Does galvanized steel rust?

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Under specific conditions, galvanized steel still has the possibility of rusting. The speed of its rusting and the specific time span will be comprehensively influenced by multiple factors. Next, we will conduct an in-depth and comprehensive analysis of this issue from multiple dimensions. The following are the detailed key points:

I. The possibility of galvanized steel rusting

   (1) Coating protection mechanism

         The reason why galvanized steel has certain anti-corrosion performance lies in the fact that the zinc layer on its surface can effectively isolate oxygen and moisture, thereby preventing the internal steel from corroding the external environment. It is worth noting that the chemical reactivity of zinc is higher than that of iron. Under the influence of this characteristic, when in a corrosive environment, zinc will undergo oxidation reactions first. This process is known as "sacrificial anode protection" in the field of metal anti-corrosion. In this way, the zinc coating can delay the rusting process of steel for a certain period of time and provide a certain degree of protection for the steel.

   However, in practical application scenarios, galvanized steel may still rust under the following specific circumstances:

   1. The coating is damaged

        During the use of galvanized steel, if the zinc layer on its surface is scratched, collided or worn due to long-term friction under the action of external forces, the steel that was originally tightly protected by the zinc layer will be directly exposed to the external environment. Once steel comes into contact with oxygen and moisture in the air, it will rapidly trigger a corrosion reaction, which in turn leads to the occurrence of rusting. For instance, during some construction processes, galvanized steel materials may suffer damage to the zinc coating due to improper handling or collision with other hard objects. If not dealt with promptly, the exposed parts of the steel are prone to rust.

   2. Extreme environment

        When galvanized steel is exposed to certain extreme environmental conditions for a long time, the protective effect of the zinc layer will gradually fail, thus exposing the steel to the risk of rusting. For instance, in a humid environment with a relatively high moisture content, this will accelerate the chemical reaction between the zinc layer and the surrounding substances, thereby weakening the protective ability of the zinc layer. In high-salt environments, such as coastal areas or regions close to salt lakes, chloride ions in the salt can have a strong corrosive effect on the zinc layer, damaging its integrity. In addition, the acid rain environment is also a factor that cannot be ignored. The acidic substances in acid rain will have a chemical reaction with the zinc layer, causing the zinc layer to be corroded, and then the steel will lose protection and rust. In addition, in areas with severe industrial pollution, there are a large number of harmful chemicals in the air, which can also cause damage to the zinc coating of galvanized steel and shorten its protective life.

   (2) Differences in galvanizing processes

         Different galvanizing processes can lead to significant differences in the corrosion resistance of galvanized steel, which in turn affects the time and speed of its rusting.

Hot-dip galvanizing

         Hot-dip galvanizing is a relatively common and effective galvanizing process. After being treated by this process, the zinc layer on the surface of galvanized steel is relatively thick, and usually the thickness can reach over 70 microns. The thicker zinc layer provides the steel with stronger corrosion resistance, enabling hot-dip galvanized steel to remain rust-free for a long time in a dry environment, with a service life of over 30 years. Moreover, hot-dip galvanized steel also has a unique advantage, namely the "self-healing property" of zinc. When the zinc coating is slightly damaged, in the natural environment, zinc will react with oxygen and moisture in the air to form a new protective layer at the damaged area, thereby continuing to protect the steel. For instance, in some outdoor steel structural components such as power transmission towers that are exposed to the natural environment for a long time, the application of hot-dip galvanizing process can effectively extend their service life and reduce the frequency of maintenance and replacement caused by corrosion.

   2. Cold galvanizing

        The zinc layer formed by the cold galvanizing process is relatively thin, approximately only 3 to 5 microns. Due to the thin zinc coating, its protective ability against steel is relatively weak. Under normal environmental conditions, cold-galvanized steel may start to rust after about one year of use. However, if the zinc layer is thickened to a certain extent by increasing the thickness of the coating, the anti-rust time can be correspondingly extended to 3 to 5 years. Overall, however, the corrosion resistance of cold-galvanized steel is significantly inferior to that of hot-dip galvanized steel. Therefore, in application scenarios with high anti-corrosion requirements, hot-dip galvanized steel is usually given priority.

Ii. Factors that Accelerate Rusting

   (1) Environmental conditions

        1. Humidity and temperature

        Environmental humidity and temperature have an extremely significant impact on the rusting rate of galvanized steel. When the environmental humidity exceeds 60%, the moisture content in the air is relatively high, which makes the zinc layer on the surface of galvanized steel more likely to come into contact with moisture, thereby accelerating the corrosion process of the zinc layer. Meanwhile, if the ambient temperature is relatively high, for instance, reaching around 70℃, the high temperature will further accelerate the rate of chemical reactions, making the reaction between the zinc layer and the substances in the surrounding environment more intense, thereby causing the galvanized steel to rust more quickly. For instance, in some tropical regions with high temperature and humidity or during the humid and muggy plum rain season, the surface of galvanized steel is prone to develop "white rust". This "white rust" is actually a product of zinc oxidation, and its appearance indicates that the corrosion process of galvanized steel has begun.

