Why Do Galvanized Pipes Rust?

Galvanized pipe is a common pipe material for construction and industry. Its surface is covered with a layer of zinc to prevent the iron elements inside the steel pipe from contacting with the outside air and moisture, thereby playing an anti-corrosion and anti-rust role. However, many users have found that galvanized steel pipes may still rust in the process of using galvanized pipes. So, why does the galvanized pipe rust? What is the reason for the rust of galvanized pipes? And what impact will its rust have on actual use? This article will explain these issues in detail, and I hope it will be helpful to you.

What Is a Galvanized Pipe?

Galvanized pipe is a pipe that forms an anti-corrosion layer by coating a layer of zinc on the surface of the steel pipe to improve the corrosion resistance of the pipe. The purpose of galvanizing is to prevent the pipe itself from oxidative corrosion due to contact with media such as air and water. Especially in the use of harsh environments such as humidity and salt, the galvanized layer provides great protection.

According to the different galvanizing processes, galvanized pipes can be divided into hot-dip galvanized pipes and electro-galvanized pipes.

• Hot-dip galvanized pipe: The hot-dip galvanizing process is to heat the steel pipe to a certain temperature and immerse it in a liquid zinc tank. Through chemical reactions, the zinc layer combines with the surface of the steel pipe to form a zinc-iron alloy layer. The zinc layer of hot-dip galvanized pipe is thicker, usually up to 60-80 microns, and has good corrosion resistance.
• Electro-galvanized pipe: The zinc layer is covered on the surface of the steel pipe by electroplating or spraying. The zinc layer of electro-galvanized pipe is relatively thin, usually between 5-15 microns. Although the anti-corrosion effect is not as good as hot-dip galvanizing, the uniformity of the zinc layer can be controlled during the processing.

The Anti-Corrosion Principle Of Galvanizing

The anti-corrosion principle of galvanized pipe relies on the "sacrificial anode" effect of zinc. The metal activity of the zinc layer is strong. When the surface of the pipe is exposed to the external environment, the zinc will first undergo oxidation reaction to form zinc rust (ZnO). This layer of zinc rust can isolate further erosion of oxygen and moisture and protect the steel pipe body from corrosion.

Why Do Galvanized Pipes Rust?

Although the zinc layer of galvanized pipes can effectively prevent corrosion in theory, rust may still occur in practice. The reasons for rusting of galvanized pipes are usually the following:

Zinc layer damage:

The surface protective layer of galvanized tubing is composed of zinc layer. If the zinc layer is affected by mechanical impact, wear, scratches and other factors during use, the zinc layer may fall off or break, and the exposed steel pipe surface is exposed to the external environment and prone to corrosion. This situation is more common during pipeline installation, especially in places such as pipe elbows and joints, where the zinc layer is damaged due to frequent operation.

Galvanizing quality problems:

The anti-corrosion effect of galvanized pipes is directly related to the thickness and uniformity of the zinc layer. If the manufacturing process of the galvanized pipe is unqualified, resulting in uneven zinc layer thickness or poor zinc layer adhesion, it may cause some areas to expose the steel pipe surface, increasing the risk of corrosion.

• Uneven zinc layer thickness: If the zinc layer thickness is uneven during the galvanizing process, weak areas are prone to corrosion.
• Poor adhesion of zinc layer: The galvanized layer is not firmly bonded to the surface of the steel pipe, and it is easy to fall off under the action of external force, causing the pipe to rust.
• Galvanizing process defects: During the hot-dip galvanizing process, if the temperature of the zinc tank is unstable, the dipping time is too long or too short, the quality of the zinc layer will decrease, and even defects will form on the surface of the pipe.

Influence of environmental factors:

The rust of galvanized pipes is also closely related to the environment in which they are located.

