13. Paint and Coatings Flashcards
Paints serve different purposes based on location and requirement.
Superstructure: resistant to atmospheric corrosion, long-lasting, easily cleaned, good colour retention, available in bright colours.
Deck: non-skid characteristics, abrasion & chemical resistance.
Cargo areas: abrasion resistance, compatibility with cargo.
Other spaces that have specific requirements:
- accomodations
- service areas (galley, mess)
- ballast tanks
- potable & non-potable fresh water tanks
- fuel and lube oil tanks
- machinery spaces
- immersed region of the hull
- topside region of the hull
- boot-top region of the hull
Properties of Paints
- Corrosion resistance
- abrasion resistance
- Chemical and oil resistance
- Long lasting
- Fire retardant & low flame spread
- Non-toxic
- Anti-fouling
- Non-skid (or slippery)
- Good adhesion to surfaces
- Compatibility with other paints, cargoes, and cathodic protection
- Availability of various colours
- Bright appearance or high shine
- Low odor
- Easily cleaned
- Easily applied
When selecting paint, be sure to look at the application characteristics
- Intended for certain substrates (steel, stainless, aluminum)
- Have specific recoat windows where subsequent coats must be applied within a certain timeframe
- Have specific atmospheric requirements (temperature, humidity)
- Have certain surface preparation requirements (removal of rust, salt and dirt)
- Have a specific curing time
- Must be placed in its working environment within a certain timeframe (ship refloated within a defined window of time, generally for anti-fouling paints)
Bottom Plating & Immersed Sides
- Corrosion protection
- Anti-fouling protection
- Low friction
- Type of cathodic protection must be considered due to the environment it creates on the hull
Boot-top Region
Area just above the waterline.
Treated similarly to the immersed plating but also consider erosion & abrasion due to wave action and floating objects.
- High gloss and colour retention
Topside Region
- Corrosion resistance (atmospheric)
- Good appearance
- Flexible
- High gloss & colour retention
- Abrasion resistance
Superstructures
- Corrosion resistance (atmospheric)
- Good appearance (gloss, colour, covering ability)
- Easily cleaned
- High gloss and colour retention
Decks
- High abrasion resistance
- Oil and chemical resistance
- Non-slip characteristics
Constituents of Paint
- Binder (top, yellow)
- Pigment (middle, pink)
- Solvent (bottom, blue)
- Binder
Purpose:
- Holds paint particles together (cohesion)
- Holds paint to substrate (adhesion)
- Disperses pigment throughout coating
- Eases application
- Provides required paint consistency
Types of Binders
- Bitumen or pitch mixed in a solvent (solvent evaporates to leave a dry coat)
- Drying oils (react with oxygen to cause the paint to harden)
- Oleo-resinous (some of the resins react with the oil to cause the paint to dry)
- Acrylic paints (fast-drying, water-soluble but become water-resistance once dried)
- Alkyd resin (use drying catalysts to accelerate paint drying)
- Chemical binders (used under severe conditions)
Types of chemical binders
- Epoxy resins
- Coal tar/ epoxy resins
- Chlorinated and isomerized rubber
- Polyurethane resins
- Vinyl resins
Epoxy resins
- Produced from petroleum and natural gas
- Usually a 2-pack formulation that must be mixed
Generally used on exterior areas of the vessel while modified epoxy coatings are used in internal areas. Modified epoxies are less abrasion resistant and less resistant to solvents and chemicals, but may be lower cost
Need to be applied to a clean blasted surface.
Good: - adhesion
- chemical resistance
- flexibility
- toughness
Poor: - Cost (high)
- Appearance (chalking aka loss of gloss)
Coal Tar/ Epoxy Resin
Gives good properties of coat tar (high impermeability) and epoxy (chemical resistance)
- Usually a 2-pack formulation
- Bitumen and pitch are often used for underwater applications due to their high impermeability as well as their ability to resist alkaline deposits caused by corrosion
Chlorinated and Isomerized Rubber
- Derived chemically from natural rubber
- Accepts a high amount of solids
- Thicker layers than most paints
- Particularly good against acids and alkalis
Polyurethane Resins
- Chemical process producing urethane
- 1 or 2 pack formulation
- Used as a cosmetic coat over high build epoxy coatings
- Need to apply on a clean blasted surface
Good: - toughness
- abrasion resistance
- Chemical resistance
- Weather resistance
- High gloss
Bad: - contains isocyanate (health hazard)
Vinyl Resins
- Chemical process using organic compounds
- Low solid content therefore offers a thin layer upon drying
- Poor adhesion to bare steel so must be used with a pre-treatment primer application
- One of the best coatings for underwater steel protection
- Pigment
- Suspended in the binder, fine powder
Provides: - Covering capacity
- Abrasion resistance
- Colours
- Corrosion Resistance (red lead, zinc chromate, zinc powder)
- Solvent
- Thinner that is used to make a paint flow more easily.
