Introduction
The war at sea against corrosion is a constant battle – the
ocean is the most testing of all proving grounds. The sea relentlessly exposes weaknesses in
structures and protective systems with uncompromising power, seeking constantly
to turn steel into rust. Ships
rust. The steel rusts before ships are
built, during construction and throughout their service lives. At the present time a liquid applied coating
system combined with proper design and cathodic protection is our most
important ammunition in this war.
When steel plate arrives at the new building yard it is
cleaned and painted with a pre-construction primer. During construction it is cleaned again and
painted with longer-term coatings. Once
placed into service it is continually cleaned and painted by an army of
crewmembers and drydocking workers, all striving to win the war against
corrosion, and preserving the ship as a productive and safe asset for many
years.
With 10’s of thousands of working vessels at sea and each
and every one of them unique in some way a single chapter in a single book
cannot cover every aspect of painting ships.
This chapter will endeavor to cover common problems encountered and the
typical painting systems in use and will also direct the reader to other
sources that may be more specific to a particular need. One of the best resources for painting
knowledge available to a ship owner is the technical department of a reliable
coating manufacture that specializes in Marine Coatings. However no owner should rely solely on one
source of knowledge, he needs to have an understanding of the forces at work
against him and the solutions available.
Corrosion science, surface preparation, coating application and
inspection are covered in other chapters of this manual and in other books
available from SSPC, NACE and many other sources including the Internet. The reader is urged to access all available
material before making decisions that may affect the life and safety of the
crew, the world’s environment, his multi-million dollar vessels and the
millions of dollars of cargo they carry.
Corrosion of ships can and has caused catastrophic failure with loss of
life, significant financial loss and serious environmental damage. All of which can be prevented with knowledge,
proper planning and follow thru with that planning.
Problems encountered
Ships are sometimes referred to as cities at sea, and this
they are. From huge aircraft carriers
and very large crude carriers to the barges that supply them and the tugs that
push them out to sea. Ships are floating factories subjected to immersion in
water and the most severe weather the earth has to offer. In addition they are subjected to bending
stress, abrasion from cargo and passage through the water, chemical attack,
dissimilar metal corrosion, radical temperature changes, extreme UV attack,
biological organisms and just about any other type of corrosion that Mother
Nature so efficiently provides.
In addition to the most extreme of problems, ships have the
most difficult situations available when it comes to coating and coating
maintenance. Coatings applied during new
construction are frequently damaged during construction and the repair, if any
is a last minute thought. Tanks and
hulls may be perfectly painted and inspected and then someone has to weld
something to the other side of the plate – the correct repair will take too
long. Ships frequently take months or
even years to build, new technology; new owners or a new purpose demand changes
during construction and just as frequently the coating specification does not
change to match the new needs and requirements.
A hopper barge designed to carry coal may be used to haul fertilizer. A cruise ship making regular runs in warm
water may shift to an artic route. It
may rain for the last 5 days of a ships 3-4 year cyclic drydocking; the plan
was to paint the hull then. To meet
competitive demands ships crews are smaller today then ever before. Fewer people mean fewer resources available
for ship preservation by painting. That
small spot of rust can become a fractured plate before it is fixed.
Ocean going ships are International travelers and must
comply with regulations worldwide. Also
the coatings they use and the manufactures technical assistance must be
available where ever they are needed. It
is not unusual for a vessel to be built in Norway, make port calls in the US,
Germany and China and be drydocked for the first time in Korea. The same paint must be available in all
locations. The local applicators in each location must be familiar with the
specified products.
Naval vessels have unique opportunities for coatings. While an ocean going vessel stays at dockside
for a very short period of time a Naval ship may spend ½ its life setting still. And then when they do steam they may do so at
twice the speed of a standard cargo vessel.
These facts impact greatly on the need for antifouling coatings,
normally designed for ships that move at a constant speed most of the
time. If the reader is involved with the
construction or maintenance of US Navy Ships he or she should acquire a copy of
Naval Ships’ Technical Manual Chapter 631 “Preservation of Ships at Sea”
Volumes I, II and III. Chapter 631 provides instructions, requirements, and
information for the prevention of corrosion and deterioration of ships, boats,
and small craft in the naval service by means of surface preparation, painting
and application of other preventive measures.
It should be noted that the US navy does a great deal of testing of
commercial coatings and has in many cases selected to use those commercially
available products instead of “mil-spec formula” products. This is especially true in critical areas
such as tanks, flight decks and underwater hulls. NAVSEA Standard Item 009-32 (current edition)
contains a chart showing locations on the vessel and the approved coating
system for them. It can be downloaded
from the Internet at www.supship.navy.mil/ssrac4/standard.htm.
