Surface Treatment for Fasteners: Zinc, HDG, Dacromet Explained

Complete guide to fastener surface treatments and coatings. Compare zinc plating, hot-dip galvanizing, Dacromet, and other coatings for corrosion protection and durability.

Why Surface Treatment Matters

Fasteners are typically made of steel, which is prone to corrosion when exposed to moisture, chemicals, or harsh environments. Surface treatment creates a protective barrier that:

Prevents rust and corrosion
Extends fastener lifespan
Maintains structural integrity
Improves appearance
Reduces maintenance costs

`nChoosing the wrong coating can lead to:

Premature failure
Costly replacements
Safety hazards
Aesthetic problems

Common Surface Treatments Overview

|———–|———–|———————|——|——————|

Treatment	Thickness	Salt Spray Resistance	Cost	Best Environment
———–	———–	———————	——	——————
Zinc Plating (Electroplating)	5-25 μm	24-96 hours	$	Indoor, mild outdoor
Hot-Dip Galvanizing (HDG)	45-85 μm	500+ hours	$$	Outdoor, industrial
Dacromet (Zinc Flake)	6-8 μm	1000+ hours	$$$	Automotive, marine
Mechanical Plating	25-50 μm	200-500 hours	$$	General outdoor
Phosphating	5-15 μm	Low (base coat)	$	Paint base, indoor
Black Oxide	1-2 μm	Very low	$	Indoor, decorative
Stainless Steel (passive)	N/A	Excellent	$$$$	Marine, chemical

Zinc Plating (Electroplating)

How It Works

Zinc plating uses an electrical current to deposit a thin layer of zinc onto the fastener surface. The zinc acts as a sacrificial anode, corroding before the steel underneath.

Specifications

Coating thickness: 5-25 μm (typically 8-12 μm for standard fasteners)
Salt spray resistance: 24-96 hours (depending on thickness and passivation)
Appearance: Bright silver, blue-bright, or yellow (with chromate conversion)
Standards: ISO 4042, ASTM B633

Types of Zinc Plating

Clear zinc (blue-bright):

Most common type
Bright silver appearance
Basic corrosion protection
Good for indoor applications

`nYellow zinc (chromate conversion):

Gold/yellow appearance
Better corrosion resistance than clear zinc
Common for automotive applications
Contains hexavalent chromium (environmental concerns)

`nTrivalent chromate (RoHS compliant):

Environmentally friendly alternative
Similar appearance to yellow zinc
Good corrosion resistance
Required for EU markets

Advantages

✅ Cost-effective

✅ Bright, attractive appearance

✅ Good for indoor and mild outdoor use

✅ Wide availability

✅ Consistent coating thickness

Disadvantages

❌ Thin coating (limited corrosion resistance)

❌ Not suitable for harsh environments

❌ White rust can form in humid conditions

❌ Coating wears off over time

Best Applications

Indoor machinery
Electronics enclosures
Furniture assembly
Light fixtures
Dry indoor environments

Hot-Dip Galvanizing (HDG)

How It Works

Fasteners are cleaned, fluxed, and immersed in molten zinc (approximately 450°C / 840°F). The zinc reacts with the steel surface to form a series of zinc-iron alloy layers, topped with a layer of pure zinc.

Specifications

Coating thickness: 45-85 μm (typically 60-70 μm for M12-M20 fasteners)
Salt spray resistance: 500-1000+ hours
Appearance: Dull gray, spangled (crystalline pattern)
Standards: ISO 10684, ASTM A153, ASTM F2329

Coating Structure

The HDG coating consists of multiple layers:

|——-|————-|———-|———-|

Layer	Composition	Hardness	Function
——-	————-	———-	———-
Gamma	75% Zn, 25% Fe	Very hard	Adhesion to steel
Delta	90% Zn, 10% Fe	Hard	Wear resistance
Zeta	94% Zn, 6% Fe	Medium	Ductility
Eta (outer)	Pure zinc	Soft	Sacrificial protection

Advantages

✅ Excellent corrosion resistance (20-50+ years outdoors)

✅ Thick, durable coating

✅ Covers all surfaces (including threads)

✅ Cathodic protection (zinc sacrifices itself)

✅ Low maintenance

Disadvantages

❌ Thick coating can cause thread fit issues

❌ Dull gray appearance (not decorative)