        2. Chemical corrosion

        When galvanized steel comes into contact with corrosive chemicals such as acids, alkalis and salt spray, these substances will undergo chemical reactions with the zinc coating, thereby damaging the structure and integrity of the zinc coating and significantly shortening the service life of galvanized steel. For instance, in some chemical production workshops or coastal areas (with high salt spray content), if galvanized steel is exposed to such an environment for a long time, the zinc layer on its surface will be corroded rapidly, which in turn leads to rusting of the steel itself. Even if exposed to these corrosive substances for a short period of time, it will cause a certain degree of damage to the protective performance of galvanized steel and increase the possibility of subsequent rusting.

   (2) Material and process defects

        1. Coating issue

        During the galvanizing process, if the coating thickness is uneven, there may be a situation where the zinc layer in some local areas is relatively thin or even absent. These weak areas will first undergo corrosion reactions when exposed to corrosive environments, becoming the starting points of rusting. In addition, if there are impurities in the zinc layer, such as iron-zinc alloy, etc., these impurities may change the electrochemical properties of the zinc layer, causing the corrosion rate in local areas to accelerate, thereby making galvanized steel more prone to rust. In addition, during the implementation of the galvanizing process, if there are areas of missed galvanizing, that is, some parts of the steel surface are not covered by the zinc layer, these areas will be directly exposed to the environment and become weak links for corrosion, greatly increasing the risk of rusting of galvanized steel.

        2. Limitations of cold galvanizing

          For the cold galvanizing process, there are some inherent limitations of itself. Under normal circumstances, cold galvanizing only achieves a single-sided coating, which means that the other side of the steel is not effectively protected. In this case, if moisture or corrosive substances in the environment seep into the steel, it will trigger an internal corrosion reaction. Moreover, as internal corrosion is not easily detectable, once it occurs, the corrosion will gradually spread from the inside to the outside, eventually leading to rusting on the surface of galvanized steel. And this kind of corrosion from the inside out is often more difficult to control and repair.

Iii. Anti-rust Maintenance Suggestions

   (1) Preventive measures

        1. Environmental control

        To prevent galvanized steel from rusting prematurely, during use, it should be kept away from long-term exposure to damp or corrosive environments as much as possible. When storing galvanized steel materials, it is necessary to choose a well-ventilated and dry place to ensure air circulation and reduce the accumulation of moisture. At the same time, the environmental humidity can be further reduced by using moisture-proof packaging materials or setting up dehumidification equipment in the storage area, providing a relatively dry storage environment for galvanized steel.

        2. Regular inspection and maintenance

Regular inspection of the surface of galvanized steel is one of the very important rust prevention measures. By carefully inspecting whether the coating is intact, and whether there are any signs of damage or corrosion, potential problems can be identified in a timely manner and corresponding repair measures can be taken. Once any damage to the coating is found, it should be repaired in a timely manner. For instance, methods such as spraying zinc-rich paint or epoxy resin can be adopted to cover the damaged areas and re-form an effective protective layer to prevent further corrosion of the steel.

   (2) Rust removal methods

    1.Mechanical rust removal

        When rust has already appeared on the surface of galvanized steel, mechanical rust removal can be adopted to remove the rust. Common mechanical rust removal tools include wire brushes, sandpaper or sandblasting equipment, etc. Rust can be removed from the surface of steel by wiping with a wire brush. Sandpaper can be used to polish small areas of rust in a local area. For large-scale rust, sandblasting is a relatively effective treatment method. By using high-speed sprayed sand particles to impact the surface of the steel, the rust layer can be quickly removed, restoring the surface of the steel to its smoothness.

     2. Chemical treatment

         Chemical treatment is another commonly used method for rust removal. Rust can be dissolved by acid washing or alkaline solution. Pickling usually employs acidic solutions such as dilute hydrochloric acid or dilute sulfuric acid. The acid undergoes a chemical reaction with the rust (mainly metal oxides) to dissolve it. However, after pickling, as the surface of the steel is in an active state, it is prone to rust again. Therefore, subsequent anti-corrosion coating protection treatment needs to be carried out in a timely manner. Alkaline solution cleaning involves using a strong alkaline solution to react with the rust, causing it to loosen and fall off. This method is relatively suitable for certain specific types of rust.

   3.Cathodic protection

        For some large-scale engineering structures, such as buried pipelines, cathodic protection methods can be adopted to suppress corrosion. By applying an electric current, the protected metal structure becomes the cathode, thereby preventing it from undergoing an oxidation reaction (i.e., corrosion). This method can effectively extend the service life of galvanized steel in harsh environments, especially suitable for large steel structures that are difficult to maintain frequently or in environments with high corrosion risks.

Iv. Summary

        To sum up, galvanized steel is not completely rust-proof as people think. Whether it will rust and the speed and duration of rusting are comprehensively influenced by multiple factors, including the integrity of the coating protection mechanism, differences in galvanizing processes, environmental conditions, and whether there are any defects in materials and processes. However, by rationally choosing the galvanizing process (and giving priority to the hot-dip galvanizing process with stronger corrosion resistance when conditions permit), effectively controlling the usage environment, and conducting regular maintenance inspections and taking corresponding anti-rust measures, the rusting time of galvanized steel can be significantly delayed and its service life can be prolonged. Once rust is found on galvanized steel, it is necessary to select an appropriate repair method based on the degree of rust and specific circumstances, and strengthen subsequent protective measures to ensure that galvanized steel can continuously and stably play its due role.

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