Introduction
Fasteners serve as the backbone of mechanical connections. Their surface treatment not only impacts their appearance but also directly determines their corrosion resistance, wear resistance, assembly performance, and even service life. Among the many surface treatment processes, galvanizing, phosphating, blackening, and chrome plating are the most commonly used. These four methods act as different “armor” for fasteners, adapting them to different working conditions. Choosing the wrong process can lead to premature rusting and failure, even compromising the safety of the entire equipment. Choosing the right process ensures fasteners remain in place in complex environments.
The following will explain the differences and selection logic of these four processes from three dimensions: principles, characteristics, and applicable scenarios.
1.Galvanizing:
The king of cost-effectiveness, the “basic model” for corrosion resistance
Process principle:
Galvanizing deposits a zinc coating (typically 5-20ฮผm thick) on the surface of fasteners through electrolysis or hot-dip galvanizing. Zinc, being more chemically active than iron, reacts preferentially with corrosive media (forming a dense protective film of zinc oxide or zinc carbonate), thereby sacrificing itself to protect the underlying steel. This is considered a sacrificial anodic coating. Common galvanizing processes include electroplating (acid galvanizing and alkaline galvanizing) and hot-dip galvanizing (which produces thicker zinc layers, 50-100ฮผm).
Core Features:
Moderate corrosion resistance: 3-5 years of rust resistance in a dry indoor environment and about 1-2 years in a humid outdoor environment; if combined with passivation treatment (such as colored zinc, blue zinc, white zinc), the corrosion resistance can be significantly improved (salt spray test can reach 48-96 hours).
Low cost: The process is mature and efficient, making it one of the most cost-effective options among all surface treatments and suitable for mass production.
Diverse appearance: Through passivation, it can present silver white, color, blue, etc., with a smooth surface, suitable for scenes with basic requirements on appearance.
Good conductivity: The zinc layer has excellent conductivity and is suitable for fasteners that require conductivity (such as electrical terminal blocks).
Average wear resistance: The zinc layer is relatively soft (hardness of about HV100-150), prone to wear due to long-term friction, and is not suitable for high-intensity friction scenarios.
Applicable scenarios:
General machinery: bolts, nuts, washers and other conventional fasteners, such as furniture, doors and windows, and pipe connections;
Mildly corrosive environment: indoor electrical equipment, non-critical parts of automobile chassis;
Low-cost demand scenarios: construction scaffolding, general hardware.
2.Phosphating
“Base Protection” Expert, Suitable for Coating and Lubrication
Process principle:
Phosphating involves immersing fasteners in a phosphate solution, where a chemical reaction forms a water-insoluble phosphate film (1-10ฮผm thick) on the surface. Common methods include zinc-based, manganese-based, and iron-based phosphating. This film is porous and inherently offers limited corrosion protection, but it can significantly improve the adhesion of subsequent coatings (spray painting, plastic spraying) or enhance lubricity.
Core Features:
Corrosion protection is “auxiliary”: the corrosion resistance of phosphating alone is weak (salt spray test is only a few hours), and it mainly relies on subsequent coatings (such as paint) to improve protection; but manganese phosphating can improve wear resistance and rust resistance to a certain extent.
Enhanced adhesion: The porous structure allows paint and grease to be “embedded”, making the coating less likely to fall off. It is a “standard pretreatment process” before painting.
Anti-friction lubrication: The phosphate film has a soft texture and can store lubricating oil, reducing the friction coefficient during fastener assembly (such as reducing sticking when tightening bolts).
Dull appearance: mostly gray, black or color film, with a dull surface. Scenes with high requirements for appearance require painting to cover it.
Applicable scenarios:
Pretreatment before painting: automobile body bolts, agricultural machinery accessories (fasteners that need to be painted later);
Connections that require lubrication: internal engine bolts, bearing seat fixings (to reduce assembly wear);
Temporary rust prevention: short-term protection of fasteners during storage and transportation (better effect with anti-rust oil).
3.Blackening
Low-key “black armor”, focusing on wear resistance and rust prevention
Process principle:
Blackening (also known as bluing) involves placing fasteners in a high-temperature, highly oxidizing solution (such as sodium hydroxide or sodium nitrite) to form a dense, ferroferric oxide film (0.5-1.5ฮผm thick) on the surface. This film, firmly bonded to the substrate and appearing black or bluish-black, primarily prevents rust by isolating them from air and water.
Core Features:
Limited anti-corrosion ability: Blackening alone has weak corrosion resistance (it can prevent rust for 6-12 months in a dry indoor environment) and needs to be used with oiling (the oil film fills the pores). It is easy to fail in a humid outdoor environment.
Excellent wear resistance: The ferroferric oxide film has a high hardness (HV300-500), which can reduce wear and seizure during fastener assembly (such as preventing seizure of high-strength bolts).
