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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