Damage Control: Stress Corrosion Cracking Assessment

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

In general terms, stress corrosion cracking (SCC) is defined as a metallurgical damage mechanism characterized by the presence of sub-critical cracking under sustained loads (applied or residual), occurring most commonly in liquid environments, but sometimes gaseous environments [1]. Furthermore, there are numerous forms of SCC that typically afflict oil refining, petrochemical, and related pressure equipment, including but not limited to the following:

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

This article will examine common engineering and fitness-for-service (FFS) methods that can be leveraged to understand and evaluate the propensity for an existing form of SCC to lead to loss of containment, whether through a leak or a catastrophic rupture or brittle fracture scenario. Specifically, determining critical flaw size is the first step in understanding the relative risk associated with existing damage. In general, critical flaw size is a function of loading (applied or residual stresses), the geometry (dimensions and orientation) of the crack-like flaw, and material properties (e.g., fracture toughness or ductile tearing resistance). The critical flaw size is usually independent of the type of SCC that may be driving crack propagation, although certain process environments can lead to degradation in material fracture toughness. Furthermore, estimating crack growth rates can also be a useful endeavor when the uncertainty associated with the predictions is realized and calculation results are coupled with appropriate periodic monitoring and risk management strategies. This article covers these topics and offers practical insight into quantifying protection against brittle fracture and ductile tearing in a variety of different steels, using modern fracture mechanics methods consistent with API 579-1/ASME FFS-1, Fitness-For-Service (API 579) [1].

Protection Against Brittle Fracture

Brittle fracture is generally characterized as the sudden rapid fracture under stress (residual or applied) where the material exhibits little or no evidence of ductility or plastic deformation [2,3]. An example of a typical brittle fracture in a pressure vessel is shown in Figure 1. A brittle fracture failure requires the presence of an initial crack-like flaw, which can be in the form of an original fabrication defect or from an SCC-related flaw that may have initiated and propagated in service. Carbon and low-alloy steels are most prone to brittle fracture, and these materials typically exhibit a ductile-to-brittle transition type of a behavior where Charpy impact energy and hence, fracture toughness, changes from an upper shelf value (at higher temperatures) to a lower shelf value (at low temperatures). Additionally, cracks initiating at or near welds are often the source of brittle fracture failures because of the presence of elevated tensile weld residual stresses and stress concentration effects associated with weld geometry or a mismatch in material properties between the weld, heat affected zone (HAZ), and adjacent base metal. Furthermore, once a crack-like flaw is identified in a piece of pressure equipment, qualification of damage via FFS, or repair/replacement is usually required. Often, flaw excavation, possible weld repairs, and complete component replacement may be time-consuming and lead to additional unplanned unit downtime, which in turn, can be economically devastating due to lost production. To this end, modern FFS methods represent a valuable and often cost-effective engineering tool for managing the risk associated with in-service SCC.