The best way to prevent the formation of deleterious oxide layers is to remove oxygen from the
piping by shielding gas flushing before welding and using backing gas during the whole welding
process and some time afterwards. The oxidisation of the weld root can be reduced also by using
root paste. This paste reacts with the welding heat and forms a slag layer on the root side of the
weld. However, the results achieved with root paste are not anywhere near the results achieved
by using backing gas. Alternatively, heat tints can be removed by pickling after welding. However,
this is not always possible when considering the welding of piping containing natural
waters. To avoid this kind of failures one should use water as clean as possible with low amounts
of solids, chlorides, manganese and iron as well as a low amount of organic substances. Additionally,
stagnant conditions or very low flow rates should be avoided as well. If continuous flow is
not possible, the water should be changed frequently enough and the amounts of bacteria should
be checked regularly. When necessary, the water should be treated to minimise bacterial activity.
In Case 1, no failures had been detected in the system using the same water with regular use
(once a week). In Case 2, the problems had been solved by using very clean demineralised water
and circulation.