Corrosion Protection Systems for Ballast Tanks and Void Spaces
This paper is written from a corrosion engineers viewpoint and reviews the factors that prolong coating lifetime and those that reduce it. This understanding is the result of a large number of inspections of vessels of all ages and includes results from comparing specifications, reviewing new building practice in many shipyards and scientifically assessing the results of crew repairs and dock-based refurbishments. Back up protection methods for both ballast tanks and void spaces, other than coatings, are also reviewed.
Each stage of a vessel’s coating lifetime has unique issues: design, new building, maintenance in service and refurbishment. The underlying corrosion driving factors, from a corrosion-engineering point of view, are similar throughout all these stages. The crucial factors are the driving forces between the cathodic and anodic sites present in the ballast tanks and how the boating acts to resist these electronic and ionic forces. The critical coating issues are its inherent barrier properties, the level of surface contamination and the adhesion to the substrate.
The first stage of the coating failure process in the presence of surface contamination under the coating from new building, as this causes osmotic micro-blistering. These tiny blisters grow with time and tank cycling and if conditions are suitable, can eventually grow to a size where electroendosmotic blistering can take over. The process is accentuated near to a source of electrons such as a corroding site or close to sacrificial anodes. This process can be seen in action on the metal surrounding the sacrificial anode shown in photograph 1.
Once new metal has been exposed to delamination (coating loss),
these areas corrode further and generate yet more electrons and the cycle
repeats itself. Good coating adhesion to the steel inhibits delamination.
The understanding of this process has been aided by the ability to measure the barrier properties of the coating and the electrochemical driving forces in the ballast tanks, using non-destructive patch probes. This is currently a manual process, but an automated system is under development that will be able to locate the onset of serious deterioration of the coating properties and tank potential.
Classification Societies have a number of requirements and recommendations with regard to ballast tanks. These have been discussed in detail elsewhere, but a hard coating is now applied to all new ballast tanks. Back up methods, such as sacrificial anodes, are optional and light coloured coatings are recommended, as these assist in the identification of corrosion and can clearly indicate when possible cracking of the underlying steel has occurred, as shown in photograph 2.
New Building Issues
Without doubt, a good specification is essential for vessel longevity. It should cover both the coatings package and the surface preparations standards. Good surface preparation is vital for all paint types and in some cases, the differences between surface preparation standards can have a greater effect on corrosion control than the choice of the coating. Many contracts are relatively inexact with regard to surface standards.
Steel types and incoming surface condition can strongly affect service lifetimes. The type of shop primer, the extent of its removal and the removal method all affect the level of coating adhesion in the mid-service lifetime period.
For the majority of vessels in service, breakdown of the coatings in the ballast tanks occurs firstly at the cut edges and welds and special attention needs to be paid to these areas during vessel construction. Edges should be well rounded and free from roughness. Welds should be even and free from porosity and spatter. A good stripe coat should be applied to both welds and edges, to increase the thickness, as spray application can result in thin coverage, particularly on edges. Conversely, over thickness should also be avoided as this can cause internal stresses in the coating and lead to cracking or disbanding of the paint.
If too many sacrificial anodes are introduced at a new building, delamination can occur and the formation of naturally protective rust scales is inhibited.
At the design stage, it is important to remember that some areas of the ballast tanks will be extremely difficult to prepare and paint successfully. The forepeak and aft peak tanks are particular areas where this commonly occurs, as shown in photograph 3.
Sometimes, deck features such as cranes, can require additional under deck stiffening. Poor anti-corrosion design in this area is frequently the cause of corrosion problems in service, as shown in photograph 4.
New building is the best opportunity to prepare and pain a surface, however shipyards differ considerably in their facilities and practices, and standards can vary over a wide range. A well equipped “ship production factory” building many vessels each year, will have enclosed abrasive blasting and sweeping facilities. Paint application and curing facilities will also be enclosed and have temperature and humidity control capabilities.
A less well-equipped shipyard may only produce two or three vessels per year, and therefore the blocks are usually prepared and coated under natural atmospheric conditions. Typical outdoor surface preparation is shown in photograph 5, with extensive power tooling preparation being carried out before coating application. Polishing of the steel rather than the production of a good mechanical key for the paint is a common problem.
Ballast Tank Corrosion
Ballast tanks do not break down uniformly in the same manner throughout the tank. Different regions behave in a different ways and each has its own, unique electrochemical loading. These regions become most differentiated when the ballast tank is empty. When the tank is full, this differentiation causes the upper regions to become anodic and the lower regions to become cathodic. The upper anodic regions tend to corrode and the lower cathodic regions tend to blister and this in turn accentuates the differences between the regions.
A ballast tank can be divided up into three distinct regions:
1. The upper areas and the under deck
2. The mid-section at the boottop region
3. The lower wings and double bottom
Different coating breakdown mechanisms occur because each area has its own temperature and humidity cycles. The anode to cathode ratios are therefore different in each of these areas and this results in different types of rust and scale forming.
The upper area of the ballast tanks can be taken to be that above the deep load line at the topsides region. Because this area is continuously under the influence of the weather, it undergoes the highest levels of thermal cycling. It is also subjected to high levels of vibration and mechanical damage. This area of the ballast tanks tend to be more anodic than the rest of the tank and this situation is made worse be the complex structure. The ullage area is a splash zone with high oxygen levels. This results in accelerated atmospheric corrosion of the under deck plating and in this section of the ballast tank, the high corrosion rates give rise to non-protective rust scales, as shown in photograph 6.
