What exactly does PREN refer to? 3

[RedVette]

Hey, guys.

Can someone please clarify whether the PREN value refers to the susceptibility of an alloy to form a corrosion (localized) pit or if it refers to the susceptibility of a given passive oxide film breaking?

The reason for my confusion is due to PREN values being used to evaluate both iron and non-iron-based alloys, interchangeably--accounting only for the percentages of the elements in the PREN equation and not taking into account the base element. This gives me the impression that only the breaking potential of the passive film is considered, since the rate of pit formation will be lower in nickel and cobalt-based alloys than in iron-based alloys (all else being the same). On the other hand, the measurement of actual pit formations makes more immediate sense, so in this scenario, I'm given the impression that two alloys with the same PREN refers strictly to their identical surface pitting potential while their passive films may not necessarily be equal in strength. An example would be comparing a low chromium nickel alloy like X-750 to a high-chromium duplex stainless steel. Using the same formula, the latter would rank much higher in the PREN value, but doesn't the significantly lower iron content in the first give enough compensation to modify its low PREN value of only its Cr content? In another example, alloy 718 has a considerably lower PREN than alloy 904L, despite having a higher Fe:Cr:Mo ratio. Is it that 718 has a stronger passive film but lower pitting resistance and 904L having a weaker passive film but with a higher pitting resistance? Or is it the case that PREN system is only designed to reflect steels and not intended to be a reliable method for predicting pitting behavior in non-iron alloys?

Any clarification is much appreciated.

 

[cloa]

Pitting is a very complicated process- its statistical- no guarantees of it happening or not happening and its mechanics are varied between metals and solutions and condition and not perfectly understood for the most ordinary combinations. PREN is only a descriptive number that is useful for stainless steels in seawater. There are lots and lots of types of solutions and alloy combinations known to have a real risk of pitting and the effect of alloying element is not simple.

Pitting consists of initiation (starting) and propagation (growing) stages. Initiation can't really be stopped as there is always many flaws in the metal surface in which the conditions for initiation might happen. Sure some alloys have thick adhering oxide layers which reduce the initial flaws but a factors such as local fluid damage such flaws are created- the part that Chromium and some other elements play.

Propagation stage is basically the stage where the elements have the most effect. For stainless its basically believed to due to Fe water complexes forming which acidic and keep the oxide layer from reforming. The effect of dissolution of elements such as nickel from the metal to poison the corrosion process so the oxide layer can reform. Basically PREN reflects the likelihood of stopping all pits- pits are very tiny so a later examination of the metal surface can easily miss them.

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[EdStainless]

The Pitting Resistance Equivalent (with Nitrogen) is based on empirical data. Many critical pitting tests are taken and the data is pooled.
In general this value is useful for relative ranking of alloys for pitting (and crevice) corrosion resistance in chloride containing environments.
What you are looking at is the breakdown of the passive film and ability to re-passivate, those are the opposite sides of the balance.
While people don't talk about limits of applicability I believe that we can make a few generalizations.
It applies to virtually any Cl level. Alloys will rank the order in 1,000ppm as they will in 100,000ppm solutions.
We can also not worry about pH. Most of the testing is done in Fe3Cl+HCl with a pH something less than 0.2. So whether you are concerned about relative performance at pH=1 or pH=8 they will still rank the same order.
I have found that it does not apply to precipitation hardened alloys, since you would need to account for the actual chemistry of the matrix, and the pitting resistance of the precipitates. (this is why you can use G48 or A923 to detect intermetallic precipitates, they corrode much easier)
two other things to watch for:
1. The values depends on alloy structure. A ferritic alloy with PRE=40 (no nitrogen, it does not aid pitting resistance in these alloys) has much better pitting resistance than an austenitic alloy with PREN=40, and a duplex would fall in between.
2. Watch out for the nitrogen multiplier. As a marketing effort to generate higher values you see some mills using 30 or even 33 for N, and in some cases 3.5 for Mo. These are not generally correct. the most accepted values are 3.3*Mo and 16*N. For alloys with W you have to watch how they figure also.

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[OGMetEngr]

Excellent summary EdStainless. What's your opinion on the variation of PREN sometimes used for Ni-Cr-Mo alloys that removes N and adds Nb? I saw this one in particular on a Special Metals Alloy 686 TDS.

