Is Your Killed Carbon Steel Resistant to High Temperature Sulfidation?

Introduction

High temperature sulfidation is probably the most well-known corrosion mechanism in the oil refining industry because it occurs in large sections of the refinery.  Sulfidation corrosion (also known as sulfidic corrosion) is a result of naturally occurring sulfur (S) compounds found in crude oil.  In the absence of hydrogen, corrosion due to sulfur compounds in the crude is thought to occur at temperatures above 500oF (230oC).  Up to that temperature, corrosion rates due to sulfidation are relatively low, even for carbon steels, unless there is naphthenic acid present in the crude.  The Modified McConomy curves used for selecting materials and also for predicting in-service resistance to sulfidation corrosion have historically served the industry well.  There are many nuances in sulfidation corrosion related to the exact types of sulfur compounds present in the stream.  Other nuances include additional corrosives such as naphthenic acid, shear stress, and, as has been shown over the years, even the amount of residual silicon (Si) present in the steel component.   

The Chevron Richmond Refinery piping failure that occurred on August 6, 2012 is just one of the latest incidents that was partially attributed to a particular section of piping with low silicon (Si < 0.10 wt %), where other piping sections contained higher Si contents in a hot sulfur containing process stream.  Although there were numerous CML’s on the piping system, one section of pipe was not monitored and thinned at a greater rate.  Sulfidation corrosion can take place in refinery units such as the crude unit, as well as the catalytic cracking unit (FCCU), delayed coker (DCU) and others.

API 939-C, Guidelines for Avoiding Sulfidation (Sulfidic) Corrosion Failures in Oil Refineries was first published in 2009 and highlights the low Si issue.  A new edition is currently being drafted to strengthen this section of the document since failures continue to occur.  Si is believed to help form a somewhat protective FeS scale.  If the Si content of the steel is below 0.10 wt % the corrosion rate can be quite variable, and in some cases can range from 2-10 times higher than surrounding steel components with greater Si content.  Therefore, materials should contain silicon in quantities of greater than 0.10 wt % to provide adequate (and more consistent) sulfidic corrosion resistance.