Solutions
All of this means that ship painting must be well thought
out and all the problems that can happen must be thought of and planned for in
advance. New construction coating
systems should be selected with the longest possible life as the top
priority. The coating systems must be
repairable, rapidly and without special equipment that may not be available
when needed. Never wait till the last
minute to paint; Mother Nature is not a controllable factor in most cases. Proper records of what was applied, not
necessarily the same as what was specified need to be kept so that maintenance
and repairs can be made intelligently.
Inspection is of paramount importance in ship painting and training of
the people responsible for maintenance is a must. Newly applied coating systems should be
inspected in the first 6 months to one year to find and correct any failures
caused by improper surface preparation and/or application. Depending on the coating system, structure
and environment as shown in ISO[i]
9223, further inspection and evaluation could be every 3rd year in a
medium corrosivity area described as category C3 or every 2nd year
in a high corrosivity area described as category C4. The reader is referred to chapter 6.4 of “The
Inspection of Coatings and Linings” by SSPC[ii]
for inspection procedures particular to ship painting.
There is no such thing as one paint or coating system that
will do everything or is correct for every situation. The closest thing to the fabled “magic paint”
is today’s surface tolerant epoxy mastic.
Used extensively in land based industrial maintenance painting for many
years and in the marine market for the past 10 to 20 years or so. This type of product gives a great deal of
flexibility to the ship painter and extended service to the owner. Although not as simple as a single package
alkyd its excellent performance and tolerance of less then perfect surface
preparation out ways the higher cost as well as the extra knowledge and work
involved. Other epoxy technology that is
gaining a foot in the marine market is the use of solvent free penetrating
sealers for maintenance painting and edge retentive materials for tank
linings. Because of environmental
concerns ships owners have to stop the use of organotin based antifouling
products. New materials using
proprietary biocides are offered by many of the major marine coating
suppliers. New high performance acrylic
coatings and blends of epoxy and acrylic resins have also seen a recent surge
in use in both the industrial and marine markets. VOC emission regulations as well as health
and safety issues in many countries have forced coating manufactures to modify
existing materials and develop new versions of older products. The reality is that many of the high
performance coatings available for use on today’s ships have been developed in
the 1990’s and could be considered “new”.
New Construction Painting
Painting
impacts every part of ship construction and frequently starts with a shop
primer. This shop or pre-construction
primer may be described as a quick drying material applied as a thin film to a
metal surface after cleaning to give protection in the period before and during
fabrication. In ship construction the
large volume of plate steel (over 8 million square feet in some vessels)
requires an economical and good method of cleaning. This is done in an automatic shop where the
plates are passed through a centrifugal blasting unit and then automatically
sprayed to a uniform film thickness. The
systems used range from alkyds to zincs and include water borne epoxies and
iron oxide PVB materials. The typical
dry film thickness is ¾ to 1 mil (20 to 25 microns). The type of product as well as the thickness
that it is applied at effects the eventual fabrication of the steel. Cutting speed, weldability, weld
contamination and fumes are all factors that must be considered when selecting
the product. Due to the very large
volume of this one product being used in a shipyard plant solvent emission
guidelines and regulations must also be considered.
Maintenance Painting
Maintenance painting of ships can be broken down into two
main subjects. Painting while underway
and painting at drydocking.
Painting while underway
“If it don’t move, paint it” this statement is as true today
as it was many years ago. Painting
onboard ships while underway is a fact of life and an unending process. The time for planning shipboard maintenance
painting is during the new construction phase.
Every vessel should have on board a manual showing the recommended
maintenance painting system with product data sheets, material safety data
sheets and a coating schedule that shows every area that might need to be
maintained. This document should show
the surface preparation necessary for various conditions and it should contain
detailed instructions on how to mix and apply the necessary coatings. Pictures of each step should be included as
the crewmembers may not be able to read the language the manual is written in. Environmental conditions for coating application
should be outlined and the crewmembers have to be made aware of curing times as
well as application temperature limitations and necessary surface
preparation. If you have taken over a
vessel and were not given the maintenance manual you will have to construct one
based on tests of the existing paint and your preferred coating manufacturers
recommendations.
In addition to the instructions you must supply the crew
with the necessary equipment to perform their job. This means having a paint locker with the
necessary paints in the correct colors and thinners. A large sign on the paint locker door
graphically showing the products that have to be mixed (part A + part B) and
showing the correct thinners will help stop curing problems. You must also have
the necessary mixing equipment onboard; power mixers are required if using any
two-component paint. Brushing and
rolling of two component paints is generally not recommended for large areas,
however it is done frequently. It should
be noted that the appearance of a brushed or rolled surface would not look like
the original spray applied surface. Also
the film thickness of high build epoxy coatings cannot be achieved with one
brush or roller coat as it can with a spray coat.
