BALLAST TANK COATINGS
Tanks, whether intended for ballast or for cargo, present some of the most corrosive environments onboard sailing vessels. Ballast tanks are regularly filled with seawater—the perfect electrolyte—and are exposed to heavy condensation on all
internal surfaces during loaded voyages. For this reason ballast tanks are problematic in view of providing long-term protection against corrosion by protective coatings. This article will discuss some factors, such as corrosive environments, influence of chemicals, surface preparation, material properties, application and curing. In addition, choice of protective systems and incorrect pre-treatment and application processes, as well as erroneous use.
All these factors may—to a greater or lesser extent—positively or negatively
influence the service life of ballast tank linings, and thus assist in or prevent the installation of coating systems, which may last throughout the design life of vessels.
stSome words on the design of tanks on modern ships: Despite being in the 21 century,
we still see ships being constructed with built-in corrosion traps. It seems as if ship designers do not have sufficient information as to how to design for corrosion protection. Frequently we find construction details which are more or less impossible to protect against corrosion efficiently—either because of their being too close to
other structural members or having a shape which really does not lend itself to normal corrosion protection methods.
In writer’s opinion the problems in the ballast tanks have been further accentuated as a result of the regulations in Marpol 73/78, Annex l, Reg. 13F, which stipulate that newbuildings shall include a double hull construction, which in turn has increased the internal areas in ballast tanks tremendously. As compared to the previous single skin—double bottom constructions, the areas of the ballast tanks on double hull vessels have increased by three to six times.
In addition we have the developments of later years, namely that the major part of the vessels structural members have been moved from the cargo tanks to the ballast tanks. This has created additional problems as concerns access as well as making life for designers more difficult.
Technical colleges and universities should, in cooperation with practical corrosion expertise, endeavor to improve the understanding of corrosion problems among their alumni.
Marine engineers and naval architects can find basic parameters for design of structures in ISO 12944-3, which describes solutions, which effectively will provide passive corrosion resistance and are suitable for application of protective coatings. In later years, corrosion protection of steel in ballast tanks has become more important—or rather the lacking protection, which frequently was the case for these vital areas---have been more focused upon. A number of incidents at sea—often with
loss of life—emerged in the late 1970s and early 1980s. This, coupled to the rather severe corrosion damages discovered, was naturally not acceptable to both authorities and classification societies.
In general, inspection frequencies will be thirty months, however, if the protective
coating in the ballast tanks has deteriorated beyond the limits set down, the inspection intervals will be shortened to twelve months (or less) until the corrosion control situation in the tank(s) is brought under control.
Deterioration (i.e. Ri 3 - Ri 4 in accordance with ISO 4628/3) of linings exceeding 20% of the total interior area is to be considered as if the lining has totally lost all effect and complete renewal shall take place. Maintenance is recommended initiated when deterioration reaches 5% of the total internal area of a tank. If this is not initiated, the inspection intervals on the part of the classification society will be shortened as described above.
Local deterioration of linings to a degree of Ri 4 in areas of great importance to the structural integrity of the ship shall be repaired at once. Examples of such vital areas are knee plates and brackets, contact areas between longitudinal and transverse girders, edges of cutouts, scuppers and other discontinuities and welds.
The basis for lining systems intended for long-term protection in ballast tanks must be a surface preparation to the highest achievable standard. Realistically this will imply a primary surface preparation to a degree of metal cleanliness of minimum Sa 2.5 in accordance with ISO 8501-1 at the time of application of the lining system. The use of prefabrication primers (previously designated shop primers) may be considered for newbuildings. In case of use, the secondary surface preparation must be very thorough as concerns repairs of all damages to the prefabrication primer. In addition, all corroded areas must be brought back to the originally specified degree of metal cleanliness. The prefabrication primer must in addition be of a quality suited for permanent inclusion in the subsequent lining system (e.g. zinc silicate prefabrication primer). If not, total removal of the prefabrication primer must take place and the steel substrate restored to the originally specified degree of metal cleanliness. For ships in service, all internal areas in the ballast tanks must be very thoroughly washed down with fresh (sweet) water in order to remove all chlorides. High pressure water jetting will be advantageous in such cases.
At time of writing, there is only one generic lining system which writer will recommend as both technically and economically acceptable for use in ballast tanks, namely zinc silicate.
A typical frame specification for zinc silicate could be as follows:
1. Stripe coating of all sharp edges, cutouts etc.
2. Application of one coat of inorganic zinc silicate coating to a dry film thickness of
3. Spot touch-up of all areas where necessary.
(Items 1 and 3 may be combined and executed after 2.)
Both water borne and solvent borne zinc silicate coatings may be used. Experience, however, has proven the water borne products to be superior to the solvent borne ones as concerns mechanical strength and service life.
If prefabrication primers are used, these must be completely removed if they are organic in nature prior to the application of the zinc silicate coating(s). Inorganic zinc silicate prefabrication primers may be given a secondary surface preparation as described above.
