H2S and CO2 corrosion

[blckwtr]

Hi, all... I am currently doing a study on various materials and their properties under influence of H2S and CO2 for downhole equipment, e.g. tools for use in an oil well. So far I have found a general guideline which states that the listed materials are accepted, but the non-listed materials have to undergo a thorough analysis to be accepted under certain conditions. I have used standards such as ISO 15156 and NACE MR 0175 to check whether or not our material is accepted in various conditions, but the results are very confusing. The oil companies are still asking for certain temperatures and concentration levels, but to find these answers quickly is a matter to experts. So I am asking this knowledge base for answers...

 

[Chumpes]

hi
several problems occur in CO2/H2S/H2O environments :
- generalized corrosion (loss of metal thickness) of carbon steel materials due to - pH lowered by dissolution of acid gas CO2 and H2S - and to scaling properties of the formed iron carbonate / sulfur scales. it exists models that describe with more or less success the corrosion rates in CO2/H2S/H2O environments : Dewaard / Nesic / ... Stainless steel and other corrosion resistant alloys (Ni-alloys) are very resistant to generalized corrosion and should be selected when corrosion rate of carbon steel goes above 0.1 to 0.25mm/y
- cracking mechanisms : wet H2S conditions lead to crack initiation and propagation (SCC : SSC, HIC, SOHIC, ...) particularly on carbon steel (refer to NACE MR0175 part 2 or to EFC 16) but stainless steel maybe sensitive in the presence of chloride ions (refer to part 3 of NACE MR0175). These mechanisms have to be anticipated as they can lead to rapid failure by thru-wall propagation of cracks.

selected material should have resistance to both types of potential failure mechanisms : generalized corrosion AND SCC

Depending on your operating conditions, carbon steel or Ni-alloy C-276 may be suitable.

regards

 

[blckwtr]

thanks, we usually use AISI 4140, limited by 22 HRC on most of the components we put into these conditions. So far questions arises, is this material, being that it is NACE "approved", covering all concentration levels of H2S (partial pressures) and tremperatures? The well itself is made up of AISI 4140 in most cases (L80 is often used). Sometimes we want to use a higher strength steel, but NACE limits the hardness to 22 HRC, so the choises are limited when it comes to carbon steels. We would rather not use stainless, since the functionality is a screw thread, and we don't want to gall the thread, then we lose all functionality.

 

[Chumpes]

There are a couple of "NACE" documents and méthodologies, thus it is always useful to refer to a specific NACE Document Number (e.g. NACE MR0175/ISO 15156) and not to a generic mention of "NACE" which can also be understood as "NACE MR0103" which is different from "NACE MR0175"

"material is NACE approved" means nothing to me
"material complies with NACE MR0175/ISO 15156" really means something

appart from that,

when a ferritic material (such as carbon steel or low alloy steel) complies to a NACE standard or another, it should mean that the material is resistant to CRACKING mechanism(s), i.e. SSC (sulfide stress corrosion cracking) of welded areas in the first place and not necessarily other cracking mechanisms (i.e. HIC of plate materials) and it does not proove at all that the corrosion rate is acceptable.

compliance with NACE MR0175 maybe necessary versus cracking in wet H2S service but this does not give any information on the suitability of the material regarding generalized corrosion.

the material may have an improved resistance to cracking (NACE compliance) but may still corrode within the first years of operation.

I don't work in E&P anymore and I don't know if the material you proposed (1%Cr) is suitable for the encountered operating conditions (vs. generalized corrosion)

regards

 

[EdStainless]

Chumpes, you are half way there. Unless people give me the NACE spec and the conditions under that spec (temp, concentration, pH, and so on) you don't really know anything.

The issue is that you have to consider so many factors, SSC, CSCC, HIC, pitting, general corrosion, weldability, and a few more.
A single spec or test does not assure you that the material is suitable for an application

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P.E. Metallurgy, Plymouth Tube