Speciality Chemicals Magazine NOV / DEC 2023 | Page 30

Figure 2 - Hot closed loop & make-up water ( below right ).

Results & discussion

A control trial was run to obtain baseline corrosion data for all metals in the hot closed loop . A pH of 8.2 was chosen as it is within the upper limit proposed by some aluminium heat exchanger manufacturers and the limits recommended by the Association of Water Technologies for mild steel protection in multi-metal systems .

Multiple trials were then performed with a variety of ‘ traditional hot closed loop treatments ( Table 1 ). A nitrite-

silicate blend was initially evaluated as a multi-metal corrosion inhibitor ( Trial 2 ) at a typical operational pH ( 9.6 ) for mild steel protection .

While mild steel and copper were well protected , nitrite failed to protect aluminium . Results showed aluminium protection is not viable with a nitrite program at its native pH and may even have had an antagonistic effect on the aluminium due to the high corrosion rate . Another traditional chemistry , molybdate-silicate , was also tested ( Trials 3 & 4 ).

Test runs were performed at both the naturally derived pH ( 9.6 ) as well as a pH-adjusted ( 8.2 ) run with citric acid . Results , as expected , clearly indicate this combination to be an effective corrosion inhibitor for all metals at pH 8.2 . While a combination

Figure 3 - Cold closed loop & make-up water ’ ( above right ).

30 SPECIALITY CHEMICALS MAGAZINE ESTABLISHED 1981