Surface Treatments - Steel Fasteners

What it does well

Appearance.
Electroplated fasteners look clean and metallic. The coating is smooth and can be passivated in bright silver, iridescent, yellow, or black. For visible applications — consumer products, interior assemblies, anything the end customer will see — electroplating is a good choice.

Dimensional precision.
Because the coating is so thin and uniform, electroplated fasteners maintain tight tolerances. Threads fit gauges and mating components without issue, even at the thinner coating thicknesses, if the plating process was done according to the standard.

Corrosion resistance (moderate applications).
A 12 µm zinc electroplate with a passivate layer provides reasonable corrosion protection — typically 96 to 200+ hours in neutral salt spray testing (ISO 9227) depending on the passivate system. This is adequate for indoor  applications.

Where it falls short

Corrosion resistance in harsh environments.
Compared to hot dip galvanizing, electroplated coatings are thin. In aggressive outdoor, marine, or continuously wet environments, that limited zinc mass is consumed relatively quickly.

Hydrogen embrittlement risk.
This is the most important limitation for high-strength fasteners. The electroplating process involves acid pickling to clean the steel surface, followed by the electrodeposition itself — both steps can introduce hydrogen ions into the steel. For fasteners above approximately 1040 MPa tensile strength (Property Class 10.9 and above), this creates a risk of hydrogen embrittlement (HE): The permanent loss of ductility in a metal or alloy caused by atomic hydrogen in combination with tensile stress, resulting from externally applied load/ or internal residual tensile stress.

ISO 4042 addresses this by requiring a post-plating bake-out (typically 200°C for 8+ hours) to drive hydrogen out of the steel. When this bake is properly performed and documented, the risk is managed. When it isn't — or when it's rushed — the risk remains. This is why Property Class 12.9 fasteners are rarely supplied with electroplated coatings, and why coating-related failure investigations so often point back to the plating process.

What it does well

Corrosion protection in harsh environments.
The sheer thickness of an HDG coating means there is significantly more zinc available to provide sacrificial protection. HDG fasteners routinely achieve 1,000+ hours in salt spray testing and are the go-to choice for structural outdoor applications: bridges, transmission towers, solar installations, agricultural equipment, and construction hardware.

Durability and impact resistance.
The metallurgical bond between the zinc-iron alloy layers and the steel substrate is very strong. HDG coatings resist mechanical damage — chipping, peeling, and flaking — far better than electroplated coatings. In rough handling and construction environments, this resilience matters.

Very low hydrogen embrittlement (HE) risk.
The galvanizing process does not introduce hydrogen into the steel in the way electroplating does — the high-temperature molten zinc bath actually has a mild outgassing effect, driving hydrogen out rather than in. Any HE risk from the acid pickling pre-treatment step is very low compared to electroplating, and the subsequent high-temperature immersion largely mitigates it. HDG is therefore considered far more compatible with higher-strength fasteners from an HE standpoint than electroplating.

Long service life.
The combination of coating thickness, bond integrity, and zinc mass means HDG fasteners can provide decades of protection in outdoor structural applications with little to no maintenance.

Where it falls short

Coating thickness and thread fit.
This is the most significant practical limitation. M8 is the minimum dimensional size for HDG due to the minimum coating thickness of 40 µm. On a standard metric thread, this consumes a significant portion of the thread tolerance.

ISO 10684 addresses this by having two systems for the thread tolerance - ISO-metric system (undersize) and Oversize thread system. This mean HDG bolts and nuts must match to the corresponding tolerance fit system.

Appearance
HDG coatings are matte grey, somewhat rough in texture, and variable in appearance. They are entirely functional but not aesthetically refined.

What it does well

Outstanding corrosion resistance for coating thickness.
This is the headline property for zinc flake coatings. The overlapping, platelet-like structure of zinc flakes creates a highly tortuous path for corrosive media - moisture and salts have to navigate through layer after layer of zinc rather than penetrating directly. A 12 µm zinc flake coating routinely achieves 720 to 1,000+ hours in neutral salt spray testing (Depending on base recipe and top coat).

Broad functional properties.
The selection of the top coat can add additional functional properties, including: electrical insulation, high resistance to chemicals, impact / abrasion resistance, thermal resistance, large selection of colours and additional lubricant to control the friction coefficient.

No hydrogen embrittlement (HE) risk..
Zinc flake coatings are applied from aqueous or solvent-based slurry without any acid pickling or electrochemical deposition. There is no mechanism for hydrogen introduction into the steel. This makes zinc flake the preferred coating for high-strength fasteners — Property Class 10.9 and 12.9 — where electroplating’s HE risk is unacceptable and HDG's thickness is incompatible with the thread tolerances.

Thin and dimensionally compatible
At 4 µm – 20 µm, zinc flake coatings are thin enough to be applied to precision-threaded fasteners without requiring special oversized thread allowances in most cases. Standard thread tolerances typically accommodate zinc flake coating thicknesses within tolerance class 6h/6H.

Where it falls short

Low ductility
Zinc flake coatings are generally not very ductile — corrosion performance can be affected when deformation occurs after coating.

Particular coating process issues related to fasteners.
For example, unintentional filling of recesses (sockets, drives) and therefore changing the size. Retention of particles in threads. Uneven coating coverage for washers — washers may get stuck together and adhered surfaces are not coated.