5 Ways to Prevent Hydrogen Embrittlement in Your Plating Process

Hydrogen embrittlement (HE) is often described as the silent killer in the metal finishing industry. Unlike surface rust or blistering, which are visible to the naked eye, hydrogen embrittlement operates invisibly within the microscopic structure of the metal. It can cause a catastrophic, delayed failure of high-strength steel components without any prior warning signs. For manufacturers and finishers, understanding and mitigating this risk is not just a matter of quality control; it is a critical safety imperative.

If you are dealing with high-stress applications and need expert guidance on developing a safe, reliable finishing process, contact our technical representatives to discuss tailored solutions that mitigate embrittlement risks. In the meantime, this guide will walk you through the mechanisms of hydrogen embrittlement and provide five concrete ways to prevent it in your plating process.

What is Hydrogen Embrittlement?

Hydrogen embrittlement is caused by the incorporation of atomic hydrogen into the grain structure of metals. During various stages of the manufacturing and finishing process, free atomic hydrogen is generated at the surface of the metal part. Because hydrogen is the smallest element in the universe, it easily diffuses into the solid metal lattice.

Once inside, these hydrogen atoms migrate towards areas of high internal stress, such as grain boundaries, voids, or micro-cracks. This increases internal stress leading to a weakening of the metal structure, significantly reducing its ductility and load-bearing capacity. When an external load is applied in the field, this weakened structure simply fractures under stress levels far below the material's designed yield strength.

It is crucial to note that metals with higher strength are more susceptible to hydrogen embrittlement. A part that has been hardened, tempered, or cold-worked possesses higher internal stresses by default, making it the perfect breeding ground for hydrogen-induced trapping and subsequent failure.

The Sources of Hydrogen in Plating

Embrittlement does not happen by accident. Hydrogen embrittlement can occur in any process where atomic hydrogen is formed at the metal surface. The electroplating line is rife with such opportunities. The primary culprits typically include:

• Acid Pickling: Often considered the major culprit of introducing free hydrogen to steel. The chemical reaction between the acid and the iron substrate directly liberates hydrogen gas.

• Cathodic Electrocleaning: When a part is made the cathode in an electrolytic cleaning bath, hydrogen gas is evolved vigorously at its surface to create a scrubbing action.

• Electroplating: Similar to electrocleaning, the cathodic reduction of metal ions (like zinc) is almost always accompanied by the side reaction of hydrogen evolution.

• Conversion Coating: Certain phosphating and passivation processes can also generate hydrogen, albeit typically in smaller quantities.

Strategic defense against Hydrogen Embrittlement

1. Material Selection: Stop It Before It Starts

Hydrogen embrittlement prevention starts before the plating process even begins. The most effective way to eliminate the risk of embrittlement is to remove the susceptibility of the substrate.

When high-strength materials are absolutely necessary, you must acknowledge the risk and plan for it. While the exact hardness value is debated among metallurgists, international standards are clear. Both ISO 9587 and ASTM B850 recommend hydrogen relief treatment for any material with a Rockwell C Hardness (HRC) value above 31. If you are plating high-tensile fasteners or springs that exceed this hardness threshold, relief procedures are mandatory, not optional.

2. Optimize Pre-Treatment

The pre-treatment line is where the battle against hydrogen often begins. As noted earlier, there are two potential major sources of hydrogen embrittlement in the pre-treatment processing: the electrocleaner and the acid pickle.

Because acid pickling is the major culprit of introducing free hydrogen to steel, extreme caution is required. You must limit acid exposure as much as possible for high-strength parts.

• Inhibitors: Always utilize high-quality acid inhibitors. These chemicals slow down the attack on the base metal and reduce hydrogen evolution while still allowing the acid to remove rust and scale.

• Alternative Cleaning: Consider mechanical descaling methods, such as abrasive blasting or tumbling, to remove heavy scale before the chemical line. This minimizes or completely eliminates the need for prolonged acid pickling.