In the mid-section of the ballast tank, in the area of the boottop, rust scales can be relatively self-healing during the early service life of the coating. This is fortunate since the major source of coating breakdown is from reverse impact damage from tugs, fendering and floating debris. The impacts shown in photograph 7 have healed well. The corrosion rates in this area of the ballast tank are lower than those at the top or the bottom of the tank.
In the double bottoms, the temperatures tend to remain lower than elsewhere as the hull is under conditions of permanent immersion. This is the region where cathodic activity is generally most intense. If there is a local anodic source such as a corroding ballast pipe, then cathodic blistering can occur, particularly where the coating is relatively young.
Retained mud can accelerate paint blistering and corrosion rates. Where the mud is not disturbed by ballast water movement, microbial corrosion of the type shown in photograph 8 can also occur. In the photograph, the hand is brushing aside the mud and water to reveal areas of freely corroding steel, which is indicative of microbial corrosion.
Void spaces are areas that do not contain ballast, cargo or other fluids such as fuel or water. They are sometimes called dry spaces, although this can be misleading as they often contain water and other liquids from new building or from leaks and other sources. Void spaces above forepeak tanks often suffer from intense atmospheric corrosion of the type shown in photograph 9.
Void spaces that have no opportunity for air changes may also contain high levels of retained solvent from the new building process. Bulk carrier stools and pipe ducts are typical examples. High solvent levels may maintain the coating in a relatively soft condition that does not resist the rest jacking caused corrosion process.
Pipers in Ballast Tanks
The introduction of noble metals such as stainless steel or copper alloys onto ballast tanks can give rise to galvanic corrosion in localised regions, as the pipes are cathodic to the steel of the ballast tank. Often the supports for vale remote control lines can corrode if both the pipes and the supports are not properly coated. Often the adhesion of the paint to the more noble metals is poor and special attention to surface preparation is required.
Galvanised pipes also suffer from poor coating adhesion, but here the problem is different. The steel is cathodic to the zinc later and the zinc corrodes sacrificially producing voluminous zinc corrosion products, which level the coating from the surface. The presence f the zinc corrosion products make successful refurbishment very difficult.
Pipes can often corrode more quickly than the remainder of the structure. This is due to their geometry and the location away from the rest of the structure.
In Service Repair Options
Once the Bessel is in service, coating repair and tank maintenance can become difficult. The major options for tank coating management are:
1. Do nothing between scheduled dry dockings and carry out repairs only under the best possible conditions
2. Carry out regular maintenance on board, by the vessel’s crew
3. Carry out extensive repairs at sea using a specialised riding crew
4. Completely refurbish the ballast tank coatings when catastrophic breakdown is just beginning. This can be detected using instrumented predictive techniques
Combinations of 1-3 are often chosen, but are very rarely successful in the long term. Most crew repairs last for a few months and then disbond from the steel and old paint, which results in corrosion breaking through the new paint. Repair failures are usually due to a combination of water-soluble species such as salt and iron ions remaining on the surface before the new paint is applied. These contaminants lead to localised micro-blistering that can easily delaminate the paint, which has only a mechanical bond to the old coating. This effect is demonstrated in photograph 10.
When repairs have been carried out effectively, new corrosion sites often quickly initiate in the near vicinity, as shown in photograph 11. This is because the barrier properties of the old paint are changed by the repairs and the next most anodic sites will become active and start to corrode.
The crew have many problems to overcome when repairing ballast tank coatings. The surface has to be cleaned and ionic contamination removed, mechanical surface preparation has to provide a good key to both the steel and the old paint, the curing conditions have to be suitable otherwise curing may not take place and intercoat adhesion may be destroyed.
Partial repairs can be problematical and a poor long-term strategy. This is because a mixture of old and new coating in a ballast tank can focus corrosion activity onto the repaired areas. Here, the coating is softer than the old paint and the pores and other defects in the new paint are not yet blocked with corrosion products. The old coating can act as a good cathodic site and drive the anodic activity onto the new, weak repair paint. This results in the osmosis electroendosmosis cycle restarting. It is a better option to judge when the barrier properties of the coating have deteriorated well into their breakdown phase and then carry out a complete refurbishment.
Coating Back-up Options
The major line of defence against corrosion in ballast tanks and void spaces is the use of a hard epoxy coating. However, other methods can be used to back this up. In ballast tanks, sacrificial anodes are commonly used and there is some interest in using inert gas atmospheres.
Sacrificial anodes are the most common additional protection
method. They work better with short trading cycles, because the residual alkalinity
on the surface can help prevent corrosion during the short empty phase. The
anode quantity and their location has to be carefully balanced to the barrier
properties of the coating, otherwise undue cathodic delamination can result.
Sacrificial anodes can be introduced at any stage of the ballast tank lifetime,
although coating damage resulting from anode welding can be a problem.
Inert gas (IG) systems in ballast tanks are sometimes used in some tankers. It is important that sulphur from the inert gas system is not introduced in to the ballast tank or corrosion can accelerate. This requires the use of a double scrubber on the IG and good maintenance. IG is not free and there is a considerable cost in its generation.
Oxygen is reduced in inert gas, but not eliminated and so corrosion can still occur. Corrosion rates are only reduced substantial when the partial pressure of oxygen is a small fraction of an atmosphere. A further concern is that the high levels of carbon dioxide that can be generated can result in the formation of carbonic acid. This will again result in increased corrosion rates.
Ballast tanks on those vessels using inert gas have to be very carefully ventilated, particularly in double bottom tanks, before inspections can take place.
Dehumidification is an option sometimes chosen for void spaces or for ballast tanks that are only occasionally used, such as the forepeak tanks of some VLCC’s and the ballast tanks of aluminium fast ferries. Figure 1 shows how corrosion rates change with relative humidity (RH).
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