Mr Jalal,
The explanations of Dr Beldyk are great and nicely explains the mechanisms of intergranular corrosion in the welds/HAZ due to sensitization. One of the popular choice avoid sensitization in austenitic S.S is to use stabilized grades such as 316-316L, 321 or 347.
321 S.S (Titanium stabilized) is prone to one type of intergranular attack known as Knife-Line Attack. The attached document illustrates the form of attack. Normally Knife-Line attack takes place in presence of very corrosive media such Nitric acids, fuming sulfuric acid or hydrofluoric acids.
During welding stabilized carbides ( TiC) dissolve at elevated temperature in and around the fusion line. Upon cooling Cr-carbide will form faster than TiC. This is so because 321 S.S contains 18% Cr as against 0.6-0.8% of Ti.
Should it happen, there would be a thin layer of Cr-carbide depleted zone running along the Weld fusion line in the HAZ and which would corrode once exposed to corrosive media such as nitric acid.
In the present context if you could organize a replication of the damaged area, followed by metallurgical analysis, the root cause investigation would be more meaningful.
As the severity of corrosion is more during shut down , alkaline washing prior to shutdown might be helpful.
The remedial measure would be to selectively weld build up the affected areas with welding electrodes of matching or superior chemistry, with very controlled heat input.
Good fabrication practices mentioned below could be useful for Newer equipments-
§ Base metal chemistry (especially C) should be should be strictly controlled. The maximum limit of "C" is 0.08%, values lower than this would be better.
§ The base metal shall be properly solution annealed as per the applicable material spec, say ASME Sec-II Part C.
§ Preferably get the base metal tested for intergranular corrosion testing, to ASTM 262 Practice-C (Huey Test), welding consumables and on weld coupons during procedure qualification.
§ Take the best possible precautions during welding, control of heat input, MAX interpass temperature of 1750C maximum.
§ Stabilize anneal @ 850-9000C for 2/4 hrs after fabrication so as to allow precipitation of TiC.
§ NACE RP-170-2004, Protection of Austenitic Stainless Steels and Other Austenitic Alloys from Polythionic Acid Stress Corrosion Cracking During Shutdown of Refinery Equipment) provides detailed guidelines for on merits of stabilizing anneal for improved corrosion resistance)
Thanks
Pradip Goswami,P.Eng
Senior Engineer/Specialist(Welding-Metallurgy)
Machine Dynamics & Component Integrity Dept.
Engineering & Training Services Division
ONTARIO POWER GENERATION Inc.
PO Box-2000, RR-55(South)
Nanticoke, Ontario,N0A1L0
Tel:-(519) 587-2201, Ext 3107--in Nanticoke
(416) 231-4111, Ext 5925--in Kipling,Toronto
Fax: (519) 587-6814
-----Original Message-----
From: pgoswami@sympatico.ca [mailto:pgoswami@sympatico.ca]
Sent: Tuesday, December 22, 2009 11:08 AM
To: GOSWAMI Pradip -FOSSIL
Subject: FW: [MW:3874] root cause analysis
> From: welding_engr_rich@charter.net
> To: materials-welding@googlegroups.com
> Subject: RE: [MW:3874] root cause analysis
> Date: Tue, 22 Dec 2009 08:49:25 -0500
>
> Without looking at the problems this is an educated guess... Weld-line
> decay though I would not eliminate an erosion-corrosion process.
>
> ________________________
>
>
> Weld decay is a form of intergranular corrosion, usually of stainless
> steels or certain nickel-base alloys, that occurs as the result of
> sensitization in the heat-affected zone during the welding operation.
>
> The corrosive attack is restricted to the heat affected zone (HAZ).
> Positive identification of this type of corrosion usually requires
> microstructure examination under a microscopy although sometimes it is
> possible to visually recognize weld decay if parallel lines are already
> formed in the heat affected zone along the weld.
>
>
>
> Cause? As in the case of intergranular corrosion, grain boundary
> precipitation, notably chromium carbides in stainless steels, is a well
> recognized and accepted mechanism of weld decay. In this case, the
> precipitation of chromium carbides is induced by the welding operation when
> the heat affected zone (HAZ) experiences a particular temperature range
> (550oC~850oC). The precipitation of chromium carbides consumed the alloying
> element - chromium from a narrow band along the grain boundary and this
> makes the zone anodic to the unaffected grains. The chromium depleted zone
> becomes the preferential path for corrosion attack or crack propagation if
> under tensile stress.
>
> Prevention - Weld decay can be prevented through:
>
> * Use low carbon (e.g. 304L, 316L) grade of stainless steels
> * Use stabilized grades alloyed with titanium (for example type 321) or
> niobium (for example type 347). Titanium and niobium are strong carbide-
> formers. They react with the carbon to form the corresponding carbides
> thereby preventing chromium depletion.
> * Use post-weld heat treatment.
>
> ______________________
> Here is an article that might help
>
> http://www.journalarchive.jst.go.jp/jnlpdf.php?cdjournal=isijinternational19
> 66&cdvol=25&noissue=5&startpage=418&lang=en&from=jnlabstract
>
> Best Regards,
>
> Richard M. Beldyk PhD, PE, CWEng
>
> Supervisor Quality Engineering
> Alstom Power, Inc.
> Chattanooga, TN USA
>
> Original Message:
> -----------------
> From: saqib jalal sakib321@gmail.com
> Date: Tue, 22 Dec 2009 08:01:50 +0500
> To: materials-welding@googlegroups.com
> Subject: [MW:3870] root cause anaylsis
>
>
> dear all
> i want to discuss one case study with you i am working in a chemical plant
> and one of our plant is Nitric acid production in it on of our equipment is
> stationary ( BLEACHING TOWER) whose function is to absorb oxides of nitrogen
> from 50-60 nitric acid at average 70-90 centigrade by spraying water from
> top and moving up the nitric acid from bottom. the main problem is that the
> material of this tower is ss 321 and in shut down we observe severe
> depletion in the thickness of shell especially near the HAZ of welds so
> would you please tell me most probable cause of this failure and its most
> possible remedy
>
> --
> with Regards
>
>
> SAQIB JALAL
> Inspection Engineer
> PakArab Fertilizer Limited
> Khanewal Road , Multan
> office number +92-61-9061 EXT 3110
> mobile number +92-321-4043556
>
>
>
> --
> with Regards
>
>
> SAQIB JALAL
> Inspection Engineer
> PakArab Fertilizer Limited
> Khanewal Road , Multan
> office number +92-61-9061 EXT 3110
> mobile number +92-321-4043556
>
> --
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