- Tends to evaporate from the binder during drying and care must be taken to ensure adequate ventilation of the fumes.
- Solvents in paint for certain areas may be prohibited
- Growing pressure to reduce the level of VOCs emitted by solvents
Paint Drying
Occurs in one of 3 ways:
- Solvent evaporation (need adequate ventilation)
- Chemical change (binder exposed to oxygen)
- Curing process (chemical reaction between 2 constituents of the paint)
Results of Improper Surface Preparation
- Poor adhesion (caused by contamination trapped under or between coats), blistering (caused by salt under the paint), peeling, and poor appearance.
Poorly prepared surfaces will not provide a secure base for paint adhesion, resulting in reduced resistance to abrasion and mechanical stresses.
Curing Process
- No oxygen needed for the chemical reaction to take place
- Paints generally have 2 components that are mixed together just prior to applications
- Curing time is short
- Specific conditions required (temperature, humidity, surface preparation)
New Steel Surface Preparation
Prior to leaving the steel mill: millscale removal by blast cleaning, pickling, flame cleaning or hand cleaning. Millscale is the result of iron oxides forming during hot working of the metal and can result in the formation of corrosion cells on the surface of the metal.
Once cleaned, steel is usually washed, dried and coated with a good quality primer.
Properties of the primer: long line, quick drying, non-toxic, compatible with welding and cutting processes, compatible with paint system.
Surface Preparation of an Existing Ship
Remove:
- Oil
- Grease
- Dirt
- Salt
- Marine growth
- Rust
- Old paint
Methods of cleaning:
- Washing with mild cleaning solution
- Washing with harsh chemical solution (remove oil, grease, old paint)
- Chipping tools and wire brushes (remove old paint and surface corrosion)
- HP water wash (remove Hmarine growth)
- HP wet abrasive blast (slurry)
- Abrasive blast (sand, metal shot, glass)
- Specialized equipment (in-water cleaning)
Hull Bio-Fouling
Common Hull Fouling:
- Algae
- Sea squirts
- Barnacles
- Bryoxoans
- Hydroids
- Mussels
- Serpulids
Consequences of Hull Fouling:
- Increased frictional resistance
- Slower speeds
- Increased propulsive power
- Increased fuel consumption
- Increased emissions
- Increased maintenance costs
- Contractual deadlines
Rate of Marine Fouling Growth
- Water temperature
- Time alongside
- Time at anchor
- Ship’s operating speed
- Stagnant water
- Geographical areas
Control of Fouling
Create an environment that is unfavourable for marine life:
- Slippery Coatings (non-toxic)
- Anti-fouling paints (poison)
- Impressed current
Anti-fouling paints
Slow, controlled release of poison (bio-active material)
Generally able to control both soft organisms (weeds, slime) and hard organisms (barnacles, mussels).
Exist as non-polishing (soluble or non-soluble) or self-polishing paints
Non-Polishing Soluble Binder Paint
Have toxin dispersed throughout the thickness of the paint layer. The rosin swells up in seawater and dissolves, which subsequently releases the toxin into the environment. The layer becomes progressively thinner until it has completely disappeared from the hull’s surface. It will continue to release toxins until the binder and poison have been completely spend.
A bit cheaper, but need to ensure adequate original thickness to predict lifespan (5 years). Rate of release proportional to speed of vessel
Non-Polishing Non-soluble Binder Paint
The release mechanism for the toxins is via a leaching action by an additive in the binder. This additive will react chemically with the seawater and change the non-soluble surface of the paint into a soluble structure. Subsequently allows for the toxins to be leached from the binder and released into the environment. The protection offered by this paint reduced through time until it is no longer effective. Constant paint thickness but surface becomes porous. Rate of release proportional to speed of vessel
Self-Polishing Paint
The binder has a chemical composition where it is both hydrophobic and hydrolysing. The hydrophobic trait limits the ingress of water into the paint structure, thus only a thin layer of anti-fouling paint is in contact with the seawater. The hydrolysis nature of the structure allows the controlled chemical reaction of the paint with the seawater. This reaction allows the leaching agent to cause the outermost layer of toxins to be released to the environment. Once this occurs, the remaining binder then becomes soluble in the water and washes off the hull. A positive side effect of this process is the development of a smoother surface over time, hence the name self-polishing. The rate of polishing as well as the rate of toxin release can be controlled by the appropriate chemical formulation of the binder. The protection level for a hull can be determined and suitably formulated paint can be applied for the specific lifespan desired. Highest value.
