Proprietary Steel Grades in Corrosive Environments

The grades for Corrosive Environments are:
• Sour service.
• High collapse and Sour service.
• Critical service.
• Steel grades with 13% Chromium.
• High Corrosion Resistant Alloys.


Sour environments: these are related to the presence of H2S. An unfavorable combination of H2S dissolved in water, low ph, a susceptible structure and high
loads will cause cracks in the steel pipe, which will propagate until a catastrophic failure occurs. This is known as Sulfide Stress Cracking (SSC), and it is catastrophic
because this type of failure occurs in a sudden way taking just a few hours ordays, unlike other types of corrosion that normally take years to evolve.


High collapse and Sour service: this is used for those oil and gas operations taking place in environments characterized by a combination of high external pressure
and the probability of corrosion cracking due to the presence of H2S. 

 

Critical service grades: in environments where CRA are not economically justified,the corrosion produced under the mild presence of CO2 can be mitigated by using
critical service grades. These grades can be divided into two basic metallurgical designs, characterized by their chromium content:
• 1% Chromium steels: these are recommended for mild sweet corrosion, with low CO2 partial pressure and medium to high water cut.
• 3% Chromium steels: which are micro alloyed materials to increase the strength.

Steel grades with 13% Chromium : this is ideal for satisfying the need for high corrosion resistance keeping excellent toughness in highly corrosive environments
with the presence of water, CO2, H2S, low ph, chlorides and high temperature.


Corrosion Resistant Alloys (CRA): another type of corrosion is related to the presence of CO2 which is known as Sweet Corrosion. We can say that high contents of CO2 under pressure, which leads to high CO2 partial pressure in the well, are normally mitigated with the use of CRA, which are very expensive. ISO 13680 is the most important international standard which specifies the technical delivery conditions for corrosion resistant alloy seamless casing, tubing and coupling stock.


This standard specifies sixteen different chemical composition ranges, classified into four main groups. The categories, each one of them represented by 3 digits,
indicate the percentage of the main alloying elements, where: the 1st digit is the nominal Chromium content, the 2nd digit is the nominal Nickel content, and the
3rd digit indicates the nominal Molybdenum content.  To sum up, in general terms we can say that the higher the alloying content is, the higher the corrosion resistance and the higher the mechanical properties are and, as a consequence, the more expensive the manufacturing costs are.

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