Damage Control: Stress Corrosion Cracking Detection

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

The propensity for these different types of SCC in pressure equipment is dependent on many specific fabrication and process operating variables, such as material properties and chemistry, weld procedures, weld geometry, weld deposit and heat affected zone (HAZ) hardness/microstructure, original heat treatment, external environmental conditions, process stream composition, pH, temperature, operating stress level, and proximity to local stress concentrations. In general, the often-confounding influence of these different variables on SCC susceptibility and on markedly unpredictable crack propagation rates, makes the above forms of SCC notably complex and difficult to manage in aging components [2,3]. Consequently, equipment failures due to SCC continue to plague numerous industries, highlighting an overall lack of understanding and awareness of these complicated damage mechanisms in many cases.

This installment of Damage Control provides an overview of some of the more common forms of SCC (listed above) that can detrimentally influence the long-term reliability of process equipment and create a notable reliability and maintenance burden for plant personnel. Furthermore, particular focus herein is placed on detecting these forms of cracking, and commentary on effective inspection methods is offered. In this article, sub-critical cracking of steels due to gaseous hydrogen or sulfide stress cracking (SSC) are generally considered to be related to hydrogen embrittlement (HE) or the presence of wet H₂S, where hydrogen permeation of steel is at the root of these damage mechanisms [4,5]. For this reason, these types of cracking are not covered in detail in this article. Additionally, sub-critical cracking due to liquid-metal environments (e.g., zinc or mercury) is also normally treated as a separate damage mechanism phenomenon from SCC and is designated as liquid-metal embrittlement (LME) [6]. To this end, LME is not covered in detail in this article, but it, along with SSC and HE, should be well-understood by engineers and inspectors such that proper delineation of any identified cracking-related damage can be accomplished in the field.

Different Forms of Stress Corrosion Cracking (SCC)

As discussed herein, the damage morphologies of different SCC mechanisms are frequently similar in nature (with common crack driving forces such as tensile weld residual and applied stresses), even though the underlying cracking susceptibility may be a function of metallurgy and is propelled by different process environments and governing chemical reactions. Being able to successfully identify the common characteristics of SCC is an essential first step in reasonably managing the risk associated with a potential in-service leak or even a sudden catastrophic fracture or rupture scenario for pressure equipment. The descriptions below highlight some of the key damage characteristics and critical factors that can dramatically increase the propensity for damage and, ultimately, failure for some of the different forms of SCC.