Carbon Steel in Caustic Service

[Guest102023]

I am looking at a case of selective corrosion in a large caustic scrubber (not in a refinery). MOC is SA-516-70N, all stress relieved. No mention of SCC at this point; I believe the temperature is not high enough. Some of the surfaces in the affected nozzle are horizontal, so deposit corrosion is not ruled out.

My basic question is whether it is hardness or residual stress that is the more significant factor in caustic corrosion. I am interested in metallurgical factors related to the weld zone which could cause selective corrosion. The repair will be a thickness restoration exercise. We will of course follow sound repair practice and PWHT the area. API 571 does not provide much guidance.

 

[metengr]

brimstoner;
There is caustic stress corrosion cracking when carbon steel is exposed to concentrated caustic solution. Caustic SCC is driven by tensile stress, susceptible material and concentration of caustic. So, lack of PWHT will increase susceptibility, mostly from residual tensile stresses, and not just hardness. This is why carbon steel tanks and fittings containing concentrated caustic solution (50%) above 120 deg F are stress relieved (aka PWHT) to remove residual tensile stresses.

Hardness plays a more dominant role in hydrogen cracking from corrosion.

Caustic gouging is typically an underdeposit waterside corrosion mechanism unrelated to hardness or residual tensile stress in carbon steel. Normally, deposits are present and when removed you have a smooth locally gouged out region. So, what do you have????

 

[SJones]

What is the temperature and what is the concentration of the caustic - process design wise? Welds may create the hydrodynamic conditions for deposition leading to caustic corrosion.

Steve Jones
Materials & Corrosion Engineer

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

 

[Guest102023]

Since getting much more detail, the question has changed by about -10 pH points! (But thanks for the caustic comments, I needed them anyway). A little background information is always helpful.

The problem is localized to a fairly large inlet nozzle, where gaseous HCl, other chlorides (including methyl), moisture, and assorted other nasties enter the scrubber. Operating temperature is moderate, 85~105°F. This explains the description of the corrosion damage much better; I am sure it was severe corrosion pitting.

The question now becomes what do do about it. Options presented for 25-year expected service life include:

(1) Magic Bullet Option: upgrade the nozzle to a resistant alloy. The downside is possible galvanic corrosion in adjacent carbon steel.
(2) Non-metallic coating of nozzle ID. Cheap and easy, but will it last? A real potential downside is if any nearby material needs PWHT following weld repair due to caustic corrosion.
(3) Electroplating with a resistant alloy.
(4) An internal concentric sleeve that channels vapour into the vessel, preventing significant exposure of the nozzle surfaces. It might be feasible to bolt this in at the external flange (a sandwich arrangement). Requires a bit of engineering, but otherwise I don't see a potential risk.
(5) Other??

Any suggestions and especially similar experiences are appreciated. Thermal expansion is not a significant issue at the given operating temperature.

 

[EdStainless]

Both slip in liners and non-metallic coatings are used for this sort of service.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[Guest102023]

I tend to favour the liner, since it is removable for any future maintenance. What is the basic design concept, and does anybody offer these ready-made?
I am thinking it would have an integral flange that would make a double-decker gasket sandwich.

 

[Guest102023]

Part 2,
A little googling revealed that a 'slip-in liner' is made of rubber, and would fit snug against the nozzle ID surface. I would be concerned about corrosion in the crevice. What would such a device look like, and where/how would it attach?

 

[metengr]

I would have a corrosion resistant weld inlay on a carbon steel nozzle substrate. This way, it can be welded into the carbon steel vessel wall.

 

[TomEun]

Before you select the solution (e.g., internal coating/lining/overlay welding materials), you have to find the root cause of the corrosion on CS. I guess the local corrosion on a nozzle may be able to be come from acid corrosion environment (e.g., HCl) and/or steaming out condition. The bulk pH value in the scrubber may be greatly different from that on the inlet nozzle section. The upset conditions on the inlet nozzle shall be considered/evaluated. Per the result of evaluation, the solution will be different.