 

[EdStainless]

In some alloys (686 and 625) there is more Nb added than is needed for stabilization.
The problem is that some of the Nb is tied up with C and N, and the rest is in the alloy.
How do you how how much of the Nb to count?
N is problematic, if it has been shown to improve pitting resistance in that alloy system then it should be added. But the multipliers may be different in different alloy systems. For example alloy 33 is a Co based alloy that uses N. But the multiplier is different than for a 6% Mo superaustenitic stainless.

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[RedVette]

Wow. You guys have given very informative responses, and I'm still in the process of absorbing these concepts in my mental visualization of the pitting process. Thanks very much to Cloa and EdStainless.

Knowing that the pitting phenomena is subcategorized into initiation and propagation helps to clarify the vantage point of metallurgists when designing corrosion resistant alloys. And further confirmation from Ed that PREN ratings represent different levels of actual resistance depending on the alloy chemistry satisfies my longheld suspicion. Now I'm just left with two things that continue to captivate my curiosity: the extent of influence imparted by Mo and Ti.

As Ed explained in another thread, Mo acts to reinforce both the lattice strain of the crystalline structure of the alloy in addition to passive oxide film, thus promoting passivation, overall, which is reflected in the PREN formula of 3.3Mo. However, another suspicion of mine is that the passivating effect of Mo in an alloy is distributed along a curve rather than being linear in effect. So what this would mean is that, apart from the metallurgical upper limit of Mo with respect to retaining a particular structure, the greatest enhancement to the pitting resistance of a stainless alloy is between a certain range of Mo content, above which the level of benefit increases at a lower rate with respect to the increasing Mo content. An example would be that, if the upper limit is--say 4% in a stainless, increasing the content to 7% would not decrease the corrosion rate much more than the 4%, despite the fact that the latter amount is nearly twice. Can you guys tell me if this is an accurate concept? Moreover, if this is correct, doesn't the PREN equation fail to account for this curvature, as a constant 3.3 is used to multiply the Mo content with?

Additionally concerning Mo, I read from some brief medical implantation article that the presence of Mo can strengthen the passive Cr oxide film up to a factor of 1000. This must be either an exaggeration or that the writer meant that Mo can strengthen the portions of the passive film that contains oxides of the other base elements by that much right? The reason why I doubt this figure is because, since the PREN formula includes, foremost, the actual percentage of Cr in the stainless alloy, a high chromium, Mo-free stainless grade such as Arnokrome 3 would about the same pitting resistance as grade 316L, which can have a Mo content of up to 2.5%, yet the first only has a Cr content about 1/3 greater than alloy 316L. (It's also worth noting that Arnokrome 3 is least not all austenitic, due to its absence of nickel and magnetic properties, so would this be a case along the lines of a ferritic pitting resistance being greater than an austenitic given the same PREN?)

Secondly, my curiosity with Ti arose from the research I did a month back on the corrosive behavior of precipitation hardenable nickel alloys in sour well and crevice. In the article, it is reported that, although the higher Mo content in alloy 625 gives it a nearly crevice corrosion-free performance in seawater (being markedly superior to 718), alloy 625 still pitted to a maximum depth of about 0.6 mm in a more "severe" crevice corrosion testing in seawater, whereas alloy 725 did not corrode at all (actually stating "0" for corrosion depth). I am not sure whether the author meant to articulate that alloy 725 never suffered initiation or that the composition of the alloy prevented propagation despite the liklihood of occasional local breakdowns of the passive film, but the author does state in a final remark on the matter that the engineer(s) believe the pitting resistance of alloy 725 was due to the greater titanium content. This therefore raised my curiosity because I never came across any indicating (certainly not from the PREN formula) that the presence of Ti to be of any enhancing agent towards greater pitting resistance. Moreover, alloy 725 has much more Fe than 625, so the Fe-to-Mo ratio is higher, yet there is evidently no detriment. Can you guys please give your thoughts on this?

Article link:

[link]http://www.pccforgedproducts.com/web/user_content/files/wyman/High%20Performance%20Age-Hardenable%20Nickel%20Alloys%20Solve%20Problems%20in%20Sour%20Oil%20&%20Gas%20Service.pdf[/url]

 

[EdStainless]

While the effect of Mo may not be linear the reality is that in most alloys you will run into phase stability issues (or formation of detrimental intermetallic phases) before you get the Mo high enough to make the 3.3 multiplier too far out of line. You can't just take an alloy and raise the Mo from 4% to 7%, you would have to lower the Cr and/or raise the Ni in order to retain balance.
I have never seen Ti credited with improving pitting resistance. The stabilizing effect of Ti can improve corrosion resistance, but you can do the same thing with Nb, or a high purity version of the alloy.

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