Hot-dip vs Cold galvanized pipe

The main difference between hot-dip galvanizing and cold galvanizing lies in the process principle and anti-corrosion performance. Hot-dip galvanizing involves immersing the entire steel pipe in high-temperature molten zinc liquid to form an iron-zinc alloy layer and a pure zinc layer that undergo metallurgical reactions with the steel substrate. The coating is thick, firmly bonded, and has a long corrosion-resistant life, making it suitable for outdoor and severely corrosive environments. Cold galvanizing usually refers to electro-galvanizing or zinc-rich coatings, which are surface covering or deposition. The coating is thin and has relatively weak adhesion, with a limited anti-corrosion life. It is more suitable for indoor use, light corrosion or on-site repair.

1. The process and principle of galvanizing

The process and principle of hot-dip galvanizing (HDG)
Hot-dip galvanizing is made by subjecting steel pipes to processes such as degreasing, pickling, flux plating, zinc immersion, cooling, and passivation. After pickling and flux plating, the steel pipes are immersed in molten zinc at approximately 450 °C, causing a dense zinc layer to evenly cover their surface. The steel base undergoes a diffusion reaction with molten zinc, successively forming a Γ/δ/ζ alloy layer + η pure zinc layer, which is metallurgically bonded to the base.

The process and principle of cold galvanizing
Electro-galvanizing (EG) involves placing steel parts into an electrolytic cell, where zinc serves as the anode and the steel part as the cathode. Through electrolytic oxidation, zinc is deposited on the surface of the steel part to form a thin zinc layer. It is mechanically adhered to the substrate, with uniform coverage, but the thickness is relatively thin.

Zinc-rich coating/cold-sprayed zinc (ZRP) : Composed of epoxy or inorganic silicate coatings with high zinc powder content (generally >80% by mass fraction), it forms cathodic protection through resin bonding and zinc powder conductivity. It can be applied by brushing, spraying and other methods, and is suitable for on-site construction and repair.

2. The thickness and quality of the zinc coating

Hot-dip galvanizing (HDG) : The common thickness is 55-100+ μm (the thicker the component and the higher the roughness, the higher the thickness). The appearance ranges from silvery white to matte gray, and there may be zinc tumors, flow marks or gray spots.

Electro-galvanizing (EG) thickness: Commonly 5-25 μm; The appearance is the smoothest and brightest, with good decorative properties.

Zinc-rich coating (ZRP) thickness: 40-80 μm per coat, with multiple coats reaching 100-160 μm. The appearance depends on the formula and application.

3. Anti-corrosion performance and durability

Hot-dip galvanizing (HDG) : It features a relatively thick zinc coating, which can effectively prevent the rusting and damage of the steel pipe base. It has better durability outdoors, and the zinc coating adheres closely to the steel pipe base, making it less likely to fall off.

Electro-galvanizing: The zinc coating applied in electro-galvanizing is relatively thin, and its durability outdoors is limited. Moreover, the zinc coating is prone to peeling off, making it unsuitable for use as water pipes or gas pipelines.

Zinc-rich paint: Its anti-corrosion performance mainly depends on the content of zinc powder and the thickness of the dry film. When used in combination with topcoat, it can achieve medium to heavy anti-corrosion, but it is prone to peeling off when subjected to impact or collision.

4. Manufacturing, environmental protection and safety

HDG: One-time film formation, long service life; The equipment and energy consumption are relatively high, and the surface appearance is easily affected by process fluctuations. The size of the plating bath limits the posture adjustment of large components.
For example: The indoor environment is controllable and the appearance is excellent. The electroplating solution and post-treatment must comply with the standards (nowadays, cyanide-free and hexavalent chromium-free systems are mostly adopted).
ZRP: Friendly on-site operation and strong mobility; It contains zinc powder and solvents. Attention should be paid to VOC emissions and construction safety.

All three welding/cutting operations must be carried out in a well-ventilated environment. Personal protection measures should be taken to avoid inhaling zinc fumes.

5. Key Points for Inspection and Acceptance

Thickness: Magnetic/electromagnetic thickness measurement methods (such as ISO 2178/ASTM E376) are adopted.
Adhesion/Bending/impact: Sampling and testing shall be conducted in accordance with the corresponding process standards.
Continuity/pinholes: Holiday detection method (commonly used for internal coatings) or copper-sulfuric acid drip method (traditional method) is adopted.
Repair: For HDG defects, welds and cuts, repair shall be carried out using zinc-rich paint or zinc spraying in accordance with ASTM A780 / ISO 1461 repair provisions.
Common benchmarks (examples, subject to contract/regional specifications) :
HDG: ISO 1461, EN 10240 (applicable to hot-dip galvanized pipes), GB/T 13912, etc.
E.g. ASTM B633, ISO 2081, etc.
ZRP: ISO 12944 system, ASTM D520 (zinc powder), coating inspection refers to SSPC/ASTM series.

Common Questions and Misunderstandings

What exactly is "cold galvanizing"?
In the field of engineering, it usually refers to zinc-rich coatings; In the electroplating industry, it might refer to electro-galvanizing. There are significant differences in the mechanisms and service lives of these two processes. Therefore, before placing an order, it is essential to clearly indicate the adopted process and the minimum dry film thickness or coating quality in the technical agreement.

It should be noted that the salt spray test hours are not equivalent to the actual service life on site. When designing, the environmental corrosion grade and the expected target service life should be taken into consideration.

For HDG (hot-dip galvanized) threaded fits, due to their high tightness, the "seizing" phenomenon is prone to occur. Therefore, special attention should be paid to interference and lubrication issues during the assembly process. For the cutting parts, necessary repair treatment must be carried out.

EG (electro-galvanizing) may involve hydrogen embrittlement issues, especially for high-strength steel materials. Ordinary low-carbon steel pipes have relatively low risks, but it is still necessary to understand the relevant process control measures and the necessary dehydrogenation baking requirements.

Read more : Galvanized carbon steel pipe