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Chemical Process Technology

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Monday, October 20, 2008

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H2S dissolved in water to form weak acid promote corrosion and form free hydrogen. Free Hydrogen will penetrate the metal, reduce ductility of metal and potentially lead to stress failure below it yield stress, results Sulphide Stress Corrosion Cracking (SSCC).

Sulfide stress corrosion cracking (SSCC) is cracking of metal involving corrosion and tensile stress (residual and/or applied) in the presence of water and H2S. SSC is a form of hydrogen stress cracking (HSC) and involves embrittlement of the metal by atomic hydrogen that is produced by acid corrosion on the metal surface. Hydrogen uptake is promoted in the presence of sulfides. The atomic hydrogen can diffuse into the metal, reduce ductility and increase susceptibility to cracking. High strength metallic materials and hard weld zones are prone to SSC.

NACE Standard MR0175 / ISO 15156 - Petroleum and Natural Gas Industries – Materials for use in H2S-containing Environments in Oil and Gas Production was established to provides limits of H2S partial pressure for precautions against sulfide stress cracking (SSC) and guidance for the selection and specification of SSC-resistant materials.

Since the released of this NACE MR0175 / ISO 15156, it has introduced a few more elements into the "sour" service criteria i.e. pH, Chloride contents, etc. This may have created some level of difficulties in understanding and usage of this standard.

Canadian Association of Petroleum Producers (CAPP) has released a document entitle "GUIDE in Use of International Standard NACE MR0175/ISO15156" to provides a supporting document, which may be used as a reference tool to :
  • provide a brief overview of the NACE / ISO publication, outlining the most significant changes and their implication to the industry,
  • provide guidance and assistance on how to apply the new publication using simple to follow flowcharts, and clarification examples,
  • provide sample forms which could be used to meet the intent of the publication.

For those engineer involved in Oil & Gas (upstream) exploration and production, this document is pretty good for understanding and reference.


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posted by Webworm, 10:49 AM


Anonymous Anonymous said...

Hi I would like to know if the H2S partial pressure is less than 0.3kPa, however client has requested to designed the system for NACE compliance, are we allowed to use SS316 piping for service temperature about 60°C? Can we provide external coating on SS316 piping to prevent from chloride stress cracking and assume no internal corrosion will be formed?

The stress corrosion crackign will only occur in the presence of water, i.e. if the system is dehydrated, do we still need to design for NACE based on H2S partial pressure?

September 11, 2009 at 1:57 AM  
Blogger Webworm said...

Please refer to the H2S partial pressure and pH chart in MR0175 / ISO15156. Low pH could lead to requirement of NACE compliance.

Chloride stress corrosion cracking (CSCC) may occur at SS316 material with elevated temperature higher than 50 degC (rule of thumb). Read more in Chloride Stress Corrosion Cracking Use correct MOC for seawater service

External coating may isolate SS316 from contact with water with Chloride content. However, coating damage due to abrasion is possible and potential localize CSCC.

Dehydrated gas with H2S may not cause Sulfide stress corrosion cracking.

September 11, 2009 at 7:22 AM  

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