Why Mild Steel (MS) tanks not used for storing de-mineralized (DM) water?

Why Mild Steel (MS) tanks not used for storing de-mineralized water? 

DM water, also known as deionized water or demineralized water, is water that has had almost all of its mineral ions removed through a process of purification. It's also called the hungry water that has high affinity to attract and dissolve minerals. It is used in various industrial applications where the presence of minerals or impurities can cause issues.

MS tanks, typically made of mild steel, are not suitable for storing DM water due to the following reasons:

  1. Corrosion: Mild steel is susceptible to corrosion when exposed to water, especially when it lacks protective coatings. DM water is highly purified and lacks minerals and ions that can provide some level of corrosion protection. The absence of these ions makes the water more corrosive towards metal surfaces, including mild steel. Over time, the tanks can develop rust and compromise the quality of the stored DM water.

  2. Leaching: DM water is often used in applications where purity is crucial, such as in laboratory experiments or semiconductor manufacturing. Mild steel tanks can leach contaminants into the water, such as iron ions, which can affect the purity of the DM water. This leaching can negate the efforts made to produce highly purified water.

  3. Maintenance: Mild steel tanks require regular maintenance and protective measures to prevent corrosion, such as coatings, cathodic protection, or frequent inspections. Using these measures to ensure the integrity of the tank can be costly and time-consuming.

Stainless steel (SS) tanks are commonly used for the storage of DM water due to their excellent corrosion resistance and hygienic properties. When selecting a suitable SS tank for DM water storage, there are a few factors to consider:

  1. Grade of Stainless Steel: Different grades of stainless steel offer varying levels of corrosion resistance. For DM water storage, it is recommended to use stainless steel grades that are highly resistant to corrosion, such as 316L or 304L. These grades have a low carbon content and contain additional alloying elements like molybdenum, which enhance their corrosion resistance.

  2. Surface Finish: The interior surface of the tank should have a smooth finish to prevent the buildup of impurities or bacterial growth. Electropolishing or mechanical polishing can be applied to achieve a smooth, non-porous surface that is easy to clean and maintain.

  3. Welding and Joints: The welding and joining techniques used in constructing the tank should be of high quality to ensure the integrity of the tank. Smooth, crevice-free welds and properly sealed joints are essential to prevent contamination or leakage.

  4. Compatibility with DM Water: Stainless steel is generally compatible with DM water, but it's important to verify that the specific grade of stainless steel being used is suitable for the application. Some aggressive chemicals or elevated temperatures may require higher-grade stainless steel or additional protective measures.

  5. Compliance with Standards: Ensure that the SS tank complies with relevant industry standards and regulations for DM water storage. This ensures that the tank meets specific quality and safety requirements.

For these reasons, alternative materials like stainless steel or non-metallic materials such as polyethylene or fiberglass-reinforced plastic (FRP) are commonly used for storing DM water. These materials offer better resistance to corrosion and leaching, ensuring the purity and quality of the stored DM water.

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