Some vessels, which were lined with water borne zinc silicate coating (1*75 microns) in the ballast tanks at newbuilding stage, have been surveyed after more than 16 years in service. The original zinc silicate lining was found to be in very good condition and these vessels exhibited minimal corrosion in the ballast tanks. With dry film thicknesses of 100 microns, which today’s products are capable of, there seems to be
no reason why inorganic zinc silicate coatings should not give effective corrosion
25 years. protection of ballast tanks for 20—
The use of zinc silicate coatings in ballast tanks will ensure safety in corrosion protection in as much as these coatings offer protection through the galvanic principle, which excludes underfilm corrosion from damages. Pitting corrosion could be a problem, however, this problem is common to ballast tanks lined with either inorganic or organic lining systems.
Repair to zinc silicate linings is relatively simple, which is yet one good reason for more widespread use of these excellent coatings.
In the group of inorganic linings, cementitious coatings must also be included. These are, in most cases, intended for corrosion protection for a limited time period. Due to their short service life, they will primarily be used on older vessels where the remaining service life to the vessels itself will not justify use of and investment in linings with long service life.
The organic lining systems provide corrosion protection mainly by the barrier principle. Thus, selection of suitable products must be done on basis of their degree of impermeability. In addition, we have seen rather serious problems connected to cracking of organic coatings in areas where stress forces from the vessels’ movement
through the sea, congregate. It is imperative that the lining in such areas has optimal adhesion as well as no excessive dry film thicknesses.
Since the early 1960s coal tar epoxy coatings have functioned well in ballast tanks with service lives up to 15 years seen. It should be noted that the longest service lives have been seen with products containing real coal tar and having been formulated to
the original receipt prescribing a ratio of approx. 50:50 between the coal tar and the epoxy. The lining must in addition be applied to a dry film thickness of minimum 300 microns with correctly executed stripe coating of all needed areas between coats. The premature breakdown of such organic linings seen in ballast tanks, may in a majority of cases be traced to use of coating products formulated in such a way that they are definitely not in conformance with the original formulation. Such changes in formulation may imply substitution of the coal tar by bituminous products, or a change of the ratio between the two main binders decreasing the epoxy content (drastically in some cases). The use of one-coat lining systems has also clearly reduced the effective service life of this class of linings.
Granted that coal tar epoxy coatings offer excellent protection, it must be admitted that they do not meet the demand for light, inspection friendly colors, being black or very dark brown in nature. In addition, there is also the problem of the carcinogenic properties of tars and bitumens.
Developing products in light colors is no problem for the paint industry. However, developing products, which can fully replace coal tar epoxy, is not all that easy.
The following generic coatings seem to have the properties needed for effective protection of steel in ballast tanks. However, there seems to be differences between products from various manufacturers although the main generic binder in their products is identical. Further, the experience base for some products is still somewhat limited. Counteracting this is the paint industry’s improved ability to predict the
protective properties of coatings.
Concerning epoxy paints, we will have to distinguish between solvent borne and solvent free products. Both types give adequate protection, however, each type has its own inherent properties, which may influence the properties of a lining system negatively.
The solvent borne products may contain relatively substantial residues of solvents for long periods of time after application if the coating is applied in too high dry film thicknesses or under marginal drying/curing conditions. This phenomenon is designated solvent entrapment. Such residues of solvents in the seemingly dry and cured film will form minute vacuoles in the coating film. When these solvent residues evaporate over time, they leave behind these vacuoles which implies a (considerable) weakening of the coating film as concerns properties such as mechanical strength and impermeability, which in turn will lead to a (substantial) reduction in the lining’s
Solvent free products are naturally nor prone to solvent retention. However, this class of materials often has poor wetting properties as compared to the solvent borne products. It is therefore often necessary to increase the surface roughness on the substrate to secure the desired adhesion.
The paint industry has more than twenty years experience with pure epoxy products for ballast tanks on ships. Even though the experience base is somewhat narrower that what the case is for coal tar epoxy products, experience show that the pure epoxy products may provide adequate protection for service periods of up to fifteen years. Basically, epoxy mastic products represent modifications of the pure epoxy coatings. In common with all modifications, a certain degree of compromise is implied, which may affect the coatings’ service life detrimentally.
The paint industry’s experience with these products is somewhat limited as the
formulations supplied today only has been in the market for 5—7 years. The earlier
formulations of the epoxy mastic coatings exhibited certain problems, however, these seem now to have been overcome. The future will show if this impression is correct. On the other hand the epoxy mastic coatings have very good adhesive properties, especially to surfaces, which have been marginally treated, making them well suited for maintenance work. However, it should always be borne in mind that the service life of coatings is dependent on the substrate’s cleanliness and the adhesion of the
Products like flotation coatings (oils), products based on sheep (wool) grease, pure bituminous coatings etc. seem to play no significant role in the market anymore. In writer’s opinion, this is a positive development in as much as these types of products only give temporary protection and need to be renewed frequently in order to give any degree of service life to their corrosion protection of the steel in ballast tanks.
It should be mentioned that the testing programme originally developed by Marintek in Sandefjord, Norway has developed into a yardstick for testing of lining systems for ballast tanks. The rating B-1 is the highest achievable.
[The Sandefjord section of Marintek has been (partly) taken over by DNV and the
testing programme for ballast tanks will be continued at DNV in Bergen, Norway.]