Ultrasonic In-Service Detection and Monitoring of Sulfidation Corrosion

Introduction

Sulfidic corrosion of steel in downstream refining assets is a ubiquitous damage mechanism occurring in sulfur-containing crudes when heated between 230°C (446°F) and 425°C (797°F). It is difficult to predict, costly to repair, and has caused catastrophic ruptures in piping, tubing, pressure vessels, and other refinery assets ^[1]. Recent updates to recommended practices require 100% inspection of components at risk, with those showing signs of accelerated corrosion requiring monitoring. Recent advances in ultrasonic testing (UT) technology now allow for inspection and continuous monitoring to be undertaken while the components are on-stream, at temperature, with high-resolution thickness data.

Sulfidation Corrosion

Corrosion in refineries and petrochemical plants can be broadly defined as low- and high-temperature corrosion mechanisms at a critical temperature of ~260°C (500°F) ^[2,3] depending on whether the damage mechanism occurs in the presence of water. At high temperatures, sulfur compounds in hydrocarbon products can react with carbon and low-alloy steels, and to a much lesser extent 400- and 300-series stainless steels, where the addition of chromium to carbon steels offers significant resistance ^[3]. The corrosion rate is typically considered by a combination of sulfur concentration, temperature, and Cr content of the low alloy steel used. 

High temperature sulfidation (sulfidic) corrosion is one of the most common internal corrosion mechanisms in the oil refining industry as nearly all crude oil feeds contain naturally occurring sulfur compounds, such as H2S and other reactive species ^[1,3]. This makes the potential for sulfidation corrosion a consideration in all crude processing refineries, affecting large sections of the plant, and is of increased concern when processing opportunity crudes with increased total acid number (TAN) and sulfur content. In the absence of hydrogen, corrosion due to sulfur compounds in the crude is thought to occur at temperatures above 260°C (500°F), forming a surface sulfide (FeS) semi-passive scale and dissolved hydrogen in the steel which can then lead to hydrogen embrittlement. Up to that temperature, corrosion rates due to sulfidation are relatively low, except for when naphthenic acid is present, and above 500°C (932°F) can lead to sulfur diffusion into the metal, displacing carbides ^[1].