Moderate cost: The process is simple, the efficiency is high, and the cost is lower than galvanizing and chromium plating, and slightly higher than phosphating.
Uniform appearance: Pure black or bluish black, no reflection, suitable for scenes that require a “low-key texture” appearance (such as internal fasteners of precision instruments).
Applicable scenarios:
Internal mechanical parts: gearbox bolts, machine tool guide rail fixings (no need to disassemble frequently, use lubricating oil to prevent rust);
Tool fasteners: connecting bolts for wrenches, screwdrivers and other tools (must be wear-resistant and have a uniform appearance);
Low corrosion environment: internal connections of indoor precision equipment and optical instruments.
4.Chrome plating
Wear-resistant and corrosion-resistant “excellent student”, the price is also “top”
Process principle:
Chrome plating involves electrolytically depositing a layer of chromium (typically 0.5-50ฮผm thick) on the surface of fasteners. This coating is categorized as either decorative chromium (a thin layer with a nickel base) or hard chromium (a thicker layer, directly plated). Chromium is chemically stable, unreactive with air and water at room temperature, and possesses exceptional hardness, providing both corrosion and wear resistance.
Core Features:
Strong corrosion resistance: The chromium layer is dense and chemically inert, and its corrosion resistance in humid, acidic and alkaline environments is far better than that of galvanizing (salt spray test can reach thousands of hours), making it suitable for harsh environments.
Top wear resistance: The hardness of hard chrome plating can reach HV800-1100, which is the best wear resistance among all surface treatments and can withstand long-term high-intensity friction (such as piston rods and bearing outer rings).
Eye-catching appearance: Decorative chrome has a silvery-white mirror-like luster and is highly aesthetic; hard chrome has a matte or slightly shiny finish and a hard texture.
High cost: The chromium plating process is complex (pretreatment and multiple plating layers are required), and chromic anhydride is highly toxic (high environmental protection costs). The price is 5-10 times that of zinc plating, and hard chromium is even more expensive.
Applicable scenarios:
Harsh environment: fasteners for chemical equipment and marine engineering (resistant to acid, alkali and salt spray corrosion);
High-intensity friction: hydraulic rods, mold guide columns, crane pins (requires long-term wear resistance);
High appearance requirements: exposed fasteners of medical devices and precision instruments (both anti-corrosion and beautiful).
5.Comparison table of core differences between the four major processes
| Dimensions | galvanized | Phosphating | Blackening | Chrome plating |
|---|---|---|---|---|
| Core Features | Basic anti-corrosion, low cost | Assist coating, enhance lubrication | Wear-resistant, basic rust-proof (oiling required) | High corrosion resistance and high wear resistance |
| Corrosion resistance | Medium (improved after passivation) | Low (depends on subsequent coating) | Low (need to use with grease) | High (suitable for harsh environments) |
| wear resistance | Low (zinc layer is soft) | Medium (porous membrane) | High (ferroferric oxide hard) | Extremely high (chrome layer hardness is top notch) |
| Appearance | Various (silver, colorful, blue, etc.) | Dark (gray, black) | Pure black/blue-black (no reflection) | Bright silver (decorative chrome) / Matte (hard chrome) |
| cost | Low (โ1-3 yuan/kg) | Very low (โ0.5-1 yuan/kg) | Medium to low (โ1-2 yuan/kg) | High (โ5-20 yuan/kg) |
| Typical scenarios | General machinery, mild corrosion | Coating pretreatment, lubrication connection | Mechanical internal parts and tools | Harsh environment, high friction areas |
Three-step selection method: matching on demand and avoiding pitfalls
1.Look at the environment: first determine the degree of corrosion
In drying room โ blackening (low cost) or galvanizing (less troublesome);
Wet/outdoor โ galvanized (passivated);
Acid, alkali/salt spray โ chrome plating (hard chrome);
Need to paint โ Phosphate (as base coat).
2.Look at the function: clarify the core needs
Anti-corrosion + low cost โ galvanized;
Mainly wear-resistant + low corrosion โ blackening (with oiling);
Wear-resistant + high corrosion resistance โ chrome plating;
To be painted/lubricated โ Phosphate.
3.Look at the budget: balance performance and cost
Batch general parts โ galvanized (price-performance ratio preferred);
Low-cost temporary protection โ phosphating;
Low to medium budget + specific wear-resistant requirements โ blackening;
High budget + high requirements โ chrome plating.
Summary: There is no “universal technology”, only “precise matching”
Galvanizing is a “popular” option, providing basic corrosion protection; phosphating is a “supplementary” option, serving subsequent processes; blackening is a “practical” option, focusing on wear resistance and low profile; and chrome plating is a “high-end” option, capable of withstanding harsh environments and high friction. When choosing a fastener surface treatment, don’t pursue the “best” option. Instead, consider the operating environment, functional requirements, and budget, ensuring every penny is spent effectively. After all, only the right “armor” can truly fulfill the fastener’s vital role.
