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.

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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