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Can External SCC be Arrested on A312 TP304L Pipe?
3

Can External SCC be Arrested on A312 TP304L Pipe?

Can External SCC be Arrested on A312 TP304L Pipe?

(OP)
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?

RE: Can External SCC be Arrested on A312 TP304L Pipe?

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.

RE: Can External SCC be Arrested on A312 TP304L Pipe?

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

RE: Can External SCC be Arrested on A312 TP304L Pipe?

(OP)
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?

RE: Can External SCC be Arrested on A312 TP304L Pipe?

Surface NDT using liquid penetrant testing to check for cracks.

RE: Can External SCC be Arrested on A312 TP304L Pipe?

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.

RE: Can External SCC be Arrested on A312 TP304L Pipe?

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

RE: Can External SCC be Arrested on A312 TP304L Pipe?

(OP)
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.

RE: Can External SCC be Arrested on A312 TP304L Pipe?

2
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.

RE: Can External SCC be Arrested on A312 TP304L Pipe?

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

RE: Can External SCC be Arrested on A312 TP304L Pipe?

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.

RE: Can External SCC be Arrested on A312 TP304L Pipe?

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

RE: Can External SCC be Arrested on A312 TP304L Pipe?

(OP)
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.

RE: Can External SCC be Arrested on A312 TP304L Pipe?

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.

RE: Can External SCC be Arrested on A312 TP304L Pipe?

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'.

RE: Can External SCC be Arrested on A312 TP304L Pipe?

(OP)
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).

RE: Can External SCC be Arrested on A312 TP304L Pipe?

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

RE: Can External SCC be Arrested on A312 TP304L Pipe?

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).

RE: Can External SCC be Arrested on A312 TP304L Pipe?

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.

RE: Can External SCC be Arrested on A312 TP304L Pipe?

(OP)
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.

RE: Can External SCC be Arrested on A312 TP304L Pipe?

The Al will provide some measure of galvanic protection, and proper insulation (with external jacket) will work fine.
Looks like you got to the bottom of this.

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P.E. Metallurgy, Plymouth Tube

RE: Can External SCC be Arrested on A312 TP304L Pipe?

(OP)
EdStainless,

We are attempting to find more information on VEDAGUARD wrap.

Have you any details on this type of cathodic protection such as material specification, thickness and how to determine the number of wraps required depending on chloride or halide concentration?

RE: Can External SCC be Arrested on A312 TP304L Pipe?

Al foil wrap has long been a back-up protection under insulation.
It is just there as a precaution, you should be able to remove 10 year old insulation and find the foil still intact.
I don't know that adding more would help.
You should have drain holes and inspection ports that you check.
Once you find moisture under insulation it is time to strip it and fix things.

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P.E. Metallurgy, Plymouth Tube

RE: Can External SCC be Arrested on A312 TP304L Pipe?

(OP)
EdStainless,

I found this 1961 Oak Ridge National Laboratory Report (attached) with some helpful advice:

"After cleaning, aluminum sacrificial anode foil shall be wrapped tightly on the surfaces of the pipelines in a helix with a 1/2-in. overlap at the edges. The aluminum foil shall conform to alloy 1100 or alloy 1145. The final aluminum wrapping shall have a minimum thickness of 0.010 in.
If multilayers are used, each succeeding layer shall cover the joints in the previous layer by starting each layer in the mid-point of the previous foil sheet. Soft aluminum alloy 1100 wire approximately 0.062-in. diameter shall be wound and twisted on the completed foil layers to hold the foil in place. Aluminum wire loops shall be placed on approximately 18-in. centers. Aluminum foil shall be fitted to the contours of the irregularly shaped components in such a, way as to give a 0.010-in. thickness, and the foil then should be secured with aluminum wire."

RE: Can External SCC be Arrested on A312 TP304L Pipe?

I have seen Al foil tape used to hold it also.
But you only use tape on the top surface, so that if any water gets in it can drain/

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P.E. Metallurgy, Plymouth Tube

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