Please find MTI MS-6 for Caustic Soda.

Thomas Eun

 

[Guest102023]

Who supplies slip-in liners?

 

[StoneCold]

brimstoner
Talk to Edlon. They make tefon slip in nozzle liners, they also make teflon coated pipe, inside and out that you can make an insertion nozzle to push your acid to the center of the tank.


Regards
StoneCold

 

[rustbuster]

You can fabricate your own liner or have any pipe vessel fabricator build it. I have done this numeous times with a sandwhich joint you mentioned. You could also go with a seal welded liner or consider metal spray applications.

Good Luck

 

[Guest102023]

Rustbuster,

Thanks, are there any Code implications for a sandwich joint? Or can I just double the gasket and reef 'er good? I assume the flange part of the liner would be about the same thickness as the existing flanges.

 

[0707]

Go to

https://dow-answer.custhelp.com/app...tion-construction-materials-for-storage-tanks

And see also the text bellow from Nace SP0472-2010

Section 3: Prevention of Alkaline Stress Corrosion Cracking

3.1 PWHT shall be used to reduce residual stresses when prevention of ASCC is specified by the user. In services
where both ASCC and HSC/SSC are concerns, weldment hardness controls shall be applied in addition to PWHT.
3.2 ASME Boiler and Pressure Vessel Code, Section VIII, allows PWHT to be performed at lower than the normally
specified temperature if it is held for a longer time. However, when PWHT is being performed for prevention of
ASCC, these lower temperatures shall not be used.
3.3 For amine and caustic cracking services, an effective PWHT procedure shall consist of heating weldments to
635 ± 15 °C (1,175 ± 25 °F) for a hold time of one hour for each 25 mm (1.0 in), or a fraction thereof, of metal
thickness, with a minimum hold time of one hour.
3.3.1 When PWHT is used for ASCC, the requirements for HAZ hardness control for SSC as defined in
Paragraph 2.3 also must be considered for services exposed to both SSC and ASCC. The allowable variation in
the chemical composition of steels could be considerable, even within the same grade. In conjunction with
welding variables, this can produce high hardness in HAZs that might not be adequately softened by this
specified thermal stress relief. Each situation should be evaluated to determine whether this thermal stress relief
is adequate.
3.4 For carbonate cracking services, an enhanced stress-relieving heat treatment should be used. The heat
treatment temperature should be 649 to 663 °C (1,200 to 1,225 °F) for a hold time of one hour for each 25 mm (1.0
in) of thickness, with a minimum hold time of one hour.
3.4.1 In addition to the higher heat treatment temperature, the guidelines provided in Paragraph 5.2.3.1 of API
RP 945 and AWS D10.1031 should be incorporated into the heat treatment procedures to minimize the residual
stresses that remain after the stress-relieving heat treatment.
3.5 When heat treatment is used to prevent ASCC, all welds and weld heat-affected areas shall receive PWHT,
including all pressure-containing welds, seal welds, internal attachment welds, nozzle-reinforcing pad welds,
temporary fabrication attachment welds, and arc strikes.
3.5.1 External attachment welds often generate residual stresses extending through the entire wall thickness.
If they do, they shall also receive PWHT. Only if an evaluation shows that the residual stresses do not extend
through wall may PWHT be considered optional.
3.6 Experience has shown that heating bands wider than required by codes (approximately > 250 mm [10 in]) are
sometimes necessary. This applies primarily to weldments in large-diameter (> 250 mm [10 in]) piping.
3.7 After PWHT, actions that reintroduce high residual stresses, such as straightening, should be avoided. If these
actions have been done, a second PWHT should be performed when deemed necessary by the user.
3.8 The shot peening process should not be used for applications in ASCC environments as a substitute for PWHT.
3.9 Alternative welding methods such as temper bead welding and controlled-deposition welding shall not be used
for prevention of ASCC.
3.10 During original fabrication, weldments should be inspected for defects such as lack of fusion, delayed
hydrogen cracking, or severe undercut. Any defects found should be removed.