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

Подождите немного. Документ загружается.

337

Organic coatings for marine and

shipping applications

J H W DE WIT

and J M C MOL, Delft University of

Technology and W M BOS and G M FERRARI,

TNO Science and Industry, The Netherlands

16.1 Introduction

Since the maritime environment is strongly aggressive, coatings needed to

ally required. This, together with safety and environmental issues, demands

high requisites in performance and reliability.

On top of performance and reliability requirements for long-term protec-

owner or operator for a well-built, long-lasting structure with minimal

shipyard’s production line. Depending on the ship building market demand

at the time of contract, the owner may have to accept a compromise against

his better technical judgement and operating know-how. The extra cost for

the desired system may be exorbitant, seen from the owner’s or operator’s

point of view, and the standard guarantee for a ship is only one year from

delivery, so the yard will rarely face a claim for coating failure if the coating

gain in terms of the time-to-delivery of a new structure, there might be a

of early and on-going maintenance and repairs over the lifetime of the

structure. The relative costs of coating systems are minor compared to

replacement of steel during operational life. However, costs for staging and

steel preparation needed to apply those coatings rise swiftly with increasing

Although there are coatings available on the market that perform satis-

factorily, and provide long-term protection against corrosion, their reli-

ability is dependent on a strict control of the application, type and thickness

of the coating as well as operating conditions. Furthermore, the past 10 to

protect steel can be classified as ‘heavy duty coatings’. In most maritime

applications because of maintenance difficulties a long service life is gener-

operating cost (long-term financial interest) and the short delivery time or

application is reasonably well done. While this may recoup some financial

more significant negative impact to the owner or operator in terms of cost

surface area. Correct selection of coating materials and their specification

at the design stage may yield financial benefits in the longer term.

15 years have shown significant changes in the coating industry. Because of

tion, there are often also conflicting interests between the desire of a ship

338 High-performance organic coatings

the awareness for the consequences for human health and for the environ-

ment, nowadays many regulations and restrictions have been decreed con-

cerning the application, content and removal (waste) of coatings. Many

traditional anticorrosive coatings are now no longer acceptable due to their

potentially toxic and carcinogenic nature, e.g. red lead, coal-tar pitch, and

more. In addition, requirements to reduce emissions to the atmosphere of

potential ozone-depleting materials have resulted in a reduction in volatile

organic content (VOC) of paints and coatings. Novel coatings should also

vated temperatures at which ships may operate.

This chapter will deal with the types of coatings and degradation mecha-

nisms as well as mitigation of degradation of coatings in marine environ-

ments and shipping applications in particular.

16.2 Types of application

16.2.1 Introduction

From the point of view of coating properties and requirements the most

important application area for marine coatings is shipping. Protection of

the ship against corrosion is mainly a matter of safety and reliability.

Optimal performance of protective measures is essential and may involve

maintenance operations and high costs.

A number of parts with totally different operating conditions and require-

ments are typical for the shipping application area. In the case of the hull,

deck and ballast tanks seawater corrosion has to be taken into account. For

other parts such as drinking water, sewage and fuel tanks together with

the corrosion protection coatings have to be compatible with the load. The

underwater part of the hull requires even more attention because of the

natural process of marine growth. This ‘fouling’ has to be completely

avoided, because it reduces the speed, in other words increases the fuel

consumption, and hinders the manoeuvrability of the ship. Moreover it can

and the cathodic protection. To combat fouling, a so-called antifouling

coating as top layer above the anticorrosive coating is required. For anticor-

rosion coatings for shipping normally a lifetime of 15 years is expected, in

which period inspections and maintenance are foreseen. Antifouling coat-

ings have a short lifetime, up to a few years depending on the type of

application and sea-going practice.

An overview of typical coating systems currently used in different critical

areas of ship painting is presented in Table 16.1 [1–3]. Please note that in

Table 16.1 coating systems designed for the underwater hull area are

not incorporated. The specific aspects of the main marine degradation

keep their properties and flexibility during harsh sea conditions and ele-

influence and even damage the anticorrosion measures such as the coatings

Organic coatings for marine and shipping applications 339

Table 16.1 Examples of typical anti-corrosive paint systems currently used in

different critical areas of ship painting [1–3]

Critical areas in

ship painting

Main requirements Typical paint

systems

Hull and

superstructures

Anticorrosive protection in

seawater

Compatibility with cathodic

protection

UV resistance

Washability

Compatibility with antifouling

(top)coat

epoxy primer/

epoxy topcoat

Aliphatic

polyurethane

Aliphatic

polyurethane/acrylic

Polysiloxane/epoxy

hybrid (anti-rust

stain topcoat may

be applied)

