Hot-dip Galvanizing vs. Cold Galvanizing: What You Might Not Know

Introduction

Galvanizing refers to a surface treatment technology that coats the surface of metal, alloy or other materials with a layer of zinc for aesthetics and rust prevention. Zinc is easily soluble in acid and alkali, so it is called an amphoteric metal. Zinc changes little in dry air. In humid air, a dense basic zinc carbonate film will form on the zinc surface. For the steel substrate, zinc coating is an anodic coating. It is mainly used to prevent corrosion of steel. Its protective performance is closely related to the thickness of the coating.

Hot-dip galvanizing and cold galvanizing are two common metal surface treatment technologies. They each have their own advantages and disadvantages and are suitable for different scenarios and needs.

Hot-dip Galvanizing

Hot-dip galvanizing is the most common and easiest method of galvanizing. First, the surface of the steel is cleaned to remove dirt and oxidation layers. The steel is then immersed in molten zinc preheated to 450-480°C, which reacts with the steel surface to form a zinc-iron alloy covering the steel surface. Finally, the steel is removed and cooled. This method can form a uniform thickness, strong adhesion and durable galvanized layer, suitable for a variety of shapes of steel.

The hot-dip galvanized layer can provide three anti-corrosion protections to the steel matrix. The first is isolation layer protection. Two important properties of an isolation protective layer are its adhesion to metal and its own wear resistance. In this regard, the hot-dip galvanized layer provides a hard, metal-bonded isolation layer that completely covers the steel surface and isolates the steel from the corrosive environment. Then is the protection of corrosion product layer. The corrosion products formed after the surface corrosion of the hot-dip galvanized layer will produce volume expansion and block the discontinuous gaps caused by the selective dissolution of the coating, thus hindering further corrosion of the coating and reducing the corrosion rate of the galvanized layer in environmental corrosive media. Lastly, it is the electrochemical protection. For any small areas exposed by accidental damage, such as bumps or scratches, etc., since the potential of zinc is more negative than that of iron, the hot-dip galvanized layer acts as a sacrificial anode and is preferentially corroded, providing cathodic protection for the steel.

Cold Galvanizing

In this method, zinc is plated on the surface of steel by electrolysis. First, the surface of the steel is cleaned to remove dirt and oxidation layers. The steel as the anode and zinc as the cathode are then placed in an electrolytic cell containing zinc salts. Through the action of electric current, zinc ions are reduced to metallic zinc, forming a uniform zinc coating layer on the steel surface. This method can control the thickness and quality of the galvanized layer and is suitable for steel with complex shapes.

The zinc coating is thicker, crystallized finely, uniformly and without pores, and has good corrosion resistance; the zinc layer obtained by electroplating is relatively pure and corrodes slowly in acid, alkali and other mist, and can effectively protect the fastener matrix. The galvanized layer is passivated by chromic acid to form white, colorful, military green, etc., which is beautiful and has certain decorative properties. Since the galvanized layer has good ductility, it can be cold punched, rolled, bent without damaging the coating.

The fields involved in cold galvanizing are becoming more and more extensive, including machinery manufacturing, galvanized chain link fence, electronics, precision instruments, chemical industry, transportation, aerospace, etc.

Hot-dip Galvanizing vs. Cold Galvanizing：Process Principles and Characteristics

Hot-dip galvanizing is to immerse rust-removed steel products in molten zinc liquid, causing the steel surface to react with the zinc liquid to form a tightly adherent zinc-iron alloy layer. This process has the advantages of uniform coating, strong adhesion, good corrosion resistance, and long service life. However, the hot-dip galvanizing process requires high energy and labor costs, and may cause environmental pollution during the production process.

Cold galvanizing uses electrochemical principles to deposit a layer of zinc on the surface of steel. This process has the advantages of simple operation, low cost, and good environmental protection. However, the coating of cold galvanizing is thinner, the adhesion is relatively poor, and the corrosion resistance is not as good as hot-dip galvanizing. In addition, the cold galvanized coating may have problems such as unevenness and easy peeling.

Hot-dip Galvanizing vs. Cold Galvanizing：Corrosion Resistance

Hot-dip galvanizing has stronger corrosion resistance due to the formation of a zinc-iron alloy layer. This alloy layer can effectively isolate the contact between steel and corrosive media, thereby extending the service life of steel products. In harsh corrosive environments, such as marine environments, industrial atmospheric environments, etc., the corrosion resistance of hot-dip galvanizing is particularly outstanding.

In comparison, cold galvanizing has weak corrosion resistance. Due to its thin coating and poor adhesion, it is easily eroded by corrosive media and causes the coating to fall off. Therefore, in highly corrosive environments, the service life of cold galvanizing is relatively short.

Hot-dip Galvanizing vs. Cold Galvanizing：Cost and Environmental Friendliness

Cold galvanizing has greater advantages when it comes to cost. Because its process is simple. And it does not require equipment and materials such as high-temperature molten zinc, leading to its low cost. In addition, the production process of cold galvanizing is relatively environmentally friendly and does not produce a large amount of waste gas, waste water and other pollutants.

However, the cost of hot-dip galvanizing is relatively high due to its complex process, high energy consumption, and high pollution. At the same time, the hot-dip galvanizing production process may produce a large amount of waste gas, waste water and other pollutants, which will have a certain impact on the environment. Therefore, in terms of environmental protection, cold galvanizing has more advantages.

Hot-dip Galvanizing vs. Cold Galvanizing：Scope and Limitations of Use

Due to its excellent corrosion resistance and long service life, hot-dip galvanizing is widely used in various steel products that require long-term anti-corrosion, such as transmission towers, communication towers, highway guardrails, bridges, building components, etc. It is an indispensable surface treatment technology especially in highly corrosive situations such as marine environments and industrial atmospheric environments.

Due to its lower cost and better environmental protection, cold galvanizing is used in some situations that do not require high corrosion resistance and long service life such as some temporary building components, agricultural facilities, light industrial products, etc. In addition, cold galvanizing is often used to repair or maintain existing coatings to extend their service life.

However, it should be noted that both hot-dip galvanizing and cold galvanizing have their limitations. Because hot-dip galvanizing requires equipment and materials such as high-temperature molten zinc, it may not be able to effectively process some steel products with complex shapes and small sizes; while cold galvanizing has a thin coating and poor adhesion, so it may not be suitable for some resistant steel products. It may not be able to meet the usage requirements in situations with higher corrosive requirements.

Summary

To sum up, hot-dip galvanizing and cold galvanizing each have their own advantages and disadvantages and are suitable for different scenarios and needs. When choosing which technology to use, it needs to be comprehensively considered based on factors such as the specific use environment, corrosion resistance requirements, cost budget, and environmental protection requirements. In situations with strong corrosion and high requirements on service life, hot-dip galvanizing technology should be given priority; while in situations with weak corrosion  and high requirements on cost budget and environmental protection, cold galvanizing can be considered.