Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:firstname.lastname@example.org
Hydrogen embrittlement can cause catastrophic failure of high strength steel and other alloys. The failure is usually cracking or complete separation. It can also cause blisters both in the basis metal and at the plating interface, reduced ductility, internal voids, and lower yield strength.
What steels are susceptible to hydrogen embrittlement?
In general the higher the tensile strength of the steel the more the susceptibility to hydrogen inclusion and embrittlement of the metal even austenitic alloys are susceptible. Maraging high-strength steels of 2740 MPa (397,000 lb/sq.in.) have higher susceptibility to hydrogen embrittlement.
Where does the hydrogen come from?
From the hydrogen that occurs in plating solutions that are not 100% efficient. Most plating solutions are not 100% efficient. The current that is used to deposit metals also deposit hydrogen. The hydrogen is absorbed into and stays in high strength metals. Electro-cleaning solutions used in the cathodic mode release hydrogen that can be absorbed into the high strength metals being cleaned. Acid solutions release hydrogen. Cathodic cleaning can introduce hydrogen. Corrosion of metals create hydrogen that can be absorbed by high strength metals.
What is wrong with having hydrogen in metals?
Hydrogen migrates to grain boundaries causing weakening of the metal that can result in catastrophic failure (cracks, breakage or weakening of metals that can break under normal use where the strength is necessary.) aircraft components, for example.
One of the most likely to create hydrogen embrittlement is the use of acid dips. For example, experiments by Paatsch (Pl. & Surf. Fin., Sept. 1996, pgs 70-72) using ultra high strength steel fuse holder rings of C75 (German standard DIN 471) and were plated in various plating solutions and under a number of different preparation cycles have shown that using 60 seconds or more pickle time failed at time intervals up to 24 hours heating at 220C (427.5F) regardless of the plating solution used. Some samples required 70 hours heat treating, after plating, to pass the test. The samples that used less than 60 seconds of inhibited HCl pickle showed no failures at all regardless of the type of plating solution used.
Electroless Nickel deposits
One exception to relief of hydrogen embrittlement by baking is the use of electroless nickel. Deposits that cover the entire surface require extended baking to relive the hydrogen because the hydrogen will remain in the metal because EN has no grain boundaries that would allow hydrogen to escape during the baking period. Hydrogen would have to diffuse through the EN deposit unless there are un-plated areas. Baking times for complete relief may be two to three times longer to achieve relief of the hydrogen entrapped by the plating process.
Why does hydrogen act this way?
Dons Theory: I think that hydrogen, as an atom (H), needs to have another electron to satisfy its only orbit. This great desire causes two hydrogen atoms to get together to share the second electron (H2) partially satisfying the need for 2 electrons. When hydrogen is delivered to a metal alloy the metal can offer an electron. In stressed areas there are more electrons available to attract hydrogen to the crystal boundaries.
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