- Binder reacts with water
- Leaching agent produced
- Releases outermost layer of toxins
- Remaining binder becomes soluble and washes off hull
- Exposes new layer of binder and toxin
- Surface becomes smoother with time (self-polishing)
Toxins
Once upon a time, tributyltin or TBT was the optimal anti-fouling poison. The tin component is an extremely effective anti-fouling poison having a very long half-life. Due to its longevity, TBT can be found in fairly high concentrations throughout the world with severe negative effects on other unintended organisms. It is felt that this poison has had a significant impact on the food chain.
TBT-free anti-fouling paints are now commonly available on the market:
- Copper pyrithione and zinc pyrithione
- Copper-based paint (cuprous oxide and copper powder)
Biomimetic antifouling coatings are being researched.
Every vessel of 400 gross tonnage or more must hold and keep on board an International Anti-Fouling System Certificate
Tank Protection
Whatever protection is to be applied, it should provide some basic desirable properties to suit the product being carried: it should withstand contact with the product, have a long life and be resistant to chemical and mechanical damage. Protective surfaces should be compatible with varying temperatures. The coating should not be toxic, flammable or explosive. The area should be bright and easily inspected for damage. Ease of application, availability and cost all play a role in choosing suitable protection for particular uses.
Tank Coating Categories
Soft coatings:
- Remain permanently wet
- Greasy characteristic
- Generally sprayed or floated onto the surface
- Limited lifespan
- Filling/emptying of the area must be done slowly to prevent disturbing the film
- Messy and dangerous during inspections
- Not generally recommended
Semi-hard coating:
- Dry but flexible
- Can be touched and walked on
- Easier to apply in adverse conditions where steel cannot be adequately prepared or when atmospheric conditions limit the use of hard coatings.
Hard coating:
- Epoxy, polyurethane, zinc silicate, vinyl
- Chemically convert during curing
Tank Coating Types
- Pure epoxy (interior use, no UV protection; high chemical resistance, used in product carriers and sewage tanks)
- Modified epoxy (additives to improve flow, contamination tolerance and chemical resistance; used in double bottom and ballast tanks)
- Amine epoxy (additives affect viscosity, mechanical strength, heat resistance; suitable for crude oil, petroleum oils and vegetable oils)
- Phenolic epoxy (easy cleaning between products giving good cargo switching flexibility; used for aggressive cargos)
- Polyisocyanate cured epoxy (tanks carrying solvents and chemicals)
- Solvent-free epoxy (fresh water tanks)
- Zinc-rich primers with epoxy or vinyl overcoats (ballast tanks; applied in 2 layers to ensure sufficient dry film thickness and avoid missed spots)
- Low VOC Zinc Silicates (chemical traders)
Tank Linings
- Vulcanized steel (natural or synthetic rubber): bonding must be done under specific conditions; easily take the shape of the surface
- Cladded steel (usually stainless steel): provides complete tank protection from reaction with the product, cladding in thin, any structural support comes from the steel understructure, easily cleaned, initial cost may be quite high but lifespan is good and subsequent treatment of the surface is not needed.
- Specialized metals (nickel-alloyed steels, invar, aluminum): initial cost is high but long life may offset this; bi-metallic corrosion must be addressed with suitable protection provided. A consideration in low temperature or pressurized tanks.
Coating Application Methods
- Spraying
- Brushing
- Rolling
- Floating
Good ventilation for fumes that may displace oxygen and/or explode
Coating Deterioration
- Loss of flexibility (increased brittleness): chemical change, loss of one or more constituents, oxidation of coating surface, cracking, disbonding, reduced corrosion protection
- Abrasion
- Improper Surface Preparation
- Mechanical damage (cargo impact or gouging)
Coating Assessment
Various standards and methodology
- Class Society driven
