DOSP-VHR-002754 | Development of an extensive model for long term immersion corrosion in seawater, with special focus on the conditions at the Paardenmarkt

Dumping of conventional as well as toxic munition just after both wars, has always been popular. It is a quick but mainly cheap solution that gets rid of the excess quantities of war material ashore. The “Paardenmarkt” is one of these many munition graveyards of WWI & II in our seas and oceans (Liebezeit, 2002; Paka & Spiridonov, 2002; Schulz-Ohlberg, Lemke, & Tauber, 2002). In itself, the “Paardenmarkt” is called an absorbant silt bank. It is situated in the Belgian part of the North Sea, off Knokke-Heist, and is the current resting place of around 35.000 tons of WWI chemical munition, where an estimated 4 million ammunition shells are buried a few meters below the seabed. Moreover, the shells are (probably) of the chemical warfare type, containing agents like mustard gas, arsenic and phosphorus components, and chloropicrin. In addition, substantial amounts of the explosives and propellants used at that time are present, such as ammonium nitrate, picric acid, centralite, nitroglycerine, and nitrocellulose, as well as the explosive 2,4,6-trinitrotoluene (TNT), meant for the explosive dispersion of the chemical agent upon impact (Van Ham, 2002). In 2019, the yearly monitoring efforts of the RBINS found, for the first time, traces of the TNT in the water adjacent to the dumpsite (Heylen, 2019). Although the measured values are still very low (around 1 microgram per kg dry weight seawater), it is a certainty that this ammunition is leaking, leading to the possibility that the other chemicals are bound to be dispersed from the shells as well, posing a definite risk for the local North Sea ecosystem as well as for the public health on the Flemish coastal region and urging the need to determine the physical condition of the shells and to propose strategies for its sanitation (Sanderson, Fauser, Thomsen, & Sørensen, 2009; Waleji, Ahlberg, Berglind, Muribi, & Eriksson, 2002). At this moment, very little is known about the actual state of the munition. After WWI, the existence of the ammunition dump site was generally forgotten, until it was "rediscovered" in 1971, during dredging maintenance works at the port of Zeebrugge (Vandeweyer, 2015). Several shells were recovered from the seabed and were described as being ‘in a remarkable good condition’. However, upon the expansion of the port of Zeebrugge in 1972, a shift in the sea currents buried the ammunition under a thick layer of silt, at a depth of between 2 to 6 meters deep (Missiaen & Henriet, 2002). The most evident approach would be to bring a sufficient number of these projectiles to the surface to study their condition. However, this is strictly forbidden by Belgian law (Koninklijk besluit tot vaststelling van het marien ruimtelijk plan voor de periode van 2020 tot 2026 in de Belgische zeegebieden, 2019), prompting a theoretical approach. Our research program is embedded in the SBO DISARM project of the Flanders Marine Institute. Not only theoretical knowledge gaps will be bridged. In order to develop and support an integrated scientific methodology to support risk assessment and management of marine chemical munition dumpsites worldwide, the “Paardenmarkt” munition dumpsite will be used as a challenging case study (Missiaen, 2018). The combination of aerobic, anaerobic, galvanic, slit, chemical and microbial corrosion makes any determination of the current condition of the gas bombs a complex task. The complexity increases due to the combination of environmental parameters at the “Paardenmarkt” (such as dissolved oxygen, temperature, salinity, pH, corrosion accelerating bacteria, presence of methane and fresh water, calcium carbonate and the formation of concretion), which have all changed over the course of the last century, not in the least after the shells became buried in the sediments. Extensive testing of the corrosion rate of the different composing elements of these toxic gas shells under those conditions is therefore indispensable to enable us to create a reliable model predicting their physical condition. The main goal of this research is to develop a model that predicts the corrosion loss of the WWI munition dumped at the “Paardenmarkt”. Until today, a number of potentially important processes have been advanced and studied individually (Loto, 2017; Wu, Pang, & Peng, 2017; Petersen & Melchers, 2018). The impact of combining those processes is still missing. To get a well-founded overall picture of what is currently happening in the sludge of the “Paardenmarkt”, extensive testing at lab level, completed with in-situ corrosion rate measurements, will be indispensable. Simultaneously and subsequently, we will try to gain a better insight into the geographical, geological and geophysical parameters that could determine the corrosion behavior of the toxic shells on the “Paardenmarkt”.