ANÁLISIS DE LAS CAUSAS

Any roughness or projection on a ship’s hull causes drag, even roughness only measuring microns! This drag requires more fuel to operate the vessel at the desired speed. The oceans contain countless organisms that seek permanent anchorage on firm struc-tures. These organisms (biofouling) adhere to the bottom of anything placed into the ocean. Some of the smallest are known as micro fouling or slime; they find and adhere to a ship within minutes of launch. When-ever a ship comes close to a shoreline and slows to less than 4 knots, other larger organisms (macro fouling) adhere to the ship. Antifouling (AF) coatings either make the hull of the ship so distasteful that bio-fouling larva reject it, or the coating makes the hull so slick the larva cannot adhere. The toxins in most AF coatings are highly regu-lated by international treaties, as well as national and local regulations.

12.3.1.1 Local and International Regulations

EPA and State Approval in US The US and many other countries have agencies that deal with environmental threats. It is the Environmental Protection Agency (EPA) in the US. Because tradi-tional antifouling coatings contain a toxin and the general environment is exposed to this toxin, the EPA is involved and writes rules regarding the use of AFs. AF coatings that do not contain toxins, such as foul release coatings, are not regulated in the same way.

The most common toxin in AF coatings is copper, in the form of cuprous oxide. Cop-per leaches out of the coating film and may cause harm to bottom dwelling ocean life.

Limits to the level of leaching have been dis-cussed, but are not yet formalized. In addi-tion to copper, many AFs contain a co-biocide, known as an herbicide, to retard the growth of marine grasses. The effective life of these AFs is limited.

The EPA must approve all products, in each available color. This approval can take many years of testing and very few newer AFs that contain toxins have been approved in the US since the 1990s. AF coatings that contain toxins are treated by regulatory agencies as pesticides, or herbicides, or both, depending on the toxins they contain.

In addition to seeking EPA approval, the coating manufacturers must register each AF product in each state where it will be used.

Country Approval

Not all port countries require the same intense study of AF coatings and those have given approval to the application of newer materials. These newer materials use the same copper and co-biocides, however, they use a different binder which depletes in a more controlled manner than the older abla-tive binder-containing materials.

IMO Regulations

In the late 1990s, the International Maritime Organization (IMO) authored a treaty ban-ning the use of organotin as a biocide in AFs. Over a period of several years, the nec-essary majority of UN member countries signed it. It went into force in 2008; at this time nearly all ocean-going vessels have had organotin AFs either removed or sealed.

12.3.1.2 Types

There are three main types of antifouling coatings; they differ in the chemistry used to control bio-fouling (Figure 12.6, Figure 12.7).

Figure 12.6 Bio-Fouling

12.3.1.2.1 Ablative

The binder in an Ablative AF slowly dis-solves in seawater, so it constantly presents a fresh layer of copper on the surface. Coating inspectors should know that during a repainting project, a leach layer of loose binder remains on the surface and must be removed by waterjetting or sweep blasting prior to over-coating.

Figure 12.7 Bio-Fouling

12.3.1.2.2 Self Smoothing

Self smoothing AFs are similar to ablative AFs; however, the rate of ablation is con-trolled and the surface of the coating system becomes smoother during use. They may each have a leach layer, but it is very thin and does not cause the same over-coating issues of a straight ablative. A tin-free ver-sion of this material is fairly new on world markets and several different chemistries are available. It is up to each coating inspector to learn from AF manufacturers the over-coating specifics of each product.

12.3.1.2.3 Foul Release

Foul release AFs do not contain certain bio-cides and work on the principle of a non-stick surface. Bio-fouling attaches to ship surfaces while it is in dock or traveling very slowly. As soon as the vessel reaches about

14 knots, the bio-fouling slides off. The neg-ative aspect of this type of AF is that it dam-ages easily. Also, micro-fouling in the form of slime can stay attached, leaving the hull with a rough finish and increasing drag.

Coating manufacturers are working on newer versions of this type of material to reduce both of these negatives (Figure 12.8).

These systems require very specific primers and intermediate coats and application is a little more complicated than typical coating application. Ensure that workers follow all recommended products and steps during sur-face preparation and application.

Figure 12.8 Comparison of Ablative and Self-Smoothing Coatings

12.3.1.3 Inspection Concerns

The film thickness of each coat of AF is very important to the life of the coating system, more so than with most typical coat-ings. Coating inspectors need to carefully measure the primer coats to ensure each coat of AF is applied at the specified thickness.

In addition, any roughness in the applied coating will add drag and reduce the ship’s efficiency. Watch for correct application techniques and any over spray on the finish coat (Figure 12.9).

12.3.1.3.1 Overcoat Times

Traditional AF coatings are single package materials that generally cure by solvent evaporation. They do not adhere well to a cured epoxy coating, which is usually an undercoat. Application must be done in a very narrow time frame, commonly within 12 hours of the final coat of epoxy. An infor-mal test to determine if the epoxy is cured enough to overcoat, is to push your thumb-nail into the coating. If this indents the sur-face and it is not sticky with wet paint, it may be the right time to overcoat. If the sur-face does not indent, then the undercoat may have cured too much, so a thin tie coat needs to be applied. If the surface is still “sticky”

to the touch, it has not cured enough. Ensure the applicator waits until cure is complete before AF application. Refer to the technical data sheets for specific recoat information.

Figure 12.9 Flaking Caused by Missed Recoat Window

A few of the major AF manufacturers have a special modified epoxy that has a narrower overcoat window to use as the intermediate coat in AF systems. Read the data sheet for each coat and never assume that the “rule of thumb” of coating while the epoxy is still soft is always true.

12.3.1.3.2 Recoating Existing AFs It is fairly common for a commercial vessel to dry dock needing a 20% spot blast to a commercial finish, a full sweep blast, two spot coats of epoxy on the commercially-blasted finish, and one or two full coats of AF applied. It is necessary to ensure that the spot-blasted areas are feathered in. Figure 12.10 demonstrates that it is possible to do, even if the contractor says it is not.

Figure 12.10 Spot and Feathered Blasted Surface