Which of the following factors does NOT affect corrosion rate?

The color of the surface does not significantly impact the corrosion rate of metals. Corrosion primarily occurs due to electrochemical reactions that are influenced by factors such as the type of metal, its heat treatment, and the grain direction. These aspects determine the metal's susceptibility to corrosion, how it reacts to its environment, and how the microstructure of the metal can affect the progression of corrosion.

The type of metal is crucial since different metals and alloys have varying inherent resistance to corrosion due to their chemical properties. Heat treatment can alter these properties by affecting the metallurgical structure, potentially enhancing or reducing corrosion resistance. Similarly, grain direction can influence how corrosion affects the metal, as the arrangement of the grains can create potential sites for corrosion to initiate and propagate.

In contrast, while surface color may have aesthetic implications and influence certain physical characteristics (like absorption of heat), it does not directly impact the metal's chemical behavior or its corrosion processes.