It is possible to have a ‘ride on’ crew from a coating
contractor to travel with the vessel and perform a larger job then what the
crew would normally undertake. This can
work out very well for many vessels that have a passage where a part of the
vessel that needs painting will not be in use.
It is also common to have a contractor supply several additional people
to ride along and perform painting that today’s smaller crews do not have time
to do.
Drydocking
This is the time when major repairs are done. Underwater hulls can be inspected and repair
work can be done. Interior areas that
require sandblasting can be completed in the yard without causing the problems
that occur when the crew is onboard. Tanks
can have holes cut in them for access and the problem of low exterior wall temperature
is drastically reduced when the ship is high and dry.
Planning and preparation is key to a successful
drydocking. The cost of having a ship
out of service added to the daily cost of the drydock is very high. All materials and equipment to perform the
job should be onsite prior to the vessel arrival. Material for each planed activity as well as
additional material for ‘unplanned’ painting should be available. Most marine coating manufacturers will supply
sufficient material for the job and will take back all unused material at the
end of the job. The owners’ port
engineer along with the coating suppliers technical service representative will
watch over the paint and he should only pay for what was applied to his vessel.
Detailed records of what happens each day must be kept. I refer you again to chapter 6.4 of “The
Inspection of Coatings and Linings” by SSPC for inspection procedures
particular to ship painting. This is
extremely important, as claims due to underwater coating failure can be very
costly. Knowing what happened, how and
when will resolve many issues and will make the next drydocking easier.
Painting Systems
Tank Painting
Tank painting is perhaps the most difficult
coating job onboard ship. It is normally
a shipyard job and requires special equipment for abrasive blasting and
dehumidification. The design of the
internals of the tank is directly related to the ability of the coating system
to protect the steel. Sharp corners,
rough welds, back-to-back angles and inaccessible areas are paint failures that
will just take a little time to show up.
Edges of steel should be rounded over, weld splatter removed and welds
ground smooth and with no undercutting.
Tank lining coatings require a set feature of environmental
conditions, air and steel temperature; airflow and humidity level all must be
within the required range. Multiple
coats and stripe coats are highly recommended in tanks, single coat materials
will almost always leave pinholes and holidays.
The one exception to this rule is the use of high build urethanes. The time interval between coats is generally
limited to both a minimum and maximum and is cortically important for intercoat
adhesion. Inspection of each step of the
way is one of the most important conditions of any tank-lining job. While water blasting is acceptable in many
areas of ship painting, tanks should be dry abrasive blasted. Even with the best design and construction
techniques close spaces are virtually impossible to avoid and can only be
cleaned with the ricochet action of dry abrasive blasting. Also removal of the water and the added
humidity can cause problems. The degree
of cleanliness of the blast and the profile required should be based on the
coating to be used and agreed to by the paint supplier, owner and applicator.
For more about ship tank lining see references 1 and 3.
Liquid Cargo Tank Coating
Tank coating of product and chemical carriers has two main
objectives. First it must protect the
steel from corrosion by the cargo. It is
important that the surface preparation, application, selection of coating
materials and the control of the work can provide satisfactory, long lasting
protection of the steel in the cargo tanks.
Second it must protect the cargo from contamination. It is of paramount importance to have a
coating that is inert towards the cargo to avoid contamination of the
cargo. As a consequence of coating
breakdown by the cargo, claims on both cargo and coating will certainly lead to
dramatic economic consequences. Without
satisfactory coatings the vessel may easily become useless for its purpose and
no longer able to serve its intention of earning money.
Epoxies, epoxy phenolic blends, urethanes and inorganic
zincs are typically used in cargo tanks, however this is one of those cases
that the ship owner and the coating manufacture must work closely
together. The United States Navy has
started using solvent free edge retentive epoxy coatings in an attempt to
combat the problem of thin areas on edges.
Factors that influence the coating selection will be:
- The types of cargo that will be carried
- The type of cleaning between cargos
- The temperature of the cargo at loading
- The storage temperature
- The length of time the cargo will be stored
Freshwater Tanks
Nasty tasting water is not something a person wants to be
stuck with for months on end. A scotch
and water on a cruise ship that tastes like zylene will not bring cruisers back
for a second trip. Potable water tank
linings must protect the steel, not impart a taste to the water and must meet
regulations in certain countries. In the
United States linings for potable water tanks must comply with ANSI/NSF
standard 61. Full curing of these
products with ample ventilation helps avoid imparting taste to the water. While a variety of coatings have been used in
the past including coal tar and cement the current choice is typically a two or
three coat epoxy system tested to meet the above standard.
Ballast Tanks
The marine industry is showing increasing concern following
major damage to vessels and the disappearance of several ships with loss of
life. As vessels built in the early
1970’s show vulnerability to cracks and fractures, ballast tanks have come
under scrutiny. It is in these areas
that corrosion can quickly result in the rapid deterioration of steel and can
lead to structural failures. Ballast
tanks are frequently used as an example of most severe corrosion service. Sometimes dry, sometime full and sometimes
only partially fill of warm salt water, agitated and mixed well with oxygen
they form the almost perfect corrosion cell.
