis

Chugoku Sea Granprix

500/700 (2nd

generation)

Zinc or copper

acrylate-based

binder integrat-

ing biocides

and others

SP. Zinc acrylate.

Hydrolysis

[29–31]

Sea Granprix

1000/2000 (3rd

generation)

Silyl-acrylate-

based binder

integrating

biocides and

others

SP. Silyl-acrylate.

Hydrolysis

[29–31]

Hempel Globic Series Synthetic colo-

phony substi-

tutes with

reduction of co-

binder (plasti-

cizer) reinforced

with mineral

product 81900–

81970 as

potential

biocide, or sea.

nine/Cu

pyrithione

SP. Ion exchange.

Fibres

R–COO–Zn–OOCR

+ 2Na

↔ 2R–

COO–Na

(s) +

Zn2

(aq)

[21–28]

Oceanic Series SP. Ion exchange.

Fibres

Olympic Series SP. Ion exchange.

Fibres

International

Marine

Coatings and

Nippon Paint

SPC

Acrylic matrix

bonded to

copper salts of

an unrevealed

organic chain.

Potential

biocide zinc

pyrithione

P–COO–CuOOCR

(s) + Na

↔ P–

COO–Na

(s) +

RCOO–Na

(aq) +

basic copper

carbonate

[21–28]

(Continued)

fibres and

Intersmooth Ecoflex

Type of paint Company Product designation Basic known

components

Proposed

mechanisms

Reference

Jotun Sea Quantum (Plus,

Classic, Ultra, FB)

SeaQueen

SeaPrince

SeaGuardian

Potential biocide

Cu pyrithione

SP. Silyl-acrylate.

Hydrolysis

SP. Copolymer

binder

SP. Copolymer

binder

SP. Copolymer

binder

Kansai Paint Exion

Nu Trim

SP. Zinc acrylate.

Ion exchange

SP. Hydrolysis

Sigma Coatings Alphagen 10–20–50

Sigmaplane Ecol

(also HA)

Potential biocide

isothiazolone

Potential biocide

isothiazolone

SP. Hydrolysis and

ion exchange

SP.

Hydrodissolution

Hybrid

systems (CDPs/SPCs)

International

Marine

Coatings

Interswift 655 Potential biocide

Zineb or Cu

pyrithione

Hybrid of CDP +

SPC

Hempel Combic Series

Table 16.2 Continued

SP + saponification