WATER TREATMENT of molybdate and silicate is known to provide aluminium protection , its high performance at pH 9.6 was unexpected .
In addition to traditional treatments , greener chemistries were evaluated . Trials 5 and 6 included a commercially available tannin product . Neither trial , with or without an azole adjunct , performed well relative to the control under the conditions tested . Interestingly , even at a pH of 9.0 , mild steel corrosion was higher than the control with a pH of 8.2 . The high aluminium corrosion rates can probably be attributed to higher pH and an increase in iron in the bulk water .
A commercially available filming amine ( Trials 7 & 8 ) product was tested and compared to a newly developed FFA formulation ( Trials 9 & 10 ). Both products were initially tested at the upper recommended dosage of 5,000 ppm as product .
There was no indication of copper or aluminium corrosion in the 14- day trial with either formulation . Mild steel rates were well within the accepted limits , all while operating at lower than desirable pH for mild steel protection . Trial 9 showed a mild steel corrosion rate of 0.03 mils / year at a pH of 7.8 , within manufacturers ’ recommended limits .
Both FFA blends were tested at the lower end of the usage range ( 1,000 ppm ). The commercially available FFAs showed some improvement on mild steel and copper compared to the control but did not provide aluminium protection at this use level . The results ( Trial 10 ) for the newly developed FFA at the lower dose were comparable to the results at the 5,000 ppm dose . A slight increase in mild steel corrosion is observed but still within the acceptable limits .
Table 2 shows aluminium corrosion coupons for select trials . Trial 3 , the molybdate-silicate blend , serves as a benchmark of traditional treatments . Coupons from this trial clearly show a clean aluminium surface in comparison to the control and tannin trials . The improved performance of the filming inhibitor ( Trial 10 ) indicates how viable this technology is .
Most available filming amine data comes from hot water- or steam-related systems . It has been previously shown that FFA adsorption increases at higher temperatures . 4 As FFA technology is being considered for other applications , a study was conducted to determine viability as a corrosion inhibitor in an open cooling circuit . Figure 4 shows the mild steel corrosion rates varying doses of filming inhibitor as active FFA .
The relatively aggressive nature of the water used in the test quickly corroded the LPR electrodes in the control run , reaching 20 mils / year in less than four hours . A dose of 5 ppm FFA significantly reduced initial corrosion rates compared to the control . However , the trend still indicates an increasing corrosion rate towards the end of the run .
The results indicate that the FFA provided some level of protection but the initial dose was not sufficient to film the entire surface adequately . The 10 ppm dose behaved in a similar manner to the 5 ppm dose up to the four-hour mark . At this point , there is a steady decrease in corrosion , indicating that there was sufficient FFA present to satisfy the surface demand and form a dense , protective film on the LPR electrodes .
The results demonstrate the need to satisfy the demand from all the system surfaces to achieve adequate corrosion protection . The initial dose and time required to achieve protection will vary , because film formation depends heavily on surface area , water volume and temperature . Systems with previous fouling , deposition or build-up of corrosion by-products may exhibit significant sloughing and deposit removal . This will also increase the initial demand of the FFA .
Corrosion data was also monitored for several days after the initial 10- ppm dose in order to determine the film stability ( Figure 5 ). NaOCl was used to treat the circulating loop and residual Cl 2 levels were maintained at 0.2-0.5 mg / L . As expected , the filming amine did not appear vulnerable to oxidation at the levels of chlorine applied . Corrosion rates were maintained at around 1.5 mils /
Trial # |
Treatment |
Product Dosage mg / L |
Azole |
System |
MS |
Cu |
Al |
1 |
None |
None |
None |
8.2 |
2.31 |
0.26 |
13.18 |
2 |
N02 / Si0 2 |
1,350 as N02 / 30 as SiO 2 |
Yes |
9.6 |
0.09 |
0.15 |
378.83 |
3 |
Mo / SiO 2 |
90 as Mo / 90 as SiO 2 |
Yes |
9.6 |
0.64 |
0.08 |
0.18 |
4 |
Mo / SiO 2 |
90 as Mo / 90 as SiO 2 |
Yes |
8.2 |
1.08 |
0.09 |
0.19 |
5 |
Tannin |
4,000 ( as product ) |
None |
8.6 |
6.23 |
0.35 |
51.86 |
6 |
Tannin |
4,000 ( as product ) |
Yes |
9.0 |
2.49 |
0.19 |
93.34 |
7 |
Filming amine comp |
5,000 ( as product ) Yes |
Yes |
8.3 |
0.49 |
0 |
0 |
8 |
Filming amine comp |
1,000 ( as product ) |
Yes |
8.3 |
1.06 |
0.09 |
13.39 |
9 |
Filming amine USW |
5,000 ( as product ) |
Yes |
7.8 |
0.03 |
0 |
0 |
10 |
Filiming amine USW |
1,000 ( as product ) |
Yes |
8.3 |
0.15 |
0 |
0 |
Table 1 - Corrosion rates in experimental trials |
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