Hydrogen Embrittlement

Wednesday, 1 May 2024

Steel fasteners exposed to hydrogen can fail prematurely at a stress level well below the fastener’s yield strength.
Hydrogen Embrittlement occurs in fasteners as a result of their being exposed to Hydrogen, usually during Pickling and subsequent Electroplating.
Hydrogen is generated during the cleaning and acid Pickling cycles, which is followed by sealing it in the steel by Electroplating.  On the other hand while Hydrogen can still be absorbed during acid Pickling, Hot Dip Galvanising temperatures rapidly expel it.  This accounts, at least in part, for Zinc Plated fasteners failing more often than Hot Dip Galvanised.  Mechanically Galvanised fasteners seldom suffer as the “Zinc” is porous, preventing Hydrogen entrapment.  
There are three common causes of Hydrogen Embrittlement:
    Hydrogen, after absorption, reacts with the carbon in steel to form methane gas, which can lead to crack development and therefore strength reduction.
    Hydrogen can also react with alloying elements such as Titanium, Niobium, or Tantalum to form Hydrides.  Because the hydrides are not as strong as the parent alloy, they reduce the overall strength of the fastener.
    Hydrogen can remain in solution between lattices in the grain structure and can cause delayed failures after proof testing.
Hydrogen Embrittlement is an insidious type of failure as it can occur without an externally applied load, or at loads significantly below yield stress, and the failure is often delayed.  Higher strength steels are more susceptible to Hydrogen Embrittlement than lower strength steels.  As a rule of thumb, steels below Rockwell C 30 are considered to be far less susceptible, while steels above Rockwell C 35 are considered to be seriously at risk.  
How does this relate to Fasteners?
Basically, normal care with Pickling and Electroplating, or Hot Dip Galvanising, means that Metric Classes 4.6, 4.8, 5.8, 8.8; and above can be plated, but subsequently “baking” to expel Hydrogen should always be applied.  The baking must commence quickly after plating.  ASTM B 850-94 recommends Class 10.9 & SAE Grade 8 Fasteners be baked at 215 degrees Celsius for 8 hours, and Class 12.9 and above SAE Grade 8 for 10 to 12 hours.  These times are well above what has been normally considered adequate.
As a rule of thumb, very high tensile fasteners should only be accepted if plating has been arranged by the manufacturer.