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Zinc surface coatings - zinc plated, hot dip galvanised, zinc flake

Author: Marc Raes                                                                               Date:27/05/2026

Choosing the right surface treatment

Ever wondered why the same bolt is available with three or four different coating options? You’re in the right place.

The main reason surface treatments, coatings, plating, paint, passivation, etc are applied to fasteners is because plain steel rusts. Expose an uncoated steel bolt to moisture and oxygen and corrosion begins almost immediately. Beyond this shared purpose, different coatings work in fundamentally different ways, perform very differently in service and come with their own trade-offs in terms of cost, dimensional impact, strength and compatibility.

With this in mind, your next question is ‘which coating is best?’ The answer- it depends on your application and there is no single best coating that is appropriate across every application.

In this article we will cover three coating systems you will encounter commonly across our product range and evaluate, what each coating does well, where is falls short and how to match it to the application at hand.

How zinc protects steel

All three surface treatments use zinc as their primary protective element, but it is worth understanding why zinc works before diving into how each process applies it. Zinc protects steel in two ways. The first is straightforward: it forms a physical barrier between the steel and the environment. The second — and more important — mechanism is sacrificial (cathodic) protection. Zinc is electrochemically less noble than steel, meaning that when both are exposed to a corrosive environment, the zinc corrodes preferentially. It "sacrifices" itself to protect the underlying steel. This is why a zinc coating can continue protecting a fastener even after it's been scratched or damaged — as long as there's fresh zinc nearby, it will corrode before the steel does. The three coatings covered here all rely on this principle, but they deliver it in very different ways.

Electroplated zinc - ISO 4042

Electroplating uses electrical current to deposit a thin layer of zinc onto the fastener surface. The fastener is submerged in a chemical bath containing dissolved zinc salts, and a direct current drives zinc ions onto the part. The result is a smooth, bright, tightly adhered coating with very precise thickness control.

Best suited for: Indoor assemblies, consumer-facing applications.

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.

Hot dip galvanising - ISO 10684

Hot dip galvanising is exactly what the name suggests. Fasteners are cleaned, fluxed and then dipped into a bath of molten zinc with a temperature range of 455°C - 480°C. The zinc metallurgically bonds to the steel surface, forming a series of zinc-iron alloy layers topped by a pure zinc outer layer. The coated is not just adhered to the surface - it is bonded into it. The minimum coating thickness is 40µm according to the ISO 10684 standard.

Best suited for: Structural and civil engineering, construction, outdoor infrastructure, agricultural and heavy equipment, any application requiring long-term protection with minimal maintenance. M12 and above is the most practical size range.

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.

Zinc flake - ISO 10683

Zinc flake coatings are applied as a liquid - a slurry containing small zinc (and often additional aluminum) flakes suspended in a binder. Fasteners are either dip-spun (dipped in the slurry and spun to remove excess) or spray-coated, then cured in an oven at 180-240°C to form a hard, thin, densely packed layer of zinc flakes.
Typical coating thicknesses range from 4 to 20 µm — similar to electroplating in thickness, but with a very different internal structure.

Best suited for: Applications requiring high corrosion, or chemical resistance with tight dimensional tolerances and no risk of HE.

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.

Choosing the best solution

There is no universal 'best' coating. Each of the three coating systems discussed has their own specific correct applications and set of conditions.
In conclusion:

  • Electroplated zinc - workhorse for moderate environments, tight tolerances and application where appearance matters - as long as the strength class/grade is appropriate and the bake out process is properly controlled.
  • Hot dip galvanising - choice for heavy structure and outdoor infrastructure applications where long-term protection and minimum maintenance is the priority and thread tolerance can be managed.
  • Zinc flake - modern solution for high-strength fasteners in very corrosive applications, and a larger variety of chemical resistance due to the myriad of top coats.

Understanding these trade-offs allows you to specify the correct coating for your application, enabling you to get the most out of your fasteners and go beyond the bare minimum.

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