Corrosion Allowance versus Design Thickness

[PermanentCharpy]

Good day all,

When corrosion allowances are specified (ie: maximum 2mm on the ID etc), is this allowance taking into account degradation of the material due to this corrosion in the form of micro structural effects (embrittlement etc).

In other words:

A pipe of 10" x 1.0" WT corrodes to 10" x 0.5" WT, is this pipe of equivalent strength to a pipe manufactured to size 10" x 0.5" WT? I understand there are many factors at play here, but I am looking to find literature that supports/disproves my belief that materials that have corroded to a specific thickness cannot be treated as if they are in the same condition as a machined part (taking into account material loss).

 

[MintJulep]

If the remaining 1/2" wall is unaffected by corrosion then it might be acceptable to consider it similarly to material originally made to that dimension, for some types of loading.

It's a really big if that depends on a whole lot of things.

State your assumptions. Identify the uncertainties. Assess the consequences of being wrong. Apply engineering judgement.

 

[SJones]

Corrosion allowance is the amount of material over and above the minimum structural integrity thickness computed by the pertinent design code. It is to account for metal loss by wastage. Generally, extra material cannot be deemed to confer protection against embrittlement which is not a wastage mechanism. When material has been lost through wastage, the integrity of the structure is assessed by "fitness for service assessment" (a useful Google search term, along with "failure assessment diagram"). It might be possible to treat the remnant material as a "new" structure with a reduced dimension, if the metal loss process has removed material in a completely uniform manner, and has not adversely affected the mechanical properties, including fracture toughness, of that remnant material. It will be extremely rare for metal loss to occur with a totally uniform morphology.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[TugboatEng]

On marine structures, the overseeing class society will require a minimum plate thickness and requires replacement of the plate at 20% wastage determined by random gauging. There is an exemption to the 20% rule if heavier plate is used. I can't remember the term they use. It seems marine classification societies are in agreement that it's only the minimum thickness that matters regardless of the beginning thickness of the plate.