Stress corrosion cracking and corrosion allowance

[svi]

A client spec for a CS piping class calls for 6 mm corrosion allowance (CA) for the Sour service. Does providing CA against stress corrosion cracking (SSC) help? My understanding is that CA will provide for pitting form of corrosion. SSC is one that needs to be addressed through material attributes, primarily through limiting weld hardness, selection of product forms and others.

The 6 mm CA is difficult to provide for smaller sizes and ratings of 600 and higher. SW end valves of sizes up to 1 1/2" and class 800 had to be changed to class 1500 for this additional CA (additional to the min thickness stated in ASME B16.34).

Would it not have been better to consider alternative material than to specify 6 mm of CA?

 

[metengr]

Corrosion allowance as used under Codes and Standards design requirements is not to account for environmentally- assisted cracking (like SCC). The corrosion allowance is for general corrosion under service conditions where the loss of wall thickness is factored into design, not service-related cracks.
Yes, selection of material to prevent cracking from exposure to corrosion in service is most important.

 

[stanweld]

The 6 mm CA is often sited for carbon steel vessels in sour service. It has nothing to do with SCC. It has everything to do with general or localized corrosion wastage. There is a significant initial capital cost to changing to a more corrosion resistant material; however, there may be significantly greater costs during the vessel's or plants's lifetime due to downtime (lack of production) when repair frequencies/replacements are factored.

Owners have been known to be short sighted in this regard.

 

[strider6]

are u sure that the 6 mm are for SSC only? i'll review the material selection an see if there is for example CO2, that could justify the corrosion allowance.

S

Corrosion & Rust Prevention Control

 

[svi]

The line is a 42" flare header leading to the flare gas recovery compressor. Co2 is normally not mentioned for other related systems. I need to check this out. Thanks for the pointer.

 

[gr2vessels]

flare gas? there could be SO2 also. In fact, the condensing water vapours mixing with CO2, SO2 or the like will generate liquid acid. It is also very corrosive during the stagnant periods, when there is a good chance of the acid concentrating due to water evaporation.

 

[TomEun]

As mentioned above by others, generally the corrosion allowance is based on the general corrosion mechanism while hardness control, PWHT, or low tensile strength materials are based on the stress corrosion cracking (including SSC, SOHIC, etc.). However some environments (i.e., sour water, caustic, etc.) have both mechanisms (SSC + severe general corrosion). In these environments, the corrosion allowance (maybe higher) should be considered the corrosion rate calculated and/or experienced.

We have normally used the 1/8 inch corrosion allowance on CS (with/without wet H2S service) in the similar circuit when the service is not exposed to acidic flue gas dew point condensation. If you cannot reduce the corrosion allowance (to 1/8 inch) and/or should be considered the upgraded materials, 304L SS or 316L SS (as solid or weld overlay/clad) may be an alternative material.

Thomas Eun

 

[svi]

Checked up the process design basis. The flare apparently is acid gas, containing up 57.54 Mol % Co2 and H2S 41 Mol %and the balance with other hydrocarbons. Other joining streams contain water. There isn't any So2. So that is the reasoning then. Thanks to all.