Damage Control: Stress Corrosion Cracking Mitigation

Editor’s Note: This regular column offers practical insights into various damage mechanisms affecting equipment in the O&G, petrochemical, chemical, power generation, and related industries. Readers are encouraged to send us suggestions for future topics, comments on the current article, and raise issues of concern. All submissions will be reviewed and used to pick topics and guide the direction of this column. We will treat all submissions as strictly confidential. Only Inspectioneering and the author will know the names and identities of those who submit. Please send your inputs to the author at damagecontrol@inspectioneering.com.

Introduction

The previous two installments of this 3-part series on stress corrosion cracking (SCC) addressed aspects of effective damage detection via non-destructive examination (NDE) and engineering analysis using fitness-for-service (FFS) methods, anchored in API 579-1/ASME FFS-1, Fitness-For-Service (API 579) [1]. Specifically, in Part 1 of this series, the following types of SCC afflicting common refining process units were discussed:

• Amine SCC
• Ammonia SCC
• Carbonate SCC
• Caustic SCC
• Chloride SCC
• Polythionic Acid SCC

This installment of Damage Control will cover damage mitigation techniques for these forms of SCC and provide practical, actionable steps to improve long-term equipment reliability through design, fabrication/welding, heat treatment, maintenance/repair, and process operating practices. Understanding and implementing these approaches to prevent SCC from occurring in the first place can offer significant long-term economic savings from an inspection and repair standpoint and can minimize costly failures that pose a safety risk to plant personnel. Unit operation and production can also be optimized when damage mechanisms like SCC are alleviated, and consequently, when unplanned equipment downtime is curtailed.

Amine SCC Mitigation

In amine units, four types of environmental cracking are commonly encountered: sulfide stress cracking (SSC), hydrogen induced cracking (HIC), stress-oriented hydrogen induced cracking (SOHIC), and amine stress corrosion cracking. The first three cracking mechanisms relate to wet H₂S damage and are discussed in detail in "A Guide to Wet H₂S Damage Management," an ebook published by Inspectioneering earlier this year [2]. The focus of this article will be on amine SCC in amine units. This type of damage was first identified in the early 1950s [3-4], and it most often occurs in carbon steel components operating in lean amine service (amine mixed with water); specifically, monoethanolamine (MEA) and diethanolamine (DEA) services, although it can also occur in methyldiethanolamine (MDEA) and diisopropanolamine (DIPA) services [5-7]. Furthermore, cracking can manifest itself in regions of corrosion or uncorroded areas, making it difficult to quantitatively correlate corrosion severity to cracking propensity. Habitually, corrosion itself (wall loss) occurs due to the presence of dissolved acid gases, such as hydrogen sulfide and carbon dioxide, or amine degradation products such as heat stable salts (e.g., thiosulfates and thiocyanates) [7]. In rich amine service, where wet H₂S results in the formation of an iron sulfide scale [2], amine SCC is not overly common because the scale acts as a protective coating for the ferritic steel substrate. As such, wet H₂S damage mechanisms are usually more prevalent in rich amine service environments [2].

Mitigating amine SCC involves different aspects of equipment general design, material selection, original fabrication methods, welding, heat treatment, repair procedures, and process controls. In general, favorable mechanical design characteristics should be specified; that is, sharp corners, notches, and other stress concentrations should be avoided, where possible. These locations often serve as crack initiation sites, and as such, represent potential SCC starter locations. For fillet welds, to minimize the potential for crack initiation, it is often recommended to blend-grind the toe of the weld to achieve a rounded contour that slightly extends into the thickness of the base metal, thus reducing local stress risers. Additionally, it is usually advantageous for attachment welds to be continuous (avoid stitch welds) and to locate welds away from abrupt thickness transitions or major structural discontinuities, if possible. For example, pressure vessel nozzle forgings with smooth inside and outside corner radii that can be butt-welded to the shell are often preferred for aggressive environmental service compared to conventional welded nozzle attachments (e.g., the weld is located directly where the nozzle intersects the shell) [8]. Additionally, socket-welded and threaded connections can introduce stress concentrations that can initiate cracks, and design alternatives should be considered, if feasible. Many of these general design recommendations go above and beyond construction Code minimum requirements and are often specified in engineering best practice documents.

Industry experience suggests that amine SCC has occurred in a variety of steels, and field encounters with SCC have not definitively rendered any significant correlation between cracking susceptibility and steel properties. Unlike SSC in wet H₂S environments [2], hardness of steel or weld deposits has little-to-no-effect on amine SCC, although the tendency for cracking increases with rising tensile stress levels (including residual stresses). Additionally, areas of deformation resulting from either cold-forming or localized high weld residual stresses are more prone to amine SCC, in general. It is worth noting that amine SCC has occurred on internal surfaces of equipment opposite external welded attachments, such as those associated with lifting lugs, due to through-wall weld residual stress fields. Generally, tensile residual stresses increase the propensity for crack initiation and propagation and increase the risk of SCC, fatigue cracking, and even brittle fracture [9,10]. Given this, perhaps no other mitigation method is more effective at preventing amine SCC on carbon and low alloy steels (and other forms of SCC) than post weld heat treatment (PWHT) and proper stress relief after cold forming [7].