- Excessive grain growth in metals primarily affects their mechanical properties by reducing the number of grain boundaries, which play a crucial role in impeding dislocation motion.
- Grain boundaries act as barriers to dislocation movement, which is essential for strengthening metals through mechanisms such as the Hall-Petch effect.
- When grains grow excessively large, the total grain boundary area decreases, making it easier for dislocations to move, resulting in decreased strength and often reduced hardness.

It is important to note the following key points:
- Grain boundaries strengthen metals by blocking dislocation motion.
- Excessive grain growth reduces grain boundary area, facilitating easier slip.
- This leads to a reduction in yield strength and tensile strength.
- Typically, ductility may increase due to the lower strength, but the primary detriment is loss of strength.
- Enhanced corrosion resistance or hardness is not a direct consequence of grain growth.

Hence, the correct answer is: Decreased strength.

Reference: Callister, Materials Science and Engineering, 10th Edition, Chapter 7, pp. 205-210