Decks Appearance

Anticorrosive protection

UV resistance

Non-slip

Zinc silicate followed

by two-pack epoxy/

polyurethane

system

Trowel- or roller-

applied elastomer

(1–3 mm) over

high build primer

Topcoat should

include aggre-

gates, e.g. alumin-

ium oxide, silica or

others to provide

non-slip properties

Tanks

Ballast

Fuel

Sewage

Drinking water

Resistance to alternating

contact with sea water and

with transported products

Compatibility with cathodic

protection (in the case of

seawater)

Aluminium-pig-

mented pure

epoxy

Solvent-free epoxy

Special waterborne

asphaltic emulsion

Cement-reinforced

acrylic

Cargo

Crude or

petroleum

oils or

hydrocarbons

Very aggres-

sive products

Resistance to contact with High-solid or solvent-

free polyamine-

cured epoxy

High-solid polyamine

epoxy

Epoxy-cyclosilicone

Pure or modified

Modified epoxy

specific cargo types

340 High-performance organic coatings

tions, are considered to be of major importance and are dealt with in

greater depth in Sections 16.3 and 16.4. In this Section 16.2, considerations

ballast tanks, potable water tanks, decks, superstructures, boot top, and

steam pipes and pipelines respectively) are presented.

16.2.2 Cargo tanks

The internal coating of a ship’s cargo tanks is a common and necessary way

to protect steel surfaces against corrosion, to avoid cargo contamination,

and to facilitate cleaning. In product and light chemical carriers, for example,

full coating of internal tank surfaces is necessary to avoid corrosion damage

and possible cargo contamination, since a smooth coating facilitates tank

cleaning between cargoes and helps prevent contamination from rust or

leftover cargo deposits. Therefore, proper tank coating is essential for effec-

For the right choice of coating system and proper handling of cargoes, a

basic knowledge of the chemical properties of the most common cargoes is

advisable, since the behaviour of products within the same groups of organic

compounds may vary to a great extent. For instance, methanol will soften

most organic coatings, but alcohols with higher molecular weight are far

less aggressive. Compounds of lower molecular weight usually are more

aggressive than their higher homologues. For instance, pure epoxy coatings

are not suitable for long-term contact with acetone, other lower ketones

(e.g., methyl ethyl ketone), or the lower esters such as methylacetate. They

are, however, suitable for higher homologues such as methyl isobutyl

ketone and dibutylphthalate.

This difference in behaviour towards coatings depends mainly on the

steric hindrance (3D structure) of individual molecules. It is easier for

smaller and linear molecules to penetrate the polymeric structures. Other

polymers are functional groups, polarity of molecules, and hydrogen bonds.

All of today’s organic coatings for cargo tanks are two-component types,

However, strong solvents can soften cured coatings, causing swelling and,

in some cases, even scaling from the substrate. An additional factor affect-

ing coating suitability is the water solubility of solvents. Most organic coat-

ings used for cargo tank protection can tolerate a certain degree of softening

absorbed solvent to evaporate and to regain their original hardness.

However, if the retained solvent is water-soluble and tanks are washed with

blistering due to osmosis may occur.

mechanisms of corrosion and fouling, specifically for underwater applica-

and dedicated coating systems for other specific ship areas (cargo tanks,

tive and profitable ship management.

resulting in a chemically crosslinked film that cannot be redissolved.

from solvent absorption, provided they are given sufficient time for the

water or loaded with an aqueous cargo before the film has completely dried,

factors influencing the solvent power of non-hydrocarbon compounds on

Organic coatings for marine and shipping applications 341

During the last 40 years, several types of coatings have been used for

tank lining service in the sea trade: vinyls, polyesters, epoxies, epoxy phe-

nolics, epoxy isocyanates, polyurethanes, alkaline zinc silicates, and ethyl

zinc silicates. Some of these coating materials have stopped being used for

tank linings. Vinyls, being thermoplastic, had limited solvent resistance.

Glass-reinforced polyesters, used for transportation of acid cargoes, were

replaced by stainless steel tanks. Polyurethanes, loudly advertised in the

1980s for their wide resistance to different types of cargoes, showed some

application problems and unsatisfactory resistance to ballast water. Today’s

state-of-the-art coatings basically fall into the following categories: pure

epoxy, epoxy phenolic, epoxy isocyanate, alkaline zinc silicate, ethyl zinc

silicate, and cyclosilicon epoxy [4–6].