❌ Coating can be uneven

❌ Hydrogen embrittlement risk for high-strength fasteners

❌ Longer lead time than plating

Thread Fit Issues

HDG coating is thick, which can make assembly difficult. Solutions:

Oversized tapping: Nuts are tapped oversize to accommodate the coating
Centrifuging: Spun to remove excess zinc (for small fasteners)
Thread rolling after galvanizing: Maintains dimensional accuracy

Best Applications

Outdoor structural steel
Bridges and highways
Utility poles
Marine structures (with proper maintenance)
Industrial plants
Agricultural equipment

Dacromet (Zinc Flake Coating)

How It Works

Dacromet is a zinc flake coating applied by dipping or spraying the fasteners with a water-based solution containing zinc and aluminum flakes, then baking at 300°C to cure.

Specifications

Coating thickness: 6-8 μm
Salt spray resistance: 1000+ hours (some formulations 2000+ hours)
Appearance: Matte silver-gray
Standards: ISO 10683, ASTM F1136

Advantages

✅ Excellent corrosion resistance (better than HDG)

✅ Thin coating (no thread fit issues)

✅ No hydrogen embrittlement risk

✅ Good for high-strength fasteners (Grade 10.9, 12.9)

✅ Uniform coating thickness

✅ Environmentally friendly (no hexavalent chromium)

✅ Good lubricity (consistent torque-tension relationship)

Disadvantages

❌ More expensive than zinc plating or HDG

❌ Matte appearance (not as bright as plating)

❌ Limited color options

❌ Specialized application process

Best Applications

Automotive (engine, chassis, suspension)
High-strength structural bolts
Wind turbine fasteners
Railway applications
Aerospace ground support
Applications requiring consistent torque

Mechanical Plating

How It Works

Metal powder (zinc, tin, or alloy) is cold-welded to the fastener surface using glass beads, water, and promotional chemicals in a rotating barrel. No heat or electricity is used.

Specifications

Coating thickness: 25-50 μm
Salt spray resistance: 200-500 hours
Appearance: Matte gray (similar to HDG but smoother)
Standards: ASTM B695, ISO 6508

Advantages

✅ No hydrogen embrittlement risk

✅ Good for high-strength fasteners

✅ Thicker than electroplating

✅ Better corrosion resistance than zinc plating

✅ Uniform coating

Disadvantages

❌ Less common than other treatments

❌ Moderate corrosion resistance

❌ Higher cost than zinc plating

❌ Limited availability

Best Applications

High-strength bolts (Grade 10.9, 12.9)
Automotive components
Spring fasteners
Applications where hydrogen embrittlement is a concern

Phosphating

How It Works

A chemical conversion coating that creates a layer of insoluble phosphate crystals on the steel surface. Usually used as a base for painting or oiling.

Types

Zinc phosphate:

Gray crystalline coating
Good paint adhesion
Common for fasteners

`nManganese phosphate:

Black/dark gray coating
Better wear resistance
Common for gears and bearings

Advantages

✅ Excellent paint adhesion

✅ Low cost

✅ Good lubricity (when oiled)

✅ Reduces galling (for stainless steel)

Disadvantages

❌ Low corrosion resistance alone

❌ Must be painted or oiled for protection

❌ Thin coating

Best Applications

Paint base for outdoor fasteners
Thread rolling lubricant carrier
Break-in coating for moving parts
Stainless steel anti-galling treatment

Black Oxide

How It Works

A chemical conversion coating that creates a black iron oxide (Fe3O4) layer on the steel surface. Usually followed by oil or wax impregnation.

Specifications

Coating thickness: 1-2 μm (essentially a conversion layer)
Salt spray resistance: Very low (hours only)
Appearance: Matte black
Standards: MIL-DTL-13924, ISO 1134

Advantages

✅ Attractive black appearance

✅ Minimal dimensional change

✅ Low cost

✅ Good for indoor decorative applications

✅ Reduces light reflection

Disadvantages

❌ Very poor corrosion resistance

❌ Must be oiled or waxed

❌ Not suitable for outdoor use

❌ Coating wears easily

Best Applications

Indoor decorative fasteners
Firearms
Optical instruments
Indoor furniture
Architectural applications (with regular maintenance)

