Corrosion Threshold of Different Stainless Steel Types 2

Could you clarify what is meant by corrosion threshold, is it the corrosion loss to cause an effect such as cracking or the chloride concentration to initiate corrosion of the steel?

Crevice corrosion in an aqueous, neutral, Cl containing environment?
Lots of that published, and even some that show shifts with temp, pH, and Cl concentration.
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/raw/upload/v1602262489/tips/pitting_curves_pu1rpx.docx[/url]

or this
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/raw/upload/v1602262691/tips/CCT_chart_fjgu05.docx[/url]

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P.E. Metallurgy, consulting work welcomed

Assuming non-carbonated concrete the pH of concrete is >12 not near neutral. Assuming chloride concentration is the subject of the original question, for corrosion of reinforcement it is usually expressed as per unit mass of cement or concrete, or as the ratio of chloride to hydroxyl ion concentration.

Cheers

GG

Thank you for the responses...

(EDIT: You beat me to the response)

GGedge... Chloride Corrosion Threshold (to my understanding) is the threshold limit in ppm of chloride concentration to initiate corrosion of the steel. The testing that I have seen done to measure this (I'm sure there is more than one way) is to cast concrete minibeams reinforced with rebar. They then will then flex the minibeam to crack it on the cover layer. They pond a 3% NaCl solution on the top of the minibeam and cycle it with a wet and dry time. They then crack open the minibeam and measure the chloride levels and observe the level of corrosion. From that they can deduce the chloride threshold level.

EdStainless.... Thank you! I took a look at the graphs. The pH is important since the service life modeling that I am interested in deals with steel cast in concrete. So looking for a pH environment around 12. That is interesting to see the regression line that is the function of Cr, Mo, and N. Both of those graphs that you sent me show 316 stainless steel to have a threshold limit of below 500 ppm at neutral pH and ambient temperature. I am curious to what it would be at a pH of 12 or 13. The research I have found on 316 stainless steel embedded in concrete has a chloride threshold limit of around 4750 ppm. Does that sound correct?

I haven't got the papers with me at home, but search for the work of Pedeferri/Bertolini(Milan Polytechnic) and more recently Randstrom (Outokumpu). In summary (from memory) the critical chloride concentration v pH plot shows a significant positive change in slope at high pH (far more than carbon steel). The critical concentration for all stainless steels is about one order of magnitude greater than that for carbon steel in the same concrete/mortar. Again from memory, Randstrom reported the value as being >2.6% by weight cement for both austenitic and duplex steels.

Cheers

GG

Usually by the time the Cl reaches the rebar the local pH is no where near 12 any more, more like 7 if you are lucky.
There is also the issue of other corrosive species, such as chloramines. These are volatile Cl containing organic compounds that can cause CSCC in SS at near ambient temps (look up history of collapse of concrete swimming pool roofs).
As far as bang for the buck goes you can't beat 2101. It is at least as pitting resistant as 316, it is much more cracking resistant, and it is much stronger (use less metal).
At higher pH and low temps (in liquid caustic) I would trust 316 at higher Cl levels, but in an environment like yours you need to think about worse case conditions.

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P.E. Metallurgy, consulting work welcomed

Change in pH of concrete occurs due to carbonation (reaction with atmospheric CO2) rather than transport of chloride through the concrete. Typically carbonation is controlled by the concrete cover and mix specification. Corrosion of reinforcement from either de-icing salts or marine environments is due to chloride transport, not carbonation, through the cover and I suspect the original question relates to modelling that transport over time to estimate a service life. For rebar corrosion due to chloride the carbonation front, and therefore reduced pH, is a long way from the bar surface.

Cheers

GG

EdStainless, I am confused on your statement about the pH of the environment being significantly lower by the time the chlorides reach the rebar. I have not heard of this... Mind you, my chemistry is pretty rudimentary but I agree with GGedge that I believed the drop in pH is due to carbonation (Carbon Dioxide) and is independent of the diffusion of chlorides. You are not the first person to tell me that 2101 is a better bang for the buck than 316. We stopped using 316 stainless steel awhile back and then started specifying 316LN. Then after some discussions with the different stainless steel rebar reps they sold us on specifying 2205 for marine applications. Most of what we do is related to neutral pH environments with chloride sources due to the ocean. No industrial applications for us.

I will have to read up on chloramines as I am not familiar with that.

