What Dacromet Is: A Zinc Armor Stacked Flake by Flake

Dacromet, commonly known as zinc-flake coating, is a non-electrolytic composite anti-corrosion film formed by coating and curing flake zinc powder and flake aluminum powder as the main body, combined with a chromate (traditional) or silicate/silane (eco chrome-free) binder. Its biggest difference from zinc plating is that the coating is not a continuous electro-deposited metal layer, but a film made of countless scale-like zinc and aluminum flakes stacked "like roof tiles", lying parallel to the workpiece surface.

It is precisely this flake structure that gives Dacromet two advantages electroplating cannot offer at the same time: first, completely no hydrogen embrittlement—it does not take the electrodeposition route, so there is no process of aqueous hydrogen generation diffusing into high-strength steel, thoroughly avoiding the hidden danger of delayed fracture in plated high-strength bolts, which makes it a mandatory corrosion-protection process for 10.9/12.9-grade high-strength fasteners; second, the film is thin yet corrosion-resistant for a long time, barely affecting the thread fit.

Because the flakes lie parallel and overlap horizontally, the path moisture must travel to reach the steel is lengthened enormously—this is the geometric reason the coating delivers far more corrosion protection than its modest thickness would suggest. A typical Dacromet film is only around 8–15 µm thick, yet it can hold off red rust for hundreds of hours where a similar thickness of smooth electro-zinc would have failed long before.

Triple Anti-Corrosion Mechanism: Barrier, Sacrificial Anode and Passivation

Dacromet's long-term corrosion protection does not rely on a single action, but on three mechanisms stacked together:

The superposition of these three mechanisms makes Dacromet resistant to white rusting in salt-spray environments, with good weatherability and a service life far superior to ordinary zinc plating.

This combination is why Dacromet behaves differently from a simple zinc layer. A plain zinc coating relies almost entirely on sacrificial protection and corrodes away relatively quickly; Dacromet adds the barrier and passivation layers on top, so the sacrificial zinc is consumed much more slowly. Even where the film is scratched down to the steel, the nearby zinc flakes still provide cathodic protection rather than letting the scratch spread as a rust pocket—a behavior engineers call "edge creep" resistance.

Process Flow: The Temperature Curve of Pre-treatment, Coating and Curing

Dacromet is a "coating-and-baking" type process, entirely non-electrolytic; the typical flow is:

1. Pre-treatment: Degreasing and shot-blasting remove oil and oxide scale, exposing a clean, active surface—this step directly affects film adhesion.
2. Coating: Dip-spin (centrifugal draining) or spray coating is used to evenly wrap the coating around threads and gaps; centrifugal spinning flings off excess coating to avoid buildup that jams the threads.
3. Curing (baking): Baked at about 300°C to cure; water and organics evaporate, and the zinc/aluminum flakes sinter with the binder into a continuous film. Control of the curing temperature curve is key to film density and adhesion.
4. Inspection: Re-check film thickness, appearance, adhesion and neutral salt spray.

Salt-Spray Performance: How to Read It per GB/T 10125

Dacromet's corrosion protection is mainly judged by neutral salt-spray hours; the test is run under the neutral salt spray (NSS) conditions of GB/T 10125-2012 Corrosion Tests in Artificial Atmospheres - Salt Spray Tests (identically adopted from ISO 9227:2006, replacing GB/T 10125-1997; published 2012-12-31, implemented 2013-10-01). Typical comparisons across film thicknesses and systems are below:

When reading a salt-spray report, distinguish two indicators: the time white rust (zinc-layer corrosion product) appears and the time red rust (steel substrate rust) appears; in engineering, the time to red rust is generally taken as the corrosion-life criterion. The thicker the film and the better the sealing, the longer the time to red rust.

Chromated vs Chrome-Free, and Selection Recommendations

• Traditional chromated Dacromet: contains hexavalent chromium; good corrosion performance but environmentally restricted, increasingly limited for EU/US export.
• Eco chrome-free Dacromet: replaces chromate with silicate/silane; no hexavalent chromium, meets ROHS/REACH export requirements, salt-spray performance close to the chromated type; the first choice for new designs.
• Where it applies: 10.9/12.9-grade high-strength bolts, wind-power equipment, PV supports, outdoor steelwork, exposed automotive chassis parts.
• Limitations: the coating surface is relatively soft with moderate wear resistance, so it is not suited to long-term reciprocating friction; conductivity is only moderate, so it is not the main choice for electrical connections.