Zinc-Nickel Abrasion and Resistant Coating

Abstract

zinc nickel platingZinc-nickel finishes with a 12-15 % by weight nickel content are widely used in the automotive, aerospace and electronics industries. In automotive applications the coating is used in both interior and exterior applications.

Automotive engineers choose zinc-nickel due to its excellent corrosion and abrasion resistance. This results in the coating retaining its corrosion protection even after handling and assembly operations. An example of this is applications such as door strikers and latches where abrasion resistance is the most important factor.

ZinKlad 1000, the highest level of performance in the MacDermid ZinKlad range of finishes, uses zinc-nickel as the base metallic coating.

Summary

Electroplated zinc-nickel coatings demonstrate the following features:

  • Exceptional corrosion resistance for relatively thin coatings
  • Hard coating, good for mechanical resistance properties
  • Suitable for electronic applications such as grounding screws
  • Galvanic compatibility with aluminum
  • Lubricated to modify friction properties

The specific properties of zinc-nickel finishes of corrosion and abrasion resistance, conductivity and adjustable friction make these coatings successful in many applications. Extensively used in the automotive industry, zinc-nickel coatings are also used in aerospace and electronics applications.

 

In Conclusion

MacDermid continue the development of zinc-nickel coatings to meet today’s industry requirements. Recent technology improvements include increasing electrolyte efficiencies, improving the aesthetics of the zinc-nickel coating and retention of corrosion resistance after post-plate deformation.

Additionally, development work includes cobalt-free trivalent passivation systems and lubricants to deliver friction properties for specific applications such as fasteners used in multiple tighten / untighten situations.

Read more

http://industrial.macdermid.com/ebooks/zinc-nickel-2013/

Coatings for high tensile fasteners

Mechanical plating

High tensile fasteners are mechanically plated

When coating performance is considered, hours to white or red rust are the normal criteria. However with high tensile fasteners, threaded or unthreaded, being free of hydrogen embrittlement is an equally important factor. For this reason components such as rivets and safety critical fasteners are zinc coated using a mechanically applied process rather than traditional electroplate. This process also offers advantages in bulk application against organic dip spin that even the smallest fastener can be treated without head / recess fill or parts sticking together.

Mechanical plating

 In the mechanical plating process the zinc is present as fine dust. It is deposited by a cold welding process. Impact energy is transferred from a rotating open ended barrel through glass beads to the zinc dust. This impacts the zinc against the steel substrate. The resulting deposit provides corrosion protection to the articles without introducing hydrogen embrittlement into the part.

The main features of the mechanical zinc process can be summarised as:

• No risk of Hydrogen Embrittlement
• Excellent corrosion protection
• Choice of zinc, zinc aluminium or zinc tin alloys
• Automated bulk process
• Room temperature, cold welding batch by batch capability
• Non-electrolytic process
• Thicknesses can range from 8µm to over 75µm

The benefits to the specifier are reliability, security and value.

Reliability

 The consistency of mechanical plating can be demonstrated by the replication in processing and the resultant coating. Additionally mechanical plating is specified by automotive manufacturers within the ZinKlad approved applicator program.

Plating process

The plating process is defined by process steps which are always the same. The only change, batch to batch, is the amount of material added (i.e. zinc dust). These quantities are correlated to the surface area being coated and the thickness required. These factors mean that the results are reproducible time and time again.

Coating performance

250 hours neutral salt spray

Protects for more than 250 hours in neutral salt spray

This reproducibility means a predictable coating is produced. Mechanical zinc plating, specified for over 30 years now by many major automotive manufacturers, delivers corrosion resistance similar to electroplated zinc for the same given zinc thickness. This means that an 8µm coating will resist more than 250 hours to red rust. Therefore exceeding requirements for:

• Non threaded fasteners such as rivets for joining metal
• Threaded fasteners used in safety critical assemblies such as seat belt mountings

Applicator program

Automotive companies often specify their surface finishing within an applicator program. This program aids coating reliability through a common audited standard. This helps to ensure the same coating performance throughout the applicator base, regardless of geographical applicator location.

Security

A common feature of the 2 component types mentioned above is that both are constructed from high tensile steel. It is generally accepted that steels having a hardness above Rockwell C-40 are susceptible to hydrogen embrittlement.

One of the issues of hydrogen embrittlement, is that the higher-strength steels which are used to bear high loads are the very steels which can fail from hydrogen embrittlement at loads much less than their design load. The role which the hydrogen plays in the failure mechanism includes factors such as internal pressure from gas formation, formation of metal hydrides, stress concentration due to interactions with metal imperfections, and micropore or microcrack formation.

This now returns to the need for a coating free of hydrogen embrittlement to prevent failure in use. For the components mentioned above, hydrogen embrittlement could mean:

• A Rivet which fails to pierce and join metal layers
• A safety critical bolt which fails under intense load

Value

Rivets in pile

High tensile self piercing rivets are also plated with mechical zinc alloys

Mechanical plating is an economical method of producing zinc coatings on high tensile steels due to:• No need for the hydrogen de-embrittlement process – Electroplated zinc coatings need to be de-embrittled within 2 hours of plating at 200ºC for 4 – 24 hours (dependant on part geometry and packing density). Additionally any passivation must be applied following this baking operation. These steps add cost due to increased process stages and energy consumption.

 • Automated equipment for bulk processing – Mechanical plating is applied in bulk processing, often in automated equipment. This increases productivity due to high loading and reduces operator costs.

• Coating uniformity and freedom from parts ‘sticking’ – These factors can reduce costs when compared to organic dip spin finishes, especially on rivets or fasteners with recessed heads.

• Low environmental impact – Finally the mechanical process results in very little waste material. This is because the majority of the material added during the plating operation is consumed to produce the final deposit.

 Summary

Mechanical plating, extensively specified for over 30 years now, is considered one of the 3 primary coating systems for threaded and non-threaded fasteners. Primarily specified to provide sacrificial protection to high tensile steel components, it offers advantages in its ability to treat small and recessed parts without sticking or filling issues. The coating is uniform and consistent and can be specified for both joining and safety critical applications. Finally the bulk nature and low waste make mechanically plated sacrificial coatings extremely cost effective.

ZinKlad…Supporting the highest quality standards

The ZinKlad™ quality assurance programme is taking on the challenges for high performance automotive coatings of the future. Introduced 10 years ago, ZinKlad has enabled many global OEM’s to specify higher standards than they achieved with previous, hexavalent chromium based coatings.

Logo for ZinKlad

ZinKlad for high performance ELV compliant coating

In the new globalising world, OEM’s must change and adapt, and ZinKlad continues to respond to these requirements worldwide. Today ZinKlad offers high performance coatings, proven consistent quality and a global network of approved applicators.

Of course, compliance with legislation, such as ELV & RoHS, and predictable torque-tension performance continue to be essential requirements. ZinKlad continues to deliver on all of these needs.

So as we move into the future, you can be sure that the high corrosion resistant coatings, freedom from proscribed materials and consistent friction control, will remain today as true as when ZinKlad was first specified.

http://www.industrial.macdermid.com/

 

Zinc plated finishes for ZinKlad

ZinKlad Finishes

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