Preventing Pitting Damage in Tank Containers

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The Causes and Consequences of Pitting Corrosion

Pitting corrosion is a highly localised form of corrosion that can rapidly penetrate the full thickness of stainless steel tank containers. It occurs when the protective passive layer on the surface of stainless steel breaks down, exposing the bulk metal underneath. This can happen due to mechanical damage, chemical attack, or imperfections in the steel itself. Once initiated, pitting tends to accelerate as corrosive attack becomes focused in one small area. The resulting pits can lead to leaks or even catastrophic failures if not detected and repaired in time.

Inspection Methods for Detecting Pitting

Tank owners currently rely heavily on visual inspection and dye penetrant testing to check for pitting damage. However, both methods have significant limitations. Visual inspection alone often cannot reliably detect pits, especially in their early stages. Penetrant testing is also prone to false negatives if pits contain residue that blocks dye penetration. There is a need for more sensitive nondestructive inspection methods to find pits reliably.

Advanced Eddy Current Inspection

Eddy current electromagnetic testing is emerging as a superior option for detecting and analysing pitting damage. Sensors induce small electrical currents in the stainless steel surface, which are disrupted by defects. Array scanners with multiple sensor coils can quickly scan large areas. Advanced imaging transforms the raw data into detailed colour-coded maps pinpointing even tiny pits. Eddy current testing excels at finding pits other methods would miss, while providing actionable data on pit size and depth to guide repairs.

Effective Mitigation Strategies

The best way to avoid pitting corrosion is proper cargo compatibility review and operational controls. This includes vetting products against the tank material, setting blankets and humidity limits, controlling heating, and promptly cleaning after discharge. Tank owners must also partner with qualified depots to map pits, weld or grind away damage within allowable depth limits, and restore the passive surface layer through pickling and passivation. With vigilance and the right tools like eddy current testing, the risk of leaks or failures from pitting damage can be greatly reduced.

Mike Yarwood

TT Club

Date31/05/2023