Ballast water filters need to withstand the most challenging circumstances in seawater filtration. Why is Cathodic protection considered the industry standard for corrosion prevention, and is it the most effective solution available?
A recent report made public by the U.S. Maritime Administration 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, they all share a single and particularly significant vulnerability. They all run the risk of falling prey to corrosion-related degradation of their ballast water filters due to the various corrosion challenges facing ballast filtration systems.
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.
After several challenging years focused on securing regulatory compliance under the Ballast Water Management Convention (BWMC) and U.S. Coast Guard’s Final Rule for ballast water treatment, shipping’s priority has shifted to operational compliance, which ensures BWMS are compliant for the lifecycle of the vessel.
At the Marine Environment Protection Committee (MEPC) 75th session, the International Maritime Organization (IMO) issued guidance to Administrations that all BWMS installations should be tested following commissioning. This requirement, as well as guidance previously issued by individual flag administrations, has provided the first glimpse into the real-world performance of ballast water management systems after installation.
One of the main reasons for BWT failures during compliance testing is organisms over 50 microns, which clog the system. This can be prevented by a strong and robust filter. Without it, the effectiveness of the entire BWTS could be compromised, leaving manufacturers open to criticism from ship owners and operators, who must repeat unsatisfactory tests at a later date.
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 is a well-tested methodology and is being successfully used in many seawater applications to negate the effects of corrosion, the time has come to ask if it is the best option for ballast water filters. This is because, though it 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 buildup of calcium carbonate. On a flat or consistent metal surface, this build-up is not as problematic as it is when it forms on complex metal structures such as woven filter screens and results in clogging of the screen pores. This is because the woven screens have a much higher surface area that is exposed and where calcium carbonate can build, but also because the increased surface area also increases the area at risk for common corrosion problems to start. These factors contribute to an environment that produces high amounts of calcium carbonate that can reduce the size of the screen’s pores, reduce the flow rate, or simply block the screen altogether. 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.
With such downsides to Cathodic protection, it would only be natural to ask: Are better options available?
Alternatives to Cathodic Protection
One of the best alternatives to Cathodic protection is using a higher-grade stainless steel for ballast water filtration. For example, 904L steel eliminates the need for Cathodic protection altogether. Traditionally used in the high-technology, aerospace, and chemical industries, 904L is also famously utilized in Rolex’s luxury watches. It has been chosen by luxury watch manufacturers thanks to its higher polish and water-corrosion resistance, enabling wearers to go about their daily business wearing the high-ticket accessory worry-free.
Austenitic stainless steel 904L has a higher percentage of nickel and chromium than 316L steel, in addition to copper. Its composition provides it with superior corrosion resistance capabilities, rendering Cathodic protection unnecessary.
Indeed, 316L steel is the traditional staple coating historically relied on by the ballast water market, and as such, it is regularly produced and widely available. But while 904L is less common, it is still relatively available, and can be supplied with excellent lead time.
Though the initial cost of 904L steel is higher than that of 316L steel, due diligence processes have found that the benefits of applying 904L steel for ship ballast filters in seawater applications far outweigh the costs. The higher durability requires less maintenance and extends the lifespan of screens and filters, ultimately saving the industry precious stakeholders’ time and money in the long term.
If better corrosion resistance can be obtained by upgrading the steel, the traditional choice of 316L steel for ballast water screens, which necessitates Cathodic protection, needs to be re-evaluated.
Bottom line
Using Cathodic protection to prevent corrosion of ballast water filter screens made of 316L steel requires more frequent maintenance and part replacements, leading to increasing costs over time. The sacrificial anodes used in Cathodic protection stimulate the formation of scale, which clogs the screens. Installation of Cathodic protection is complicated, and the anodes need to be immersed in an electrolyte for a significant portion of the voyage. Overall, Cathodic protection is complicated to install and is a less durable corrosion prevention approach than other existing solutions available in the market today, such as using a higher grade stainless steel.
Several parameters have been used to evaluate corrosion prevention approaches for ballast water filtration systems, including Pitting Resistance Equivalent Number (PREN), Pitting Resistance Accelerated Test, corrosion rates in acidic conditions, and natural seawater tests. A new comparative analysis of corrosion resistance approaches is now available for download in a special free white paper.
Download it now: Overcoming Corrosion of Stainless Steel Screens in Seawater Applications.
