The details of the ASTM A500 standard for square tubes are as follows:

Formation mechanism of white rust: When galvanized parts are closely stacked and placed in damp air, due to the absence of freely flowing air on the surface between the plated parts, electrochemical corrosion will occur on the local surface of the coating, forming white rust. After the layer of condensed water adhering to the surface of the galvanized layer interacts with oxygen, carbon dioxide, hydrogen sulfide, sulfur dioxide, oxides and other chemical gases, a corrosive aqueous solution will be generated and adhere to the zinc layer, forming an electrolyte. This electrolyte and the zinc layer produce electrochemical corrosion, thus producing the white dotted and powdery substance (zinc oxide) white rust visible to the naked eye. The secondary and tertiary oxidation products of white rust forming a local black appearance are all collectively referred to as the alkali return phenomenon.

The choice of 304L stainless steel material instead of 304 and 316 may be because 304L has some advantages in specific applications. Here are some possible reasons: Corrosion resistance: Compared with 304 and 316, 304L stainless steel has better corrosion resistance. The 304L material has a low carbon content, which makes it more suitable for use in high-temperature environments, especially in the presence of corrosive media. It can better resist intergranular corrosion, which is an important advantage in some applications, such as hospital hot water systems.

The biocompatibility of water pipes is usually not the primary consideration factor because water usually does not come into direct contact with the human body unless it is used for special purposes such as in pharmaceuticals or laboratory applications. However, in some cases, biocompatibility is still an important consideration factor, depending on the usage scenario of the water pipe and the purpose of the water being transported.

For chilled water circulation pipelines, materials suitable for low-temperature applications need to be selected to ensure that the pipelines can maintain durability and stable performance during the chilled water circulation process. The following are some materials commonly used for chilled water circulation pipelines: Stainless Steel: Stainless steel is a common choice, especially in cases where high corrosion resistance and biocompatibility are required. 316 stainless steel is usually used for chilled water pipelines because it has good corrosion resistance at low temperatures. Stainless steel pipelines can also withstand thermal contraction and expansion under low-temperature conditions and have high cold temperature resistance.