Caustic Vessel Protected After Repeated Failure

Caustic Corrosion

Caustic conditions, within MEROX and other demercaptanization processes can rapidly attack steel’s protective magnetite layer and cause material wastage. Following destruction of the protective magnetite layer, NaOH can additionally react with exposed elemental iron to form atomic hydrogen, leading to hydrogen embrittlement, decreased wall thickness, and caustic stress corrosion cracking (CSCC) or caustic embrittlement. These phenomena have led to multiple plant failures under different caustic conditions.

PROBLEM

A caustic wash settler was suffering pitting corrosion and weld cracking caused by the corrosive caustic environment. The organic coating application had failed and a lasting solution was required.

SOLUTION

Upon stripping the organic coating, IGS HVTS alloy cladding was applied to the vessel shell. 20 nozzle inserts were also installed into small bore nozzles. Rigorous testing has been carried out to ensure HVTS will be fit for purpose and protect the vessel until the next shutdown in 2025/26.

Material Selection

Material susceptibility to caustic corrosion and CSCC is heavily dependent on concentration, temperature, and alloy type. Carbon steel is the most common material used for infrastructure exposed to caustic environments and is susceptible to CSCC and general corrosion or pitting at elevated temperatures above 50-60°C and NaOH concentrations below 20 wt. %. Stainless steel provides more protection but is at risk of CSCC and aggressive corrosion above 120°C.

Caustic Wash Settler | Trial and Error

The original vessel was installed in 2011. Following significant failure, the caustic settler was replaced in 2015 and an organic lining was installed to the entire internal area of 351m². The organic coating failed, which led to the selection and installation of IGS HVTS CRA barrier for long-term performance in 2020.

After the HVTS process was completed, and the asset owner was satisfied with the cladding application, cladding thickness readings were taken, and a full thickness map was compiled.