A report made public by the U.S. Maritime Administration recently stipulates that, as of January 1st, 2015, there were 41,674 ocean freight merchant vessels (weighing in at 1,000 gross tons and over) registered with an International Maritime Organization number and sailing through waterways across the globe. Among the ships in this worldwide fleet are vessels ranging from container and general cargo ships to tanker ships.
While these ships run the gamut with respect to size and functionality, the vast majority of vessels must ensure they are compliant with the International Maritime Organization’s (IMO) International Convention for the Control and Management of Ships’ Ballast Water and Sediments, 2004. The regulation, which is commonly known as The Ballast Water Management Convention or BWM Convention, came into force globally on 8th September 2017 and requires ships in international traffic to manage their ballast water and sediments to a certain standard, according to a ship- specific ballast water management plan. There are two ballast water management standards, D-1 and the more stringent D-2. Eventually, most ships will have to conform to the D-2 standard, which specifies the maximum amount of viable organisms allowed to be discharged. For the majority of ships, this means a ballast water treatment system (BWTS) must be installed to neutralise organisms.
The most important foundation for an efficient BWTS is its filter, which must be able to remove large organisms. It is imperative that the filter does not clog, as this would limit or even prohibit water flow through the BTWS, ultimately impacting the overall effectiveness of the entire system and even causing it to fail commissioning testing.
All vessels installing BWTS must select their filter carefully, or they will run the risk of falling prey to corrosion-related degradation of their filters. Corrosion can be caused by various corrosion challenges within BWTS. Taking in seawater for ballast naturally invites a host of microorganisms that ballast water filters need to treat, such as Sulphate-reducing bacteria and phytoplankton, which stimulate microbiologically influenced
corrosion (MIC). The filters not only need to protect the ballast water tanks from MIC, but they are also subject to rust themselves and need adequate protection against it.
In an ideal situation, to ensure uninterrupted operation and protect BWTS, the recommended practice of leaving the ballast water filters full at all times or emptying and drying them would help prevent corrosion and its subsequent degradation from taking root. However, the need to take on more ballast water to accommodate the absence of cargo during particular portions of a given voyage and unexpected rough operational conditions during loading and unloading procedures often makes it difficult to successfully and satisfactorily complete the process on a thorough and regular basis. The result: sedimentation accumulates on the ballast water filters, leading to reduced service life, the risk of failures, and high, short- and long- term maintenance costs.
To ensure uninterrupted operation regardless of varying water conditions and other unpredictable factors, ship owners need effective and dependable filtration systems that protect their entire ballast water management systems. The filters remove as much matter from the water as possible before it goes on to secondary treatment, reducing the amount of chemicals needed and time required to neutralize living organisms in the water. If the filters themselves are not adequately protected from corrosion, they might not filter water effectively, incurring frequent maintenance and replacement costs.
CATHODIC PROTECTION FOR BALLAST WATER FILTERS IN SEAWATER APPLICATIONS
Today, most ballast water filter screens in the world are made of 316L stainless steel. Cathodic protection is the most common corrosion protection method for this type of steel, making it widely used in vessels worldwide. Cathodic protection safeguards the metal against corrosion by connecting the at-risk steel to a highly active “sacrificial metal” acting as an anode. The anode introduces free electrons to the space and relinquishes its ions. In doing so, the formerly active 316L steel areas on the screen’s metal surface become passive, and the new, more active metal coating ultimately and safely corrodes instead.
While Cathodic protection has long been recommended by filter and screen manufacturers to provide the necessary protection for ballast water screens made of 316L steel, this corrosion protection approach has some significant drawbacks.
A sacrificial anode can be used for protecting the filter’s screen, but the anode is consumable, and its dissolving leads to the formation of a hard scale on the screen surface. This scale is caused by the build-up of calcium carbonate and results in clogging of the screen pores. In high consumption rates of the anode, this becomes a critical risk and requires regular cleaning of the screen. The economy of Cathodic protection is less-than-ideal. Although it is cheaper than other alternatives, installing Cathodic protection is complicated. Once it is up and running, ongoing electricity supply and periodic inspection and maintenance fees add to the cost.
While Cathodic protection is a viable solution, its durability may call its high investment requirements into question. In particular, the sacrificial anodes’ limited available current and their vulnerability toward rapid corrosion lead to a shockingly limited lifespan.
Furthermore, sacrificial anodes need to be immersed in an electrolyte for a minimum of 24 hours before Cathodic protection can be applied. If filters are not regularly kept full as recommended by manufacturers, the sacrificial anodes need to be immersed for 25% of the voyage.
The Report • June 2021 • Issue 96 | 93
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