Can External SCC be Arrested on A312 TP304L Pipe? 3

[Bambie]

At our plant, external examination of a section of A312TP304L boiler blowdown pipe detected the onset of stress corrosion cracking.
Is there anything that can be done to arrest this cracking mechanism or is whole sale replacement the only option?

 

[metengr]

You have to determine the contaminants that caused the SCC - most likely it was chloride-assisted SCC. If you know the source and remove the contaminant(s), the mechanism will stop.

 

[EdStainless]

No, you can't arrest it unless you can keep it perfectly dry and cold.
There is enough Cl deep in the cracks to keep the process going.
When you replace it look into coatings, either epoxy or a Zn or Al bearing coating.

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P.E. Metallurgy, Plymouth Tube

 

[Bambie]

In order to determine the extent of replacement, is there a simple test to detect SCC micro cracking or is the appearance of thin spidery black lines enough to condemn the line?

For example, is the surface hardness of the line measurably affected by micro crack initiation?

 

[metengr]

Surface NDT using liquid penetrant testing to check for cracks.

 

[metengr]

The more I think about this, why is this material even being used in boiler blowdown piping? Have you confirmed the cracking is not from the ID out? Boiler blowdown contains contaminants, which can contain chlorides. I would replace the blowdown pipe with carbon steel piping; there is no need for stainless steel pipe. Save yourself a ton of money.

 

[EdStainless]

When you pick a CS for the replacement just make sure that there is enough Cr in it to help resists erosion.

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P.E. Metallurgy, Plymouth Tube

 

[Bambie]

metengr,

The inside surface is crack free - confirmed by LP examination. Stainless was intended to avoid liquid impact, EC and MIC which it has done very well.

If EdStainless is correct, and if there are visual indications of micro-cracks, which cannot be cleaned out and that will eventually go through-wall even if the contamination source is removed, then LP would not be required to condemn the pipe (we have lots of suspect pipe).

Are you suggesting LP regardless whether it passes visual?

I was hoping a simple hardness test on the pipe OD might give a go-no-go indication of impending SCC.

 

[metengr]

I am suggesting replacement of the piping with current cracks AND if you cannot eliminate the source of chlorides for this material. You don't need austenitic stainless steel for blowdown piping. ASME Section I prohibits use of this material in water-wetted service. 304L stainless does not afford MIC resistance. Low alloy Cr-Mo steel (P11) will afford all of the resistance you need against FAC or EC. I would replace the blowdown piping with P11 or even P22 material, which is common in Power Generation.

 

[EdStainless]

In an austenitic stainless there is no correlation between hardness and SCC, unless you are dealing with heavily cold worked material (which you are not).
Intermittent weted service on 304 is a recipe for disaster.
A CrMo grade (I like the idea of 22) should have good enough erosion resistance.
There should be little risk of MIC in boiler blowdown since it should be sterile.

If you are set on using 304 for the replacement you must remove the Cl source. Many chemical plants on the Gulf Coast externally coat 3xx SS to protect from SCC. These days a lot are using cold spray Al with a suitable resin sealer (urethane, epoxy, or silicone).

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P.E. Metallurgy, Plymouth Tube

 

[MagBen]

Great commments!
Just a supplement: if replacing SS with CS, you may consider thermal stress when having bimetal welding, CS will have less CTE then SS (about 70%). SCC may be more concerned.

 

[weldstan]

I very high velocity liquid impact on blow down lines, 5% Cr alloy (P5) has been used effectively where P22 was previously employed.

 

[Bambie]

Just to re-iterate...the only evidence of SCC has been detected under the insulation which has been attributed to chloride bearing moisture ingress during boiler maintenance. These stainless sections replaced A106B at the tail end of blowdown where EC at elbows, liquid impact at laterals and under insulation MIC (during maintenance) had resulted in through wall failures.

 

[metengr]

Are you sure? Have you checked the entire (100%) run of BD piping? Where is all of this chloride-bearing moisture coming from that is soaking external insulation resulting in SCC and MIC?

So you de-insulate, check the BD pipe and replace the existing insulation with new insulation. What is to prevent SCC from re-occurring after the pipe has been checked? Are you prepared to monitor this piping each year? Again, remove the source of the moisture, if you can't, replace the piping with less susceptible material to SCC. This is a very straight forward approach.

 

[Duwe6]

Another MAJOR variable in your testing for the presence of Chloride SCC is the grade of Dye Penetrant used. Was it plain 'ole red dye or was it fluorescent? Many years ago, the aircraft industry established "Levels" of penetrant sensitivity; 1 thru 4. Red visible dye is about a 1/2. I would not hunt SCC with anything less than a Level 2 penetrant. My current favorite is a water-washable Level 3 that can be bought in a 12oz spray can -- ARDROX 970P25E. It will reliably find cracking 50 microns deep, or less. And for a sensitive penetrant, it is very forgiving and easy to work with. The old ZYGLO Level 3 and 4 penetrants were extremely hard clean off, and get a 'readable' background. [too much residual dye and you cannot see the indications from real flaws]. One major problem with fluorescent penetrants of any sensitivity Level -- red dye quenches the glow of fluorescent dye. If the part has had any red dye on it, fluorescent cannot be used until the red is burned out [think 800°F or possibly even higher]. I got good results on a Rx head that had run at 900-1200°F for over a month, after being drenched in red dye. YMMV.

Theoretically, you can sand off the cracks. At the incipient stage, where they are just visible with a Level 3 dye, they are less than 1 mil deep; easy to 'buff' off. I would buff out the cracks on your line that are visible, or findable with plain red dye. This will buy you some time. And then start fabbing a P22 line to replace your 'problem child'.

 

[Bambie]

Thanks Duwe6, I'll ask our inspection group what they used.

metengr, Warm moist air can find many ways to get under the insulation of a cold pipe (vents and supports to mention a few).

 

[metengr]

Bambie;
True. Which is why 304 ss blow down piping in a Power Plant was the wrong material.

 

[mrfailure]

Correct me if I'm wrong, but it sounds like you got chloride SCC from contact with wet insulation. Your insulation needs to be certified to pass testing to ASTM C692 if you were to use it with austenitic stainless steel. Testing involves a drip test on 304 bend samples for 28 days, if I recall correctly.

Also note that even if you got rid of all chlorides (and I agree that is doubtful), you still would most likely continue to propagate the preexisting SCC cracks under continued cyclic stresses (i.e. fatigue propatation).

 

[weldstan]

Chloride SCC at welds in austenitic stainless steel pipe has been common under insulation in marine, coastal environments. Whereas austenitic stainless steel is not necessary in blow down service, change the metallurgy to P5 as was previously recommended.

When stainless is required for the service, painting of the welds has proven effective to mitigate SCC under insulation; however, the proper coating must be applied for the temperature conditions and expected thermal cycling. Periodic coating maintenance will be required.

 

[Bambie]

Gentlemen,

A review of the blowdown maintenance history has revealed:

1) The replacement of A106B with A312TP304L happened in 1995
2) VEDAGUARD aluminum foil wrap was installed under foam cell insulation
3) De-insulation and external inspection in 2012 revealed no evidence of SCC or wall thinning
4) Re-insulation without VEDAGUARD and with foam cell insulation that had come in contact with road salt
5) Inspection in 2015 revealed significant SCC

It would appear that, if the proper corrosion protection had been applied in 2012, SCC would not have gained a foothold.