[Answer] Does Coatings Help in Scrubber Corrosion?


Scrubbers corrosion and malfunction have become a critical issue in recent times and we are trying to address this with adequate information. Scrubbers material is important in preventing this as we had highlighted in previous corrosion updates. This time we are dealing with issue of coatings and how they can protect scrubbers, as reported by an article published in Riviera Maritime Media. 

Scrubbers Scenario Now

Let us first take a look at the scrubbers scenario now.

To meet the requirements of the 2020 sulphur cap, some 4,000 ships will run scrubbers next year, the majority of which are open-loop hybrid systems. These scrubbers discharge all washwater overboard.

DNV GL estimates that less than 2% of these units will be closed-loop systems, while 20% are likely to be hybrids – a combination of open-loop and closed-loop systems to ensure compliance in areas where the discharge of scrubber washwater is banned.

Any SOx trapped in the exhaust gases dissolves in the washwater and can form strong acids, lowering the pH of the washwater. As reports come in about scrubber washwater pipelines developing cracks and leaks, the selection and specification of scrubber material and coatings is becoming a major industry concern.

Corrosion and high temperatures

While washing the exhaust gas of the engine, the sulphurous exhaust gas stream is introduced into the lower portion of the scrubber as the washwater is introduced as a spray. The acidic washwater flows down the walls of the scrubber tower.

Bottom and Top Portion Vulnerable

In a typical scrubber tower, the bottom portion can be subjected to high levels of acidic corrosion. The top portion also experiences acidic corrosion, though to a lesser degree, since not all the SOx is removed. Residual sludge removed from the washwater is fed to a tank for eventual disposal to the shore, while the associated pipework can experience acidic corrosion.

Closed Loop Scrubber Corrosion Area

In a closed-loop system, the washwater is re-circulated. The make-up water is dosed from the alkali tank and there is a risk of alkaline corrosion as a result.

Diluted Water Prevents Corrosion?

IMO guidelines require the washwater to be neutralised and diluted with additional seawater so the pH level is more than 6.5 before discharge. The proper operation of these systems is critical given reports of corrosion damage stemming from scrubber washwater.

Coating Causing Corrosion?

In addition to corrosion concerns, a technical paper by Safinah Group titled, “The coating challenges posed by scrubber system installation”, points to potential damage to the anti-biofouling coatings on the hull. IMO guidelines require the pH to be 6.5 at4 m from the discharge point when the ship is stationary. The paper says the binder existing in some biocidal antifouling systems could be damaged if the washwater is not neutralised to levels close to 6.5.

Key Areas of Concern

The paper lists three key requirements for the materials that go into scrubber systems: corrosion resistance; acid resistance; and high temperature performance. Corrosion is an electrochemical process that degrades the material. Corrosion can occur in all seawater pipelines, such as in the water supply to the scrubber tower.

Acid attack occurs in environments with lower pH. The scrubber tower is an area where acid attack can be severe. Acid attack is a challenge across the overboard pipeline system too. In these regions, corrosion is an additional challenge.

How Appropriate Coatings Can Help?

In the scrubber tower, the temperature could be in the range of 50°C to 60°C, whereas in the overboard pipeline it could be less than 40°C, according to the Safinah Group paper. If scrubber systems do not have an exhaust gas bypass, the tower would be subjected to higher temperatures. The paper recommends the use of appropriate coatings in addition to the proper selection of scrubber system materials.

The stainless steel alloy mix should be carefully selected for various parts of the scrubber tower.

Material selection and good quality fabrication

Despite the concern highlighted above, Exhaust Gas Cleaning Systems Association director Don Gregory has accused the marine press of scaremongering over the issue of corrosion and acid attack. He believes a quality scrubber supplier will ensure the right materials are chosen for the scrubber system and sees a need for coatings only in the overboard discharge section, where the washwater could impact on the hull coating.

Mr Gregory says that scrubbers have been part of inert gas systems on tankers for many decades and the industry has learned to design and operate them safely. He does add however, that many of the scrubbers fitted on tankers use diesel exhaust from boilers, which has far less sulphur content and corrosive properties.

Stainless Steel Spells Hope

Mr Gregory says scrubber towers should be made of stainless steel, which is among the most corrosion- and acid attack-resistant materials available. If towers are made of stainless steel there is no need for coatings. Mr Gregory, however, notes the fabricator should know how to deal with stainless steel, since the layer of passivity that provides the resistance, though powerful, is only a few nanometers thin and can be destroyed by fabrication mistakes. “There is a prescribed process to handle stainless steel which should be adhered to,” he says.

Welding Issues

A white paper by Sverdrup Steel titled “Scrubber material selection and fabrication challenges” illustrates Mr Gregory’s point. The paper highlights the significance of mistakes made during fabrication, especially during welding. Welding defects may introduce crevices, destroy the passive layer and introduce contaminants, all of which could contribute to the failure of scrubber material.

Correct Alloy Mixture, the Key?

An appropriate mix of the alloy composition of the stainless steel is crucial. Nickel, chromium, molybdenum and nitrogen content must be correctly balanced. For instance, in the lower part of the scrubber, the paper recommends using special stainless steel with high temperature acid- and chloride-resisting alloys, such as nickel. “The lifespan of a marine scrubber system will depend significantly on the chosen material, its corrosion resistance and the manufacturing process performed,” the paper says.

Glass Reinforced Polyester Piping

Mr Gregory recommends the use of Glass Reinforced Polyester (GRE) piping for the washwater discharge section from the scrubber tower outlet, explaining no coatings are required for GRP piping. He says the use of such materials is not allowed in the discharge section. The steel sections there would need coatings, he adds.

Mr Gregory says GRP’s coefficient of expansion is different from steel, so GRP pipes should not be clamped with steel piping, which could result in premature failures.

That’s how coatings can be useful for scrubber corrosion. Follow this section for more such insights every Wednesday.

Meanwhile, do let us know what else you want to know about by writing to us at experts@mfame.guru

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Source: Riviera Maritime Media