Hydrogen Embrittlement: What It Is and How to Combat It
How Hydrogen is Introduced During Zinc Plating
There are two stages that could result in hydrogen entering the substrate of steel during zinc electroplating: the cleaning stage and the zinc plating bath. In the beginning of the zinc electroplating process, product is first subjected to a muriatic acid, otherwise known as hydrochloric acid, where hydrogen atoms and chloride atoms create chemical reactions to clean the product and remove surface contaminants. This process activates the steel surface to better accept zinc. The main ingredient here is hydrogen. The second stage that introduces hydrogen when parts are in the zinc plating bath. The application of zinc is an electrochemical reaction which allows for hydrogen ions to dissolve into steel in a process called electrolysis.
Within both the acid cleaning and plating stages, product is submersed into heated tanks which expands the material and allows hydrogen ions to impregnate into the surface at a molecular level, diffusing into microscopic traps in the substrate. As product leaves the heated tanks and enters cooled rinses, these traps contact and capture the hydrogen ions, locking them into the substrate. This weakens the overall steel component and introduces brittleness.
Not every part is susceptible to hydrogen embrittlement; it depends on the hardness level of the steel. The hardness of a part is synonymous with its tensile strength, otherwise described as how compact the material is. The more compact or hard a part, the smaller the microscopic traps in the substrate and the stronger the overall part. Product that is of a low hardness level are less compact and hydrogen can move around more freely without compromising the integrity of the steel. In general, steel with a tensile strength of 1000 MPa or HRC value of 32 or lower is not considered susceptible to hydrogen embrittlement, but the stronger the steel, the more susceptible it is to hydrogen embrittlement.
How to Combat Hydrogen Embrittlement
To eliminate hydrogen embrittlement there is a process called hydrogen embrittlement relief which involves “baking” the product. In this process, steel is heated to a certain temperature and held at this temperature for a certain amount of time to relieve hydrogen from the microscopic traps within the substrate. Similar to when product enters heated tanks in the plating process and hydrogen enters the substrate, heating the product again in the baking process allows the traps to expand and hydrogen ions to diffuse out of the product. For this relief to occur, though, there is a specific temperature and amount of time that steel must be held at; every specification has different requirements depending on the tensile strength/hardness of product and material type, but it is most common to bake product at a temperature of 400 degrees for a minimum of four hours.
The hydrogen embrittlement bake oven for many platers resembles a large industrial oven; suppliers open a door, place a heaping pile of steel parts in the middle of the oven, close the door, set the temperature, and wait four hours for the bake process to be complete. However, we at Plateco have found that this is not the most ideal way to relieve hydrogen for two reasons:
- Product is not actually held at 400 degrees for a full consecutive four hours. Instead, these four hours include bringing the parts up to temperature as well as a cooling off period.
- Product is put into a heaping pile that does not allow for proper airflow to evenly distribute heat. It is similar to baking a cake; the edges are always finished before the middle.
At Plateco, we handle the hydrogen embrittlement relief process a little different than our competition. Instead of a large industrial oven with doors, we use a conveyor belt system that slowly moves product through our oven over an eight-hour period. These eight hours include bringing the parts up to temperature, holding them at 400 degrees for four hours as well as a cooling down period. Also, the opening to our oven is less than a foot tall to ensure we evenly distribute parts across the conveyor belt, allowing for proper air flow and an even distribution of heat. For quality control, we verify temperature zones by running a calibrated temperature gage through our bake oven on a bi-annual basis to ensure that we are bringing product up to temperature and holding them at temperature for the specified amount of time.
The Quality Issue with Baking Product
While many zinc specifications require a hydrogen embrittlement relief bake to ensure the integrity and strength of steel products, there is one downfall. By heating product after zinc plating, there is a risk of dehydrating the passivate topcoat and causing premature white corrosion.
We will discuss this topic and how to combat premature white corrosion in our next Plateco Pointer, “Plateco, Inc. Case Study: To Passivate or Not to Passivate.”