Austenitic & Nickel ( IGC & ClSCC)

[Corroded_Eng]

Experts,

What is the relation between Austenitic & Nickel alloys in term of Intern-granular corrosion and Chloride Stress Corrosion Cracking ?

Why these mechanism occurs only in these specific alloys ?

 

[EdStainless]

There are environmentally assisted mechanisms for all known materials (even gold).
Different environments, but they all have ones.
IGC will happen in nearly all alloys. If you miss-heat treat them you will form grain boundary phases.
Either the secondary phases will be susceptible to corrosion or the chemical depletion adjacent to them will be what gets corroded (as in sensitized stainless steel).


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P.E. Metallurgy, Plymouth Tube

 

[Chumpes]

hi
from my point of view, Cl-SCC is a cracking mechanism that affects austenitic stainless steel. Nickel alloys are almost immune (immune above 42%Ni)

for IGC, Edstainless is rigth, most metal can be affected but the corrosion mechanism can be very different from a metal to another
for austenitic stainless steels, IGC is usually the consequence of sensitization by chromium carbide precipitation at grain boundaries above 400°C that is a concern for austenitic microstructures (austenitic stainless steels and Ni-Alloys)

the diffusion coefficient (ficks's law) of species in the austenitic phase is small. Chromium carbides precipitate at grain boundaries leaving Cr depleted zone around the precipitate particle. Slow diffusion avoid chromium to refill depleted zones. It promotes corrosion of grain boundaries with lower chromium content.

 

[Corroded_Eng]

Thanks Ed and Chumpes,

but is there a relation between nickel and chloride ? what is the main factor so that 42% Ni are immune ? is there a passivation layer ? nickel chloride solubility ? FCC structure ?

Please help

 

[EdStainless]

There are multiple effects going on at once, it isn't just structure or just Ni.
If you go back to the old Copson and Chang CSSC work those were all Fe-Ni-Cr alloys, but if you use real alloys they follow the same trends. A ferritic stainless with little or no Ni will not CSSC, while a ferritic alloy with higher Ni levels will. Austinitc alloys with low Ni (8%) crack quickly, while increasing the Ni makes them more resistant to CSSC.
There are a few basics, CSCC cannot happen unless there is active corrosion, so if an alloy is truly immune to corrosion in an environment then if cannot crack.
Secondly there are known environmentally assisted cracking mechanisms for all alloys, the cracking of SS and Ni alloys in chloride environments is just one small aspect of EAC.

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P.E. Metallurgy, Plymouth Tube

 

[Chumpes]

I guess this 45% (not 42%^^ but i was close) is experimental and based on feedbacks, i think there is no academic model demonstrating 45% is a particular limit for Cl-SCC of Ni-alloys / austenitic SS (yes CRA passivate !)
Very few corrosion mechanisms are fully well understood !
Other parameters such as vaporization / concentration of water, presence of oxygen, on heat treatment history are of great importance

for info :
extracted from API RP 571 about Cl-SCC of corrosion resistant alloys with austenitic structure :

Nickel content of the alloy has a major affect on resistance. The greatest susceptibility is at a nickel content of 8% to 12%. Alloys with nickel contents above 35% are highly resistant and alloys above 45% are nearly immune.

 

[SJones]

The nature of the question really demands that the OP starts a little personal research into corrosion mechanisms in order to pose a less nebulous problem.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.