• Humidity and climate: High humidity and humid climate conditions accelerate the oxidation process of the zinc layer, especially under frequent temperature changes, the surface of the pipe is prone to condensation and form water droplets, further accelerating corrosion.
• Salt spray corrosion: Pipes near the sea or saline-alkali areas are particularly susceptible to salt spray corrosion. Salt spray adheres to the surface of the pipe, forming an electrolyte environment, making the zinc layer easily corroded, causing the base metal to be exposed and then rusting.
• Acidic and alkaline environment: Environments containing acidic and alkaline gases such as industrial areas and chemical plants will also accelerate the corrosion of galvanized pipes. Corrosive substances such as acidic gases, sulfides, and chlorides have a strong corrosive effect on galvanized pipes.

Influence of water quality:

The quality of water is directly related to the corrosion rate of galvanized pipes. In some highly corrosive water environments (such as water sources containing more acidic substances), even galvanized pipes are difficult to avoid rust.

• Water pH: The pH of water directly affects the corrosion rate of galvanized pipes. Acidic water (pH value below 7) has a strong corrosive effect on the zinc layer, causing the zinc layer to dissolve.
• Hard water and soft water: Hard water contains more minerals (such as calcium and magnesium ions). Long-term transportation through galvanized pipes can easily form scale on the inner wall of the pipe, promoting corrosion. Soft water is usually more active and easily reacts with zinc, accelerating the dissolution of zinc. 
• Corrosive substances: When the water contains corrosive substances such as sulfide and ammonia, the corrosion rate of galvanized pipes will accelerate, and even cause serious pipeline leakage.

Electrochemical corrosion:

The electrochemical corrosion phenomenon is caused by the potential difference generated when different metals come into contact in an electrolyte environment, resulting in metal corrosion. In the galvanized pipe system, electrochemical corrosion mainly occurs in the following situations:

• Contact of different metals: When the galv pipe is in direct contact with other metals (such as copper, aluminum, etc.), due to the existence of potential difference, zinc, as a sacrificial anode, will corrode before other metals, resulting in accelerated corrosion of the galvanized pipe.
• Pipeline joints: At the joints of the pipeline, cable protection pipes and other places, current or potential difference often accumulates, aggravating the corrosion reaction.

Weakened sacrificial anode effect of zinc layer:

Although the sacrificial anode effect of zinc can effectively prevent corrosion, when the zinc layer is too thin or consumed prematurely in a corrosive environment, the sacrificial anode effect will be weakened, resulting in the exposure of the steel pipe matrix, thereby increasing the risk of rust.

• Insufficient zinc layer thickness: If the zinc layer of the galvanized pipe is too thin, the protective effect of zinc will quickly fail, and the corrosion process of the steel pipe will be accelerated.
• Premature consumption of zinc layer: In long-term use, especially in a highly corrosive environment, the zinc layer is consumed too quickly, the sacrificial anode effect is weakened, and the steel pipe matrix is gradually exposed.

Effects Of Rust On Galvanized Pipes

1).Reduced durability of pipes:Once galv pipes are rusted, their bearing capacity, pressure resistance and impact resistance will be reduced, and their service life will be greatly shortened. Pipe aging caused by corrosion makes it unable to withstand the stress under high-pressure water flow or other working conditions, and it is easy to rupture or leak.

2).Water pollution:Zinc pollution caused by corrosion of galvanized pipes may have a negative impact on the environment. Especially in municipal water supply systems, zinc leakage may cause water pollution and affect drinking water safety. In addition, leakage from rusted pipes may cause certain pollution to the surrounding soil and vegetation, affecting the ecological environment.

3).Threats to structural safety:Corrosion of galvanized pipes not only affects the pipes themselves, but also may affect the bearing structure of the pipes. For example, in the fields of electricity and communications, galvanized pipes are often used to support and protect cables. If galvanized pipes are severely corroded, it may cause the breakage of cables or damage to the outer protective layer, which in turn affects the stability and safety of the entire system.

Anti-Corrosion Measures For Galvanized Pipes

Improving the quality of galvanizing

Ensuring that the uniformity, thickness and adhesion of the zinc layer meet the standards can greatly enhance the anti-corrosion ability of galvanized pipes. During the production process, the temperature, concentration and other process parameters of the galvanizing solution should be strictly controlled to ensure the quality of the zinc layer.

