Under what conditions can stainless steel become active?

Stainless steel is known for its corrosion-resistant properties, which largely stem from the formation of a passive layer of chromium oxide on its surface. However, this passive layer can be compromised under certain conditions, leading to what is known as the "active state."

When stainless steel is covered in scale or has deep scratches, the protective oxide layer can be disrupted. Scale buildup can trap moisture and contaminants against the metal surface, facilitating an environment that promotes corrosion. Deep scratches can also remove the passive layer, exposing the underlying metal to corrosive elements. This exposure can result in pitting, crevice corrosion, or rust formation, making the stainless steel active and susceptible to damage.

The other options do not typically lead to stainless steel becoming active. Polishing may actually enhance the protective layer by improving the surface finish. Exposure to air does not inherently cause the stainless steel to become active, as air is necessary for the formation of the passive layer. Being in water can also be misleading; while water alone does not activate stainless steel, the presence of salt or other corrosive elements in the water can lead to activity if the passive layer is already compromised. Thus, the conditions described in the correct answer are the most directly related to stainless steel becoming active.