Zinc Plating: Practical Guidance for Design Engineers

Zinc plating is one of the most common corrosion-protection finishes used on steel components, especially fasteners, brackets, and general hardware. It's often specified because it's familiar, inexpensive, and “good enough” for many indoor or light industrial environments.

Where zinc plating tends to trip up newer designers is in the details: how it differs from galvanizing, how much protection it really provides, how chromate finishes affect performance and appearance, and how quickly expectations can outpace reality if the environment is harsher than assumed.

This page explains how zinc plating actually works in production, what it protects well, and where its limits are.

What Zinc Plating Actually Is

Zinc plating is a sacrificial metallic coating applied to steel or iron. The zinc corrodes preferentially, protecting the underlying steel even if the coating is scratched or damaged.

In most machine-shop and hardware applications, zinc is applied via electroplating, which deposits a thin, controlled layer of zinc onto the part's surface using electrical current.

That sacrificial behavior is the key reason zinc is so widely used—it protects steel even when the coating isn't perfect.

Zinc Plating vs. Galvanizing

Zinc plating and galvanizing are often confused because both involve zinc, but they serve different roles.

Zinc Plating

• Thin coating (typically ~0.0002-0.0005 in)
• Smooth, uniform appearance
• Good for threads and close tolerances
• Common on fasteners and precision hardware
• Limited corrosion life without chromate

Galvanizing (Hot-Dip Galvanizing)

• Very thick coating (often 0.002-0.005 in or more)
• Rough, spangled appearance
• Significant dimensional impact
• Not suitable for tight fits or threads
• Intended for outdoor and structural use

If a part needs to assemble cleanly, fit accurately, or look controlled, zinc plating is usually the right process. If it needs to survive outdoors for years, galvanizing may be more appropriate.

Why Chromate Is Applied Over Zinc

Bare zinc corrodes relatively quickly in the presence of moisture and oxygen. To slow that process, zinc-plated parts almost always receive a chromate conversion coating on top of the zinc.

The chromate layer:

• Passivates the zinc
• Greatly improves corrosion resistance
• Affects color and appearance
• Adds very little thickness

When engineers talk about “clear zinc,” “yellow zinc,” or “black zinc,” they are usually describing the chromate, not the zinc itself.

Hexavalent vs. Trivalent Chromate

Hexavalent Chromate (Hex-Chrome)

• Traditional chromate system
• Yellow/gold iridescent appearance
• Excellent corrosion resistance
• Strong self-healing properties
• Increasingly restricted due to toxicity (RoHS, REACH)

Hex-chrome is still used in some industrial and military contexts, but availability is declining. Despite tightening restrictions, hex-chrome continues to be the cheaper option.

Trivalent Chromate (Tri-Chrome)

• Modern, environmentally compliant alternative
• Clear, blue, black, or slightly iridescent finishes
• Slightly less corrosion resistance than hex-chrome
• No hexavalent chromium (RoHS compliant)

Tri-chrome is now the default for most commercial and industrial zinc-plated parts where regulations prevent hex-chrome. It is typically more expensive than hex-chrome.

Dimensional Impact

Zinc plating adds material to the surface.

Typical zinc thickness:

• ~0.0002-0.0005 in per surface

That means:

• Outside diameters increase
• Inside diameters decrease
• Threads tighten slightly

In most cases, zinc's thickness is small enough to be ignored on general hardware. On tight-tolerance or finely threaded parts, it should be considered.

What Zinc Plating Does Well

Sacrificial corrosion protection

Zinc protects steel even if the coating is scratched.

Cost-effective

One of the lowest-cost corrosion protection options for steel.

Compatible with fasteners

Thin enough for threads, holes, and assemblies.

Wide industry acceptance

Standardized, familiar, and easy to source.

Limitations Designers Should Be Aware Of

Limited lifespan in harsh environments

Zinc plating is not intended for long-term outdoor, marine, or chemical exposure.

Appearance degrades over time

White corrosion products (“white rust”) will form as zinc sacrifices itself.

Hydrogen embrittlement risk

High-strength steels may require baking after plating.

Chromate choice matters

Not all “zinc plated” parts perform the same.

Design Considerations for Engineers

Specify the chromate

Understand the environment

Indoor machinery and dry environments are ideal. Wet or outdoor exposure shortens service life significantly.

Watch high-strength steels

Fasteners and parts above ~Rockwell C 35 may require hydrogen embrittlement relief baking.

Avoid relying on zinc for precision fits

While thin, zinc still adds material. Critical fits may need masking or post-plate chasing.

Typical Applications

• Fasteners and hardware
• Brackets and sheet metal components
• Machine frames and guards
• General industrial steel parts
• Consumer and commercial assemblies

When Zinc Plating Is the Right Choice

Zinc plating is a good choice when you need:

• Low-cost corrosion protection for steel
• Compatibility with threads and assemblies
• Sacrificial protection against scratches
• A widely accepted, standardized finish

When parts will see outdoor exposure, salt, washdowns, or chemicals, thicker coatings (galvanizing) or alternative finishes (electroless nickel, powder coating) may be more appropriate.