Prevent mechanical damage

Mechanical damage and wear should be prevented during the transportation, installation and use of the pipeline. Once the surface of the galvanized pipe is impacted, scratched or collided, the zinc layer is easily damaged, exposing the base metal of the steel pipe, which is easy to cause corrosion. Therefore, be careful during the installation process to avoid damage to the zinc layer. Mechanical damage can be reduced by the following measures:

• Use soft packaging materials: During transportation and handling, use soft protective materials such as foam, plastic film, etc. to wrap the pipe to reduce collision and friction.
• Avoid direct knocking during installation: Use professional installation tools such as pipe brackets and clamps to avoid direct knocking on the pipe.

Anti-corrosion coating and supplementary galvanized layer

For places where the galvanized metal pipe is used in a harsh environment, additional anti-corrosion coatings can be applied to the outside of the pipe. These coatings are usually made of materials such as epoxy resin, polyurethane, polyethylene, etc., which can further enhance the anti-corrosion performance and extend the service life of the galvanized pipe.

• Epoxy coating: It has good adhesion and corrosion resistance and is suitable for demanding industrial environments.
• Polyethylene coating: For buried pipes, polyethylene coating can effectively prevent the intrusion of moisture, acidic and alkaline substances and prevent corrosion.

In addition, in some special cases, when the zinc layer is partially damaged, the method of re-galvanizing can be used to repair the damaged area and restore the anti-corrosion ability of the pipe.

Strengthen environmental control

When selecting galvanized pipes, it is necessary to consider their installation environment. For pipes exposed to humid, high salt spray, acidic or alkaline environments, pipes with higher corrosion resistance should be selected as much as possible, or some measures to control the environment should be taken.

• Reduce exposure to high humidity areas: For example, in humid areas or open-air environments, try to take measures to reduce the time and area of pipe exposure, such as setting up shelters, roofs, etc.
• Improve ventilation conditions: By strengthening air circulation, reduce moisture retention on the surface of the pipe and slow down the corrosion reaction.

Regular inspection and maintenance

Through regular inspections, corrosion problems can be discovered in time and repair measures can be taken. The inspection content includes the integrity of the zinc layer, whether the surface is peeling or cracking, and whether there is water accumulation or scaling inside the pipe.

• Regular inspection: For exposed galvanized pipes, inspections can be carried out every six months or every year, especially at the pipe joints, elbows, flanges and other parts to check for corrosion or damage.
• Internal inspection: For galvanized pipes that are buried or cannot be directly observed, the condition of the inner wall of the pipe can be understood through endoscope inspection, ultrasonic testing and other means.

Use corrosion inhibitors

In some pipeline systems, especially water pipes or gas pipe systems, corrosion inhibitors can be used to reduce the occurrence of corrosion. These inhibitors usually inhibit the progress of corrosion reactions by changing the chemical environment inside and outside the pipe.

• Water quality treatment: For galvanized pipes in water pipes, water softening, removal of dissolved oxygen and corrosive ions in water and other methods can be used to reduce the possibility of corrosion.
• Gas protection: In gas transmission pipelines, some protective gases can be injected to change the composition or pressure of the gas and slow down corrosion.

Select suitable pipeline materials

For pipeline systems exposed to extreme environments for a long time, in addition to galvanized pipes, some other pipeline materials with stronger corrosion resistance are also available. For example, PVC pipes, PE pipes, stainless steel pipes, etc., these materials have better corrosion resistance than galvanized pipes, and are especially suitable for some special applications.

• Stainless steel pipes: Stainless steel has strong corrosion resistance and long service life, and is suitable for occasions requiring extremely high corrosion resistance.
• PE pipes, PVC pipes: These plastic pipe materials have good corrosion resistance and are particularly suitable for conveying liquid media such as water and gas.

Rusting of galvanized pipes is a common problem, but through reasonable material selection, construction and maintenance, the service life of galvanized pipes can be effectively extended.

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