Comparison Chart

|———-|————–|—–|———-|————|———–|————-|

| Thickness | 5-25 μm | 45-85 μm | 6-8 μm | 25-50 μm | 5-15 μm | 1-2 μm |

| Salt spray | 24-96h | 500-1000h | 1000h+ | 200-500h | Low | Very low |

| Cost | $ | $$ | $$$ | $$ | $ | $ |

| Hydrogen risk | Yes | Yes | No | No | No | No |

Property	Zinc Plating	HDG	Dacromet	Mechanical	Phosphate	Black Oxide
———-	————–	—–	———-	————	———–	————-
Thickness	5-25 μm	45-85 μm	6-8 μm	25-50 μm	5-15 μm	1-2 μm
Salt spray	24-96h	500-1000h	1000h+	200-500h	Low	Very low
Appearance	Bright silver	Dull gray	Matte silver	Matte gray	Gray/black	Black
Cost	$	$$	$$$	$$	$	$
Thread fit	Good	Poor (oversize needed)	Excellent	Good	Good	Excellent
Hydrogen risk	Yes	Yes	No	No	No	No
High-strength OK	No	No	Yes	Yes	Yes	Yes

How to Choose the Right Coating

Step 1: Determine the Environment

Indoor, dry? → Zinc plating or black oxide
Outdoor, mild? → HDG or mechanical plating
Outdoor, harsh? → Dacromet or stainless steel
Marine/chemical? → Stainless steel (A4) or Dacromet

Step 2: Consider the Fastener Grade

Grade 4.8, 8.8? → Any coating is suitable
Grade 10.9, 12.9? → Avoid zinc plating and HDG (hydrogen embrittlement risk). Use Dacromet or mechanical plating.

Step 3: Check Appearance Requirements

Bright, decorative? → Zinc plating (clear or yellow)
Industrial look OK? → HDG or Dacromet
Black finish needed? → Black oxide (indoor only)

Step 4: Evaluate Cost vs. Performance

Budget priority? → Zinc plating
Long-term value? → HDG or Dacromet
Critical application? → Stainless steel or Dacromet

Environmental Considerations

RoHS Compliance

The EU Restriction of Hazardous Substances (RoHS) limits hexavalent chromium [Cr(VI)] in coatings. This affects:

Yellow zinc plating — Traditional chromate conversion contains Cr(VI). Trivalent chromate alternatives are RoHS-compliant.
HDG — Generally RoHS-compliant (no Cr(VI) in the process).
Dacromet — Most formulations are RoHS-compliant.

REACH Compliance

The EU REACH regulation restricts certain chemicals. Most modern fastener coatings are REACH-compliant, but verify with your supplier.

Lead and Cadmium

Some older coatings contained lead or cadmium. Modern fasteners should be lead-free and cadmium-free. Always specify “RoHS compliant” or “cadmium-free” when ordering.

Frequently Asked Questions

Q: How long does hot-dip galvanized coating last?

A: In rural environments, 50+ years. In industrial/marine environments, 20-35 years. The coating thickness determines lifespan—thicker coatings last longer.

Q: Can I paint over galvanized fasteners?

A: Yes, but the surface must be clean and dry. Use a primer designed for galvanized surfaces. Dacromet-coated fasteners accept paint better than HDG.

Q: What’s the difference between zinc plating and HDG?

A: Zinc plating is thin (5-25 μm) and bright; HDG is thick (45-85 μm) and dull gray. HDG lasts much longer outdoors but has a rougher appearance.

Q: Why can’t I use zinc-plated high-strength bolts?

A: Zinc plating and HDG processes can cause hydrogen embrittlement in high-strength steels (Grade 10.9, 12.9). The hydrogen atoms diffuse into the steel and cause delayed cracking. Use Dacromet or mechanical plating instead.

Q: What is white rust?

A: White rust is zinc oxide that forms on freshly galvanized surfaces when exposed to moisture. It’s usually cosmetic but can reduce coating life if severe. Prevent by storing in dry conditions.

Q: Is Dacromet better than HDG?

A: For corrosion resistance, yes (1000+ hours vs 500 hours salt spray). For cost, no (Dacromet is more expensive). For high-strength fasteners, Dacromet is the only choice (no hydrogen risk).

Need Fasteners with the Right Coating?

Chaoshuo Trading supplies fasteners with all surface treatments:

Zinc plating: Clear, yellow, trivalent (RoHS compliant)
Hot-dip galvanizing: ISO 10684, ASTM F2329
Dacromet: Zinc flake coating for high-strength bolts
Mechanical plating: For hydrogen-sensitive applications
Stainless steel: A2 (304), A4 (316) — no coating needed

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