Guidelines for Providing Process Conditions for RBI - Part 7: Environmentally Assisted Cracking and RBI

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Tags: Damage Mechanisms Data Collection Risk Based Inspection


The 7th installment in our series covers environmental cracking. James Leonard, RBI Team Leader, discusses the additional process data that may be needed to determine environmental cracking damage potential when environment, stress, and metallurgy are so alligned so as to indicate susceptibility to this damage mechanism.

Guidelines for Providing Process Conditions for RBI - Part 7: Environmentally Assisted Cracking and RBI

Background

Environmentally Assisted Cracking is a term that covers multiple forms of cracking. The environment the crack occurs in defines the type of cracking. Environmental Cracking is and has been a major problem for many facilities around the globe. A damage mechanism review (DMR) with accurate data will provide information to the owner/user on what kinds of cracking are suspected and where the cracking will most likely occur.

Environmentally assisted cracking can occur when three factors are aligned: environment, stress and metallurgy.

  • Environment - A corrosive environment is the main reason for environment-assisted cracking. A specific type of a corrosive contaminant will dictate the classification of environmentally assisted cracks. This is what sets apart the different forms of environmentally assisted cracking. The concentration of the contaminant inducing cracking typically affects the severity of the damage. Other contaminants present can affect the occurrence of the damage. Environmentally assisted cracking can also be affected by pH of the service, temperature, and exposure time, among others. Damage is typically enhanced by high levels of impurities, dissolved oxygen, and wetness in the stream.
  • Stress - Environmentally assisted cracking occurs preferentially where the local stress is high. Residual or applied stresses created from fabrication or from an intermittent process are some of the most common examples of stress found in industrial environments.
  • Metallurgy - the environmentally assisted cracking results from interaction between the environment and a “specific alloy”. Typical examples include environmental cracking of copper alloys in ammonia, carbon steel in caustic soda, and stainless steels in chloride environments (under certain conditions). Impurities in the material and high tensile strength can increase the susceptibility to cracking.

Environmentally assisted cracking can be avoided by proper material selection, use of coatings, and application of stress relieving treatments.

As discussed previously in Part 4 of this series, the basic process information required by most Risk Based Inspection (RBI) software is, in some cases, not enough to calculate the probability (damage potential) of loss of containment for the high temperature damage mechanisms that are expected, and this is also the case with the environmental cracking damage mechanisms. Since additional process data will be required in most instances, the corrosion specialist should request the process engineer to provide any additional process data needed to determine the damage potential.

A general list of some of the key data needed to determine susceptibility to environmental damage is provided below. Specific process data required to determine susceptibility to several different environmental damage mechanisms is listed in the Table 1.

  • Corrosive Contaminants - The concentration and type of chemical contaminants will dictate the classification of environmentally assisted cracks. This is what sets apart the different forms of environmentally assisted cracking.
  • Temperature - Temperature, in some cases may not play a role in creating the potential for cracking but can also be the most critical factor for cracking to occur. For example, Ammonia Stress Corrosion Cracking can form at low or ambient temperatures, however increasing temperature is a driving force for Chloride stress corrosion cracking to occur.
  • Stress - Stresses, residual or applied, created from fabrication, high operating pressure, or from an intermittent process are some of the most common examples of stress found in industrial environments.
  • PWHT - Post Weld Heat Treating is one of the most common practices that can be utilized to relieve stress created from the fabrication processes. However, this practice can also leave some materials more susceptible to cracking if not done properly.
  • Metallurgy - Proper design of equipment needs to account for potential cracking. Upgrading the metallurgy can be one of the costliest paths to helping prevent cracking but in some cases will eliminate the potential for cracking all together. Upgrading the metallurgy is helpful in preventing cracking however if done improperly it can be a major factor in causing cracking to occur.

Once the risk analysis has been performed, mitigating actions can be determined and prioritized. These mitigating actions can address probability of failure, consequence of failure, or both. The most common mitigating actions for cracking mechanisms are inspections. The amount, type, and extent of inspection can be determined based on the risk associated with each cracking damage mechanism.

Implementing an RBI program will not only make the user-owner more aware of what type of risks the equipment is susceptible to, but it can help in creating a strategic approach to mitigating the risks.

