A New Risk Assessment Tool for Determining the Likelihood of Chloride Promoted Stress Corrosion Cracking in Austenitic Stainless Steels

Introduction

Chloride promoted stress corrosion cracking (ClSCC) is a damage mechanism that everyone in the inspection business has likely heard about.  However, historically, if asked when or how likely an austenitic series stainless steel component is to crack, corrosion engineers usually answered “very” or “maybe” or, later, “I didn’t see that coming.”  With the introduction of API’s formal risk assessment methodology, that situation has changed over the past 20 years. 

The purpose of API RP 581, 3^rd Edition Risk-Based Inspection (RBI) Methodology is to provide “quantitative procedures to establish an inspection program using risk-based methods for pressurized fixed equipment, including pressure vessels, piping, tanks, pressure relief devices (PRDs), and heat exchanger tube bundles.” [1] The essential companion to RP 581 is API RP 580 Risk-Based Inspection.  API RP 580, 3^rd Edition “provides guidance for developing RBI programs on fixed equipment in refining, petrochemical, chemical process plants and oil and gas production facilities. The intent is for API RP 580 to introduce the principles and present minimum general guidelines for RBI” [2].  Alternatively, API RP 581 “provides quantitative calculation methods to determine an inspection plan.”

A key component of this quantitative calculation method is the determination of the likelihood that a pressure-containing part (such as a pipe, pressure vessel, storage tank, pressure relief valve, etc.) will fail within the time period under consideration.  The likelihood is derived from a Damage Factor (DF) based on a generic failure frequency (taken from industrial experience) which is adjusted to account for damage mechanisms (DM) that may be active in a component.  The active damage mechanisms are specific to the materials of construction and the process service.