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.

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Improving your friction control coatings

The extensive use of topcoats for fasteners began in the 1980’s. The primary need was to enhance corrosion protection and were generically known as leach and seal, due to their ability to turn yellow passivates to an almost silver colour. An improvement to leach and seal was to incorporate dry film lubricants in the coating to lower the coefficient of friction (CoF) of a fastener. With the introduction of high performance trivalent chromium passivates in 2000, the technology evolved into processes applied at room temperature. These were more sympathetic to the underlying passivates, giving high quality black finishes. One major global automotive OEM followed this evolution, moving from hexavalent passivates with leach and seal, to trivalent passivates with a thin film topcoat and lubricant combination.

Leach and Seal

Arguably the best known leach and seal process is the MacDermid JS500 system. Used alone, it reduces the CoF range of pure zinc from >0.4 to 0.22 +/- 0.08. Combined with an integral lubricant it reduces the CoF to 0.12 (+/- 0.02). This integrated process (known as JS600) provided the required protection, improvement and lubrication for the majority of their fasteners.

Change instigates higher performance requirements

Around the year 2000, the ELV directive drove many automotive companies to upgrade their existing plated fastener finishes requirements. Typically this was the new specification:

  • Higher corrosion resistance
  • Compatibility with trivalent passivation
  • CoF 0.15 with a deviation of +/- 0.02
  • New CoF requirements for different fastener innovation
  • Identification with an integral UV tracer

The answer was a new breed of topcoats. These mixed inorganic and organic compounds gave thin topcoats which adhered to and respected the underlying passivate, gave significant improvements in neutral salt spray (corrosion) resistance and a very predictable CoF of the desired 0.15. As the topcoat is so thin and transparent, its application can be verified by the presence of tracers, which maybe seen under a UV lamp. Let us review how these new topcoats achieve these performance enhancements.

Higher corrosion resistance

Three effects are taking place to increase the overall protection: (i) water resistance (ii) corrosion inhibition (iii) adhesion to the passivate layer. The first line of defence is that the coating performs as a barrier layer. The topcoat prevents water reaching the surface by providing a strong hydrophobic layer. The homogeneity of the coating also prevents premature swelling of the coating by water absorption. The second defence is the presence of corrosion inhibitors throughout the coating. These help to seal the coating in any areas where minute discontinuities in the film might occur. Thirdly the adhesion to the passivate layer is so strong that any ‘undercutting’ of the film is prevented. This is particularly important on sharp profiles (where the coating will typically be thinner).

Compatibility with trivalent chromium passivates

The original leach and seal coatings were designed around hexavalent chromium passivates. The leach process ensured exceptional adhesion by combining the passivate with topcoat layer. Trivalent passivates are homogenous layers and not so easy to leach. So the new products had to adhere to a smooth and pore / crack free coating. They also had to be compatible with various types including thin film (typically blue) and thick film (iridescent or black) passivates.

Narrow range coefficient of friction

All fasteners have a designed maximum proof load. Creating the correct torque-tension relationship achieves maximum joint security without exceeding the proof load of the fastener. Zinc and zinc alloys have a relatively high and variable coefficient of friction. This can adversely affect the torque-tension properties of fasteners. Additionally passivates offer different levels of CoF. For example, it was noted that hexavalent passivates have an average CoF of 0.4, whereas a high build trivalent could be as high as 0.5.

If fasteners are used without a friction control fluid, the increase in friction results in lower bolt tension for a given torque, resulting in a joint weakness, which leads to poor clamping, insecure joints and, possibly premature bolt fatigue failure. Conversely too much can lead to bolt fracture and thread stripping. This factor becomes even more crucial in safety critical applications, such as wheels, seat belts, steering and suspension component system.

Torque n Tension for zinc nickel coatings

Torque n Tension for zinc nickel coatings

Therefore lubricated topcoats provide both a lower friction coating than simply metal to metal joints; and also makes the relationship more predictable, avoiding too low or too high clamping forces. Returning to our OEM, they changed to the newer topcoats in order to consistently achieve this predictable surface CoF. Another consideration was to ensure that all applicators, across an increasingly global supply chain, conformed to the same standard. Incorporation of the UV tracer permits verification that the right topcoat has been applied.

New range coefficient of friction requirements

As new fastener technology is introduced, new CoF ranges are demanded, whilst still maintaining the corrosion resistance and compatibility with trivalent passivates. The most current dry film lubricant systems can be tailored to meet these new CoF demands, while still returning low variability.

Summary

Dry film lubricants have evolved from leach and seal processes, designed primarily for hexavalent chromium passivates, to non leach systems compatible with trivalent chromium passivates. Coupled with outstanding corrosion resistance, the non-leach technology delivers exceptionally predictable torque-tension relationships without interfering with the dimensional tolerances. Additionally they can be modified to meet newer demands for friction ranges. This technology allowed a major global automotive OEM to improve the effectiveness of the fastener assembly operations by consistently returning desired corrosion and coefficient of friction on zinc and zinc alloy plate and trivalent passivation systems.

http://www.industrial.macdermid.com

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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