What is Hydrogen Embrittlement?
Hydrogen embrittlement is an engineering term that refers to a compromise in the tensile strength of a molded metal or alloy due to an infiltration of gaseous or atomic hydrogen. In short, hydrogen molecules occupying the metal react in a way that renders the material brittle and prone to cracking. Obviously, hydrogen embrittlement presents significant problems in terms of being able to rely upon the structural integrity of bridges, skyscrapers, airplanes, ships, etc. In fact, this natural phenomenon leads to a condition known as catastrophic fracture failure and is the direct cause of many mechanical disasters that have taken place on land, as well as in the air and sea.
The process begins with exposure to hydrogen, which can occur while a metal undergoes certain manufacturing processes, such as electroplating. Successful plating relies upon preparation of the metal with an acid bath before it can accept layers of chrome. The electricity used during the “pickling” and plating process initiates a reaction called hydrolysis in which water molecules are broken down into positively charged hydrogen ions and negatively charged hydroxide anions.
Hydrogen is also a by-product of corrosive reactions, such as rusting. Hydrogen decomposition can also be triggered by the very measures taken to prevent it, if improperly applied. For example, hydrogen embrittlement can sometimes be attributed to cathodic protection, which is intended to increase corrosion resistance of coated metal by modifying the hydrogen-vulnerable components of the material. This is accomplished by introducing an opposing current to cause the “sacrifice” of metallic anodes that possess a lower corrosion potential than the metal itself. In effect, the material becomes polarized.
Once hydrogen is present, however, single atoms begin to disperse throughout the metal and accumulate in tiny spaces in its microstructure, where they then regroup to form hydrogen molecules. The absorbed hydrogen, now trapped, begins to seek an escape. It does so by creating internal pressure, which permits the hydrogen to emerge in blisters that eventually crack the metal’s surface. To counteract this process, the metal must be baked within an hour or less after electroplating to allow the trapped hydrogen to escape the layers of plating without creating cracks or stress points.
While hydrogen can invade most metals, certain metals and alloys are known to be more susceptible to hydrogen embrittlement, namely magnetic steel, titanium, and nickel. In contrast, copper, aluminum, and stainless steel are least impacted. However, steel and oxygen-containing copper can become vulnerable to embrittlement if subjected to hydrogen exposure under high heat or pressure. Respectively, these materials are affected by hydrogen attack or steam embrittlement generated by reactions between hydrated molecules and carbon or copper oxides.
Discuss this Article
Post your comments