Discussions on the need for improved safety at sea and
better protection of the environment have led to IMO[iii]
regulations that address the treatment of water ballast tanks. The SOLAS Regulation II-1/4-1 requires
corrosion prevention systems to be fitted in all oil tanker and bulk carrier
seawater ballast tanks. Extracts for the
“Guidelines for the selection, application and maintenance of corrosion prevention
systems of dedicated seawater ballast tanks’ state that:
-
The owner should select and maintain a system to ensure
an adequate level of corrosion prevention of the seawater ballast tanks.
-
Coatings manufacturers should give evidence of the
quality of the product and its ability to satisfy the owner’s requirements.
-
The shipyard and/or its sub-contractors should provide
clear evidence of their experience in coating application. The coating standard, job specification,
inspection, maintenance and repair criteria should be agreed by the shipyard and/or
its subcontractors, owner and manufacturer, in consultation with the
administration or an organization recognized by the administration, before the
ship’s construction.”
New Construction Ballast Tanks
The newbuilding stage offers the ideal opportunity to invest
in long-term protection of ballast tanks and avoid the high cost of eventual
remedial work caused by poor preparation and ineffective coating
protection. As a general rule it will
cost a minimum of ten times as much to repair ballast tanks to the original
standard, as it would cost to apply a solid newbuilding system.
In coating ballast tanks color is a consideration, due to
the typical very close areas involved it is necessary to use light colors for
easier application and improved conditions for inspection. Also stripe coatings are always
necessary. They should be applied to
difficult areas such as edges, welding seams, holes etc. A stripe coat should be applied prior to the
first full coat as well as between the first and second coat.
|
Coating type
|
No. of coats*
|
DFT mils/microns
|
Lifetime (years)
|
|
Coal tar epoxy
|
1
|
8/200
|
5±3
|
|
Epoxy Mastic
|
2
|
12/300
|
10±3
|
|
Epoxy Mastic
|
3
|
16/400
|
15±3
|
*Assumes brush applied stripe coat on areas difficult to
achieve required DFT.
Solvent free, edge retentive epoxies applied in a two or
three coat system, with stripe coats are being used by some vessel owners
including the US Navy. Solvent free,
high build, instant set urethanes are also being used by some ship owners. It should be noted that there are other “soft”
single package coatings on the market however these products seemed to have
lost favor over the past few years and are not normally recommended if long
term corrosion protection is necessary.
They are fine for a temporary fix and sometimes are used in dry voids.
Another ballast tank lining system, a single coat of
inorganic zinc with back up anodes has proven to work extremely well in certain
cases. It is an especially useful system
in a double bottom vessel that handles heated cargo. Do not use a topcoat and do not double coat
the zinc; one 3-mil coat applied in a single application is the recommendation.
Cathodic protection can be successfully used in ballast
tanks and is recommended to extend the live of the tanks and coating
system. Cathodic protection will only
work when the anodes are submerged so this is not normally used as a standalone
protection system. The design of a
cathodic system in a ballast tank will be based on several things:
- General arrangement, midship and longitudinal sections plan and capacity plan.
- Tank type: Ballast only, cargo-ballast or segregated ballast tanks in tankers.
- Wetted surface including supports, pipes, ladders etc.
- Current density (should be evaluated by a corrosion engineer/CP designer.)
- Expected lifetime of anodes in years.
- Condition of the tank coating and degree of deterioration
- Ballast period – percentage of total time in service and average ballast time in days.
Maintenance of Ballast Tanks
The maintenance of a ballast tank can take several forms,
depending on the tanks previous history, its current condition and the vessels
operating situation. On a vessel that
had a coating system applied during new construction the tanks should be
inspected at the end of the first year and then every 2nd year. The repairs should follow the recommendations
of the manufacture of the existing coating system.
On tanks that were not coating during construction or where
the system has been allowed to fail completely you will be faced with a very
difficult and costly surface preparation task.
Heavy scale, deep pits, salt contamination, difficult accessibility,
poor lighting, damp conditions, temperature extremes and sweating are just the
top of the list in the difficulty column.
If you are faced with a tank that has been in service, and has failed,
be prepared to power wash, hand clean, power wash again, abrasive blast and
possible power wash again to remove the scale and salt. Another option is to use electrolytic
descaling with magnesium strips to clean badly scaled tanks. This only works when at least 70% of the
paint has failed. Electrolytic descaling
only removes rust, not paint. It also
requires the tanks to be full of seawater (not brackish) for 8 to 14 days. This causes an electrolytic reaction to take
place, which causes a breakdown of the oxide (i.e. the scale and rust). During
this process a soft calcareous layer forms on the surface of the steel. This forces the rust to loosen and fall off
the steel and drop to the bottom of the tank.
