Final answer:
Metals or alloys that are sensitive to chloride ions must use low chloride hydro test water and must be dyed after hydro test to prevent corrosion and check for any leaks or cracks.
Step-by-step explanation:
The material that must use low chloride hydro test water and must be dyed after hydro test is metals or alloys that are sensitive to chloride ions. Chloride ions can cause corrosion in metals, so it is important to use low chloride water during hydro testing to prevent this corrosion. After the hydro test, the material is dyed to check for any leaks or cracks that may have occurred during the test.