Amine reboiler cracking 2

[Corgas]

Colleagues,

Equimpemnt was bulit back in the 60"s and is SA-201B (PWHT).

DEA = 7.5% (< 30wt%)
H2O = 89%
CO2 = 1%
H2S = 2.5%
HSAS < 2wt%
Acid loading < 0.1mol/mol
Operating temp = 244 °F

Cracking was found in one area(inside shell)during WFMT in BASE METAL.
Yes, base metal.
-No welds (from nozzles) are close.
-No external attachments nearby (ladders, lifiting lugs,etc).
-No inlet opening is facing the cracked area.
-No evidence of arcs strikes(from welding) are present.

Cracking looks like typical SCC (or ASCC), it's been ground 3/16" and it's still there. Unfortunately, I don't have a picture to share.
Since I don't see the where the stress is coming from, Can I rule out any type of stress corrosion cracking?
Also, internal surface is showing pitting all over.

Can it be HIC?

We're getting ready to do a full UT scan (shear wave).

I've already betted a bottle of 18 years old scotch that is not ASCC.

Any takers?

Thanks again,

CORGAS

 

[SJones]

Deduction would lead to a strong case for HIC or, more likely, SOHIC, particularly with the steel vintage. SSC could be an outside contender, and may have actually been an initiation site for SOHIC, but the temperature is a little high. I think your water of life is safe!

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[Corgas]

Thanks Mr. Jones.

Can turbulent flow cause high enough wall shear stress inside this vessels as to provide a source of stress for ASCC to occur?

Generalized pitting on ID surface leads me to think about high velocity.

I know inside pipes, wall shear stresses can be high in turbulent flow. I remember reading NORSOK standard M506 (for CO2)where they take wall shear stresses into account to calculate corrosion rates.

 

[TomEun]

Normally the velocity is not a critical factor for amine cracking. High velocity with turbulent flow promotes the localized corrosion (i.e., at elbow) and severe metal loss (due to removal of FeS film). Once the failure is cracking mode, the root cause may be SSC (sulfide stress corrosion cracking), HIC (hydrogen induced cracking), or ASCC (alkaline stress corrosion cracking). The common root cause of them is a highly stressed metal surface. The wall shear stress is based upon the friction coefficient and mixed flow velocity/density on the pipe surface. It means the wall shear stress is not a primary cause of the cracking because the stress is not under tensile stress mode at the pipe line, but the friction stress may promote the erosion corrosion/metal loss of the pipe surface. So the shear stress is a major factor for the corrosion rate calculation (not for crack evaluation) in piping system (including reboiler circuit).

In this case, the root cause may be come from a highly stressed metal surface during long term operation (under fatigue stress) and/or maintenance (repair welding). Please measure the hardness on the cracked area. The residual stress/hardness value will be the most critical factor for the crack. Max. 200 BHN is the most preferable guidance.
Also, I believe there are a lot of hidden histories (for highly stressed condition) for 40+ years old facilities.

PWHT is required after repair welding.

Thomas Eun

 

[Corgas]

Thanks Tom,

I thought about measuring hardness but I read this in API 945(Appex A).

"A.5.....ASCC has occurred in a variety of steels. Field experience to date has not indicate any significant correlation between susceptibility to ASCC and steel properties. Hardness of the steel has virtually no effect on ASCC."

Similar statement in NACE RP0472 6.2 "Weldment hardness usually have no effect on ASCC susceptibility".

What will be your approach to calculate "fatigue stress"?

Also, I read in a publication (attached):"Corrosion pits are common sites for the initiation of stress corrosion cracks".

your thoughts?

 

[TomEun]

The highly stressed area near the HAZ is the most common place of the ASCC. Also, in many cases, the amine service is exposed to SSC as well as ASCC at the same time.
The material may be also under the fatigued condition which has lower threshold tensile strength than the initial tensile strength value. Even though API RP581/579 have some guidance for the fatigue evaluation, I am not sure it can be applicable to your situation. The most recommended way for the evaluation may be to take a test coupon from the equipment (i.e., by scooping).

Meanwhile the pits before the cracking of carbon steel are a common. It means the crack on carbon steel normally occurs at the stressed area (especially residual tensile stressed surface) directly without pits. Corrosion pits as an initiation site of stress corrosion cracking are normally shown in stainless steels.