Seeking help regarding the mechanism of dezincification corrosion

Brass dezincification corrosion is the most typical form of selective corrosion; this type of corrosion involves the removal of a certain component from the alloy as a result of corrosion. There are two types of brass dezinching: one is the plug type and the other is the regular type. The former resembles many small holes blocked by zinc-removing plugs; zinc dissolves away, leaving behind loose copper ； The latter features a continuous layer of corrosion surrounding an uncorroded brass core; zinc dissolves into the solution, and copper also dissolves, but then deposits back on the surface of the alloy. Increasing the copper content can prevent zinc loss, and adding 1% tin to the alloy, or small amounts of arsenic, antimony, and phosphorus, can also improve resistance to zinc loss.

Dezincification corrosion refers to a corrosion phenomenon in brass in which the alloying elements do not dissolve in the corrosive medium in proportion to their amounts in the alloy; instead, it is the zinc element, which has a lower potential and is relatively more reactive, that is selectively dissolved due to electrochemical effects. There is currently no definitive explanation for the mechanism of zinc loss corrosion in copper pipes. One theory suggests that zinc loss occurs as a result of the selective preferential dissolution of zinc within the alloy, that is, a process in which zinc dissolves preferentially ； Another theory suggests that copper and zinc in the alloy undergo oxidative dissolution simultaneously, and copper can then precipitate from the water and deposit at the site of corrosion, forming a layer of red copper; this is what is known as the dissolution-redeposition process. The acute zinc corrosion of the condenser brass tubes occurs on the cooling water side, and it takes two forms: layered zinc loss and plug-like zinc loss (see Figure 2.5). When stratified zinc removal occurs in brass tubes, a large reddish copper layer appears on the water-side surface. A distinct stratification can be seen in the cross-section of the corroded area of the copper tube; on the golden-colored copper alloy matrix, there is a layer of purplish-red red copper. The wall thickness of the corroded copper tube remains unchanged or only decreases slightly, but its mechanical strength is significantly reduced. Layered zinc removal corrosion occurs most frequently in water with low hardness and pH, as well as high salt content, especially high chloride levels, such as brackish water and seawater. Figure 2.5 shows the morphology of copper tubes after layerwise zinc loss and plug-like zinc loss corrosion. Plug-like zinc loss corrosion in brass tubes is a more typical form of localized corrosion; it can progress to a considerable depth perpendicular to the tube wall, and even penetrate the wall, leading to condenser leakage. Therefore, it is more dangerous than layered zinc removal corrosion. In the areas of plug-like zinc loss corrosion on the surface of copper pipes, small white bulges formed by the accumulation of corrosion products often appear; these white substances are mainly zinc chloride, zinc carbonate, and zinc hydroxide. After removing these corrosion products from the surface, a spongy red copper plug embedded in the wall of the copper tube can be seen, with a diameter of about 1–2 mm. Brass plug-like dezincification occurs most frequently in alkaline water with high hardness, such as natural fresh water; it particularly tends to happen in areas where the protective oxide film on the surface of copper pipes is incomplete or where there are porous deposits on the surface. Typically, the pH value of circulating water and make-up water is in the range of **～8.5, which generally does not cause severe zinc loss corrosion in copper pipes. However, when the surface of the copper tube is covered with deposits, the pH value of the water beneath those deposits can drop below 6 (in actual measurements abroad it can fall to around 3), thereby causing severe zinc loss corrosion in the brass tube located beneath the deposits. This will be covered in sediment corrosion.