The tank is then emptied and washed with freshwater as soon as possible
to remove the rust scale, salt and grayish jelly-like calcareous layers. This calcareous layer is easy to remove when
wet, but hard and difficult to remove when dry.
No matter what surface preparation system is employed salt
contamination testing should be done and the tank rewashed until the level of
salt is acceptable to the manufacture of the coating to be applied. There are
chemicals on the market that can help to lower the salt content to an acceptable
level. However make sure the manufacture
of the lining system to be applied approves the use of any chemicals used
during the cleaning and decontamination process. Salt left on the surface will cause
blistering and paint failure. Once clean
the tank will then have to be dried with dehumidifying equipment before the
coating system is applied. A two or
three coat epoxy mastic system as shown above under new construction can then
be applied.
Accommodations
Accommodations are where the people live, work, eat, sleep
and play. In the olden days of sailing
accommodations were about the last thought in designing a vessel, how times
have changed! Now we recognize that
employees have rights and one of those rights is to have a livable healthy
working space. The coating systems for
accommodations should be bright, cleanable and in most cases must have a low
flame spread.
New Construction, Accommodations
Since these spaces are enclosed the system should be
designed with potential application problems of the maintenance coating in
mind. Products with low flash points
and/or high levels of solvents should be avoided. Water based materials can be used but require
a high level of fresh air being blown into and removed from the painted area so
that the water will evaporate from the film in a timely manner. While the use of zinc/epoxy/polyurethane may
be thought to be the best system it is not really the most cost effective nor
is it the easiest to repair or recoat.
Typically an alkyd or acrylic system will give the necessary protection
in these areas and is easy to clean and repair and a new coat can be applied to
change color or freshen the appearance.
Keep in mind that you cannot apply an alkyd coating over an inorganic zinc
preconstruction or standard primer. To do so will cause sponification and
failure of the alkyd. Another excellent
choice is to use an epoxy system, typically one of the surface tolerant types,
two 4 to 6 mil coats with the final one being the color coat. This system should be used for decks and
other areas that may be exposed to frequent wetting from splash or spillage.
|
Suggested systems for
newbuilding in interior accommodation areas
|
|||
|
No. of coats
|
Product Generic Type
|
Approximate DFT
(See specific product)
|
Comments
|
|
2
1
1
|
Alkyd Primer
Alkyd Undercoat
Alkyd Topcoat
|
2 mils (50 mm)/coat
2 mils (50 mm)
2 mils (50
mm)
|
Typical, well proven system. Easy to apply and economical
|
|
1
1
1
|
Acrylic Primer
Acrylic Primer
Acrylic Topcoat
|
2 – 3 mils (50 –75 mm)
2 – 3 mils (50 –75 mm)
2 – 3 mils (50 –75 mm)
|
Easier to clean then the alkyd. Higher cost. Better color stability.
|
|
1
|
Epoxy Mastic
|
6 – 8 mils (150 – 200 mm)
|
May be top coated with an acrylic for easier cleaning.
|
|
1
1
|
Epoxy Mastic
Epoxy Mastic
|
6 – 8 mils (150 – 200 mm)
6 – 8 mils (150 – 200 mm)
|
Use in areas below gratings or that may be frequently
wetted or immersed.
|
Maintenance, Accommodations
The maintenance should be based on the new construction
system and should follow the instructions of the coating manufacture that
supplied it. If you don’t know what you
have then you must do a little testing.
First do a solvent wipe, take a fairly strong solvent – epoxy thinner
works very well – place it on a white rag and wipe the surface with a medium
pressure 20 to 30 times. Look at the
surface of the rag, if you took paint off then you probably have a single
package material such as an alkyd or a chlorinated product. If this is the case you should use a single
package material over it. If no paint
comes off on the rag then your have a solvent resistant surface and you will
need to sand it to form a suitable surface for the next coat to adhere to. If it is glossy you will also need to lightly
sand it to remove the gloss and roughen the surface. If you have some rust showing then that needs
to be removed with a needle gun or sand paper.
Do not use a power wire brush, it will remove the rust but it shines the
surface and the next coat will not adhere very well at all. Loose paint can be removed with a
scrapper. Damaged areas should have the
coating system tapered back from bare steel to the finish coat with at least an
inch or two of each coat exposed. To
keep the new coating from curling up at the edges of the interface you can use
a 100% solids epoxy penetrating sealer.
These products are designed to go over just about any existing coating
and have very low stress during curing so they do not pull up those edges as
other coatings do.
