Brass parts with high residual tensile stress at the surface are susceptible to season cracking (i.e. spontaneous cracking occuring on exposure to atmospheric corrosion), if its zinc percentage is more than

ANS:C - 20

zinc percentage exceeding 35% are more prone to season cracking, especially if they also have high residual tensile stress at the surface. A zinc percentage of 20% is less likely to cause season cracking compared to compositions with higher zinc content, such as those exceeding 35%. Here's an explanation:

1. Composition: Brass is an alloy primarily composed of copper and zinc. The properties of brass, including its susceptibility to season cracking, can vary significantly based on the proportion of zinc in the alloy. Higher zinc content can lead to increased susceptibility to season cracking.
2. Susceptibility to Season Cracking: Season cracking, also known as stress corrosion cracking (SCC), occurs when a material is subjected to both tensile stress and a corrosive environment. This type of cracking typically occurs along the grain boundaries of the material and can be accelerated by the presence of residual tensile stresses.
3. High Zinc Content: Brass compositions with a zinc percentage exceeding 35% are particularly prone to season cracking. This is because brasses with high zinc content are more likely to form a brittle zinc-rich phase along the grain boundaries, making them susceptible to cracking when exposed to tensile stress and a corrosive environment.
4. Intermediate Zinc Content: While brasses with a zinc content of 20% may still exhibit some susceptibility to season cracking, their risk is generally lower compared to compositions with higher zinc content. However, the risk of season cracking increases as the zinc content exceeds 35%.

In summary, brass parts with a zinc percentage exceeding 35% are more prone to season cracking due to the formation of a brittle zinc-rich phase along the grain boundaries. A zinc percentage of 20% is less likely to cause season cracking compared to compositions with higher zinc content.