In the first segment of the scrubbers corrosion issue, we discussed how corrosion occurred and what are the basic steps of prevention as outlined by Norshipping 2019 Press Release. Now, we are discussing how different corrosion occurs and how the scrubbers material selection accelerates this process as illustrated by the Norshipping press release.
How different types of corrosion happens?
Primarily there are 3 different types of scrubbers corrosion – uniform, pitting and crevice.
As the name suggests, uniform corrosion happens at a constant rate in constant conditions across the whole surface. It is predictable corrosion which can be prevented if the thickness of the material is less than 0.1mm/year
Pitting corrosion, on the other hand, is a small and fast-growing deep surface corrosion caused by chloride salt attack. This can be easily prevented by adding Chromium, Molybdenum, and Nitrogen to the surface.
Crevice corrosion occurs in crevices and under deposits which are caused by chlorides on ’occluded’ surfaces. Same corrosion mechanism as in pitting but more aggressive where resistance increased by Chromium, Molybdenum and Nitrogen addition
How Alloys Can Help Resist Corrosion?
This brings us to the next question on role of these alloys in preventing corrosion and how far these are effective.
- Chromium (Cr): Increasing content increases corrosion resistance and promotes a ferritic structure
- Nickel (Ni): Slows the corrosion in acidic environments and promotes austenitic structure
- Molybdenum (Mo): Increases strongly resistance against uniform corrosion, acts as a
Ferritic promoter and increases risk of precipitates
- Nitrogen (N): Very strong austenite former which increases resistance against localized corrosion
How To Select Scrubbers’ Material?
Now, the pertinent question is how to select a proper scrubbers’ selection material. Well, it’s a well balanced calculated measure as depicted by the charts given below.
How To Know Your Scrubber Material’s Potential?
The scrubber material’s corrosion potential is determined based on a graph drawn by comparing the critical crevice corrosion temperature (CCT) vs critical piting pitting temperature (CPT).
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