Galvanized steel has long been considered an optimal solution for protecting metal structures from corrosion and rust. With a sacrificial zinc layer wrapped around the outside, this material is expected to have a superior lifespan, especially in harsh environments.
However, in reality, there are many cases where galvanized steel pipes still exhibit rapid rusting, sometimes after only a short period of use. What causes this seemingly perfect protective layer to fail? Has the sacrificial protection mechanism been “betrayed” ?
What is Galvanized Steel ?
Galvanized steel is steel that has undergone a galvanizing process to enhance its corrosion resistance. This protective zinc coating helps prevent the steel from rusting, thereby extending its lifespan, especially in harsh environmental conditions.
The main structure of galvanized steel consists of two primary components:
- Steel
This is the core material, mainly composed of an iron (Fe) and carbon (C) alloy, which provides strength and load-bearing capacity for the product. - Zinc Coating
This outer layer is applied to the steel through two common methods: hot-dip galvanizing and electro-galvanizing. The zinc coating acts as a protective shield that prevents the steel from corrosive elements.
Corrosion Resistance of Galvanized Steel
Galvanized steel pipes offer superior corrosion resistance compared to conventional black steel pipes. Under normal environmental conditions—such as exposure to UV rays, rain, and wind—the zinc coating works effectively to prevent corrosion, helping maintain durability and appearance over an extended period.
The Zinc Coating Protects The Steel From Corrosion In Two Ways
- Physical protection
The zinc layer forms a barrier that separates the steel from the environment, preventing direct contact with oxygen and moisture. - Electrochemical protection
Zinc is more reactive than iron; therefore, when exposed to the environment, the zinc layer oxidizes first, protecting the underlying steel from corrosion.
Sacrificial Protection: How it wokrs
The rust-resistance of galvanized steel is achieved through a mechanism known as sacrificial protection. This is a corrosion-prevention method based on the chemical properties of metals.
Zinc is a more active metal than iron in the electrochemical series. When the surface of a galvanized pipe comes into contact with moisture or electrolytes, the zinc layer “sacrifices” itself—it oxidizes first to protect the steel core from rusting. Even if the coating is scratched or partially peeled, the surrounding zinc continues to protect the exposed steel, maintaining the integrity of the pipe.
This process generates a protective electric current that shields the iron/steel surface, preventing rust formation on the steel core.
Why Does Galvanized Steel Still Rust?
Despite having a strong sacrificial protection mechanism, galvanized steel can still corrode quickly under harsh conditions, especially in environments with high temperature and humidity.
In environments with both high heat and moisture—such as agricultural greenhouses—corrosion can accelerate. For example, at contact points between the rolling shaft and the cover sheet, where water may accumulate and temperatures can reach nearly 100°C, the corrosion-resistance performance of the zinc coating can deteriorate.
Sacrificial Protection Reaction Reversal Phenomenon
At this point, a “reversal of the sacrificial protection reaction” can occur due to:
High temperature and humidity promote the formation of compounds such as zinc oxide (ZnO) and zinc hydroxide (Zn(OH)₂) on the surface.
The formation of these substances destabilizes the zinc coating, causing a polarity reversal between zinc and steel.
Instead of acting as the sacrificial anode that protects the steel, the zinc becomes the cathodic element that accelerates the corrosion of the steel. This causes the steel core to rust more quickly, reducing the corrosion resistance of the galvanized steel pipe.
How To Protect Steel From Sacrificial Reversal and Optimize Corrosion Resistance
Anticipating the possibility of a reversal of the sacrificial protection reaction in harsh environments such as agricultural greenhouses or heating systems, we manufacture highly corrosion-resistance plated pipes such as Daiwa Color Pipe which was developed for greenhouse winding shafts - an environment that frequently hot and humid, where the sacrificial protection reversal phenomenon is likely to occur. Thanks to the high-performance polyester coating, the reverse sacrificial - protection reaction is effectively prevented. The combination of a thick galvanized layer and a polyester coating ensures that the Daiwa Color Pipe is not only durable but also guarantees a long lifespan in environments that typically cause severe metal corrosion.
Similarly, the Heating Pipes used in heating systems also benefit from this polyester coating, which forms a perfect physical barrier, preventing high temperature and moisture from degrading the galvanized layer, thereby optimizing the pipe’s corrosion resistance under harsh operating conditions.
Conclusion
In summary, although galvanized steel is an effective anti-corrosion solution, it still poses a risk of rapid rusting in harsh environments, especially in areas with high temperature and humidity such as agricultural greenhouses. The main reason is the “Reversal Phenomenon of Sacrificial Protection” where the protective zinc layer switches from an anode to a cathode, accelerating the corrosion of the steel core.
To combat this phenomenon and optimize corrosion resistance, the optimal solution is to apply a dual-protection mechanism. Daiwa Lance highly recommended our product as Daiwa Color Pipe made from galvanized steel pipe which has been developed to thoroughly address this issue. This durable coating forms a perfect physical barrier, completely eliminating the possibility of the “sacrificial protection reaction reversal phenomenon” and ensuring superior durability and maximum safety for the structure in the most challenging environmental conditions, including heavy industrial, coastal and humid areas.
