CSCC of austenitic SS 1

[bob330]

Greetings All,

Although it is well known that electrolyte turbulence and flow help to mitigate pitting in austentitic stainless steels, little to nothing is ever said about the influence of flow or flow rate on CSCC susceptibility. Is there evidence to believe that flow rates can have a significant effect here in the area of CSCC? One interesting thought/observation along these lines is that 316SS shows a significant increase in pitting resistance over 304SS but fails to show a significant increase in CSCC resistance. I have never heard an explanation for this behaviour.

Also, is it believed that CSCC cracks in austenitic SS grow more or less continuously by slow anodic dissolution at the crack tips (assuming exposure conditions are constant) or is it believed that the cracks grow in sudden bursts with the crack lengths increasing in small discrete amounts at very high rates?

Bob330

 

[metengr]

bob330;

One interesting thought/observation along these lines is that 316SS shows a significant increase in pitting resistance over 304SS but fails to show a significant increase in CSCC resistance. I have never heard an explanation for this behaviour.

The explanation is that the addition of Molybdenum (Mo) to 316 SS increases resistance to pitting. The Mo has no effect on CSCC resistance.

Once a crack develops from either a surface flaw or corrosion pit, you are now dealing with a very tiny crevice that will concentrate contaminants over time. For SCC to occur you need a threshold tensile stress, susceptible material and local (in the crevice) environment. I don't see how bulk flow rate would impact chloride stress corrosion crack propagation after contaminants have been introduced. You will not be able to wash away the chloride ions once mass transport has enabled the contaminants to enter the crevice. Just my opinion, and past experience in dealing with CSCC in heat exchanger tube material.

Also, is it believed that CSCC cracks in austenitic SS grow more or less continuously by slow anodic dissolution at the crack tips (assuming exposure conditions are constant) or is it believed that the cracks grow in sudden bursts with the crack lengths increasing in small discrete amounts at very high rates?

First off, stress corrosion crack propagation rates can be determined by testing. Thus, crack propagation models can be developed based on measured crack propagation rates. I have seen reports where crack growth occurs by start/stops. Regarding high rates, this is a relative term.

The mechanism of crack propagation regarding SCC is another matter and more complex because of various mechanisms. For example three mechanisms (there are others) that have been widely published and discussed are active path crack tip dissolution, hydrogen embrittlement (which in of itself can be described by several theories of crack propagation), and film induced cleavage. There are plenty of technical papers on the subject of SCC in metals.

 

[EdStainless]

The biggest difference between 304, 316 and various 'N' grades of these alloys relates to the strength of the passive film. This passive layer must fail in order for any corrosion to proceed, pitting or CSCC.
Pitting attack is sensitive to flow conditions because the surface electro chemistry is altered in turbulent flow. Flow has little impact on the electrochemical state in a pit, crevice, or crack.
Once the corrosion starts there isn't a big difference in the rate of corrosion for these alloys. If you look at tests where CSCC is forced so that incubation and repassivation are not issues, you will see that Ni content of the alloy is the dominant material factor in CSCC.
Whole books are written on this subject. From a practical engineering standpoint I use the rule of thumb that CSCC is very fast and if it starts the part has failed. If I find it before there is actual failure I just got lucky.

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Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[bob330]

Thanks for your responses.

MetEngr, since I agree with EdStainless that once SCC starts, you are in big trouble and have no reason to expect crack cessation (in the absense of a bulk environmental change, my question was totally geared towards the effect of flow on SCC initiation only.
That question was not answered/adressed at all.

Also, since SCC cracks are so often associated with propagating from pits, is it wise to completely separate the two phenomena? If CSCC cracks often initiate at pits, it may seem logical to believe that a situation (like higher flow rates)that improves pitting resistance could improve CSCC resistance. I suspect at least with 304SS that avoiding stagnant conditions can lead to dramatic improvements in the levels of chloride content/temperature that it can handle without experiencing CSCC cracking. That is what I am asking about.

Ed, I agree with most of what you say but I don't know if you are correct about flow condition effects after pitting has started (during more stagnant conditions). If you have a pit (with unique lower pH and higher chlorides inside of it) and then crank up the turbulence, you may be able to change the localized chemistry back to the bulk chemistry thus stopping the pitting (so long as the pit is not under a deposit where flow cannot clean it out). It would only seem logical that stagnancy may be necessary to maintain different solution chemistries on a steel surface. With a crack already present, I agree that the game is over as long as chlorides are still present in bulk solution. Ed, you also addressed my question on CSCC susceptibility from the perspective of already assuming a crack is present but failed to comment on whether you thought flow affected bulk CSCC susceptibility with no cracks present. What are your thoughs on how flow conditions affect crack initiation capability and the breakdown of the passive film necessary for it? You said that pitting attack is sensitive to flow conditions because the surface electro chemistry is altered in turbulent flow. My question all along was whether or not this altered surface electro chemistry improves resistance to CSCC such that flow conditions should be considered when addressing CSCC in the equipment design stage.

Thanks,
bob330

 

[metengr]

bob330;

If CSCC cracks often initiate at pits, it may seem logical to believe that a situation (like higher flow rates)that improves pitting resistance could improve CSCC resistance.

Disagree with this logic. Pitting and SC cracking are two separate damage mechanisms. If there is no tensile stress, you will not initiate or propagate SCC.

 

[bob330]

I agree with you Metengr that these are different and distinct damage mechanisms but they are not totally unrelated. In fact they are intimately related as follows:

A) As EdStainless notes, they both involve the local breakdown of passivity and they are both induced by the presence of chlorides so they are related in that they are caused by the same environmental conditions.

The fact that one of these damage mechanisms is stress sensitive as you noted is perhaps the biggest difference.

B)But considering this topic of stress, the pit will indeed act as a stress raiser further relating the two damage modes.

C)The presence of a pit increases the local chloride concentration and reduces the pH which agin enhances suspectibility to CSCC. Again, different damage mechanisms indeed as you say but ones that that ar ehighly related by so often working hand in hand together.

D) All of the above is affected by flow and thus my original question on flow effects on CSCC which still has not been addressed yet!!!!

 

[EdStainless]

All CSCC initiates at active corrosion sites, in most cases they are so small that they can not be identified as pits. If you can increase flow so as to reduce the risk of passive film failure, then you will reduce the risk of CSCC.

Both mechanisms are sensitive to micro-surface finish conditions that we normally only control in lab samples. I have seen CSCC on electropolished tubing, it does look different. There is almost no near surface branching.

I have seen cases of changed flow conditions impacting the pit growth rate. The pits formed in stagnant conditions. The tubes were then well cleaned. Under low flow they grew at 50%/yr. Conditions were changed and flow increased about 33% and the pit growth rate dropped to about 25%/yr.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[metengr]

bob330;
I would recommend you consider contacting NIDI for specific information.

http://www.nidi.org/index.cfm/ci_id/80.htm

They may have or know of specific research performed on this topic. If they can't help you, suggest you consider a PhD thesis.