Hydrogen corrosion: Under what conditions does it take place?

In the corrosion of ferrous metals, two different reactions can occur: oxygen corrosion and hydrogen corrosion. Both reactions take place under different conditions. What are the basic conditions for the expiration of hydrogen corrosion, and what causes them, you can read here.

Requirements for hydrogen corrosion

When oxygen in the form of air and water is available in the vicinity of metals, especially ferrous metals, the electrochemical reaction is usually oxygen corrosion, which oxidizes the iron to rust.

However, if the oxygen is missing, the so-called acid corrosion or hydrogen corrosion takes place instead. The end product is pure hydrogen, the metal is oxidized. However, the second part of the redox reaction is the reduction of oxonium ions to hydrogen. The metal goes into solution as ions. This leads to a removal of the material in a nearly uniform amount

Hydrogen corrosion always occurs when there is a lack of oxygen. Both reactions, both hydrogen and oxygen corrosion, are responsible for the formation of rust.

Hydrogen embrittlement

As a further consequence of the reaction, hydrogen embrittlement can also occur. Part of the released hydrogen, which was formed by the redox reaction, now diffuses into the metal lattice of the material.

There it settles preferentially at the grain boundaries of the material, and leads to a kind of chemical material fatigue. Some metals, such as titanium, can also form special metal hydrides that weaken the material. Titanium hydride is a common cause of weakening of the material.

Hydrogen embrittlement in steel

Especially with steel, hydrogen embrittlement is common. It leads there to a significant weakening of the material and also to defects in the material structure. This reduces the strength of the steel and also its corrosion resistance.

Only austenitic steels (chromium-nickel steels) are completely insensitive to hydrogen embrittlement. For all other types of steel so-called delayed brittle fractures are to be feared. They happen suddenly and unexpectedly and also with minimal deformation of the workpiece. The process has a close resemblance to a stress crack.

Tips & Tricks

Unlike oxygen corrosion, hydrogen corrosion is very common in many metals. The erosion of the metal surface occurs in hydrogen corrosion - in contrast to the oxygen corrosion - almost completely uniform.

Video Board: What is stress corrosion? Stress corrosion cracking/Stress cracking: Electrochemical Corrosion