Engine & Machinery Spaces
Think white, the brighter the better. Most marine coatings manufactures have
“engine room white” or something similar as a standard color choice. This is to give the ships engineers a high
level of reflected light. Keeping them
clean is also required so a coating that is resistant to lubricants, fuels and
cleaning solvents is necessary. Dropped
tools as well as rolling drums and moving equipment can damage the
surface. High temperatures can yellow
the paint and will shorten the expected life of many products. Each of these has to be addressed and this
will require several different coating systems in the engine room.
The bulkheads and overhead spaces should be coated with a
two or three coat epoxy system. If the
epoxy you select is not a high gloss you may want to add a coat of alkyd or
acrylic as a third coat. If initial
economy is a high factor in the coating selection equation a three or four coat
alkyd system is an option, but expect more maintenance and physical damage to
the coating. The deck areas will be a
three-coat epoxy and should have non-skid additive broadcast between the second
and third coats. You may want to
consider the use of a glass fiber reinforced epoxy coating for the deck if you
expect a high level of physical abuse to occur.
High temperature equipment and piping require coatings that match the
highest temperature the item will reach.
You will need to inform the coating manufacture of the normal
temperature, the peak temperature and the frequency of the temperature
changes. Keep in mind that most standard
epoxies are not recommended on surfaces that exceed about 200F on a regular
basis. Insulated carbon steel piping
should be coated, the system will depend on the temperature and to some degree
the frequency of cool down. There are
numerous papers written on this subject and the reader is directed to:
|
Suggested systems for
newbuilding in engine rooms
|
|||
|
No. of coats
|
Product Generic Type
|
Approximate DFT
(See specific product)
|
Comments
|
|
Decks
|
|||
|
3
|
Epoxy or Epoxy Mastic
|
6 – 8 mils (150 – 200 mm) per coat
|
Add non-skid to 2nd coat in traffic areas.
|
|
Bulkheads and Overheads
|
|||
|
2 or 3
|
Epoxy or Epoxy Mastic
|
6 – 8 mils
(150 – 200 mm)
per coat
|
Add an optional 3rd coat of high gloss alkyd or
acrylic if necessary
|
|
2
1
1
|
Alkyd Primer
Alkyd Undercoat
Alkyd Topcoat
|
2 mils (50 mm)/coat
2 mils (50 mm)
2 mils (50 mm)
|
Typical, well proven system. Easy to apply and economical
|
|
High Temperature
|
|||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Insulated Carbon
Steel Pipe
|
|||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Dry Void/Cofferdams Spaces
There are three things to keep in mind when designing and
building a vessel that will have a dry void:
- Dry voids are not dry
- Steel corrodes in dry voids
- Mil scale will not protect the steel
When the
carbon steel for your vessel was manufactured several layers of ferric oxide
were formed on the surface. These layers
are referred to as mil scale and they do temporarily protect the steel from
corrosion. However mil scale is very
brittle and will crack during the cooling process, by any flexing during
handling and by thermal expansion and contraction. The small cracks in the scale allow moisture
to penetrate to the steel and you have one of the worst forms of corrosion
possible on a vessel – pitting! The
reason for the pitting is that the break in the scale surface becomes the
anode; the remainder of the mill scale, which is many times larger then the
break becomes an immense cathode. This
small anode, large cathode situation causes accelerated corrosion of the anode
and a pit in the steel starts to form.
All it takes is the small amount of moisture that exists in the air
trapped in the void during manufacturing.
The effects of mil scale in a marine atmosphere are well documented an
excellent reference is Charles Munger’s “Corrosion Prevention by Protective
Coatings”.
Unless you
are building a vessel with a very short expected life span it is false economy
to not blast and paint dry voids. In a
recent drydocking of a 5-year-old dry cargo ocean barge the steel repairs and
painting to the “dry” double bottom cost over $700,000 US. Several bottom plates were so badly pitted
they had to be replaced. The owner had
saved approximately $300,000 US by not blasting and painting the voids during
new construction.
If you make
the decision not to blast I recommend that you at least apply a long oil type
of coating to give some protection and extend the life of your vessel. This type of material, generally a soft oily
or waxy product is temporary but may be all you need in the void of a platform
type barge or other small working vessel.
Otherwise you should remove the mil scale by abrasive blasting and apply
two coats of epoxy if you expect the surface to get wet or three to four coats
of a good quality alkyd if you expect it to be dry.
Superstructures
The superstructure, along with the boottopping is the first
thing someone sees on your ship and it can give a good, or bad first
impression. So not only do you have to
protect it, you have to make it look good!
In today’s new construction you will find many owners using a
zinc/epoxy/polyurethane system for the superstructure. However as stated previously this is not the
easiest system for shipboard crews to repair.
Experience has shown that single package systems such as acrylics and
alkyds used as the finish coat give good economical protection. The US Navy is currently using silicone
alkyds and has several special blends that both help reduce the solar
absorption and the appearance of rust bleed.
