Scrubber corrosion: addressing the issues

Q&A with M.Sc. Chem. Eng. Anders M. Sørheim, Yara Marine Technologies

How important is material quality when selecting a scrubber system?

If the intention of a scrubber installation is to operate the scrubber for the lifetime of the vessel, assuming a minimum of 10 years, then the material quality is imperative. A study performed by the American stainless-steel manufacturer ATI titled “Evaluation of Alloys for Marine Exhaust Scrubbers – Effect of Welding and a Crevice” confirmed the importance of correct material choice for scrubbers. The weight loss and corrosion rate of five corrosion resistant alloys commonly used in marine scrubbers were tested in a simulated scrubber environment. The results revealed that for a severe corrosion case, the alloy 254SMO had a crevice corrosion rate of approx. 0.7 mm per year, while a corrosion rate close to 0 was found for Alloy59.

What are the worst-case consequences of choosing poor quality equipment?

Over the course of 10 years these crevice corrosion rates could in theory grow up to 7 mm deep, which in some cases is more than the thickness of the scrubber wall. Such weak spots in the scrubber tower structure can lead to cracking or other fatigue issues caused by the constant mechanical vibration and the thermal expansion-contraction cycles onboard a vessel, which would result in scrubber leakages. Hopefully such corrosion damages would be detected and repaired before any leakage occur, but such expensive repair work would drastically increase the OPEX of scrubber produced in lower quality materials.

What are the most important factors in building a corrosion-resistant scrubber, in addition to alloy quality?

Welding, welding and welding. Even if a scrubber was built in pure nickel, it could be susceptible to corrosion damages if the welds were not properly performed by approved welders, following an approved welding procedure using approved welding materials. A good rule of thumb is that the welding material should be of a higher grade than the material being welded together, also known as “overmatching the filler material”. In addition, the amount of heat applied during welding should be kept to a minimum to avoid hot cracking caused by the formation of carbides and intermetallic compounds in the weld. Furthermore, the welds should be properly pickled and cleaned, and finally subject to 100% NDT testing to ensure the absence of crevices caused by poor welding, including visual surface examination and ultrasonic testing. A high focus on quality is as important in the scrubber material selection as in the post-manufacturing inspections.

What is Yara Marine’s preferred choice of alloy?

Equipped with a broad base of maritime experience, Yara Marine was very conscious of the corrosion challenges in the extremely corrosive environment inside the scrubber when developing its pilot scrubber back in 2009. For this reason, the company chose the material Hastelloy for the first few scrubbers installed. It has excellent corrosion-resistant properties but is more sensitive to welding and is not very compatible with other materials. For this reason, Yara Marine Technologies decided to go for a similar nickel-based alloy for our scrubbers. Based on ten years of operational experience supported by simulations, we know where conditions are worst inside the scrubber, especially in the bottom part of the scrubber, and in these areas we design for the use of the material Alloy 59. In other areas where the temperature is lower, we use the material AL6XN or similar, which, although not as resilient as Alloy 59, has great corrosion-resistant properties matching the corrosive environment in the upper part of the scrubber tower.

How can quality be guaranteed?

The simplest ways to guarantee quality is to purchase the nickel-alloys from world-renowned high-quality steel suppliers, and to have the scrubbers manufactured in high quality weld shops. In addition, Yara’s quality department does a thorough job of checking that the quality is ensured at Factory Acceptance Inspection, where 100% of the internal welds undergo rigorous NDT testing and any deviations found are reported, double checked and corrected, if deemed necessary.

Are there any significant developments underway in scrubber materials or construction methods?

Over the last few years we have seen a shift in our competitor’s material choice towards more corrosion-resistant alloys, although not to the extent we would have hoped. We have seen players in the market experimenting with ceramic materials for the scrubber, which is an interesting approach, as ceramics do not corrode, but they are unfortunately susceptible to other types of damages due to their brittle nature. Various kinds of plastic composites such as GRE has been tested but are not a real option due to their low melting point. Some manufacturers have even tried different coatings inside the scrubber, but unfortunately the coating does not last long in the extreme environment. The next developments we see will be about the continued optimizing of the scrubber tower design, as long as it does not increase the risk for corrosion.

What are some of the most common reasons that shipowners opt for lesser quality in scrubbers?

Different ship owners have different reasons for opting for lower quality scrubbers, and we cannot know for certain the true reasons behind these decisions. Some ship owners may not realize the extent of the corrosive environment, while others may not believe that it is necessary to use such high-grade nickel-alloys. But then again, dealing with sulfuric acid in high temperature environment is a new challenge for most ship owners. There are also speculations that some ship owners who have high rates of asset flipping do not appreciate the lower OPEX of a more corrosion resistant scrubber, as the ships will only stay a few years in their fleet before being sold.

In your opinion, are the rules and regulations for scrubber manufacturing strict enough?

In general, the rules set by the class societies such as DNV GL cover the structural strength, hull integrity and the safety and availability of the main functions in order to maintain essential services on the ship. Unfortunately, the scrubber system is not considered a main function, and therefore has less stringent rules for manufacturing. When it comes to corrosion protection, DNV GL amended their rules for scrubber systems in 2017 to include the text: “The exhaust gas cleaning unit and exhaust piping exposed to the cleaning water or treated exhaust shall be suitable for the corrosive properties of the two medias.” How well this rule is interpreted and enforced, however, is a different question. As we have seen, not all corrosion resistant alloys are sufficiently resistant to corrosion in the scrubber environment.