Zinc and zinc alloy systems
Automotive approved, hexavalent chromium-free zinc ans zinc alloy coatings for EV, hybrid and ICE powered vehicles
Steel components including fasteners, battery trays, brakes parts and fluid handling tubes need protecting. But how do you choose the optimum coating, achieve consistency in your global supply base and avoid using proscribed materials? Here is how ESI Automotive have helped automotive design and corrosion engineers overcome their issues.
High tensile steels
High strength steel fasteners are replacing mild steel fasteners. As the strength of steels increases, the susceptibility to hydrogen embrittlement (HE)1 increases. High strength steel fasteners (>1000 N/mm²) 10.9 and higher are more susceptible to embrittlement. For hydrogen embrittlement to occur, a combination of three conditions are required;
- The presence and diffusion of hydrogen
- A susceptible material
How does zinc-nickel and passivation help reduce hydrogen embrittlement?
MacDermid Enthone Enviralloy zinc-nickel provides a micro-cracked surface, preventing a local high concentration of hydrogen (it can readily escape). Zinc plated fasteners require a heat treatment (typically 8 hours at 210 °C) to avoid the risk of hydrogen embrittlement. Heat resistant passivation can be applied before this treatment is carried out and the coating still retains its color and corrosion protection.
1 HE also known as hydrogen assisted cracking (HAC) and hydrogen-induced cracking (HIC), describes the embrittling of metal after being exposed to hydrogen.
Joining aluminum surfaces
The use of aluminum components is increasing in lightweight vehicle construction. Using zinc plated fasteners for joining aluminum components will result in galvanic corrosion 2 (contact corrosion). For that reason Enviralloy zinc-nickel is recommended. The low potential difference between zinc-nickel and aluminum alloys reduces the corrosion current when these are joined in component assemblies. In contrast, zinc plated fasteners corrode 5 times faster when in contact with aluminum surfaces.
Coefficient of friction
Additionally, within the component assembly, friction properties are a safety critical safety feature. Specific coefficient of friction targets (as set by automotive manufacturers) have to be met against various material surfaces such as; aluminum, painted steel, steels, and diecast.
For aluminum surfaces; Enviralloy zinc-nickel and the use of Torque and Tension dry film® lubricated sealers are proven to give the desired coefficient of friction properties. This avoids galling and therefore aids optimum tightening of the joint.
To compliment the light weighting efforts optimized regenerative braking 3 increases driving range in electrical vehicles.
In addition to improving the overall efficiency of the vehicle, regeneration can significantly extend the life of the braking system as the mechanical parts will not wear out very quickly. As a direct result of this cast iron brakes as well as brake pads need to be protected from corrosion much longer. Enviralloy zinc-nickel is recommended over zinc as it provides superior long-lasting corrosion protection.
2 Galvanic corrosion (also called bimetallic corrosion or dissimilar metal corrosion) is an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another.
3 Regenerative braking is an energy recovery mechanism that slows down a moving vehicle by converting its kinetic energy into a form that can be either used immediately or stored until needed. In this mechanism, the electric traction motor uses the vehicle's momentum to recover energy that would otherwise be lost to the brake discs as heat.