For a 6" x 8" water pump impeller, you have a choice of materials to consider:

1. Cast Iron: Cast iron impellers are known for their strength, durability, and resistance to wear. They are suitable for handling water with moderate levels of solids or abrasives and are commonly used in various industrial and agricultural applications.

2. Stainless Steel: Stainless steel impellers offer excellent corrosion resistance, making them suitable for pumping corrosive fluids or water with high chloride content. They are often used in applications where hygiene and sanitation are critical, such as in the food and beverage industry or marine environments.

3. Aluminium: Aluminium impellers are lightweight and offer reasonable corrosion resistance. They are commonly used in applications where weight is a concern, such as portable or mobile pump units. However, they may not be as durable as cast iron or stainless steel impellers, particularly in demanding environments.

When selecting the material for your water pump impeller, consider factors such as the type of fluid being pumped, the operating conditions, and the specific requirements of your application. Consulting with a pump manufacturer or supplier can help you choose the most suitable material for your needs.