Dissolution of corrosion product over inconel 600 and SS304

[soman_kps]

I would like to know why alkaline permanganate (potassium permanganate in sodium or potassium hydroxide) is used for dissolving corrosion products formed over nickel alloys such as inconel 600 (composition of alloy is 72% Ni, 16% Cr, 8% Fe) while acidic permanganate (potassium permanganate in nitric acid) is more effective to dissolve corrosion products formed over iron based alloys such as SS304 (composition of the alloy is 10% Ni, 18% Cr and 70% Fe). I presume that the corrosion product over both the surfaces will be oxide of Ni-Cr-Fe.

 

[Gokul43201]

Just a few pointers here (no thinking has been applied yet )

1. SS corrosion is essentially rust while corrosion in Ni alloys is, I believe, primarily oxides/hydroxides of Ni (in monel, for instance, I believe it is a mixture of the oxides of Ni and Cu).

2. Permanganate itself is merely an oxidizing agent.

3. Rust is usually dissolved by HCl.

 

[Astronuc]

The difference in behavior between oxides on Inconel vs SS has to do with the oxide composition, which is predominantly NiO on Inconel (72% Ni) vs Cr₂O₃ on SS, and the stability of NiO/Ni vs ₂O₃ and the chromate ions.

I believe there are some deffinitive papers, but tracking them down will take a little time and effort.

Inconel-600 has had a notoriously problematic history in the commercial nuclear industry. It was the primary alloy used in steam generator tubing in PWRs, and it suffered from IGSCC which limited its life to something like 15-20 years, or much less than the 40 years it was supposed to achieve.

SS304 is more or less the standard 18-8 SS, and it's corrosion resistance is related to the Cr oxides formed on the surface in contact with the acqueous environment.

Actually, I am interested in the same issue.

SS304 has a problem, that being IASCC in a radiolytic environment where the ECP > -0.23 V.

 

[Gokul43201]

Astro,

Cr2O3 provides the passivation over stainless steels, but the corrosion product (when corrosion happens) is still Fe2O3, is it not ?

 

[Astronuc]

Astro,

Cr2O3 provides the passivation over stainless steels, but the corrosion product (when corrosion happens) is still Fe2O3, is it not ?

Correct, it's the layer of Cr2O3 that covers (protects). In addition to the Fe2O3, there is also Fe3O4 or NiFe2O4 (and sometimes CoFe2O4), and I seem to remember a FeCr2O4 oxide as well. The iron oxides (or their proportions) depend on pH, and the Fe2O3 is usually hydrated.

More to your point, iron oxides are usually removed by pickling, i.e. more acidic solution.

I am also wondering if it is deposited corrosion product or simply oxidation of those surfaces (Inconel-600 or SS304).

Most of the issues I deal with are the crud (metal oxide corrosion products) on Zr-alloys (heat transfer surfaces). The stainless steels and Inconels are relatively clean, but then I am not usually concerned with the entire circuit.

I have seen some badly corroded SS cladding material that had both orange, brown and black oxides, as well as shiny grey coloring.

Here is some examples of chemical cleaning in the nuclear industry - a lot of stainless steel and Ni-bearing alloys involved.
http://www.eu-decom.be/europe/otherprojects/framatome/decontamination.html

 

[soman_kps]

Thanks for your reply. I am a Phd student.

by the way, i agree with you that dissolution of corrosion product on the surfaces depend mainly on their nature which inturn is dependent on the surface itself.
on SS304, the protective oxide is Cr2O3 while the outer layers are predominantly iron rich oxides such as Fe3O4, NiFe2O4, FeCr2O4 and mixed FeNiCr spinels.

on the other hand, inconel 600, also contains Cr almost to the same extent as in SS304 and therefore the protective oxide should be Cr2O3 (?) and the corrosion products should be (as i think) NiO, NiCr2O4 (is it possible to exist) and Fe deficient ferrites (NiFe2O4) and Fe deficient chromites (as %Fe is very less)

Thus, difference between the corrosion products on SS304 and inconel 600 narrows down to FeCr2O4 in the case of SS304 and NiO and NiCr2O4 in inconel 600.

Does it mean the reduction potential of acidic permanganate (around +1000mv) is favourable to destabilise FeCr2O4 and release Cr.

and the reduction potential of alkaline permanganate (around +400mv) is favourable to
destabilise NiCr2O4 to releases Cr. i would also want to know if there will be any influence of NiO on alkaline permanganate in destabilising NiCr2O4 lattice.

pardon me if my reasoning is wrong

 

[Astronuc]

soman, have you looked at Pourbaix diagrams for Fe and Ni systems. That's a starting point, but not the complete system since it does not cover alloy systems, which are orders of magnitude more complicated. There is a book on corrosion by Pierre Roberge, "Handbook of Corrosion Engineering", which is a good start.

Another reasonable overview is found in ASM's Metals Handbook, Ninth Edition, Vol 13, "Corrosion". It has chapters on stainless steels and Ni-bearing superalloys.

The "Corrosion" book mentions an example that shows SS304 is stable at a pH of about 9, but corrodes if pH is reduced to 6. However, that is at 25°C and temperature, as well as other chemical species in the water, will have a significant effect.

One key is the solubilities in the Ni/NiO system and NiFe2O4, and others, as functions of pH, temperature, and chemical species.

One particular area in which I am interested is the Ni/NiO system, particularly the behavior of NiOOH as a possible precursor to Ni(OH)₂. It's a pretty obscure area in electrochemistry, because it involves microchemistry near the grain level.

 

[soman_kps]

thanks astronuc, i will look for the "corrosion" chapter and the pourbaix diagram and try to correlate.

 

[Gokul43201]

soman, please do not double-post (I have deleted the other copy of the OP in Mat Engg).

I'm going to leave this thread here for a few more days to see if we can get some definitive responses from the chemists, specifically on the solubilty of the oxides (and hydroxides) in acidic/basic media and on the specific role of permanganate. If nothing comes out of waiting, I'll move this over to Mat. Engg.