Inspectioneering Journal

The Hype about Hydrogen Bake-Outs!

Part 1

By Marc McConnell, P.E., Metallurgy and Fixed Equipment Engineering Coordinator at Pro-Surve Technical Services. This article appears in the May/June 2013 issue of Inspectioneering Journal.
This article is part 1 of a 2-part series.
Part 1 | Part 2


With 30 years of refinery experience, I have been through many turnarounds and been involved with a lot of repairs. When I started in the business, we would have inspectors that “owned” their specific pieces of equipment. They would conduct a complete internal inspection on all equipment items on a fixed interval of every two years. At that time, the same Inspector would enter, inspect, and “document” the same piece of equipment on a regular basis. That inspector would inevitably know his pieces of equipment through and through.

After a long history of involvement, that inspector would be completely acquainted with his specific area/equipment, yet there was always one question that would come up when repairs were needed: “Do we need to complete a bake-out on this equipment before we perform our weld repairs?” Why did this question perpetuate itself? Why was this subject so difficult to understand? To this day, this question still follows me, so let me explain.

A bake-out is used to drive hydrogen out of the steel, as trapped hydrogen can cause cracking in the weld. Actually, trapped hydrogen can cause cracking in the entire vessel, but let’s focus on weld repairs first. This type of cracking has many names, and is often called delayed cracking, cold cracking, hydrogen assisted cracking, hydrogen induced cracking, and hydrogen embrittlement.

It is quite well known that cracking can occur immediately during welding, or there can be some delay between the completion of the weld and the formation of hydrogen cracks, but usually within 48 hours. Therefore, if traditional inspection is carried out too soon after welding susceptible material, these cracks may not be detected and as a result, a faulty weld is put into service. On the other hand, excessive delays after welding prior to inspection can seriously impact project planning and delay the return of equipment to service.

Currently, there are rules-of-thumb and/or “in-house” recommendations for weld inspection delays between 16 – 48 hours in various standards, but there is no firm basis for these times. Furthermore, there is generally no discrimination between different materials, joint geometries, or welding conditions. For simplicity, and lack of detailed knowledge, most folks have one delay time recommended for all circumstances.


Hydrogen cracking in ferritic steels only occurs when a critical combination of the four basic factors involved is exceeded. These factors are:

  1. Hydrogen content,
  2. susceptible microstructure,
  3. stress, and
  4. temperature.

Atomic Hydrogen

First off, let me start with an explanation of the term “hydrogen.” When we think of hydrogen, we think of molecular H2. This is the gas that is in the pipeline. However, the hydrogen that gives us problems in steel is atomic hydrogen, or H+. In everyday life on Earth, isolated hydrogen atoms (usually called “atomic hydrogen” or, more precisely, “monatomic hydrogen”) are extremely rare. Instead, hydrogen tends to combine with other atoms in compounds, or with itself to form ordinary (diatomic or molecular) hydrogen gas, H2. The problem presented with atomic hydrogen (H+) is that it is a small atom, and has the ability to travel into steel. When the hydrogen is molecular hydrogen (H2), it becomes too large to migrate through steel. Therefore, from a cracking standpoint, we only care about atomic hydrogen (H+).

So this is what happens: In certain conditions, hydrogen (atomic hydrogen) can diffuse into the steel. Hydrogen uptake by steel can occur at specific conditions at both low temperature as well as high temperatures.

  • Low Temperature - At low temperatures atomic hydrogen forms as a result of:
  1. Corrosion involving hydrogen promoters, such as H2S and hydrofluoric (HF) acid
  2. Cleaning & pickling.
  • High Temperature – At high temperature atomic hydrogen forms as a result of:
  1. Welding – wet electrodes will charge the steel with hydrogen
  2. Service at high temperatures - a small amount of hydrogen gas will dissociate to form atomic hydrogen that can diffuse into the steel.

When atomic hydrogen enters the steel and causes cracking, it is referred to as Hydrogen Embrittlement or Hydrogen Stress Cracking. This specific type of embrittlement is produced when atomic hydrogen (H+) diffuses into the metal to highly stressed sites (e.g., notches, inclusions, weld defects or cracks). Where there is enough concentration, these hydrogen atoms will exert stresses within the metal structure, reducing the threshold stress for crack initiation and propagation and reducing the ductility (Reference: Figure 1).

