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Inspection and Performance in Ballast Tanks

Dr. Les Callow
Amtec Consultants Ltd.

Introduction.
This paper deals with the corrosion of marine ballast tanks and the lifetime assessment of their coatings. The techniques and mechanistic models have been developed to enable a clear and quantitative assessment of the remaining useable ballast tank lifetime to be reached. The ability to make an accurate decision in this area enables:

a) A cost effective maintenance and repair strategy.
b) Expensive repairs and steel renewals to be avoided.
c) Clear pre-purchase assessments of vessels to be made.
d) Sale or repair decisions to be made on an informed basis.

Breakdown Mechanisms.
Ballast tank steel corrosion and the breakdown mechanisms of coatings are all electrochemical in nature with a large cathode driving a small anode. The large areas of the flat surfaces are usually the cathodes with the welds and edges being the anodic sites. This results in a strong tendency for the corrosion current to focus on any areas of weakness or non-uniformity in the coating.

The corrosion situation is at it’s most severe when the tank is drying out, as the transport of oxygen through the residual moisture film on the surface of the steel or paint is at it’s greatest. This situation can be changed dramatically by the presence of sacrificial anodes in the ballast tank.

Coating barrier properties are affected by the electrochemical processes occurring in the tank. The resistance of the coating to corrosion remains high for several years over the majority of the surface, but as some areas begin to decline in their corrosion resistance with time, this increases the level of electrochemical stress. After a period of time, there is a rapid loss of barrier effect in the flat areas. This generates the large and invisible cathode.

Quantifying Breakdown.
The normal Classification Society method of quantifying ballast tanks gives three categories: “good”, “fair” and “poor”. However, when a ballast tank has reached a “fair” condition the useable lifetime of the coating has probably been exceeded and steel repair will become inevitable.

It is important to be able to detect coating breakdown before the point that extensive refurbishment becomes necessary. In order to do so, it is important to classify the breakdown in a manner that enables the breakdown mechanism itself to be understood.

The vessel structure also needs to be understood in terms of how the breakdown on the edges and welds interacts with that on the flat areas. The breakdown phenomena is either driven by calcareous scale formation or by rust jacking which is corrosion driven.

Each of these phenomena behaves differently with time. Scale formation tends to be self-limiting whereas corrosion breakdown is not.
Coating breakdown is usually attributable to surface preparation and coating application factors such as blast or power tooling quality, edge preparation, stripe coating and paint quality. Once breakdown has initiated it is important to quantify its level and severity. Two methods are available; visual and instrumented assessments.

Visual methods require a categorisation method whereby corrosion and coating degradation on the welds, edges and flat surfaces are clearly quantified either as a percentage value or as a rating of severity. However, once serious visually apparent corrosion and coating breakdown has occurred, then the paint is usually past economic repair.

Instrumented methods of measuring coating barrier properties give more reliable and consistent results. Electrochemical patch probes designed for coating assessment allow measurements of the current flowing through the coating to be used to give a quantitative measure of the quality of the coating.

Measurements of the substrate potential also provide information on the type of corrosion reaction occurring. The combination of both current and potential methods allows areas of weakened coating to be located for repair.

Comparisons over a large number of vessels between visual and instrumented methods gives a very good correlation between the two methods.

Predicting Failure.
Coatings tend to lose their barrier properties very suddenly over a period of around two years and that this usually starts after a period of between six and nine years in service. Some areas of the tanks begin to fail earlier than others and picking up this failure at the earliest possible time enables preventive remedies that can extend the coating lifetime. Failure shows first in instrumented measurements, then in sacrificial anode consumption rates and finally in visual observations of coating breakdown at edges and welds.

The first point at which sites of failure can be detected is usually at the one year guarantee inspection. The first Intermediate survey after 2.5 years in service provides another opportunity for monitoring the condition at those locations and also the detection of other weak sites. The most crucial time is that of the first Special survey after 5 years of service, when coatings often look good visually but their corrosion resistance may have declined dramatically to the point where total repair is unavoidable at the next dry docking.