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Inspectioneering Journal

Hamstrung by Hydrogen Damage Mechanisms

By David A. Hansen, PhD, PE at Metallurgical Consulting, Inc., and Marc McConnell, P.E. at Pro-Surve Technical Services. This article appears in the November/December 2017 issue of Inspectioneering Journal

Introduction

Over a long history of involvement in equipment repairs, many have asked: “Do we need to complete a hydrogen bake-out on this equipment before we perform our weld repairs?”   This is always an engaging subject when teaching a refinery corrosion course.

Let’s start with an explanation of the term “Hydrogen.” When we think of hydrogen, we think of molecular H2. This is the gas in the pipeline. However, the hydrogen that gives us problems in steel is atomic hydrogen, or H(sometimes called nascent hydrogen). Only atomic hydrogen (H0)* can penetrate steel, accumulating in the crystal structure and affecting the steel’s mechanical properties.

Note that, in most corrosion reactions that produce atomic hydrogen, the atoms recombine almost immediately to form hydrogen gas at the corroding surface; the recombined H2 goes to atmosphere. However, there are a few corrodents, such as H2S, HF, arsenic, and cyanide that interfere with the recombination of the nascent hydrogen. In such cases, the atomic hydrogen dissolves into the solid metal instead of forming H2.

Hydrogen-induced damage mechanisms become possible when hydrogen atoms begin to dissolve into a metal or alloy. Hydrogen gas is insoluble in metals and alloys, but hydrogen atoms are soluble. The dissolving hydrogen atoms usually come from either corrosion reactions or high temperature gaseous hydrogen services.   Hydrogen dissolved in some metals and alloys can cause various types of cracking problems, some of these problems are associated with welds, while others can affect parent metal.

Common materials of construction susceptible to hydrogen damage include carbon steels and low-alloy Cr-Mo steels. Martensitic 400-series stainless steels are subject to precipitation hardening. Duplex stainless steels are also susceptible to at least some hydrogen related damage mechanisms.

  • Corrosion, such as that which occurs when hydrogen sulfide reacts with the surface of a steel, produces atomic hydrogen (sometimes called nascent hydrogen). In some corrosion reactions, the atomic hydrogen generated by corrosion dissolves into the steel rather than departing into the atmosphere as hydrogen gas.
  • Hydrogen gas (H2), as discussed below for high temperature hydrogen attack (HTHA), dissociates into hydrogen atoms (H0) and can then diffuse into a steel surface. The rate of this reaction is very slow at temperatures less than about 400°F (205°C); accordingly, thermally produced dissolved hydrogen is not active at process temperatures less than about 400°F (205°C).

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Comments and Discussion

Posted by Sam Miller on January 8, 2018
Outstanding summation of the various... Log in or register to read the rest of this comment.

Posted by Juergen Deininger on January 9, 2018
Great summary with a very good Explanation of... Log in or register to read the rest of this comment.

Posted by Ibrahim Kodssi on March 28, 2018
Simple introduction to hydrogen damage with a... Log in or register to read the rest of this comment.

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