16.2.3 Ballast tanks

Coating systems for ballast tanks should be resistant to (polluted) seawater,

corrosion inhibiting, free from pores, and resistant to the side-effects of

most life-determining factors for a ship is the condition of its ballast tanks.

The inner water ballast tank area of a ship can be extremely large. For a

single-hull very large crude carrier (VLCC), the water ballast tank area

might be 140 000–160 000 m

; for a modern double-hull design, it could be

240 000–280 000 m

or even larger.

Besides exposure to a corrosive environment during service, the ballast

tank coating is subjected to various sources of stress that may lead to crack-

ing of the coating:

• Mechanical stress due to vibration and movement of the ship

• Thermal variations due to loading and unloading of ballast water or

product in adjacent cargo tanks

• Wet/dry cycles due to loading and unloading of ballast water

• Reverse impact from ice trading, tug boats, fenders, collision or loading

of cargo in adjacent cargo holds

• Internal stress due to curing shrinkage and solvent evaporation.

International regulations have had a clear and positive effect on the

structural protection of ships. They deal with the duty to coat ballast tanks

(with light-coloured, hard coatings combined with cathodic protection), the

minimum width of double hulls, the height of double bottom tanks, the

deletion of the allowance for reduced scantlings when tanks are coated with

Modern systems for ballast tanks normally consist of at least two coats

cathodic protection. According to the classification societies, one of the

an approved system, and a harmonized system of survey and certification

from the classification societies.

of straight, modified or solvent-free epoxies with a total dry film thickness

of at least 250 µm for straight and modified epoxies and 300–350 µm for

342 High-performance organic coatings

solvent-free epoxies. Reinforcement of an epoxy coating with (synthetic)

reduce possible cracking in water ballast tanks in ships [4, 7].

16.2.4 Potable water tanks

Compared to water cargo tanks or ballast tanks, drinking water tanks on

ships are relatively small. Cargo ships usually have two or three tanks with

an overall volume of about 100 m

, while passenger ships require larger

tanks. A suitable coating for potable water tanks should last, with minimal

maintenance, for the service life of the ship, up to 25 years without prob-

lems of corrosion or water quality. Coatings applied to potable water tanks

are usually two-component pure epoxies, either solvent-free (100% solids)

or solvent-borne. Most pure epoxy coatings that are suitable for immersion

service are also suitable for potable water contact, provided they do not

contain pigments with heavy metal compounds such as chromates. Before

prolonged contact with drinking water. There are several standards for

testing such coatings, but all test criteria address water taste and extraction

of possible harmful substances. Most epoxy coatings, if formulated prop-

between solvent-borne and solvent-free coatings during laboratory testing,

because test panels are prepared under controlled conditions that avoid

entrapment may play a major role in coating performance. Therefore,

solvent-free coatings are better suited for potable water tanks than those

containing solvents, which require careful ventilation and proper thermo-

hygrometric conditions to avoid solvent retention. Also, small percentages

of solvent retained in a coating system for potable water tanks may be a

serious problem, since they can affect water taste and smell. Moreover,

once solvent has been entrapped, it might be impossible to remove without

reblasting and recoating the tank [8].

16.2.5 Decks

ences of weather. They should be non-slip (even when the decks are wet)

and resistant to UV, impact, scratching and abrasion, as well as resistant to

(sea) water, fuel oils, lubricating greases, cleaning agents and cargo spills.

To minimize damage that may occur to a deck coating before delivery, the

best procedure may be to apply a recoatable epoxy holding primer during

fibres helps to significantly improve mechanical coating properties and

accepting a coating for use on board ships, classification societies require

certification from a recognized laboratory that the coating is suitable for

erly, will pass these test criteria. There are usually no significant differences

possible solvent entrapment. However, under field conditions, solvent

construction and the final coating system as soon as possible before deliv-

Paint systems for decks should be very resistant to corrosion and the influ-

Organic coatings for marine and shipping applications 343

ery. The most common deck coating systems are two-component epoxies,

epoxy/polyurethane systems and 75–100 µm for zinc silicates. Epoxy/poly-

urethane systems often consist of a primer, a thick midcoat, and an easily

recoatable topcoat, preferably all high-solid products. The topcoat can be

made anti-skid by adding an aggregate such as non-sparking silica, pumice

powder or aluminium oxide. Due to their limited resistance to acids and

alkalis, zinc silicates should not be used on the decks of chemical tankers.