Weatherdecks
Topsides
Boottopping
Underwater Hull
The coating system on the underwater hull of a steel vessel
has to do two things, reduce the corrosion rate of the steel and keep the hull
smooth. On non-metallic vessels it may
only be required to keep the hull smooth.
The coatings main part in keeping the hull smooth means stopping the
attachment (fouling) of some 4,500 to 5,000 species of plant and animal life
that live in the world’s oceans.
Roughness comes in two forms, temporary and permanent. Permanent roughness includes welding seams,
valve openings and bulging plates. These
irregularities are part of the design of the vessel and generally are not
changed once built. Temporary roughness,
which can be corrected, results from corrosion, flaking of old paint, the
porosity of spent conventional and long life antifoulings, bad workmanship in
paint application, external hull mounted cathodic protection systems, mechanical
damage incurred during service and, of course, fouling. Compared to a smooth hull, a rough hull will
increase friction, slow the vessel down or require additional fuel consumption
to maintain speed. Control of roughness
therefore has a major influence on operation costs.
Purpose of antifoulings
- To prevent or reduce growth
- To provide better fuel economy over the sailing period
- To avoid growth penetration through the coatings and thus extend corrosion protection
A 10% increase in fuel costs is the sort of penalty to be
paid by a ship owner if fouling occurs.
There are 80,000+ sea-going vessels representing 460 million gross tons
in operation in the world today.
Assuming an average size of 6,000 gross tons, each equipped with a 5,000
hp engine, fuel consumption would be 15 tons per day per ship, or for the
80,000 ships it would be 1,200,000 tons per day. If the ships operate 250 days a year then the
total consumption of fuel would be 300 million tons. Since most of these ships drydock at 2.5-year
interval the total fuel consumption for this period would be 750 million
tons. The proper use of hull coating
systems can save 5% to 10% of fuel costs - $3,750,000,000 (5% savings at year
2000 prices) and 37,500,000 tons of heavy fuel exhausts discharged through the
smokestack into the environment.
Composition of antifoulings
In their simplest form antifoulings comprise a binder,
pigments and solvent. The binder type
determines the nature of the antifouling.
Pigments include the antifouling agents or biocide and various
fillers. The solvents provide a workable
body and application characteristics.
The biocide is a chemical substance which is released at
very low rates and which kills or inhibits growth. One of the most common antifouling biocides,
still used today, is cuprous oxide.
Organotin compounds started to be used in the 1960’s and became the
predominant biocide in the late 1970’s.
Due to environmental concerns the use of organotin has been restricted
in several countries and based on a recent international agreement will be
phased out world wide by 2008. The
coating industry is working to replace this product and has several new
materials that are claimed to provide equal service. See the section on aluminum vessels
in this chapter for more about the special limitations of antifouling on them.
Types of antifoulings
There are four choices of antifoulings, conventional,
longlife, selfpolishing and fouling release.
Conventional antifoulings also known as ‘soluble
matrix’ or ablative paints, have a natural product – rosin – as a binder. This rosin slowly dissolves in seawater. When the coating is immersed in seawater the
biocide leaches out of the paint. This
release rate however, soon falls to a level below which the fouling can be
controlled. Effective life is generally
short, approximately 12 months.
Longlife antifoulings referred to as ‘insoluble
matrix’ paints do not dissolve in seawater.
Instead the biocide leeches out of the film leaving a porous skeleton on
the surface. As the porous layer
increases, the rate of biocide release reduces.
Eventually no more biocides can be released and antifouling performance
drops dramatically. Effective life is up
to 24 months and a relatively porous layer remains on the surface and must be
dealt with prior to recoating.
Selfpolishing antifoulings can be broken down into
two classes: containing tributyltin (TBT) and tin free. Both work a little differently and the
industry is still working on the tin free so expect more changes in the
future. Because TBT will be banned from
use in the very near future we will not discuss it in depth in this paper. Just remember that tin-free selfpolishing
products on the market at this writing do not work under the same principle as
those that contained TBT. TBT was not
just the biocide; it was a part of the binder as well. Manufacturers are now using various blends of
water-soluble and water sensitive binders along with newer forms of
biocides. In essence early ‘ablative’ or
physically deteriorating paints (shown above as conventional) have been
refined. The polishing effect is similar
to those containing TBT but not identical in performance. Manufacturers are claiming up to 4 years life
with the tin free selfpolishing materials.
However this may change with newer technology as many companies are
spending a great deal of their research budgets looking for a better
antifouling.
Fouling release products are relatively new and are a
product of research done to replace tributyltin materials. They work on the simple principle of
supplying a smooth non-stick surface thereby stopping the adherence of
fouling. Two negatives must be considered
regarding these products, they are typically easy to damage and difficult to
repair; at this writing their impact on the market is minimal.