Figure 1
Figure 1

In reference to Figure 1, notice that hydrogen cracking in ferritic steels only occurs when a critical combination of the four basic factors involved is exceeded. The four basic factors have to combine in the very narrow band to cause embrittlement.

Hydrogen Generation

Now that we have a basic understanding of how atomic hydrogen can be generated, let’s delve into a little more detail.

Figure 2
Figure 2

Low Temperature Hydrogen Generation

Under most conditions and at low corrosion rates, molecular H2 forms at the surface of the steel, and it harmlessly dissipates into the surrounding process environment. However, when sulfide scale is present, the sulfide acts as a negative catalyst and discourages the reaction of two atomic hydrogen molecules joined to become molecular hydrogen.

H+ + H+ → H2

As a result, the atomic hydrogen penetrates the steel, accumulating in the crystal structure and affecting the steel’s mechanical properties. Compounds, such as sulfide, cyanide (HCN), phosphorous, antimony, selenium, and arsenate (which are called recombination poisons) also interfere with the conversion of atomic hydrogen to molecular hydrogen. In the presence of a recombination poison, the concentration of atomic hydrogen rises, and a corresponding increase occurs in the amount of atomic hydrogen diffusing into the metal.

Hydrogen damage in wet H S service is caused by the generation of atomic hydrogen as a by-product of the corrosion reaction, and the subsequent diffusion of the atomic hydrogen into the steel. Atomic hydrogen (H+) and molecular hydrogen (H2) are produced in the corrosion reaction of steel with aqueous H2S as follows:

Reaction 1: Fe + H2S → . FeS + 2 H

Reaction 2: 2 H+ . → H2

As shown in Reaction 1, atomic hydrogen is generated. Since sour waters from both the Catalytic Cracking unit (FCCU) and the Delayed Coker Unit (DCU) both contain the recombination poison cyanide, these waters are prone to produce an increased amount of atomic hydrogen, and therefore promote the increase of the amount of atomic hydrogen diffusing into the metal. Now Reaction 2 continues because the hydrogen naturally wants to exist as H2, but not nearly as fast as it normally would if the recombination poisons were not in the picture. (Reference: Figure 2) The cyanide can be thought of as a barrier that prevents the atomic hydrogen from combining. As a result, the atomic hydrogen is available to migrate through the steel.

Figure 3
Figure 3

Figure 4
Figure 4

High Temperature Hydrogen Generation

It is well known that generation of hydrogen at high temperatures comes from the two sources that were mentioned above. They are:

  1. Welding – wet electrodes will charge the steel with hydrogen, or
  2. Service at high temperatures - a small amount of hydrogen gas will dissociate to form atomic hydrogen that can dif- fuse into the steel.

Welding – The level of hydrogen in weld filler metal is low enough to preclude adverse effects in the welds, but greater quantities of hydrogen can be present in the weld region from the breakup of water/moisture (H2O) in hygroscopic welding fluxes or from adsorption on metal surfaces if the welding fluxes and surfaces have not been properly dried before weld deposition. The result is that the H2O will now form molecules of Hydrogen and Oxygen.

Service – Several refinery units operate in hot hydrogen service and high pressures. These units are hydrotreaters, reformers, and hydrogen plants. It is in these units, hydrogen is part of the process, and as a result of the process itself, the hydrogen atom is generated. Where the hydrogen atom exists, and there is adequate temperature and pressure, the atomic hydrogen has the opportunity to leave the process and migrate into the steel.

Weld Cracking as a Result of Hydrogen Charged Material

Okay, so the ferritic steel is hydrogen charged; this is what happens when it is welded. Trapped hydrogen from whatever source presents a threat to the integrity of the weld, due to the large temperature gradients prevailing within the weld. Thermal energy from the weld releases hydrogen from existing traps in the metal. The hydrogen then migrates toward the weld pool because of hydrogen’s substantially increased solubility and increased diffusivity at increased temperatures. The escape of weldment hydrogen into the adjoining base metal, heat affected zone (HAZ), and the atmosphere is slow in comparison to its relatively rapid rate of cooling, particularly for weldments of more than 1 inch (25.4 mm) thick. At points within the weld metal and HAZ, particularly those under high stress, the hydrogen content will increase for a period of time because of stress-assisted diffusion. Consequently, within one or two hours of welding, a large weldment can contain hydrogen at concentrations far exceeding its low solubility at ambient temperatures, leading to hydrogen stress cracking or hydrogen embrittlement.

