The ideal pH for passivity of metals in heating systems
If layers have formed on the metals in the heating system and if they are stable the metal becomes passive to corrosion and oxygen attacks. Thus this process is called passivation or passivity of metals.
The picture shows the pH ranges for common metals in heating systems when these react with passivation or corrosion.
The pH for aluminium is said to be 4-9 but it is better to create a buffer, most common is a limitation of 8.5.