In-Service Damage Mechanisms and the Nondestructive Examinations that Aid in their Identification

Introduction

Organizations such as ASME, CSA, and API publish codes, standards, and practices that provide instructions and guidelines for the nondestructive examination (NDE) of as-fabricated equipment. However, for in-service damage, reliability personnel must have an understanding of the damage mechanisms that could credibly affect equipment in specific services, including the locations where damage is expected, the anticipated shape of the damage, and the specific operating conditions and practices that may cause the damage. This requires personnel to be suitably versed in today's rapidly evolving NDE landscape to select the most appropriate examination methods for the task at hand.

In this article, various NDE techniques used to examine damage in pressure equipment are described. The cases discussed are for hydrofluoric (HF) acid and somewhat acidified damage, temper embrittlement, downtime stress corrosion cracks, stress relaxation cracks (SRC), and pitting. They underscore the in-depth knowledge needed to select and use NDE effectively.

Low Alloy Steel Acid Corrosion

In HF alkylation units, "Corrosion by water containing dilute hydrofluoric (HF) acid can result in high rates of general or localized corrosion …" [1]. Residual elements have an effect on this galvanic corrosion attack exemplified in the Philadelphia 2019 refinery explosion and numerous publications [2, 3]. Residual elements are trace elements in steel, primarily (in this context) carbon, copper, nickel, and chromium. Figure 1 shows the metallographic cross-section of a reducer to pipe weld where the reducer and its heat-affected-zone (HAZ) had extensive losses. The reducer had lower residual elements than the pipe and deposited weld metal (DWM).

Beyond alkylation units, this galvanic corrosion attack also occurs in somewhat acidified cooling waters in various industries. Figures 2 and 3 illustrate the pitting attack in piping components in a petrochemical plant. The fittings with lower residual elements are pitted, while the pipe is not. Since recycled steel and its associated residual elements have become the norm, this damage is becoming more frequent.