Corrosion allowance for Bare Carbon Steel in Caustic Service of Alumina Refinery

[Kashif456]

Hi Members,

I am working in one of the Alumina refinery where corrosion allowance for bare carbon steel in the following service is considered zero

- BAUXITE SLURRY (BSL): SAND SLURRY (SSL): MUD SLURRY (MSL)
- HIGH TEMPERATURE SPENT LIQUOR (SLHT)
- COLD SPENT LIQUOR (SLC): COLD GREEN LIQUOR (GLC): HYDRATE SLURRY (HSL):FILTRATE (FLT): FILTER FEED (FF): WASHER OVERFLOW (WO): LIQUOR TURNBACK (LTB)SODIUM OXALATE SLURRY (OSL): CAUSTIC WASH (CW) (e): DORRCO FILTER AID (FA): FLOCCULANT (FL) PRECOAT (PC): HIGH DENSITY OXALATE SLURRY (HDO): SAND SEPARATION OVERFLOW (SSO) (f): CALCIUM OXALATE SLURRY (COS): DRAIN PIPING >50NB (DL)
- HOT SPENT LIQUOR (SLH): HOT GREEN LIQUOR (GLH): HOT HYDRATE SLURRY (HSLH)
- LIME SLURRY (LSL)

Can someone please explain why corrosion allowance is considered as zero while there are chances that the above caustic solutions may cause stress corrosion cracking, then why taking a risk of having zero corrosion allowance?

 

[EdStainless]

When cracking is the failure mechanism it is addressed through chemistry and residual stress control.
Making things thick may actually increases stresses and make SCC risks higher.
It is similar to pitting corrosion, you can't fight it by using more thickness. Since it is a highly localized event bulk material can't help you.

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P.E. Metallurgy, consulting work welcomed

 

[BenMacier]

It's almost zero because the pipe schedule will include some sacrificial allowance.
In caustic environment (pH > 8) carbon steel will passivate by creating a stable oxide. More stable than in acidic environment.

* Finding a solution is great * Knowing how to implement it is fantastic * Believing it is the only one and best is naive ?