How long is the hydrazine/morpholine lay-up valid for?

The hydrazine/morpholine lay-up is considered valid indefinitely, as long as the required concentrations of the chemicals are maintained. This is crucial in preserving the integrity of the system being protected from corrosion and other detrimental effects while it is not in operation. Regular monitoring and maintenance of the chemical concentrations are essential to ensure that the lay-up remains effective over time.

In contrast, the other options suggest fixed durations for the lay-up validity, which do not account for the capability to maintain the protective conditions indefinitely as long as the necessary concentrations are upheld. The indefinite validity reflects a key principle in chemical lay-up practices where the focus is placed on maintaining specific conditions rather than adhering to a strict time frame.