How to Avoid Hydrogen Embrittlement Issues in High-Strength Fasteners

When placing your order for high-strength fasteners, you’re worried about several things. Order minimums, shipping costs and delivery times just to name a few. But if you’re thinking ahead, you’re also concerned about hydrogen embrittlement issues. 

Fasteners play a critical role in OEM, agriculture, military, automotive and distribution markets. However, they’re also known for their susceptibility to hydrogen embrittlement (HE). If you’ve been in the industry long enough, you’ve run across it. 

First discovered in the late 1800s, the industry has been taking steps to reduce its impact. But HE still causes problems, and is an important concern for joint designers, fastener manufacturers and application engineers.

What is Hydrogen Embrittlement?

Also known as hydrogen-induced cracking or hydrogen attack, HE is the process by which metals such as steel become brittle and fracture due to the introduction and subsequent diffusion of hydrogen into the metal. The fasteners then fail when placed under high load.

For hydrogen to do damage, it must enter steel in its atomic form, typically introduced during production at pickling, cleaning, electroplating, or phosphating. However, it can also come from the environment, normally from corrosion.

The progression of hydrogen embrittlement is fairly straightforward. Hydrogen enters the fastener and is diffused to high-tensile stress regions such as grain boundaries, inclusions, dislocations and other traps. Then once the hydrogen concentration reaches a critical value, cracks develop.

Some common characteristics of HE failure are:

• Occurs with high-strength steel components
• Components subjected to protective coatings such as Zinc electroplating are more sensitive
• Parts in contact with acid during manufacturing or service
• The fasteners hardened to at least Rockwell Rc 39 may subject to HE failures. Unhardened fasteners never suffer from hydrogen embrittlement.
• The appearance must be that of an “inter-granular” failure.

All metals can be affected by HE, but some are more impacted than others. High-strength steels, titanium and aluminum alloys are the most affected.

Types of Hydrogen Embrittlement

There are two types of hydrogen embrittlement: internal and external. 

Internal HE happens at the point of production and is caused by residual hydrogen emanated from processing and manufacturing methods. The production process literally instills the hydrogen into the metal.

The environment the fastener operates in can play a role in HE because of the amount of hydrogen present. External HE is when the environment instills hydrogen into the metal causing corrosion. Situations such as cathodic protection, electroplating, or arc weld exposure can accelerate HE. 

An Ounce of Prevention

At Auto Bolt, we take hydrogen embrittlement seriously. Our processes and materials are chosen with great care to mitigate the impact of HE. Some methods to avoid hydrogen embrittlement are:

• Selection of the coating for the raw material. If possible, phosphate coating should be avoided for high strength fasteners (Hardness > RC 39). Phosphate has been associated with associated with hydrogen embrittlement in high strength fasteners.
• If phosphate is used for a high strength fastener it must be removed prior to heat-treat.
• Following hydrogen embrittlement guidelines, such as baking from organizations such as the ASTM Committee BO8 on Metallic and Inorganic Coatings or the ISO Technical Committee ISO/TC2, Fasteners, Subcommittee SC 14 Surface Coatings.
• Selection of another coating, such as a phosphate or zinc flake coating, which has a very low risk for hydrogen embrittlement.

If you have specific questions or concerns about HE, please don’t hesitate to contact us.