Galvanizing and cadmium plating for seamless steel pipes

Galvanizing and cadmium plating are two important surface treatment processes used to enhance the durability and corrosion resistance of seamless steel pipes. Each method offers unique advantages, making them suitable for different environments and applications. This article will explore the features and uses of galvanizing and cadmium plating for seamless steel pipes.

Galvanizing of Seamless Steel Pipes

- Features:

Galvanizing involves coating the surface of steel pipes with zinc, which is stable in dry air and resistant to discoloration. In humid environments, zinc reacts with oxygen or carbon dioxide to form a protective oxide or alkaline zinc carbonate film. This film prevents further oxidation, providing a protective barrier against corrosion. However, zinc is highly susceptible to degradation in acidic, alkaline, or sulfide-rich environments.

The galvanized layer is often treated with passivation using chromic acid or chromate solution. This process forms a passivation film that resists reaction with moisture in the air, significantly enhancing its anti-corrosion properties.

For components such as springs, thin-walled parts (with a wall thickness of less than 0.5mm), and steel parts requiring high mechanical strength, a hydrogen removal process is necessary to avoid embrittlement. This is not required for copper or copper alloy parts. Galvanizing is a cost-effective and easily processed treatment method.

- Applications:

Galvanizing is widely used for seamless steel pipes in atmospheric and other mild environments, providing effective protection against corrosion. However, it is not suitable for parts subject to friction, as the zinc layer may wear off under constant abrasion.

Cadmium Plating of Seamless Steel Pipes

- Features:

Cadmium plating is highly effective for steel pipes used in marine environments, hot water systems above 70°C, or parts exposed to seawater. The cadmium coating is stable, offering strong corrosion resistance and excellent lubricity. It dissolves slowly in dilute hydrochloric acid and is highly soluble in nitric acid, but it is insoluble in alkalis and water.

Compared to zinc coatings, cadmium coatings are softer, less prone to hydrogen embrittlement, and exhibit stronger adhesion to the base metal. Additionally, under specific electrolysis conditions, cadmium coatings can have a more attractive finish than zinc coatings. However, cadmium is toxic, and the fumes released when it melts, as well as soluble cadmium salts, pose health risks.

In normal conditions, cadmium acts as a cathodic coating on steel, but in marine environments and high temperatures, it serves as an anodic coating, offering enhanced protection.

- Applications:

Cadmium plating is primarily used to protect components from corrosion in marine environments, saltwater, and saturated seawater vapor. It is commonly applied to parts in the aviation, navigation, and electronics industries, as well as for springs and threaded components. Cadmium coatings can be polished, phosphated, and used as a base for paint. However, due to its toxicity, cadmium-plated items are unsuitable for use as tableware.

In conclusion, both galvanizing and cadmium plating provide seamless steel pipes with valuable protective qualities, but their applications vary based on environmental conditions and performance requirements.