Special, heavy-duty systems based on thick, solvent-free elastomeric coat-

of 1–3 mm also are available. In addition, water-borne systems consisting

of an alkali zinc silicate primer and an epoxy emulsion mid- and topcoat

are used for decks [3, 9].

16.2.6 Superstructure

For the topside and superstructure, an aesthetic topcoat of aliphatic poly-

urethane or aliphatic polyurethane/acrylic may be used. Isocyanate-free

although they generally have reduced gloss and colour retention compared

to polyurethanes. Silicon-based inorganic coatings are much more resistant

cantly enhanced durability when compared with conventional polyurethane

extent with organic resins to enable other coating performance properties

while, at the same time, not detracting from the polysiloxane properties.

e.g. acrylic, these materials can provide an inorganic backbone that pro-

against coating breakdown caused by weathering and exposure to aggres-

sive environments. Polysiloxane epoxy hybrid coatings also can be used as

aesthetic topcoats.

with rust to produce a colourless, water-soluble material. As a result, rust

tions, water-borne coatings based on alkali zinc silicate, styrene acrylate

dispersion, or epoxy or alkyd emulsion are suitable for the topside and

superstructure. However, their use generally is limited to the mid- and

topcoat layers. Suitable products include acrylic dispersions and epoxy

acrylic or (silicone) alkyd emulsions. However, their gloss levels are usually

lower than those of solvent- borne paints. Water-borne systems can be suc-

polyurethanes and zinc silicates with a dry film thickness of 250–300 µm for

ings applied by trowel or roller over a thin primer to a dry film thickness

alternatives include epoxy/acrylic or other modified epoxy coatings,

to these degradation mechanisms, and polysiloxane coatings offer signifi -

coatings. Polysiloxane coatings must, however, be modified to the right

By carefully choosing the organic modification of the polysiloxane polymer,

vides not only enhanced finish coat aesthetics but also long-term durability

A special antirust-stain finish may be applied to the topside and super-

structures. This finish contains an active pigment that chemically combines

stains are not visible. Although not often mentioned in painting specifica-

such as flexibility, toughness, adhesion to primers and cost to be obtained

344 High-performance organic coatings

cessfully applied, especially for ships built completely under cover. However,

this requires at least moderate temperature and humidity levels and strict

planning. Also, painters experienced in applying water-borne paints are

needed [3].

16.2.7 Boot top area

Modern paint systems typically include a two-pack epoxy primer.

Polyurethanes and coal-tar epoxy have been banned for environmental

reasons. Apart from good photostability and surface tolerance, they gener-

ally should have long maximum overcoat times and good recoatability.

Anticorrosive properties are obtained by barrier protection, which means

that the water vapour transmission of the system should be very low.

commonly used protective coatings in these extremely aggressive areas and

are hard, tough materials with good chemical, solvent and abrasion resis-

tance. They can also be used in association with cathodic protection systems

[3].

16.2.8 Steam pipes and pipelines

The problem of protecting deck steam lines and pipelines from corrosion

has plagued operators, owners and coating suppliers for years. While taking

up a very small space, these components of the vessel’s engineering system

are vital to the operation of the ship. Steam is used to power deck winches

in some older vessels and to heat coils in the cargo tanks of vessels carrying

liquid product. Failure to heat the liquid cargo increases its viscosity to the

point where it is too thick to pump or transfer. This condition is unaccept-

able. Steam lines and deck pipelines have traditionally been an enigma to

protect because of their unique environment. In addition to the obvious

problem of heat, which often can range from 150 to 180°C, depending on

the pressure of the system, the line can be constantly awash with cooling

salt water or salt spray. In addition to the obvious corrosion problem this

causes, it creates a thermal expansion and contraction cycle that puts sig-

tive system on the pipes. In the past, concoctions of enamel paints and oils

were used to provide an inexpensive topcoat that offered some protection

and was easy to apply. Today, a common method of maintaining the deck

lines is to constantly repaint them with a heat-resistant aluminium coating,

as time and schedule permit, and then to replace the coatings as they fail.

Another approach is to use surface-tolerant epoxies, but because of the

heat, they offer only short-term protection [10].

