Introduction

Mechanical zinc coating allows mass production with a consequently excellent quality-price ratio, combined with the absence of hydrogen embrittlement and alteration of the hardness of the material.
Overcoatability, low environmental impact, film uniformity with film thickness from 5 to 110 µm, good corrosion resistance and the intrinsic compatibility with application of zinc and aluminium alloys or lubricating systems complete the many benefits offered by this type of treatment.

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.

This section specifies mechanically applied zinc coating. This coating shall be used on steel fasteners including bolts, screws, nuts and washers. Anchor bolts are coated as specified in Section 05910. Electroplated corrosion protection is not an acceptable substitute for mechanical zinc coating

Zinc Coating Thickness Coating thickness shall be Class 50 as specified in ASTM B695

Inorganic Zinc Coating Statistics

Use Temperature) 800°F max
Chemical Resistance (ASTM D543) good
Salt Spray Resistance (ASTM B117) 4300 hrs @ 2 mils DFT
Water Absorption (ASTM D570) < .01%
Color >Flat Gray
Thickness .001″ – .003″
Adhesion (ASTM D4541) 1500 psi
Slip Coefficient (ASTM A325) 0.668
Hardness (ASTM D3363) H pencil

Coating Performance

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

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.

  1. 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.
  2. 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.
  3. 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.