T-Bolts for Outdoor Roof Racks: Why Black Zinc-Nickel Replaced the Ones That Rusted

Short answer: For T-bolts on an outdoor roof rack, the finish decides how long they last as much as the steel does. A black finish chosen for looks can rust where the bolt is exposed. In this case, finding out where the bolts were used led to black zinc-nickel in place of plain black zinc, and the customer then ordered the matching nuts and washers in the same finish.

The case: T-bolts for a 4×4 accessories maker

A maker of 4×4 and pickup accessories, such as roof racks and side steps, asked for custom M8 T-bolts in four lengths, 16,000 pieces in total, black, in property class 8.8 or 10.9. The piece price was acceptable, but the tooling charge was high for their volume.

We compared cold heading with CNC machining for their quantity. Cold heading still came out cheapest, and we showed that the tooling charge was what the die actually cost.

The question that changed the finish

The customer then mentioned that their earlier bolts had rusted, and asked how the quoted finish would hold up. We looked at their products and confirmed with them where the bolts go: on roof racks, outdoors. We quoted two options, black zinc and black zinc-nickel, and the customer chose black zinc-nickel, with a coating of 5 µm or more.

After that the customer added cap nuts, flange nuts and flat washers, all in the same black zinc-nickel, so the whole fastening set on the rack would weather the same way.

What to plan for

• Ask for the finish by use, not by colour. “Black” covers finishes with very different corrosion resistance.
• Salt spray hours depend on the coating and its thickness. State the hours and the acceptance criterion you need, and the supplier confirms against them.
• Leave room in the schedule for a new finish. On this order, a problem in the black zinc-nickel plating delayed delivery.
• Class 10.9 with an electroplated finish needs the hydrogen embrittlement risk handled on the drawing.

If you need T-bolts or other fasteners for outdoor equipment, tell us where they go when you send the drawing through our RFQ form. We will suggest finishes for that use.

Related: black zinc or black oxide · plating high-strength bolts and hydrogen embrittlement

How this was written: the case comes from our sales records and order history (2025–2026). Plating is done by a plating partner. The customer is not named.