Environmental Damage MechanismProcess Variables Affecting SusceptibilityDamage Mechanism Description
Chloride Stress Corrosion Cracking (Cl-SCC)
  • Aqueous phase present
  • Chloride content
  • Temperature
  • Presence of oxygen
  • pH
Cracking of austenitic stainless steel under the combined action of tensile stress, temperature and an aqueous chloride environment. The presence of dissolved oxygen increases potential for cracking.
Caustic Stress Corrosion Cracking (Caustic Embrittlement)
  • Caustic concentration
  • Metal temperature
Surface initiated cracking occurring in piping and equipment exposed to caustic. Cracking can occur in carbon steel, austenitic stainless steel, or nickel alloys.
Ammonia Stress Corrosion Cracking
  • Copper Alloys
    • Aqueous phase present
    • Oxygen present
    • pH
  • Carbon Steel
    • Anhydrous ammonia (< 0.2wt% water present)
    • Oxygen present
Some copper alloys are susceptible to SCC in the presence of aqueous streams containing ammonia.Carbon steel is susceptible to SCC in anhydrous ammonia. Industry experience shows that failures have occurred in pressure equipment carrying aqueous ammonia with < 0.2wt% water present in the vapor space.
Ethanol Stress Corrosion Cracking (SCC)
  • Fuel grade ethanol
  • Oxygen present
  • Water content
Ethanol SCC has been observed in carbon steel materials in some services containing fuel grade ethanol.
Sulfate Stress Corrosion Cracking
  • Presence of sulfates
Environmental cracking of copper alloys in sulfate solutions over many years.
Polythionic Acid Stress Corrosion Cracking (PASCC)
  • Sulfide scale present
  • Moisture present
  • Oxygen present
Cracking is due to polythionic acids forming from sulfide scale, air and moisture acting on sensitized austenitic stainless steels and some nickel alloys such as Alloy 600/600H and Alloy 800/800H. Damage typically occurs during startups / shutdowns. Sensitization can occur during welding or operations, typically, in the 750-1500°F range.
Amine Stress Corrosion Cracking
  • Temperature
  • Amine type
  • Amine concentration > 2 wt%
Cracking of carbon and low alloy steels resulting from the combined action of tensile stress and corrosion in lean aqueous alkanolamine systems used to remove/absorb H2S and/or CO2 and their mixtures from various hydrocarbon streams.
Wet H2S Damage
  • H2S content
  • Aqueous phase present
  • pH
  • Presence of cyanides
  • Temperature
Damage in the form of blistering, hydrogen induced cracking (HIC), stress oriented hydrogen induced cracking (SOHIC), and sulfide stress cracking (SSC) due to presence of H2S in an aqueous environment.
Hydrogen Stress Cracking - HF
  • HF content
  • Aqueous phase present
A form of environmental cracking that can initiate on the surface of high strength low alloy steels and carbon steels with highly localized zones of high hardness in the weld metal and HAZ as a result of exposure to aqueous HF acid environments.
Carbonate Stress Corrosion Cracking (ACSCC)
  • Aqueous phase present
  • pH
  • Carbonate content
  • H2S content
  • Ammonia content
  • Cyanides present (informational)
A form of Alkaline Stress Corrosion Cracking (ACSCC) that occurs in systems containing carbonate in an aqueous environment. H2S is also typically present. Damage typically occurs adjacent to non PWHT'd carbon steel welds.

Summary

Proper material selection, use of coatings, and application of stress relieving treatments may help alleviate environmental cracking that can occur when the environment, stress, and metallurgy are aligned to promote cracking. Mitigating actions can be determined and prioritized when taking into consideration several key data points that indicate a susceptibility to environmental cracking. Implementing and RBI program will increase awareness of what type of risks equipment is susceptible to as well as help create a strategic approach to mitigation of such risks.

Stay tuned for our concluding remarks in this eight-part series covering guidelines on assigning process conditions for RBI efforts:

  1. Guidelines for Providing Process Conditions for Risk Based Inspection (RBI) Implementation and Revalidation (Introduction)
  2. Corrosion Under Insulation (CUI) and How it Relates to Risk Based Inspection
  3. Process Fluids and Consequence Models
  4. High Temperature Damage Mechanisms
  5. Low Temperature Damage Mechanisms
  6. High Temperature Hydrogen Attack
  7. Environmental Cracking Damage Mechanisms (this article)
  8. Concluding Remarks

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