Corrosion resistant alloys for use in oil and gas plants

Corrosion resistant alloys are important for offering prolong resistance to corrosion for various components used in oil and gas production processes. These components include downhole tubing and safety critical elements, wellhead, valves, pipelines, piping, heat exchangers and various other pieces of equipment in facilities. There are different corrosion resistant alloys to choose and they can be featured by their resistance to specific conditions.

Major environmental parameters influencing the corrosion characteristics of these alloys are:

·         Temperature

·         Magnitude of chloride ions

·         Partial pressure of Carbon dioxide

·         Partial pressure of H2S

·         Environment pH

·         Sulfur

Above factors influence

a.       Stability of the passive layer

b.      Ease of repassivation of created pits

c.       Dissolution rate of metal from pits

d.      The chance of stress corrosion cracking initiating and propagating

The selection of corrosion resistant alloys for development and transport of corrosive oil and gas can be a complicated procedure and if not done correctly can cause problems in application as well as misguidance about the performance of a CRA in a specific service condition.

There are several ways companies and people choose Corrosion resistant alloys for the required well and flowline conditions. With extensive research facilities start a test program that involves simulating the particular application environment. A group of alloys using the information is chosen that shows a possible range of alternatives. Instead testing alloys, it is more economical and less time consuming to test some alloys that are possibly eligible. This method easily takes 1 – 3 years to accomplish at a large cost.

Another method is to check the corrosion data that applies to the anticipated field conditions. It can help in eliminating those materials that do not perform well and hence shortlist the number of testing alloys.

Austenitic stainless steel 316L

It shows the resistance of AISI 316L to deaerated, non-H2S containing conditions. It is widely used in surface piping, linepipe, cladding etc. In the presence of oxygen, stainless steel 316 will develop pits if exposed to cold seawater. In H2S condition, the performance of this alloy is very sensitive to the presence of chloride ions. In chloride-free conditions, this steel gives reliable service in sour gas handling facilities, however pitting readily develops in presence of chloride ions. So you should evaluate completely for the specific application.

Alloy 825, 625 and C276

The increased resistance of Inconel 625 wire to corrosion in H2S with increasing temperature as well resistance to all concentrations of CO2 make it fit to use in oil and gas applications. These alloys are not much susceptible to chloride concentration except at very high chloride levels. Although this factor should be more explored.

A common component of gas streams that has a profound effect on alloys is elemental sulphur that is called as free sulphur. It causes pitting and catastrophic damage to alloys in specific conditions but Hastelloy C276 is the most resistance however it cannot prevent this kind of corrosion and damage.