Zinc-Flake Coatings for Fasteners: The Two Numbers That Matter After Salt Spray

Most conversations about zinc flake coatings focus on salt spray hours. Since corrosion resistance is one of the main specification’s companies look at, it often gets the most attention. However, engineers who have dealt with a stripped nut, a cross-threaded bolt, or a joint that loosened in service know that salt spray hours don’t tell the whole story.

A coating needs to do more than prevent corrosion. It also needs to allow fasteners to thread properly and maintain the required clamping force.

That’s why we’re focusing on two important questions: Does it thread, and does it clamp?

Question 1: Does it thread?

Screw geometry and pitch diameter

While zinc flake coatings are thin and add minimal thickness, even a small amount of coating buildup can affect a screw’s pitch diameter. Also known as the effective diameter, the pitch diameter determines whether mating threads fit together properly and securely. For standard 60° threads, the pitch diameter can change by approximately four times the coating thickness. While generally this is true, thread geometry and coating distribution play a crucial role in the actual estimate. 

Thread tolerances must be planned, not assumed. Fasteners are often threaded with coating buildup in mind to ensure the finished parts meet the dimensional specifications. ISO 10683 provides requirements for zinc flake coating systems, including coating thickness and corrosion resistance testing.

Gauge testing after coating is essential. Once the coating has been applied, fasteners should be checked with the appropriate thread gauges to verify that they meet the required tolerances and fit specifications.

Coating application can affect consistency. Many fasteners are coated using a dip-spin process, in which parts are placed in a wire basket, submerged in coating, and spun at high speeds to remove excess material. Although this process is efficient for high-volume production, coating buildup may vary depending on part geometry and process conditions. When dimensional precision and coating consistency are critical, DECC recommends rack spray application.

Internal threads present additional challenges. While coating external threads on bolts can be challenging, coating nuts and other internally threaded components can be even more complicated. Coating may accumulate within the threads, making it difficult to achieve consistent coverage while maintaining the required dimensions. Careful process control and post-coating inspection are essential to ensure proper fit and function.

Question 2: Does it Clamp?

Controlled Coefficient of Friction

As you tighten a bolt, the threads engage and stretch the bolt slightly. This stretch is called preload. The bolt naturally wants to return to its original, unstretched shape, creating a clamping force that holds the connected parts together.

The torque-tension equation is:

T = K × d × F

• T = Tightening torque
• K = Nut factor, which accounts for friction and other variables
• d = Nominal bolt diameter
• F = Clamp force

The challenge is that not all the applied torque goes toward stretching the bolt. Only a small portion, often around 10%, goes toward preload. Roughly half goes toward friction under the bolt head or bearing surface, while the remaining is used to overcome friction in the threads. These percentages are approximate and can vary depending on the fastener and application.

This means that any change in friction can change the resulting clamp load, even when the same tightening torque is applied. A coating that reduces friction too much can cause excessive preload and potentially stretch the bolt beyond its elastic limit. A coating that creates too much friction can result in insufficient clamp load, increasing the risk of joint failure.

This is why zinc flake coatings often have a specified and controlled coefficient of friction. Industry-recognized coating manufacturers, including Magni, Dörken, and NOF Metal Coatings, offer coating systems designed to provide both excellent corrosion protection and predictable friction performance.

For example, certain automotive applications may require a coefficient of friction between 0.09 and 0.14. But how can a coating consistently meet such precise requirements?

• Basecoat: usually a mix of zinc and aluminum flakes bound together through an inorganic matrix. These materials actively protect components from environmental and chemical breakdown, creating a high level of cathodic protection.
• Topcoat: complements the basecoat, providing added corrosion protection and determining the controlled coefficient of friction.

Together, the base and topcoat provide a layer that protects against corrosion while also providing a predictable coefficient of friction. This allows coatings to meet performance requirements in demanding environments while also accounting for corrosion and clamping force.

Take Away

Corrosion resistance is only part of the whole picture. Especially with fasteners, the coating can meet its salt spray requirements and still create problems if it affects thread fit. The coating has to leave the fastener usable and the friction predictable.

At DECC, we understand that choosing the right coating and application process means more than passing a single test alone. Our goal is to protect your parts while making sure they are performing to the highest level.

Want to learn more about zinc-flake coatings and how they work? Explore these past articles.

• Zinc-Flake Coating
• Magni 565: The Gold Standard of Zinc-Flake Coating Systems