1、 ourMISSIONquality real life examples are shown and discussed later in chapter 13.3.1 Blistering via osmosisBlistering is usually caused by the presence of ionic contamination or water soluble species at the interface between thecoating and the steel. Water is drawn from the environment through the
2、coating from an area of low ionic concentrationto an area of high ionic concentration by osmosis. The coating domes upwards as a result of the pressure difference, asshown in Figure 2-3.Figure 2-3. Schematic osmotic blister formation.3.2 Blistering via electro osmosisOsmotic blisters are usually sma
3、ll and relatively closely spaced. Larger blisters are the result of electro osmosis. When anormal blister grows so that it encompasses a pore in the coating, electro osmotic blistering can take place. The differencein potential between the anodic site and the local cathodic site that occurs beneath
4、the blister drives ions into the blister.These then result in further growth of the blister via the mechanism shown in Figure 2-4.In Figure 2-4 an actively corroding site or an external sacrificial anode would provide the same driving force forblistering. Osmosis and electro osmosis tend to occur ea
5、rly in the lifetime of a coating while it retains a degree ofplasticity.Figure 2-4. Schematic electro osmotic blister formation.CHAPTER 2: COATINGS AND CORROSION3.3 Rust jackingRust jacking or rust leverage is the predominant mechanism of coating failure during the late stages of the service life of
6、the coating. It occurs at defects in the coating, cut edges, welds and at sites of mechanical damage. It is a result of thechange in volume that occurs between iron in the steel and iron in the corroded condition. Typically, a volume increase ofbetween eight and 12 times is involved when iron become
7、s rust. This volume change occurs beneath the coating andmechanically levers it from the surface in the manner shown in Figure 2-5. Good mechanical strength in the coating anda good mechanical key to the substrate both help to resist this occurrence.Figure 2-5. Rust jacking.3.4 Calcareous deposit ja
8、ckingCalcareous deposit jacking is similar to rust jacking in that the coating is levered from the surface mechanically by adeposit growing beneath it. In this case, the white calcareous deposits grow at the interface of the steel and coating as aresult of electrochemical polarization by the cathodi
9、c protection system. Hydroxyl ions are generated at the cathodic sitebeneath the coating. These ions change the pH of the environment, which in turn changes the solubility of componentsof the sea water and cause precipitation to take place in this area. Dissolved carbon dioxide can also diffuse into
10、 the gapunder the coating and react with the hydroxyl ions, giving rise to chalky deposits in the manner shown in Figure 2-6.Calcareous deposit jacking and rust jacking usually occur together (sometimes in alternating layers) due to the cyclicconditions found in ballast tanks.It should be noted that
11、 the calcareous deposits which grow, help to protect the steel from further corrosion by theformation of a barrier layer on the steel. This is particularly of benefit in the ballast tanks of bulk carriers which sufferfrom impact damage from grabs, etc., on the opposite side of the plate in the cargo
12、 holds. The paint in the ballast tankscan crack or become detached and the calcareous deposits assist in the prevention of rapid corrosion at these areas untilrepairs to the coating can be carried out.Figure 2-6. Calcareous deposits.ABS GUIDANCE NOTES ON THE INSPECTION, MAINTENANCE AND THE APPLICATI
13、ON OF MARINE COATINGS SYSTEMS14CHAPTER 2: COATINGS AND CORROSIONABS GUIDANCE NOTES ON THE INSPECTION, MAINTENANCE AND THE APPLICATION OF MARINE COATINGS SYSTEMS 154 Anti-corrosion protection by coatingsIn corrosion prevention by paints, three main principles are employed, either alone, or in various
14、 combinations:1) Create a barrier that keeps out charged ions and retards the penetration of water and oxygen.2) Ensure metallic contact between the steel and a less noble metal, such as zinc in the paint, which providescathodic protection of the steel utilizing the galvanic effect.3) Ensure that wa
15、ter on its passage though the paint coating takes on special properties or compounds inhibiting itscorrosive action.4.1 Barrier effectThe barrier effect may be obtained by applying a thick coating, typically 200H9262m to 350H9262m. This barrier effect is the mostcommonly used type of anti-corrosion
16、mechanism. Typical paints employing this mechanism include epoxies andpolyurethanes.By adding flake pigments, such as leafing aluminum, an improved barrier effect can be achieved. The flake pigments areoriented parallel to the steel surface and water trying to pass through encounters a more complica
17、ted and longer passagearound the pigment particles, as shown in Figure 2-7.For permanently immersed steel, the first and often the only choice in coating protection is to utilize the barrier effect.However, if a barrier coating is damaged, the damaged area is open for corrosion to begin. Corrosion c
18、an then proceedinto the steel substrate and outwards under the intact coating, known as rust jacking or under film rusting. Where thereis a risk of mechanical damage, additional protection such as cathodic protection is sometimes provided.Figure 2-7. Complex pathway produced by lamellar pigments.4.2
19、 Galvanic effectProtection of steel through the galvanic effect (cathodic protection) can be achieved with paints containing large amountsof metallic zinc or aluminum. A condition for effective protection is that the paint is formulated to give metallic contactbetween the individual metal pigment pa
