Material Selection For The Oil And Gas Industry

Most process engineers are unaware of material selection and shy away from the task of providing material selection saying that a corrosion specialist needs to do the job.

What if you do not have the services of a corrosion specialist and you need to provide some quick answers for material selection as a process engineer. This blog entry attempts to give some insight into issues such as corrosion, it’s impact and material selection in context to the upstream oil and gas industry.

Before we look into the material selection part let us understand the various corrosion mechanisms and their definitions in the upstream oil and gas field.

Below are some definitions and the mechanism descriptions:

Sulfide Stress Cracking (SSC):
Cracking of metal involving corrosion and tensile stress (residual and/or applied) in the presence of water and H₂S
Mechanism of SSC:
SSC is a form of hydrogen stress cracking (HSC) and involves the 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.

Stress Corrosion Cracking (SCC):
Cracking of metal involving anodic processes of localized corrosion and tensile stress (residual and/or applied) in the presence of water and H₂S.

Mechanism of SCC:
Chlorides and/or oxidants and elevated temperature can increase the susceptibility of the metals to this mechanism of attack.

Hydrogen-Induced Cracking (HIC):
Planar cracking that occurs in carbon and low alloy steels when atomic hydrogen diffuses into the steel and then combines to form molecular hydrogen at trap sites.

Mechanism of HIC:
Cracking results from the pressurization of trap sites by hydrogen. No externally applied stress is required for hydrogen-induced cracks. Trap sites capable of causing HIC are commonly found in steels with high impurity levels that have a high density of planar inclusions and/or regions of anomalous microstructure (e.g. banding) produced by segregation of impurity and alloying elements in the steel. This form of hydrogen-induced cracking is not related to welding.

Hydrogen Stress Cracking (HSC):
Cracking that results from the presence of hydrogen in a metal and tensile stress (residual and/or applied).

Mechanism of HSC:
HSC describes cracking in metals that are not sensitive to SSC but which can be embrittled by hydrogen when galvanically coupled, as the cathode, to another metal that is corroding actively as an anode. The term “Galvanically Induced HSC” has been used for this mechanism of cracking.

Stress-Oriented Hydrogen Induced Cracking (SOHIC):
Staggered small cracks formed approximately perpendicular to the principal stress (residual or applied) resulting in a “ladder-like” crack array linking (sometimes small) pre-existing HIC cracks.

Mechanism of SOHIC:
The mode of cracking can be categorized as SSC caused by a combination of external stresses and the local strain around hydrogen-induced cracks. SOHIC is related to SSC and HIC.It nhas been observed in the parent material of longitudinally welded pipe and in the heat-affected zone of welds in pressure vessels. SOHIC is a relatively uncommon phenomena.

Sour Service:
Exposure to oilfield environments that contain sufficient H₂S to cause cracking of materials by the mechanisms described above.

Corrosion-Resistant Alloy (CRA):
An alloy intended to be resistant to general and localized corrosion of oilfield environments that are corrosive to carbon steels.

Carbon Steel:
An alloy of carbon and iron containing up to 2% mass fraction carbon and up to 1.65% mass fraction manganese and residual quantities of other elements, except those intentionally added in specific quantities for deoxidation (usually silicon and/or aluminum).

NACE MR-0175 / ISO 15156-1 – “Petroleum and natural gas industries – Materials for use in H₂S-containing environments in oil and gas production” is a standard that all corrosion and material specialists refer to for H₂S corrosion related phenomena and material selection for the oil and gas industry. The definitions mentioned above are referenced from this document.

NACE MR-0175 / ISO 15156-1 is an extensive document divided into 3 parts. Part 1 is "General principles for selection of cracking-resistant materials". Part 2 is "Cracking-resistant carbon and low alloy steels, and the use of cast iron". Part 3 is "Cracking-resistant CRAs (corrosion-resistant alloys) and other alloys". Of particualr interest are the annexures of part 2 and 3 which provide actual corrosion data on the various grades of carbon steel and CRAs as well as give material selection guidelines for the various grades in a defined oilfield environment.

Process engineers in the oil and gas industry should also refer this standard since many process data sheets require the input for material of construction (MOC) in the data sheets and knowledge of the subject will ensure that you can complete your datasheets even if you don’t have a material and corrosion specialist at hand.

A very good and useful free reference for material selection provided by NORSOK (Norwegian Industries Standard) for the oil and gas industry is available in the Norsok Standard "M-001 - Material Selection" at the following link:

http://www.standard..../1194/M-001.pdf

It is important for process engineers to remember that a wrong selection of material in corrosive oilfield environments can cause pre-mature material failure, which would not only lead to containment loss and production loss, but could also endanger health, safety and environment.

Hope this blog entry is liked by the readers and members of "Cheresources". Looking forward to comments from all of you.

Regards,
Ankur.