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HT sulfidation of Carbon Steel & Si content : H2-free vs. H2/H2S environments

HT sulfidation of Carbon Steel & Si content : H2-free vs. H2/H2S environments

HT sulfidation of Carbon Steel & Si content : H2-free vs. H2/H2S environments

Dear Specialists and Experts

High temperature sulfidation stands for the oxydation of metal (carbon steel is here under consideration) by sulfur species (H2S, R-SH, R-S-S-R', ...) at high temperature (above roughly 200°C). Even if this corrosion was extensively studied, the complete understanding of the corrosion mechanisms remain unclear. Reference documents (API RP939C, NACE 34103, API 581) describe 2 distinct mechanisms, but I believe these 2 mechanisms are 2 aspects of a single general mechanism of HT sulfidation :

- HT sulfidation in H2-free environments :
In absence of H2, chromium additions in the steel renders the material more resistant (9Cr >> 5Cr >> CS) because chromium may act as a delaying factor for recombinaison of sulfur to H2S, H2S being believed to effectively attack metal.
Also, reference documents demonstrate that silicon content is of great influence on corrosion rate of CS, i.e. high corrosion rates are expected for low Si content below 0.10%.
Corrosion rates may be evaluated using McConomy curves in H2-free environments, but uncertainities remain because corrosion rates maybe under-estimated (separation section of hydrotreating units)

- HT sulfidation in H2/H2S environments :
H2S is said to be the most corrosive among all other sulfur species regarding HT sulfidation.
When H2 is present, H2 may act as a inhibitor of the delaying effect of chromium (see above), which would suppress the corrosion mitigating effect of chromium additions up to 9% chromium, and would render materials from 1.25Cr to 9Cr not more resistant to HT sulfidation than CS material in presence of H2.
When H2 is present, it is also said to partially inhibit HT sulfidation corrosion mechanisms and tend to lower corrosion rates.
Corrosion rates may be evaluated using Couper&Gorman set of curves in H2/H2S environments.

Corrosion rates of carbon steel evaluated with Couper&Gorman and McConomy are very consistent. So are corrosion rates for stainless steel. Differences between Couper&Gorman and McConomy may mostly come from estimated behaviour of alloyed and low-alloyed steels (1.25Cr to 9%Cr) that corrode as fast as CS in H2/H2S environments (no Cr effect due to presence of H2), and that corrode slower than CS in H2-free environments (mitigating Cr effect active in absence of H2).

My question deals with the influence of the silicon content on HT sulfidation of carbon steel :
Our Company has internal documentation asking for 0.10% minimum sulfur content of CS material in order to control "HT sulfidation in H2-free environments" ONLY.

I cannot understand why this recommendation on minimum 0.10% silicon content of carbon steel materials does not have to be considered for "HT sulfidation in H2/H2S environments"... In my opinion, minimum Si content should also be useful versus HT sulfidation in H2/H2S environments, because Silicon may increase stability of iron sulfide scales and mitigate corrosion by HT sulfidation with or without H2, whatever the corrosive sulfur species is...

In other words, can I say : minimum O.10% Si content is not required for carbon steel to be operated in H2/H2S environments ?
(I would say no : minimum Si 0.10% should probably be also considered for H2/H2S environments)

Any reference on this specific subject of Si influence on HT sulfidation in H2/H2S environments would also be useful.

Thanks a lot and see you soon


RE: HT sulfidation of Carbon Steel & Si content : H2-free vs. H2/H2S environments

That's not the only consideration in determining the desired quantity of Si.

Silicon increases strength and hardness but to a lesser extent than manganese. It is one of the principal deoxidizers used in the making of steels to improve soundness, i.e. to be free from defects, decays or damages. Si is detrimental to surface quality, especially in the low carbon, grades. It aggravates cracking tendencies when the carbon content is fairly high. For best welding condition, silicon content should not exceed 0.10%. However, amounts up to 0.30% are not as serious as high sulphur or phosphorus content. Carbon steels with low-silicon (<0.10%) content can corrode at an accelerated rate. Given that, I would not say that there is not a minimum 0.10% Si content that should be established based on H2/H2S exposures.

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RE: HT sulfidation of Carbon Steel & Si content : H2-free vs. H2/H2S environments

thanks Biginch for your remarks on Si content vs. weldability of CS. I was not looking at it from this point of view.

if anybody else has something on HT sulfidation vs. Si content...

thx again

RE: HT sulfidation of Carbon Steel & Si content : H2-free vs. H2/H2S environments

I would recommend reading API 939-C.

If I had to guess, I'd say that the reason you don't see that same requirement in hydrogen service is that you're probably not going to be using carbon steel in a H2/H2S service, either due to HTHA concerns, or corrosion rate. I know several major refiners require the use of austenitic stainless steel in H2/H2S environments.

Nathan Brink

RE: HT sulfidation of Carbon Steel & Si content : H2-free vs. H2/H2S environments

I wondered what's the process conditions you have for the CS material with min 0.1% Si content per API RP 939-C. I know that the material design of the HT H2/H2S corrosion is gaining 300 SS per API RP 941.

RE: HT sulfidation of Carbon Steel & Si content : H2-free vs. H2/H2S environments


I am a corrosion specialist for a licensor of refinery technologies (hydrotreatment, CCR, FCC etc) with more than 12 years experience and I know by heart both API 939C (sulfidation) and API 941 (HTHA). I asked the question on the influence of Silicon on HT sulfidation of CS in H2/H2S service because there is no information in API and I would appreciate valuable inputs only (sorry if I am being rude).

@NBrink : I confirm that there are conditions for which carbon steel (with PWHT) is selected in high temperature hydrogen service (see Nelson curve below the curve for 1.25Cr-0.5Mo material...)

@mk3223 : selection of SS upon CS or LAS dépends on many parameters such as the temperature. Usually SS 321 or 347 CS are selected vs. CS and LAS above 250 to 300°C only, because of HT sulfidation, and carbon steel can be selected below 250°C...

RE: HT sulfidation of Carbon Steel & Si content : H2-free vs. H2/H2S environments


You are not being rude. You are only asking questions and giving answers. Carbon Killed steel above 200ºC in the petroleum industrie shall be used carefuly and allways according to the contaminants involved in the process, that´s all!

Good luck!

luis marques

RE: HT sulfidation of Carbon Steel & Si content : H2-free vs. H2/H2S environments

Available references highlight that failures of carbon steel materials by high temperature sulfidation (either H2/H2S or H2-free or...) in refinery systems are usually connected to low Si carbon steel materials. indeed, silicon prbably reinforces HT iron sulfide scales whatever the HT sulfiding environment is.
Thanks for this additionnal reference mk3223 !

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