The number of coatings available to automotive fastener engineers keeps growing. Drivers such as increased performance, friction properties, robustness in use and environmental compatibility have led to more choice; but is more choice always a good thing? How does an engineer make that choice? Does a vehicle need so many different fastener coatings? Perhaps the opposite is now more preferable, less choice and one coating which meets all the needs!
Zinc-Nickel, the coating which is gaining new advocates
High alloy (15% Ni) zinc-nickel’s popularity has grown to such an extent that an engineer will now consider it directly alongside contemporary finishes such as organic (paint) and (pure) zinc electroplate coatings. Whereas these two coatings have limitations when assessed against the primary drivers, zinc-nickel meets all the requirements.
Electroplate, long considered as the bulk sacrificial coating of choice, has a real advantage in delivering a coating which respects even the finest dimensional tolerances of fasteners. However pure zinc electroplate does not meet the higher corrosion (neutral salt spray) requirements of many modern automotive specifications.
On the other hand painted coatings, which meet the tougher neutral salt spray demands, are not usually employed when coating the smaller bolt or screw sizes due to the risk of head and thread fill.
So we come back to zinc-nickel. With a neutral salt spray resistance in excess of 1000 hours, it competes with any other commercially available mass fastener coating. Being electroplated it respects even the finest dimensional tolerance, has a high hardness (450HV) and has exceptional adhesion. The growing use of self threading fasteners means that a coating which retains adhesion during assembly process is critical. Unlike other coatings, zinc-nickel won’t gall or lift during the tapping and tightening process. Additionally it can be specified in both black or silver (without the need for a heavy painted topcoat).
Application of a thin film dry film lubricant provides friction properties which can be tailored to the OEM specification demands. This dry film (often referred to as a topcoat) also provides abrasion resistance. Good abrasion resistance is required to maintain the coating properties during vehicle assembly, where processes such as vibratory automated line feeds subject fasteners to high abrasion. Finally being free of hexavalent chromium compounds these coatings meet ELV and RoHS directives.
In summary some engineers are looking at zinc-nickel as being an effective way to reduce the number of finishes required to achieve high fastener service life performance and reduce coating variability by streamlining their fastener supply chain.