• Anodic Cleaning: Where possible, switch from cathodic to anodic (reverse) electrocleaning for sensitive high-strength steels. Anodic cleaning evolves oxygen at the part surface instead of hydrogen, eliminating that specific source of embrittlement entirely. To explore these options deeply, review our available pre-plate solutions specifically formulated for sensitive substrates.

3. Coating Material Selection

An often-overlooked aspect of preventing embrittlement is the physical structure of the plated layer itself. Some coatings can physically trap hydrogen inside the base metal, acting as an impermeable barrier. Others are better at allowing hydrogen to escape during the subsequent relief baking process.

The plated-metal's grain structure can affect the ability for hydrogen to escape, providing a better coating where hydrogen embrittlement is a high risk. This is significantly influenced by the chosen plating bath chemistry.

For example, the columnar grain structure of plated alkaline Zn-Ni coatings is more amenable to hydrogen relief than the lamellar structure of acid-chloride Zinc deposits. The columnar structure provides microscopic pathways that allow atomic hydrogen to effuse out of the steel substrate when heat is applied. In contrast, the overlapping, tight lamellar structure acts as a lid, trapping the hydrogen within. Therefore, when specifying a finish for critical high-strength components, transitioning to specialized plating technologies like alkaline zinc-nickel can significantly improve the success rate of your embrittlement relief efforts.

4. Optimize the Plating Bath Efficiency

The relationship between plating efficiency and hydrogen evolution is direct and inverse. In an electroplating bath, the electrical current (electrons) supplied by the rectifier does two things: it reduces metal ions to form the deposit, and it reduces water (or acid) to form hydrogen gas.

Maintaining high bath efficiency is an excellent preventative strategy. Keep the bath chemistry perfectly balanced, control the temperature tightly, and ensure optimal metal concentrations. Using brighteners and additives correctly ensures the current goes toward building the coating rather than pumping your high-strength fasteners full of destructive hydrogen gas.

5. Mandatory Post-Plate Hydrogen Relief Baking

While the first four steps aim to prevent hydrogen from entering the steel, the reality of electroplating is that some hydrogen absorption is inevitable for high-strength materials. The final, non-negotiable step is getting that hydrogen back out before it causes damage.

This is achieved through a controlled, prolonged heating process known as Hydrogen Relief Baking. The heat increases the mobility of the hydrogen atoms, allowing them to diffuse out of the steel lattice and escape through the plated layer before they can migrate to areas of high stress and form damaging gas pockets.

• Timing is Everything: To be effective, the parts must go into the baking oven within a strict timeframe after the plating process is completed—typically within one to three hours. Waiting too long allows irreversible micro-cracking to begin.

• Do Not Passivate First: Never apply a passivate, chromate, or topcoat before the relief bake. These films act as additional barriers, sealing the hydrogen inside the part. Parts must be plated, dried, baked, and only then returned to the line for post-plate finishing. If you need robust passivates after the bake, modern post-plate solutions can be applied over the aged zinc surface.

• Temperature and Duration: Follow the guidelines established in standards like ASTM B850. The specific temperature (usually around 190°C - 220°C) and the duration (ranging from 3 to 24 hours) depend entirely on the tensile strength of the steel and the thickness of the coating.

Ensuring a Strong Finish for Your Plated Components

Hydrogen embrittlement remains a formidable threat in the metal finishing industry, capable of turning high-strength components into fragile liabilities. However, by understanding the mechanisms of failure and the sources of hydrogen evolution, finishers can implement robust, multi-layered defense strategies.

By making smart material selections, minimizing acid exposure during pre-treatment, choosing coatings with amenable grain structures like alkaline zinc-nickel, maximizing bath efficiency, and executing mandatory, timely relief baking, the risk of embrittlement can be virtually eliminated.

Do not leave the safety of your components to chance. Implement these five strategies rigorously, document your processes carefully, and ensure your team understands the critical importance of hydrogen management.

For the chemistries and expertise needed to support these preventative measures, explore our products to find reliable, high-performance solutions for your exact specifications.