How Cr3+ Zinc Plating Protects Against Rust

Zinc electroplating deposits a thin zinc layer on steel. Zinc acts as a sacrificial anode — it corrodes instead of the steel, so the base metal stays rust-free even if the coating is scratched.

That is why zinc-plated screws, nuts and bolts resist corrosion economically — ideal for general indoor and protected outdoor use.

After plating, a passivation step adds a thin chromate film that boosts corrosion resistance and sets the color.

• Yellow/Iridescent — high corrosion resistance; popular in industry.
• Clear/Blue — clean appearance; moderate corrosion resistance.
• Black — for appearance and reduced glare.
• Olive Drab — highest corrosion resistance in the group; military/heavy use.

Passivation once used Hexavalent Chromium (Cr6+), a carcinogen banned under RoHS and REACH.

Today we use Trivalent Chromium (Cr3+), which gives comparable or better corrosion protection while being safe for people and the environment. V.S. Heat Treatment runs an all-Cr3+ line and offers clear/blue, yellow iridescent and black zinc to match each customer’s use and budget.

The problem with Cr6+ is health and compliance. Hexavalent chromium is a confirmed human carcinogen and an environmental hazard, restricted by the EU RoHS directive, the REACH regulation and the End-of-Life Vehicle (ELV) directive. Parts finished with Cr6+ can be stopped at customs or rejected by a customer's incoming-quality team, and the spent chemistry is hazardous and expensive to treat.

Why it matters for small parts: screws, nuts and bolts are high-volume items that feed regulated assemblies, and a single non-compliant coating can fail an audit for the whole product. Moving to Cr3+ removes that risk, future-proofs your exports, and keeps your documentation clean.

How to switch: ask your plater to confirm a Cr6-free (Cr3+) process and to provide salt-spray and RoHS documentation per lot. If a part needs a specific salt-spray rating, specify it up front so the right chromate and sealer are chosen.

V.S. Heat Treatment migrated entirely to Cr3+ trivalent zinc in 2020. Every barrel and rack zinc job we run is Cr6-free and RoHS-ready, with test and process documentation available on request.

A top coat or sealer over the passivation multiplies salt-spray life and helps control thread friction (torque-tension) on fasteners needing consistent clamp load. The deciding factor is process control and the right sealer — not the valence of the chromium on its own.