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INTRODUCTION
ABSTRACT: An in situ study of more than 100 ballast tanks of merchant marine vessels looks to the corrosion process in these tanks from another perspective. The developed corrosion model shows major similarities with earlier studies based on laboratory experiments. The field work exposes the influence of ship construction parameters such as land of construction, coating type and the presence of sacrificial anodes on the corrosion process in the ballast tanks. Possible alternatives for vessels constructed with ordinary grade A steel and coated according to IMO PSPC standards are presented, even though further research is required to come to final conclusions.
The degradation of metallic surfaces due to atmospheric corrosion is a well-known problem for many exposed steel structures such as bridges, storage tanks and pipelines. Bringing seawater into this equation causes an even more aggressive environment, and an increased corrosion effect. Merchant navy vessels sail across the world's oceans and in the absence of cargo or when the ship is only partly loaded, she carries seawater in het ballast tanks to ensure maneuverability and to control draft, stress and stability. As necessary as ballast tanks are for the operation of a ship, though, the fact that they are prone to corrosion poses a distinct problem for a ship. First of all, corrosion is expensive. For the U.S. economy alone, the 1998 cost of corrosion amounted to $275.7 billion/year1. Production interruptions, incidents and repairs provoked these economic losses. On board of a ship these elements were boosted by the omnipresent safety aspect. Moreover, the problem of ballast tank corrosion was exacerbated by the introduction of the double hull tanks. In 1989 the Exxon Valdez polluted Prince William Sound and consequently the USA government imposed a new ship design for all tankers carrying oil in US territorial waters, called double hull. IMO followed a few years later and the double hull design became obligatory by the MARPOL convention in 1993 for newly built ships The purpose of this design was to protect the cargo tanks with a double barrier, to increase ship safety and to minimize pollution in case of a calamity. Indeed, today, all tankers (and most other vessels as well) have their ballast tanks wrapped around the cargo tanks, serving as a protective barrier. To facilitate tank washing and maintenance, all structural elements were excluded from the cargo tanks and transferred to the ballast tanks, resulting in very unfriendly labyrinth-like structures. Even more so - reality proves that maintenance of ballast tanks does almost not exist. Chipping, grit blasting and airless paint spaying in these enclosed spaces is cumbersome and very expensive. Recent studies 2, 3 show corrosion algorithms and functions, given a generalized corrosion model. All these models are the result of prolonged laboratory studies. In this paper, the opposite approach has been followed. Taking advantage of our geographical position in the centre of the international port of Antwerp, we have surveyed a number of ships to assess the corrosion damage in situ.
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