Guidelines for Selecting Surface Treatment Processes for Fasteners:Galvanized vs phosphate coated fasteners vs Blackening of fasteners – A Comprehensive Comparison of Corrosion Prevention, Cost, and Applicable Scenarios

Why is surface treatment of fasteners crucial?

The service life and reliability of fasteners (anchor bolt, rods, nuts, screws, washers, etc.) as the basic components of mechanical connections largely depend on the selection of surface treatment processes. Surface treatment achieves three core objectives by forming a functional thin film on the surface of a metal substrate:
1. Anti corrosion protection – isolate water vapor, salt, and chemical media to delay the formation of red rust
2. Improve assembly performance – stabilize friction coefficient, prevent thread biting, and facilitate torque control
3. Decoration and Identification – Unified appearance color for easy product classification and brand recognition
Choosing the wrong surface treatment can result in early rusting and rework, or serious consequences such as connection failure, equipment shutdown, and even safety accidents. The three most commonly used processes in industry are galvanizing, phosphating, and black oxide

Type	Process method	Typical zinc layer thickness	Neutral salt spray (NSS) no red rust time
Electrogalvanized (cold galvanized)	Electrolytic deposition	5–15 μm	Blue and white zinc ≥ 48h; color zinc ≥ 72h; black zinc ≥ 72h (with sealing)
Hot dip galvanized (hdg)	High temperature immersion plating	30–50 μm	≥500h， High quality products can last up to 1000 hours+

 Zinc Plated Galvanized / Hot-Dip Galvanized

Deposit a layer of metallic zinc on the surface of steel fasteners using electrochemical or hot-dip methods. The electrode potential of zinc is lower than that of iron, and when corrosion occurs, zinc is preferentially oxidized as a “sacrificial anode” to protect the substrate steel.

 

 Zinc Plated Galvanized / Hot-Dip Galvanized Fastener Core advantages:

Strong anti-corrosion ability, suitable for various environments such as outdoor, humid, industrial atmosphere, etc
Electroplated zinc has a bright appearance (white zinc/color zinc/black zinc optional) and is also decorative
Thick hot-dip galvanized zinc layer, resistant to mechanical wear, suitable for heavy corrosion scenarios such as bridges, electricity, photovoltaics, etc

Zinc Plated Galvanized / Hot-Dip Galvanized Fastener Limitations:

Hydrogen embrittlement risk : Hydrogen atoms infiltrate during the galvanizing process. Ordinary galvanizing is prohibited for high-strength bolts of grade 10.9 and above, and dehydrogenation treatment must be carried out

The accuracy of hot-dip galvanized threads is affected, and large-sized nuts need to be threaded back
The friction coefficient is unstable, and key connections require the use of lubricating grease to control the preload force

phosphate coated fasteners

Immerse the fasteners in an acidic phosphate solution and generate a water-insoluble phosphate crystal transformation film (approximately 1-5 μ m thick) on the metal surface through chemical reaction. The phosphating film itself is a porous structure and usually needs to be used in conjunction with oil immersion or wax coating.

 

system	Film layer color	Features	Applications
Zinc phosphide	Gray to dark gray	Good corrosion resistance, often used as a base coating for painting	Blue and white zinc ≥ 48h; color zinc ≥ 72h; black zinc ≥ 72h (with sealing)
Manganese phosphide	Grey black to black	Dense film layer, excellent wear resistance and friction reduction performance	High strength bolts, gears, engine fasteners
Hot dip galvanized (hdg)	High temperature immersion plating	30–50 μm	Ordinary bolts, screws, pre-treatment before painting

 phosphate coated fasteners Core advantages:

Zero hydrogen embrittlement risk : Pure chemical conversion film, no electrolytic hydrogen permeation process, is the preferred process for high-strength bolts of grade 10.9 and 12.9
Stable friction coefficient, smooth thread assembly, and less prone to tearing
The cost is lower than galvanizing, and immersion in oil after phosphating can significantly improve the anti-corrosion ability
-Strong adhesion to paint and powder coatings, making it an ideal coating substrate

phosphate coated fasteners Limitations:

Bare film has weak anti-corrosion ability and must be used in conjunction with anti rust oil/wax
Phosphating+ordinary anti rust oil salt spray only takes 10-20 hours; with advanced anti rust oil, it can take 72-96 hours
The appearance is not as shiny as galvanized, and the decorative value is average

Black fasteners

Place the steel fasteners in a high-temperature alkaline oxidizing solution to form a very thin layer of Fe∝ O ₄ magnetic iron oxide film on the surface (with a thickness of about 0.5-2 μ m). The film layer is tightly bonded to the substrate and hardly changes the size of the parts.

Core advantages:
Lowest cost , simplest process, high production efficiency
Does not affect thread accuracy and dimensional fit, suitable for precision parts
Matte black appearance, non reflective, professional visual effect
Good high temperature resistance and can be used in an environment of 107-204 ℃
No hydrogen embrittlement problem
Limitations:
Worst anti-corrosion ability : Neutral salt spray only takes 1-2 hours in oil-free state; with anti rust oil, it can take 12-120 hours
Once the oil film is depleted, it quickly rusts
Only suitable for dry indoor environments or short-term rust prevention
Typical applications:  engine connecting rod bolts, cylinder head bolts, main bearing bolts, flywheel bolts, mold fasteners, precision instrument internal connectors

Select by strength level

Grade 4.8/5.8/6.8 ordinary bolts: priority should be given to electroplated zinc (moderate cost, good appearance, sufficient anti-corrosion)
8.8 grade medium strength bolt: electroplated zinc (requires dehydrogenation) or phosphated+oil immersed
Grade 10.9/12.9 high-strength bolts: Phosphating (manganese based phosphating+immersion in oil) is preferred, and ordinary galvanizing is strictly prohibited
Grade 12.9 or above ultra-high strength : phosphating or blackening+molybdenum disulfide lubrication