Polythionic Acid Stress Corrosion Cracking (PASCC)

Polythionic acid stress corrosion cracking (PASCC) occurs in certain materials, such as austenitic stainless steel, under specific environmental conditions. It is caused by the interaction of the material with a corrosive environment containing chlorides, such as seawater, and high levels of hydrogen sulfide. In addition, the presence of Polythionic acid, formed by the reaction of hydrogen sulfide with oxygen and water, further exacerbates the corrosion process.

PASCC can result in the formation of microcracks in the material, which can lead to sudden failure under stress, even at stresses below the yield strength of the material. This makes it a severe concern in applications where the material is exposed to such an environment, such as in the oil and gas industry, chemical processing, and marine applications.

Preventing PASCC involves careful material selection, designing to minimize stress concentrations, and controlling environmental conditions, such as reducing chlorides and hydrogen sulfide levels.

Austenitic stainless steels are the most commonly known materials susceptible to Polythionic acid stress corrosion cracking (PASCC). This includes grades such as 304, 316, and 321 stainless steel, which are widely used in various industries due to their excellent corrosion resistance, mechanical properties, and weldability. However, other materials, such as nickel-based alloys, aluminum alloys, and some high-strength steels, can also be susceptible to PASCC under certain environmental conditions. Therefore, it is essential to carefully consider the materials selection and operating conditions when designing structures or equipment exposed to a potentially corrosive environment.

In the oil and gas industry, equipment most susceptible to Polythionic acid stress corrosion cracking (PASCC) includes components exposed to sour environments, such as those found in wells containing high levels of hydrogen sulfide and chlorides.

Some examples of equipment that may be susceptible to PASCC in the oil and gas industry include:

• Tubing and casings: These are often made of austenitic stainless steel and are exposed to the corrosive environment of the wellbore.
• Heat exchangers: These are used to transfer heat between fluids and are often made of austenitic stainless steel. They may be exposed to sour gas, seawater, or other corrosive fluids.
• Piping: Piping systems used for transporting fluids, such as oil, gas, and water, may also be susceptible to PASCC if exposed to a sour environment.
• Pressure vessels: These are used for storing or transporting fluids under pressure and may be made of austenitic stainless steel or other materials susceptible to PASCC.
• Valves and fittings: These components control the flow of fluids and may be exposed to a corrosive environment.

Preventing PASCC in oil and gas equipment involves careful material selection, design, and operating conditions to minimize the risk of corrosion and cracking. This includes using materials with improved resistance to PASCC, such as duplex stainless steels or nickel-based alloys, controlling the concentration of chlorides and hydrogen sulfide, and avoiding high-stress conditions.

Inspecting for Polythionic acid stress corrosion cracking (PASCC) typically involves a combination of non-destructive testing (NDT) methods and visual inspections.

NDT methods used for PASCC inspection may include the following:

• Ultrasonic Testing (UT): This method uses high-frequency sound waves to detect internal defects or anomalies in the material. UT can be used to detect the presence of cracks or other discontinuities caused by PASCC.
• Radiography: This method uses X-rays or gamma rays to create an image of the material's internal structure. Radiography can detect cracks or other anomalies caused by PASCC and different types of corrosion.
• Eddy Current Testing (ECT): This method uses electromagnetic induction to detect surface and subsurface defects or anomalies in conductive materials. ECT can be used to detect the presence of cracks or other discontinuities caused by PASCC.

Visual inspections may include:

• Surface inspections: This involves visually inspecting the material's surface for signs of cracking or other damage caused by PASCC, such as pitting or discoloration.
• Microscopic inspections involve using a microscope to examine the material's surface for signs of cracking or other types of damage at the microscopic level.

It is important to note that PASCC can often be challenging to detect because it can occur internally and may not be visible on the material's surface. Therefore, regular inspection and monitoring of the material and its operating environment are crucial to prevent catastrophic failures.

Inspecting for Polythionic acid stress corrosion cracking (PASCC) in a laboratory typically involves a combination of specialized testing procedures designed to simulate the environmental conditions that can cause PASCC.

One standard laboratory test used for PASCC inspection is the Slow Strain Rate Test (SSRT), which involves exposing a small material specimen to a solution containing Polythionic acid, hydrogen sulfide, and chlorides at a slow strain rate.

The specimen is typically subjected to a constant load, and the strain rate is controlled to mimic the slow deformation rates that can occur in real-world applications. The test is conducted in a specialized testing apparatus, such as an environmental chamber or a test cell that can maintain the desired temperature and pressure conditions.

During the SSRT test, the material is monitored for signs of cracking, such as elongation or fracture. In addition, the testing conditions are carefully controlled and monitored to ensure accuracy and repeatability.

Other laboratory tests used for PASCC inspection may include electrochemical tests, such as potentiodynamic polarization or electrochemical impedance spectroscopy (EIS), which can provide information about the corrosion behavior and susceptibility of the material under various environmental conditions.

It is important to note that laboratory testing is typically conducted as part of a broader testing and inspection program, which includes field inspections and monitoring, to ensure that the material and equipment are performing as expected and to detect potential issues before they can lead to catastrophic failure.

The American Petroleum Institute (API) provides a set of codes and standards commonly used in the oil and gas industry to prevent and manage various types of corrosion, including Polythionic acid stress corrosion cracking (PASCC).

Some of the relevant API codes and standards for preventing and managing PASCC include the following:

• API RP 571: This code provides guidance on damage mechanisms affecting fixed equipment in the refining industry, including PASCC.
• API RP 578: This code guides material verification programs for new and existing equipment used in the oil and gas industry, including measures to prevent PASCC.
• API RP 939-C: This code provides guidelines for materials selection and corrosion control for oil and gas production equipment, including measures to prevent PASCC.
• API 5L: This specification covers seamless and welded steel line pipe used for transporting fluids in the oil and gas industry and includes material properties and testing requirements to prevent PASCC.
• API 6A: This specification covers requirements for wellhead and christmas tree equipment used in the oil and gas industry, including measures to prevent PASCC.
• API 6X: This specification covers requirements for subsea pipeline valves used in the oil and gas industry and includes measures to prevent PASCC.

By following these API codes and standards, operators in the oil and gas industry can minimize the risk of PASCC and other types of corrosion and ensure their equipment's safe and reliable operation.

The National Association of Corrosion Engineers (NACE) provides a set of codes and standards commonly used in the oil and gas industry to prevent and manage various types of corrosion, including Polythionic acid stress corrosion cracking (PASCC).

Some of the relevant NACE codes and standards for preventing and managing PASCC include:

• NACE MR0175/ISO 15156: This code provides guidelines for selecting materials resistant to sulfide stress cracking (SSC) and other forms of corrosion in oil and gas production environments. It includes specific recommendations for controlling Polythionic acid-induced PASCC.
• NACE SP0472: This code provides guidelines for corrosion control in oil and gas production and processing facilities, including measures to prevent PASCC.
• NACE SP0176: This code provides guidelines for applying internal coatings to prevent corrosion in oil and gas production and processing equipment, including measures to avoid PASCC.
• NACE TM0177: This test method guides laboratory testing procedures for detecting and characterizing PASCC in metallic materials.
• NACE RP0188: This code provides guidelines for using cathodic protection systems to prevent corrosion in buried or submerged metallic structures, including pipelines and storage tanks.

By following these NACE codes and standards, operators in the oil and gas industry can minimize the risk of PASCC and other types of corrosion and ensure their equipment's safe and reliable operation.