20、rticles and between these particles and the steel. The very nature of these paints requires an absolutely clean steel surface and, especially for zinc silicates, a well definedsurface profile for a lasting coating system. When applied, zinc silicates are initially porous. After a while the porosity
21、isfilled with corrosion products from the zinc and a barrier is formed. When damaged, the galvanic effect is re-establishedat the damaged area and the steel is protected effectively against rust creeping.CHAPTER 2: COATINGS AND CORROSION4.3 Inhibitor effectA corrosion-inhibiting effect is achieved b
22、y using primers containing inhibitors. These are soluble or basic pigmentsdesigned to suppress the corrosion process. Inhibitors work by reducing the rate of either the anodic or the cathodicprocess, or by depositing a high resistance film onto the corroding surface. The corrosion current flowing be
23、tween theanodic area and the cathodic area is usually reduced by at least an order of magnitude. To prevent them from beingwashed out of the primer coats, top coats without inhibitors are applied to provide the barrier necessary for the inhibitiveprimer to last. However, due to the water solubility
24、of the pigments used, inhibitive primers are not suited for prolongedimmersion, as they suffer from blistering and subsequent early breakdown of the coating system can occur.When damaged, reasonable protection against rust creeping or under rusting is provided if the damaged area is not toolarge. Wh
25、en the inhibitor has been used up, corrosion will occur.4.4 Surface tolerant coatingsAfter a vessel enters service, corrosion will begin to occur under coatings in cargo tanks and holds at areas of damage orat regions where good surface preparation was not initially carried out. Maintenance of the c
26、oating is essential if its targetservice life is to be achieved. Any damages to the paint or any areas of rust jacking must be repaired as quickly aspossible. Under service conditions, it is not always possible to achieve a very high standard of surface preparation,although some vessels have small s
27、cale abrasive blasting equipment on board.The application of a surface tolerant paint product can be useful in the repair coating process. However, it should beremembered that no paint will perform adequately if it is applied onto heavily rusted or contaminated surfaces and thatsteel preparation sho
28、uld always be carried out to the proper or highest possible standards, to avoid the necessity to repairthe same area many times.Outer hulls of vessels suffer from mechanical damage from fendering, tugs, etc., and with time, this can result in pittedsteel which is difficult to clean by spot abrasive
29、blasting. The steel looks clean visually but ionic contaminants, such assalts, can be trapped under rust scales or in pits as shown in Figure 2-8. Washing the surface with fresh water can help to reduce the residual contamination. The use of surface tolerant coatingsin this situation can also be ben
30、eficial, providing that the levels of contamination are not excessive. Paint manufacturersspecify the maximum contamination which can be tolerated by their products on blasted surfaces (i.e. see IACS Rec.87.3a.2.)Figure 2-8. Residual contamination in pits.5 Coating compatibilityThe compatibility of
31、coatings with a specific type of paint and between different types of paint varies considerably.Coatings such as epoxies have very specific over-coating time intervals (sometimes called the over-coating window) andthese times must be strictly followed if the individual layers are to adhere to each o
32、ther. Incompatibility between coating types, such as epoxy anti-corrosive coatings with some types of anti-fouling paints, canbe overcome by the use of a tie coat, which has good adhesion to both paint types and is therefore applied onto the anti-corrosive layer before the anti-fouling layer is appl
33、ied.If paints from different manufacturers will be used, then advice should be sought from the manufacturer of each of thedifferent paints. Coating compatibility is important in new building and its approval is specifically addressed in theAppendices of these Guidance Notes.Coating compatibility is
34、also important when maintenance and repair work is carried out, to ensure that the repair coatwill adhere to the original paint, or failures will occur between the individual layers (inter-coat adhesion failure).ABS GUIDANCE NOTES ON THE INSPECTION, MAINTENANCE AND THE APPLICATION OF MARINE COATINGS
35、 SYSTEMS16CHAPTER 2: COATINGS AND CORROSIONABS GUIDANCE NOTES ON THE INSPECTION, MAINTENANCE AND THE APPLICATION OF MARINE COATINGS SYSTEMS 176 Stripe coatsStripe coats are generally applied during the new building process as the blocks are being coated. They are also appliedduring maintenance and r
36、epair/refurbishment. Spray application processes and the inherent nature of paints in the liquidstate cause the coating to pull back from sharp edges and this results in the formation of a thin film at edges, as shown inthe first coat portion of Figure 2-9. Figure 2-9. Schematic showing the need for
37、 stripe coatings.Figure 2-9 shows the first layer of the scheme coat with the first stripe coat applied. For ballast tanks, a second schemecoat and a second stripe coat will be added, as shown in Figure 2-10. The purpose of the stripe coat is to add an extrathickness (preferably about 30H9262m) of c
38、oating around vulnerable areas such as cut edges, welds and drain holes. Duringroutine maintenance on board the vessel, the application of stripe coats during repair work (particularly if the vessel wasconstructed without stripe coats) will prolong the life of the coating scheme.Figure 2-10. Schematic of a full stripe coat scheme.
