The corrosion protection offered by zinc or zinc alloy layers can be considerably improved by passivating the products. In the classical form of this follow-up treatment, an extremely thin chromate layer, approximately 0.1 µm, is formed. This layer seals the pores in the zinc or zinc alloy layer and also binds oxygen to it.
Traditional chromate layers often contain hexavalent chromium, Cr(VI), and therefore possess a unique self-healing property. In the event of mechanical damage, soluble hexavalent chromium salts within the passivation layer repair, or re-passivate, the exposed areas.
The downside of this post-treatment is that Cr(VI) is considered hazardous and environmentally unfriendly, as it is toxic and carcinogenic. Its use in the EU is limited, and Cr(VI) has been gradually banned by various regulations, including RoHS and ELV. Details about chromate conversion coatings can be found in the international standard ISO 4520.
Depending on the thickness and composition of the passivation layer, the colour can vary from transparent, also known as white, through blue and yellow to black.
White or Blue Passivation
This is the most common type for fasteners. It offers low corrosion protection and is therefore advised for indoor applications.
Black Passivation
Black passivation provides the same level of protection as white or blue passivation and is typically chosen for its black colour. Traditional black passivation may contain Cr(VI).
Yellow Passivation
This type of passivation provides much better corrosion resistance and has commonly been recommended for outdoor applications. However, it continues to decline in popularity due to its significant Cr(VI) content.
Olive Green Passivation
Olive green passivation types are mainly used for military applications. Their corrosion resistance is comparable to, or slightly better than, yellow passivation. Traditional versions may also contain Cr(VI).
Trivalent Chromium Cr(III) Passivation
As a result of hazardous substances legislation such as RoHS, REACH and ELV, a new type of passivation layer without Cr(VI) had to be developed. Instead of Cr(VI), which is present in traditional black, yellow, brown and green chromate layers, most newly developed passivation layers use trivalent chromium, abbreviated as Cr(III).
Some Cr(III) passivation types can offer better corrosion resistance than Cr(VI) passivation. They are often referred to as thick-layer passivation. A thin layer may measure around 0.08 to 0.1 µm, while a thick layer is approximately 0.2 to 0.3 µm.
Transparent thin-layer passivation is most commonly used. Thick-layer passivation is often iridescent, appearing bluish-yellowish-greenish on zinc layers and yellowish-greenish on zinc alloy layers. It offers superior corrosion resistance compared with Cr(VI) yellow passivation. To further improve corrosion resistance and/or enhance the appearance of the coating, a sealant can also be applied.
The electroplating process uses electricity to precipitate zinc or zinc alloys. The current also causes the water in the bath to partially electrolyse into hydrogen and oxygen.
The oxygen disappears from the liquid in the bath, but the hydrogen ions may diffuse into the fastener material and bind to form hydrogen molecules. This process is accompanied by an increase in volume, which causes high stress in the metal structure. In the presence of external tensile forces, this stress can lead to delayed and spontaneous brittle fractures.
Hydrogen embrittlement can also be induced by pickling, which is used in the hot-dip galvanising process, when inhibitors are not used. It can also result from unskilled quenching and tempering of steels with high mechanical properties.
Products Most at Risk
The danger of hydrogen embrittlement applies mainly to products with one or more of the following characteristics:
- Tensile strength ≥ 1,000 MPa
- Hardness ≥ HV320
- Case-hardened products
Reducing the Risk
To minimise the risk of hydrogen embrittlement, these products must be reheated, or baked, after the electroplating process for a defined period of time and at a defined temperature. The international standard for electroplated coatings on fasteners, ISO 4042:1999, states that electroplated parts should be baked to a part temperature of 200°C to 230°C within four hours of electroplating, preferably within one hour, and before chromating. The maximum temperature should be determined with consideration for the coating material and the type of base material.
As coating thickness increases, removing hydrogen becomes more difficult. However, introducing an intermediate baking process when the coating is only 2–5 µm thick may reduce the risk of hydrogen embrittlement.
ISO 4042 does not provide exact baking conditions. Eight hours is considered a typical example of baking duration. However, baking durations of 2 to 24 hours at 200°C to 230°C may be suitable, depending on the part type, size, geometry and mechanical properties, as well as the cleaning and electroplating processes used.
For critical components, it is recommended that the temperature and time are determined experimentally. The reheating temperature must never exceed the tempering temperature. The reheating time begins as soon as the products have reached the minimum temperature.
Despite all the care taken during the process, current electroplating techniques can only reduce the risk of hydrogen embrittlement. They cannot eliminate it completely. For critical applications where this risk is unacceptable, another coating method should be chosen, such as zinc flake coatings.
