I have recently encountered a failure in the copper tube on a water cooling circuit.
Circuit contained glycol which had stripped out all the zinc coating off the galvanised pipe the unit was piped in. This was passivated using Sodium Tetraborate, Sodium Metasilicate, Sodium Molybdate and the sludge created flushed prior to treatment. This sludge had been in the system for a number of months.
The tube failed during a cold spell (-1 to -2°C) with circulation with about 5% glycol in system (had not been checked by operators) but seems to be to warm for frost failure. Wind quite light. Glycol had been much higher 40%+

Can anybody help to indicate if the failure could potentially be by a chemical reaction between the galvanised sludge or chemicals used to treat the zinc stripped pipework. I am thinking galvanic corrosion? This really is not my area of expertise hence this post.
Operating temperature of water is 70-80°C

There are further copper components in the system which have not failed so I wish to prevent such potential failure.
Thanks For any help