GGedge, you are correct! This is for service life modeling based on Fick's Second Law of diffusion. Considers only chloride diffusion in a perfect uncracked concrete scenario. Definitely, unrealistic, but seems to be the industry standard. On a side note, I am changing the parameters in Life365 (i.e. the chloride exposure, concrete diffusion coefficient, chloride threshold, and propagation period). I have lots of great information concerning different admixtures and types of rebar and how to manually modify the inputs but currently Outokumpo has not been very helpful to me in supplying some of the test information I had requested from them. Do you know of another service life modelling program other than Life365 for reinforced concrete? I was told that there was another one that was going to be coming out of the UAE and would be able to account for sulfates and cracked concrete conditions. Thanks for the lead on Pedeferri/Bertolinni.

Your memory of it being an order of magnitude higher seems to be spot on! Black Bar threshold is 0.0005% and 316 stainless steel is at 0.0048% according to Life365 and many other independent sources.

I'm not involved on the modelling aspects, I get dragged in if/when chloride get to the bar and the "what happens?" question. As far as stainless goes the usual answer is not very much at all for any of the steels you listed.

The only reason I can think of for using 2205 reinforcement are commercial, it might be more economic than the 316 and derivatives. For practical purposes with typical design lives (say 50 to 120 years) durability will be similar unless you are looking at crazy long life (hundreds of years). Our standard specification, for stainless steel reinforcement on structures in a splash zone, is 1.4307 (304L) or 1.4162 (2101) for 120 year life; the choice is usually a commercial one for the supply chain. The grade might get bumped up in very hot climates, for example the Middle East. There might also be a case for areas with de-icing salt on bridges without waterproofing.

Assuming the reinforcement is produced to a recognized standard, ASTM A955 or BS6744, all the alloy grades will need to meet a common strength grade to comply. In Europe that is 500 MPa, I think the ASTM is similar but with some lower strength options.

Chloroamine is a problem related to the particular environments created in swimming pool hall atmospheres (not in the water itself). It is interesting to know about generally but is not something to worry about for stainless reinforcement in concrete in marine environments.

CHeers

GG

2101 is less expensive than 2205 (no Mo) but about the same strength. With lower Ni and Mo than 316LN 2101 should be less expensive also and stronger.
That said if I was doing a bridge I would look at 2205, andy other marine structure and 2101 would be my choice.
As a family the duplex grades are more resistant to CSCC than the austenitic grades.
I guess that the cases that I have been drug into have been unusual then. The ones that I have seen have had extensive carbonation and were more permeable than they should have been.
I have also seen tidal environments where chloramine generation was detected and blamed for ambient temp CSCC in 316L.
It would figure that I have only been involved in some weird cases, sorry for generalizing.

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P.E. Metallurgy, consulting work welcomed

For avoidance of doubt:

"I guess that the cases that I have been drug into have been unusual then. The ones that I have seen have had extensive carbonation and were more permeable than they should have been.""I

is that referring to stainless steel or carbon steel reinforcement?

"I have also seen tidal environments where chloramine generation was detected and blamed for ambient temp CSCC in 316L."

Likewise was that CSCC of 316L Reinforcement in concrete?

GGedge and EdStainless, Thank you both for your help!

GG, SS rebar, 316L in both cases.

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P.E. Metallurgy, consulting work welcomed

Thanks for clarifying. As both are rather unusual (particularly the second) are there any further details in the public domain?

Cheers
GG

GG, Given that it was not public infrastructure I doubt it. I'll double check. Ed

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P.E. Metallurgy, consulting work welcomed

EdStainless said:

Crevice corrosion in an aqueous, neutral, Cl containing environment?
Lots of that published, and even some that show shifts with temp, pH, and Cl concentration.

https://res.cloudinary.com/engineering-com/raw/upload/v1602262489/tips/pitting_curves_pu1rpx.docx

or this

https://res.cloudinary.com/engineering-com/raw/upload/v1602262691/tips/CCT_chart_fjgu05.docx

Click to expand...

Ed, do you have a reference or soruce for those 2 files/figures? I found them incredibly useful in a general sense, and like to use them for my personal dossier.

Huub
- You never get what you expect, you only get what you inspect.

I'll apologize in advance but I don't think that I have the original reference, it was a European paper that I excerpted the data from. I'll hunt my files.

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P.E. Metallurgy, consulting work welcomed

Thanks, would be nice and much appreciated!

Huub
- You never get what you expect, you only get what you inspect.