Total dry film thickness of boot top systems ranges from 250 to 400 µm.

nificant stress on the coating material. Thus, many factors affect the protec-

Increasing use of glass flake in coatings for underwater hulls and especially

for boot top areas is evident. Epoxy-based glass- flake coatings are the most

Organic coatings for marine and shipping applications 345

16.2.9 Surface preparation

The aim of surface preparation is to remove any surface contamination such

as mill scale, corrosion products, old coatings, grease and detritus and salt

which to apply the paint system. The principal methods of surface prepara-

tion are:

• Mechanical preparation using hand or power tools. These methods are

such as access, prohibition of blasting or simply cost may dictate their

use.

• Dry abrasive blasting using a variety of abrasive types (e.g. grit, shot,

sand, garnet, etc). The most common method in use today, dry blasting

removal of spent abrasive, and requirements for containment on site.

• Use of water as a blast medium is still increasing, either as wet abrasive

slurry blasting, or ultra high pressure (UHP) water jetting. UHP jetting

• Flame cleaning and acid pickling are older-style methods, but are still

areas where blasting is impractical.

According to Almeida et al. [1], the anti-corrosive protection of steel

ships starts in the shipyard, which is now normally equipped with automatic

Here, the different steel surfaces are blasted and painted with shop primer,

in accordance with the ship’s construction schedule [12, 13]. Following a

shop where, depending on the ship’s design, they are subjected to various

operations such as marking, forming, cutting and welding. In most cases,

practicalities prevent full blasting of the join-up welds and these tend to be

unitary blocks or pre-blocks, and later in large blocks or superblocks, which

are then transported to the site where the ship is under construction. Finally,

appropriate surface treatments and painting procedures are applied to each

transported to the construction site [1].

Fabricators using this process work very much on a production line basis:

each coated block must be in the right place, at the right time, and there

contamination, thereby presenting a clean (and ideally profiled) surface on

the most basic but least efficient means of preparation; however, factors

offers efficient removal of contaminants to present a good profiled

surface appropriate to the paint specification. Drawbacks include noise,

offers very efficient removal of contaminants, especially water-soluble

salts, though it will not create a profile in steel although it will reveal

any previously created profile.

used in specific instances. Chemical strippers can be useful for small

blasting and painting plants for both steel plates [11] and steel profiles.

suitable drying time, the prepainted plates and profiles pass on to the plate

ground and power-disked [14]. After this, they are mounted, firstly in

of the ship’s specific exposure/operating areas. Nowadays these blocks can

be treated and fitted out in blasting/painting cabins, after which they are

346 High-performance organic coatings

can be no hold-ups, a number of vessels are in the dry-dock at any one

In many cases, to ensure production schedules are met, coating application

are met, even at the risk of over-application and over-usage of paint. This,

together with the use of stripe coats on welds/edges, etc., can lead to

[14].

16.3 Coatings for corrosion protection

16.3.1 Coating failure and corrosion mechanisms

The corrosion protection of metal substrates by organic coatings is attrib-

uted to a barrier mechanism, resistance inhibition, and/or an active inhibi-

tive mechanism. Each of these mechanisms is discussed extensively in the

literature [15, 16] and in preceding chapters in this book. This paragraph

deals with the mechanisms of corrosion and protection against corrosion in

marine environments.

Formation of conductive pathways

the degradation process. This process causes the opening of conductive

pathways in the coating, allowing ions to reach the metal surface (Fig.

16.1).

Nguyen and Hubbard [17] assume that the formation of conductive path-

ways in an initially intact organic coating during water uptake (Fig. 16.2:

step 1; note that the consecutive steps are indicated by the circled numbers)

by interconnections of these regions.

The presence of macroscopic defects such as craters and pinholes, air

bubble inclusions, poor wetting between pigment and binder, or mechanical

damage in the coatings would accelerate the pathway connections. Swelling,

stress relaxation and conformational changes in the coatings during

ex posure may all contribute to the formation and enlargements of such

pathways.

Hydrophilic regions contain low molecular weight/low-crosslinked

(LMW/LC) materials. Ionogenic groups, like soluble pigment components

and ionizable resin functional groups, facilitate the formation of the con-

ductive pathways. These materials take up large amounts of water, have a

low resistance to ion transport and are susceptible to water attack e.g.

hydrolysis and dissolution.

is directed at being absolutely certain that required dry film thicknesses

extremely thick areas up to 3–4 times the specified dry film thickness

For coated materials, water uptake is often mentioned as the first step of

is due to an attack by water in the hydrophilic regions in the film, followed

time, and progress on all of these must be synchronized to allow flooding.