Why Are Brass And Copper Easily Anodized Turn To Black

Having 20 years CNC machining experience, we customized a lot of Aluminum, Brass, Copper, Stainless steel, Steel, Titanium and other plastic parts for our customers and many customers always complain why my Brass and Copper parts turned to black/green after a long time, here I’d like to let you know the details.

The main reasons why brass and copper are prone to rust include environmental factors and chemical reactions.

The main reason brass and copper rust is that they react with oxygen, water vapor and carbon dioxide in the air. Specifically, copper reacts with oxygen, water vapor and carbon dioxide in the air in a humid environment to form basic copper carbonate [Cu2(OH)2CO3], we called copper rust, which is something like black and green. This reaction occurs not only on brass, but also on copper, because their main component is copper. The formation of copper rust is a chemical reaction process in which copper combines with oxygen, water vapor and carbon dioxide in the air, causing rust to appear on the copper surface.

In addition, the rusting of brass and copper is also affected by environmental factors. For example, high temperature and high humidity environment will accelerate the oxidation and discoloration process of copper. Under high humidity conditions, the surface of copper easily adsorbs or condenses trace amounts of water molecules to form a water film. The corrosive gases in these water films such as carbon dioxide, sulfur dioxide, nitrogen oxides, etc. dissolve in the water film to form a conductive liquid, thereby promoting the oxidation and corrosion rate of copper parts. Especially today when air pollution is becoming more and more serious, the content of corrosive gases in the air has increased, and the corrosion of the copper surface has become more and more serious.

In order to prevent brass from rusting, some measures can be taken, such as immersion treatment with chromium-free passivation solution, which can form a chemical film on the surface of brass, effectively blocking oxygen and moisture in the air, and avoiding the formation of visible rust. This method reduces the occurrence of rust by forming a protective film on the surface of brass.

So, please kindly note this next time you order Brass, Copper CNC part.