No impact on corrosion or coatings by CleanBallast system

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Bremen-based RWO GmbH, a Veolia Water Solutions & Technologies company in cooperation[ds_preview] with on of a leading European corrosion institute (SWEREA-KIMAB) and the classification society Germanischer Lloyd have carried out thorough accelerated corrosion studies in treated full-salinity seawater with the CleanBallast ballast water treatment system. The tests simulated operation over an approximate entire lifetime of a ballast water tank / piping structure (approx. 40 years).

CleanBallast operates readily in waters with low and full salinity. The natural corrosiveness of those environments differs significantly, with, for example, full salinity (> 32 PSU) being a very corrosive media to common construction materials. It is also well known that a larger quantity of active chlorine has a further negative effect on corrosion, increasing the wear rate of non-passivated metals, etc.

The disinfection unit »EctoSys®« utilised by the CleanBallast system is based on electrochemistry, however operating very differently compared with, for instance, conventional chlorination or electrolysis systems using salt water (containing chloride), where a maximum production of active chlorine is desired. Instead, EctoSys® produces short-lived mixed oxidants which together have a more striking and powerful effect compared to active chlorine. Thus, the EctoSys® is not dependent on chloride content (salinity), but produces oxidants directly from the water. The negative effects of active chlorine on corrosiveness can effectively be avoided.

In natural brackish and full-salinity seawater, besides the short-lived oxidants hydroxyl radicals the disinfection unit EctoSys® will produce only low levels (up to maximum 2 mg/l) of more persistent oxidants, summarised as TRO (Total Residual Oxidants). Being oxidizing agents, such substances in higher concentrations are relevant for corrosive properties of water. TRO will decay via interactions with, for example, dissolved organic matter. Fig. 1 below illustrates a typical decay curve of TRO, showing that the natural blank level of 0.2–0.3 mg/l of TRO is reached within approximately two hours.

These studies were later recommended by the IMO technical group GESAMP-BWWG, as part of the guidance for other vendors developing ballast water treatment studies, to be included in their respective approval process (ref. MEPC 59/2/16, § 4.5.1).

The tests included accelerated comparative studies (treated and untreated seawater) using both uncoated steel test specimens but more importantly test specimens with 2-coat paint systems according to NORSOK Coating 3B approved according to DNV Classification Note 33.1 class B1, common and approved for use in ballast water tanks, for instance, the Jotun system »Balloxy HB light«. The tests included parallel tests with both continuous exposure to the water and intermittent cyclic exposure of water and air. Intermittent exposure resembles better the real conditions in ballast water tanks and a worse corrosive case than continuous exposure. The tests were accelerated, that is, the exposure of the test panels was set to simulate an approximate entire lifetime of a ballast water tank / piping system, regarding initial maximum concentration of TRO and natural decay.

The evaluation of the exposed test panels was performed according to the following standards:

• SS-EN ISO 9227:2006 (salt spray 1440 h)

• SS-EN ISO 6270-1 (condense 1440 h)

• SS-EN ISO 2812-2:2007 (immersion 3000 h)

• EN ISO 15711:2004 (cathodic 3000 h)

Saskia Skovdal