Key factors in choosing a underwater hull coating system
The selection of the hull coating system should be made
after consulting with a well-known and experienced manufacturer of them. The underwater hull system is comprised of
both the anticorrosive coats and the antifouling coats. The entire system should be from the same
supplier and the specification for the number, type and DFT of each coat should
be followed exactly. Full time
inspection by someone from the coatings suppliers technical service department
is necessary. Some of the things you
will need to make known to the coating supplier are:
- What will be the speed of the vessel?
- Where will the vessel be trading?
- What will the drydocking interval be?
- What will the sailing duration be per year?
- Where will the vessel drydock?
- What are the economic risk factors?
Speed
High, low or slow? The typical speed of the vessel will
determine whether or not a self-polishing AF can be used and if so what type
and how thick. Slower moving vessels may
not be able to benefit from a self-polishing antifouling.
Where
The trading area is very important – different types of
fouling occur in different parts of the world.
A ship moving from warm to cold water on a regular basis will require a
different antifouling then one staying in mostly warm water.
Dry Docking interval
Smaller working vessels such as tugboats may have to be
dry-docked every year for some sort of repair.
This allows time for inspection and repair of a coating system on a
frequent basis. Less expensive, shorter
life antifouling are typically used in these cases. The US Navy has recently announced that they
are looking for a 10-year plus drydocking cycle; this will require an
antifouling that will give service for 10 years. At the present time five years is about the
maximum drydocking interval that most antifouling manufacturers currently
suggest under typical conditions, with their best product.
Days at sea
A ship that sits still a lot will require more frequent
dry-dockings and a different type of antifouling then one that is underway for
most of its life. Self-polishing
products work to their best advantage when the water flowing over the hull
removes layer after layer of the coating, continually exposing a fresh layer of
antifouling.
Location of Dry Dock
Climatic and environmental factors are particularly
important. Make sure the dry dock you
choose can apply the antifouling coating you require.
Maintenance of the hull
The use of antifoulings provides some unique maintenance
problems. In a perfect world you would
just be able to clean and apply another coat, however this is not a perfect
world. Some antifoulings lose the
biocide but leave the skeleton of the binder; most antifouling materials are
single package products that are not the best under coating for the required
anticorrosive. So this leaves us with
the problem of how to repair areas that may be damaged to the substrate and how
to apply another coat of antifouling without removing the existing
coating. Sandwich coatings, layers of
anticorrosive and antifouling over and over are to be avoided if possible. For a small vessel it is generally
recommended to remove all the left over antifouling prior to repairing the
anticorrosive and reapply the antifouling.
For a larger vessel with usable antifouling left on the majority of the
hull the damages should be spot repaired and enough antifouling applied on the
surface to build the total film thickness back to the originally specified DFT.
This repair must be done carefully so that the ‘sandwiched’
coatings do not grow to the point where cracking, alligatoring and eventual
flaking of the antifouling occur. This
requires a careful inspection of the vessel after water cleaning in the
drydock. Only large rough areas should
be blast cleaned, smaller areas should be mechanical cleaned to avoid damaging
more then you need. The anticorrosive coating
should be applied as specified only to the bare steel with as little overlap as
is possible. Each touch up coat should
overlap slightly more until the specified DFT is achieved.
One important step to take when performing maintenance on a
ships hull, no matter the size of the vessel, clean with fresh water BEFORE you
do any other mechanical or abrasive blast cleaning. This is true no matter what type of water the
vessel is in but is most important when it has seen service in salt water. If you blast before you clean you will drive
salt and other contaminants into the surface.
If salt is present between the substrate and the paint film you will
have blisters with pits formed underneath them.
Aluminum Vessels
Aluminum vessels are coated for two reasons: to provide a
color coat and to protect against fouling.
GRP Vessels
References
- Surface Preparation and Painting of Tanks and Closed Areas, SSPC 00-09
- B. R. Appleman and J.L. Heisel, Improving the Quality of Preservation Work on U.S. Navy Ships, Technology, JPCL, August 2001
- E. D. Thomas, The U.S. Navy’s Advances in Coating of Ship Tanks, Technology, JPCL, February 2001
- L. Smith, What You Should Know About Applying Water-Borne Coatings, Applicator Training Bulletin, SSPC 92-03
- K. Kelleher and D. Meyer, What You Should Know About Two-Pack Epoxies and Polyurethane’s, Applicator Training Bulletin, SSPC 92-03
- General Dynamics (Electric Boat), Guidance Manual for the Application of High-Solids Coatings, MARITECH/ASE Project 3-98-3 for NSRP Technical Panel SP-3, Surface Preparation and Coatings, December 2000.
- M. Candries, D. Anderson, and M. Atlar, Foul Release Systems and Dray, JPCL, April 2001