Prevention / Mitigation

Pre-weld hydrogen bake-outs (dehydrogenation) are considered a necessary measure to eliminate hydrogen from steel that has been subjected to the uptake of diffusible hydrogen during prior service. The question is, what temperature is required and how long should it be held to remove all of the atomic hydrogen? Embrittlement of the charged steel can be avoided by “low-temperature” heat treatment once the com- ponent is removed from the hydrogen-generating source. Molecular hydrogen (H2) trapped in steel cannot be removed unless very high temperatures are used.

Traditional methods for bake-outs vary, but the typical procedure is to “bake” out residual atomic hydrogen in the steel by heating it to 400-600°F (204-315°C) and holding for 2-4 hours, depending on the thickness of the material that the severity of the exposure. Bake-out temperatures up to those required for full post weld heat treatment (PWHT) may be used for holding times shorter than specified for PWHT.

So for the purpose of eliminating hydrogen from the ferritic steel to make repairs, let’s look at and address what may, and may not, need hydrogen bake-out. One easy way to determine if the steel will crack is to simply run a weld bead and wait 24 hours to evaluate with NDE and determine if the material is subject to cracking. If the ferritic steel is hydrogen charged from the process itself (not from welding), then there are a couple of things that will determine the need for bake out:

  1. Is the steel in hydrogen charging service, as defined in either the Low Temperature or High Temperature service previously described? If not, then no bake-out required.
  2. If the steel is in hydrogen charging service, we now have several decisions to make to determine our course of action; decisions that are primarily based on material type and thickness.
  • ASTM A-106 piping less than 1⁄2 inch (12.7mm) thick is unlikely to be recommended for bake-out, even if it is in wet H2S service. There simply has not been an issue with this in the industry, because it rarely accumulates enough hydrogen to become a problem.
  • Welded piping made from plate, such as A-516 piping is different. This material is a little “dirtier”, and as a result, a bake-out is recommended.
  • In general, always plan bake-out thick walled vessels, or at least build that step into the repair plan. NOTE - Maintenance will like it a lot better if it is built into the plan, rather than adding this time-consuming step during the repair.

Thin walled vessels, on the other hand, require more of a judgment call. The questions become are they thin enough and are they in mildly hydrogen charging service? Different sour waters have different propensities to promote hydrogen charging.

So now that we have determined that we need to bake-out, take a look at the specifics in this chart. This was developed using Fick’s Law to calculate the rate of diffusion. There are still a few uncertainties and a few assumptions in this “one size fits all” chart, but it will put the reader in the range where needed.

Notice that increasing from 600°F to 800°F (315 to 426°C) shortens the required time. This is because the thermal energy releases hydrogen. Increased temperature increases hydrogen solubility and diffusivity. Notice how this chart starts at 600°F (315°C), and yet the prevailing thought is to use 400°F (204°C) “or higher”. There is no doubt that heating to 400°F can do some good, but heating to higher temperatures better insures that sufficient hydrogen has been removed so the repair can be made.


Hydrogen bake-out in preparation for weld repairs is not always needed. When it is recommended, the traditional “standard” of 400-600°F (204-315°C) will not necessarily achieve the desired results. By elevating the metal temperature up to 600 – 800°F (315 – 426°C) degassing is accomplished. At 800°F (425°C), the hydrogen will move through the steel at almost twice the rate of 600°F (315°C). 

Rolled plate such as 516 that is formed into a pipe is a candidate for bake-out when used in sour service. However, the thinner it gets, the less likely it needs this hydrogen removal.

Always remember that hydrogen cracking in ferritic steels only occurs when a critical combination of the four basic factors involved is exceeded. When you eliminate the hydrogen content out of the equation, your material will not crack. Figure 1 illustrates the four basic factors required for cracking.

  1. Hydrogen content,
  2. material - Susceptible microstructure (hardness, grain size)
  3. stress, and
  4. temperature.

As a final comment, some people suggest that when in doubt, run a test bead.

Continue to the next article in this series.

Comments and Discussion

Posted by anonymous on June 12, 2013
I've been involved in creating a lot of... Log in or register to read the rest of this comment.

Posted by Tim Rudd on June 18, 2013
We've had a lot of good experience using... Log in or register to read the rest of this comment.

Posted by Marc McConnell on July 9, 2013
Tim, you really bring up a good point. I should... Log in or register to read the rest of this comment.

Posted by Krishna Prasad Jonnalagadda on July 8, 2015
All said and done can this tests simulate site... Log in or register to read the rest of this comment.

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