Introduction

In many technical and engineering applications, it is of particular importance to understand the effects and mechanisms of hydrogen action. Atomic hydrogen produced through cathodic reactions penetrates metals and causes them to crack and fracture. The presence of hydrogen in steel affects its mechanical properties and generally reduces ductility and increases the tendency to brittle fracture and crack formation in the metal. Hydrogen reduces the load-bearing capacity of the metal, although this phenomenon is common in carbon and low-alloy steels, many metals and alloys are sensitive to hydrogen. Hydrogen damage is a general term that refers to mechanical damage to a metal caused by the presence or reaction with hydrogen. Hydrogen causes metal failure and hydrogen damage in various ways, including the following:

1. Hydrogen blistering
2. Hydrogen embrittlement
3. Decarbonization
4. Hydrogen attack

A brief description of each of the hydrogen damage is given below.

Hydrogen blistering

Blistering is caused by hydrogen penetration into the metal, which usually results in local deformation in the form of blisters on the metal. Hydrogen blistering can cause metal destruction in certain cases.

Atomic hydrogen is very small in size and easily penetrates into metals, and its penetration is even significant at low temperatures. The source of hydrogen can be from corrosion in acids, pickling and cleaning operations, electroplating, etc. Suppose we have an environment that contains hydrogen ions, and these hydrogen ions, which have high permeability, can be reduced to hydrogen on the metal surface. Now, if there is a hole in the metal as a defect, a hydrogen atom can enter the hole and form molecular hydrogen.

When atomic hydrogen is converted to molecular hydrogen, it can no longer leave the hole and this process is repeated continuously. Hydrogen gas is formed inside the hole and the pressure inside the hole increases, this pressure can rise to the point where it destroys the metal. The gas can produce an excess pressure of up to 300 atmospheres, which stretches the metal and creates blisters on the surface. Therefore, the production of molecular hydrogen inside the steel creates a lot of pressure in that area, which ultimately leads to blistering or destruction of the steel and hydrogen damage.

Hydrogen embrittlement

As mentioned, atomic hydrogen is the only element that can penetrate into the metal or steel, but molecular hydrogen cannot. Hydrogen embrittlement, which is a type of hydrogen damage, causes brittleness and a decrease in the flexibility and strength of the metal, which often occurs in steel-containing stresses.

The solubility and permeability of hydrogen decreases sharply with decreasing temperature. Therefore, when a large section of hydrogen-containing steel is cooled from high temperature to ambient temperature, hydrogen penetrates the steel and remains in the form of a gas. The pressure of hydrogen gas is often sufficient to cause internal cracking.

Hydrogen may be formed on the surface of the metal in aqueous environments and during corrosion reactions. Some of the atomic hydrogens on the metal surface combine to produce hydrogen gas that enters the environment, but another part of the atomic hydrogens penetrates into the metal and, if the metal is under tensile stress, hydrogen embrittlement occurs.

Cracks that occur as a result of hydrogen embrittlement may be intergranular or intergranular, and it is very difficult to distinguish whether stress corrosion cracking or hydrogen embrittlement has occurred.

The presence of various chemicals in the environment has a great effect on hydrogen embrittlement. For example, sodium chromate slows down the absorption or diffusion of hydrogen, while the presence of sodium silicate increases the absorption of hydrogen.

Note: Hydrogen blistering and hydrogen embrittlement usually occur at ambient temperature and are very similar to each other. Hydrogen blistering is more common in low-strength steels and hydrogen embrittlement in high-strength steels.

Decarbonization

Decarbonization of steel is often caused by contact of wet hydrogen with the metal at high temperatures, and as a result of this phenomenon, the tensile strength of the steel decreases.

In most cases, hydrogen gas is accompanied by water vapor, which can cause decarbonization of steel according to the following reaction.

H2+CO→ CFe+H2O

The carbon in the steel reacts with water vapor and produces hydrogen and carbon monoxide. The speed and extent of this reaction depends on the activity of the carbon in the alloy and the ratio of water vapor, carbon monoxide, and hydrogen in the environment.

Whenever steels are exposed to hydrogen at high temperatures, the following reaction occurs.

CH4+O2→CFe+2H2O

In this case, the carbon in the steel reacts with water to produce methane. Since atomic hydrogen easily and quickly penetrates steels, it causes cracks in the internal parts of the metal. By adding chromium and molybdenum to steels, their resistance to decarburization and cracking can be increased.

Hydrogen attack

This type of corrosion refers to the reaction between hydrogen and one of the alloying elements or components of the metal at high temperatures.

Methods for preventing hydrogen damage

Hydrogen blistering can be reduced by using one or more of the following methods:

1. Use of clean steel: Frame steels have many pores, so replacing them with dead steels that are free of pores will increase resistance to hydrogen blistering.
2. Use of coatings: To prevent hydrogen blistering, metallic, inorganic and organic coatings are used.
3. Use of inhibitors: Inhibitors can reduce the corrosion rate and the rate of hydrogen ion reduction, thus preventing hydrogen blistering.
4. Removal of harmful substances (hydrogen producers): Hydrogen blistering often occurs in corrosive environments that contain harmful substances for hydrogen production such as sulfides, arsenic compounds, cyanides, etc. and rarely occurs in pure acidic environments. Therefore, in oil refining operations, such harmful substances are present and cause hydrogen blistering.
5. Alloy change: Nickel-containing steels and nickel-based alloys are less resistant to hydrogen penetration and these alloys are often used to prevent hydrogen blistering.

Hydrogen embrittlement can be reduced by using one of the following methods:

1. Use of inhibitors: Hydrogen embrittlement often occurs in pickling operations where corrosion releases hydrogen. Therefore, by using inhibitors carefully, the corrosion of the metal in the pickling stage can be reduced and, as a result, the penetration of hydrogen into the metal can also be reduced.
2. Annealing: Annealing can be used to remove hydrogen from steel. For steel, three hours of baking at a low temperature of 175 degrees Celsius is usually sufficient.
3. Alloy change: High-strength steels are more sensitive to hydrogen embrittlement. Therefore, by adding nickel or molybdenum to steels, their sensitivity to hydrogen embrittlement can be reduced.
4. Correct welding: If hydrogen embrittlement occurs in welding operations, high-quality and suitable electrodes should be used in welding. It is important to keep the weld area and surroundings dry during welding operations. Because water and water vapor are the main factors that create hydrogen.