Why does the 304 stainless steel pipe turn yellow?

304 stainless steel pipe is widely used due to its excellent corrosion resistance, smooth surface, and hygienic properties. Made from high-quality 304 stainless steel, it contains around 18% chromium and 8% nickel, which gives it the ability to resist corrosion in various environments, including humid air, weak acids, and alkalis. It’s commonly used in industries like food processing and healthcare to transport liquids and gases, ensuring product safety. It also performs well in high-temperature environments, typically withstanding temperatures up to 800°C.

However, a common issue that arises is the yellowing of 304 stainless steel pipes, which may occur under certain conditions. Below, we explain why this happens and how to address it.

Reasons for Yellowing of 304 Stainless Steel Pipe

- Exposure to Polluted Environments:

Yellowing often occurs in areas exposed to highly polluted environments, such as near vents or oil fume outlets. The accumulation of pollutants, including oil fumes and smoke, can lead to the discoloration of the steel surface over time.

- Uneven Annealing During Manufacturing:

In some cases, yellowing can happen due to manufacturing defects, such as uneven annealing during the rolling process. Specifically, the head or tail of the steel strip may not undergo uniform heat treatment, leading to slight discoloration in individual products. Typically, these defective pipes are discarded by manufacturers.

- High-Temperature Use:

When 304 stainless steel is exposed to high temperatures, alloy elements like chromium (Cr) and silicon (Si) are added to improve its resistance to oxidation and corrosion. These elements form a protective oxide film on the steel surface, which helps prolong its life and enhance its heat tolerance. However, if the alloy content is low, the oxide film may not form as effectively, causing the pipe to yellow, especially when exposed to high temperatures or in hot weather.

What to Do if the 304 Stainless Steel Pipe Turns Yellow?

- Understanding the Chemical Process:

304 stainless steel has a low carbon content, and during the oxidation process in high-temperature environments, carbon atoms can diffuse into the stainless steel matrix. This results in a carburizing process, which can lead to oxidation and yellowing.

- Choosing the Right Materials:

To reduce oxidation, manufacturers can select steels with higher chromium, nickel, and silicon content for better high-temperature performance. High-temperature cycle tests can be conducted to measure the carburization effect and determine the best materials to minimize yellowing.

- Enhancing the Oxide Film:

The oxide film on stainless steel acts as a barrier, preventing further oxidation. To ensure that the oxide layer remains dense and effective, the steel can be treated using methods like forced oxidation or continuous pickling, which improve the film’s integrity. This treatment can prevent the film from loosening and thickening, which otherwise leads to more discoloration.

- Surface Roughness and Color Resistance:

The surface roughness of the steel pipe significantly impacts how much heat it absorbs. Stainless steel pipes with a smooth surface (roughness less than 0.022μm) absorb less heat, which helps reduce the likelihood of further oxidation and minimizes the yellowing effect. Keeping the surface smooth helps maintain the protective oxide layer, preventing it from thickening under high temperatures.

- Quality and Manufacturing Variances:

While all 304 stainless steel pipes share the same basic alloy composition, variations in smelting technology, chemical composition, and manufacturing processes can impact their resistance to yellowing. Different manufacturers may use different quality control measures, leading to variations in performance and corrosion resistance. Therefore, the performance of 304 stainless steel pipes after use may vary depending on the quality of the material and production process.

In conclusion, while yellowing of 304 stainless steel pipes can be a common issue, it is often related to environmental exposure, manufacturing inconsistencies, and high-temperature use. By choosing the right materials, improving surface treatment, and ensuring consistent manufacturing quality, the yellowing problem can be minimized, extending the life and performance of the steel pipe.