OVERCOMING THE CHALLENGES OF HYDROGEN EMBRITTLEMENT
WHAT IS HYDROGEN EMBRITTLEMENT?
Hydrogen embrittlement is a chemical process that causes certain metals to become brittle and highly susceptible to fracturing. This occurs when fasteners are subjected to excess hydrogen absorption during the finishing or manufacturing stage or during operational use. Hydrogen atoms are smaller in size than the other atoms that compose the metal. This causes them to relocate into the crystal lattice of the base metal and to get between the individual atoms in the metal. As a result, there is a significant escalation of stress within the base metal which leads to fracturing.
There are two types of hydrogen embrittlement: Internal Hydrogen Embrittlement and Environmental Hydrogen Embrittlement. Internal is caused by introducing susceptible parts to hydrogen. Environmental is caused by galvanic corrosion and stress corrosion cracking.
WHAT FASTENERS ARE VULNERABLE TO HYDROGEN EMBRITTLEMENT?
Hydrogen embrittlement can affect various types of metals but it is most commonly seen in high strength fasteners in environments that contain high levels of hydrogen. These processes include:
- Cathodic protection
- Acid Pickling
- Arc welding
It is critical to take additional provisions to ensure that fasteners in these environments are not being negatively impacted by embrittlement or to use fasteners that are resistant to hydrogen embrittlement by nature. Understanding the need to use high strength fasteners, coating solutions provide a much safer alternative to zinc plating.
HYDROGEN EMBRITTLEMENT SOLUTIONS
Once hydrogen embrittlement sets in and causes fasteners to crack they must be replaced. However, it is also possible to reverse the process of hydrogen embrittlement by eliminating the hydrogen source. This allows the hydrogen in the metal to disperse without harm. This process can be expedited by introducing higher temperatures to the fasteners.
It’s also important to not over tighten fasteners and understand the proof load of the fasteners that are being installed. Over tightening could cause stress cracking which continues to grow until embrittlement takes place.
STOPPING HYDROGEN EMBRITTLEMENT IN ITS TRACKS
There are numerous approaches that can be utilized to prevent hydrogen embrittlement from taking place:
- Heat can be applied to fasteners that were used in processes that produce hydrogen such as chemical or electrochemical procedures.
- Remove hydrogen from any fastener by preheating and post heating.
RIE COATINGS SOLUTON
Avoiding zinc electroplating can significantly reduce any risk of hydrogen embrittlement. Post-baking for relief of hydrogen embrittlement on zinc electroplated fasteners isn’t always effective as hydrogen can still remain trapped under the surface. Using a RIE zinc flake coating system is the safest option in preventing hydrogen embrittlement. The porous nature of coatings allows anything under the surface of the coating to escape during the coating cure process.
The more sacrificial/active a coating is, the more hydrogen is generated during the sacrificial corrosion process. With zinc being the most sacrificial coating, zinc electroplating is a major concern for environmental hydrogen embrittlement. Zinc flake coatings utilize a duplex top coat, creating a barrier between the zinc rich base coat and the substrate the fastener is mated to. The barrier layer significantly slows down the sacrificial corrosion process (white corrosion), reducing risk of environmental hydrogen embrittlement.