4140 is a type of steel that contains chromium, molybdenum, and other alloying elements to improve its strength and hardness.

Some people gargle pickle juice for its purported health benefits, such as relieving muscle cramps or improving digestion.

It is not recommended to pickle copper and silver together as the acid solutions used for pickling can cause damage to the metals and may create a risk of fire or explosion.

Passivation can fail due to a number of factors, including inadequate cleaning or preparation of the surface, exposure to corrosive environments, and improper application of the passivation treatment.

Passivation is not necessarily permanent and may need to be periodically reapplied to maintain the protective oxide layer on the surface of the metal.