🎨 How to choose surface treatment for fasteners? Galvanized/hot-dip galvanized/blackened/Dacromet

Surface treatment does not change the strength of the body, only solves rust prevention, appearance, and adaptability to special working conditions (hydrogen embrittlement/temperature resistance). Wrong selection=early rusting or assembly issues.

1、 Comparison Table of Four Mainstream Processing Methods

Surface treatment, anti-corrosion ability, coating thickness, hydrogen embrittlement risk characteristics, typical applications

Electroplated zinc (blue white zinc/color zinc) ★★ 3-5 years (indoor) 5-15 μ m ⚠️ There are (high-strength parts need to be dehydrogenated) inexpensive, good appearance, basically unchanged threads, indoor machinery, electrical cabinets, and general steel structures

Hot dip galvanizing (hot-dip) ★★★★ 10-20 years (outdoor) 45~85 μ m ✅ No thickness, strong weather resistance, thread needs to be enlarged/drilled for transmission towers, bridges, photovoltaic brackets, outdoor steel structures

Blackening (oxidation blackening) ★ Almost no rust prevention<1 μ m ✅ No need to apply anti rust oil, beautiful black and shiny machine tool interior, engine, drying room equipment

Dacromet (chromium free zinc aluminum coating) ★★★★★ High anti-corrosion and temperature resistance of 6-20 μ m ✅ Hydrogen free, brittle and expensive, salt spray resistant for over 500h, temperature resistant to ≈ 300 ℃. Automotive chassis, high-speed rail, wind power, military, new energy

💡 Hydrogen embrittlement risk reminder: 10.9/12.9 grade bolts must undergo hydrogen annealing (200~230 ℃ × 2~4h) after galvanizing to prevent delayed fracture! Dakro/hot-dip galvanizing does not have this issue.

2、 Select directly from different scenarios ✅

-Indoor drying, ordinary equipment, cost sensitive
→ Electroplated zinc (blue white zinc/color zinc), most commonly equipped with 8.8 grade bolts

• Outdoor exposure · Steel structure · Long term sun and rain exposure

→ Hot dip galvanizing (note: nuts need to be equipped with “hot-dip galvanized special enlarged tapping nuts”, ordinary nuts cannot be screwed in!)

-Inside the machine, not in contact with water, requires light absorption/black and shiny appearance
→ Blackening+application/immersion of rust proof oil, regular oil replenishment and maintenance

High anti-corrosion+no hydrogen embrittlement+important stress (vehicle/rail/new energy)

Dacromet or zinc nickel alloy (Zn Ni) coating, although expensive, reliable

-Seaside/chemical plant extreme corrosive environment
Prioritize 316 stainless steel body+passivation, or Dacromet carbon steel (ordinary galvanizing is not recommended)

3、 Common pitfalls ⚠️

1. Hot dip galvanized thread interference
After hot-dip galvanizing with bolt diameter ≥ M12, the outer diameter increases, and the hole diameter of the connected part needs to be enlarged (generally+1~2mm). The nut must be a hot-dip galvanized special nut

2. Blackening=non rust proof
Blackening itself has almost no anti-corrosion effect and must be combined with anti rust oil/grease. It is prohibited in humid environments

3. Stainless steel does not need to be galvanized again
Re electroplating 304/316 stainless steel will damage the passivation film and is unnecessary, making it prone to contamination

4. The appearance of Dakro is silver gray/light gold stripes
This is a normal phenomenon, not a quality defect

4、 Quick selection mnemonic ✅

-Want cheap indoor → Electroplated galvanizing

-Long term outdoor use → hot-dip galvanizing (pay attention to threads)

-Black machine interior → Blackening+Oil coating

-High strength+high anti-corrosion → Dacromet/Zinc Nickel