– Zinc Plating and Hydrogen Embrittlement
Origin of Hydrogen Embrittlement
Zinc electroplated steel is widely used in rigging industry to protect rigging hardware from corrosion. In addition, Zinc coating adds the brilliant shining look to steel components.
The success of Zinc plating attributes to the invention of acid bath. Acid bath technology developed in 1970s. The simplified process of application greatly lowered the cost of Zinc plating. Zinc plating became popular since then.
Hydrogen Embrittlement Definition
However, during the process of acid pickling, a phenomenon called Hydrogen Embrittlement could happen. Hydrogen Embrittlement could cause cracks to the high-strength metal during or after the manufacturing. It is one of the toughest factors affecting the performance of high-strength steel components. Therefore, Zinc electroplating process needs to be handled extra carefully.
Hydrogen Embrittlement Mechanism
When high-strength steel are being formed or finished during the heat treatment process or zinc plating stage, hydrogen, often exists as a diffusible small atom, gets released. Condition of high temperature can cause a rise on hydrogen solubility level. If not treated carefully, too much hydrogen diffuses into the high-strength steel, makes the steel to become brittle and fractured; therefore, causing cracks to the high-strength metal during the processes.
Hydrogen Embrittlement can also be developed after the processes. As time goes by, diffused hydrogen atoms could reassemble and form hydrogen molecules in voids of the metal matrix. With the increases of hydrogen molecules, internal pressure (produced from the cavity where hydrogen lives in) keeps rising as well. Once the structural balance is broken, inner cracks will happen. If this trend continues, the whole component will break apart over time.
Hydrogen Embrittlement Testing
Recently, hydrogen embrittlement has become one of the toughest factors affecting the performance of high-strength steel components. Once inner cracks happen, it is impossible to detect hydrogen embrittlement from outside, early stage hydrogen embrittlement can be only tested with the help of specialized devices. People started to realize the harmfulness of hydrogen embrittlement back from 1875. Despite a large number of efforts done to eliminate the damage caused by hydrogen embrittlement, the complete solution remains unknown.
How to Prevent Hydrogen Embrittlement in Zinc Plating Stage
After carefully examine how hydrogen is generated during zinc plating stage, it is found that:
- Most hydrogen are produced in the process of acid pickling.
- Acid pickling is used to chemically clean the surface of steel before electroplating.
This shows acid pickling playing a key role in causing hydrogen embrittlement. To control hydrogen embrittlement, have to control the process of acid pickling at first. The popular way to do this is to keep the time of acid pickling at a minimum level.
Method 2 – Use Non-Cyanide Instead
Electroplating solution is another major source of hydrogen. In recently years, the most popular eletrolyte used for plating bath is cyanide which is well-known for its affordable price and easy to apply solution. But cyanide has a major issue. The efficiency of chemical reactivity will go down as the consumption of the plating solutions, while hydrogen are released in proportion. Non-cyanide electrolyte was then invented to fix this issue. The efficiency of chemical reactivity can be raised to 95% or above. This greatly reduces the generation of hydrogen.
Method 3 – Baking
Inevitably, there will still be a certain amount of hydrogen accumulates inside the metal with all the the above efforts. Fortunately, the process of invading is reversible. Baking the rigging hardwares at an elevated temperature (around 200 ℃) for 4 – 8 hours, within 2 hours after zinc plating. Condition of elevated temperature could cause hydrogen diffuses out from the parts. This can also help rigging hardware getting rid of Hydrogen Embrittlement.