New Corrosion Inhibitor Resists Chlorine
By Roger C. May, Longchun Cheng, Kurt M. Given and P. Rick Higginbotham, BetzDearborn Water Management Group, and Wayne Gurganious, Carolina Power & Light
A New Halogen Resistant Azole (HRA) for Protection of Copper and its Alloys is Much More
effective than Tolyltriazole (TTA) in the presence of oxidizing biocides: it demonstrates excellent copper corrosion inhibition in chlorinated systems. As a result, less copper is dissolved and the water becomes much less aggressive toward steel. Because this new molecule has very low reactivity with oxidizing biocides, it also decreases the amount of oxidizing biocide required to maintain good microbiological control, an important environmental benefit.
Copper alloys are widely used in cooling systems because of their excellent heat transfer properties. Azoles are commonly used to protect these alloys from corrosion1-3 and can be very effective at dosages of as little as 1 to 3 ppm. TTA is by far the most extensively used azole and has become the industry standard for copper alloy protection.4,5
While the use of azoles is widespread, they have important drawbacks when used in combination with oxidizing halogens. Oxidizing halogens, such as chlorine, bromine, their hypohalous acids and their alkaline solutions, are the most common materials used to control microbiological growth in cooling water systems. When copper alloys that have been protected with azoles are exposed to an oxidizing halogen, the corrosion protection breaks down. TTA-treated systems that are chlorinated are particularly susceptible.
Chlorine reacts with TTA in cooling systems and drastically reduces its effectiveness as a copper corrosion inhibitor. Copper corrosion rates increase, often accompanied by severe mild steel corrosion in the form of pitting caused by the galvanic reaction between dissolved copper and the steel.6 When corrosion protection is lost, plant operators usually increase TTA feedrates in an attempt to overcome the reaction with chlorine and maintain a high enough residual to protect the copper surfaces.
The first set of laboratory tests on HRA was conducted in a beaker corrosion test apparatus (BCTA). In the BCTA tests, 2-liter beakers containing test specimens were immersed in a water bath to control temperature. The water flow past the specimens was regulated, and the beakers were continuously sparged with either air or a mixture of CO2 and air. Tests were conducted for 18 hours and corrosion rates were determined electrochemically from potentiodynamic polarization resistance measurements. The test water contained 500 ppm Ca, 250 ppm Mg and 25 ppm M alkalinity (all as CaCO3). The pH was 7.2 and the temperature was 120 F. The baseline treatment was stabilized phosphate, based on an orthophosphate dosage of 15 ppm. The azole was either TTA or HRA. One hour after the test began, a 2.0 ppm shot dose of chlorine was administered. Admiralty brass (ADM) corrosion rates were monitored. As Figure 1 shows, admiralty corrosion rates were significantly lower with HRA than with TTA.
A second set of laboratory tests was conducted using bench-top (BT) units. These units, described in detail elsewhere,7 can accurately duplicate actual cooling system conditions. The tests were conducted with the same water and baseline treatments as the BCTA tests. Both ADM and low carbon steel (LCS) coupons were exposed along with an LCS heat transfer tube with a heat flux of 8,000 Btu/ft2-h. The bulk water temperature was 120 F and the tube skin temperature was 135 F. The water velocity past the coupons and tube was 3.0 ft/sec. Continuous chlorination was administered using hypochlorite, beginning after 18 hours. Free Cl2 residuals varied from 0.1 to 0.3 ppm. The retention time (to 75 percent depletion) was 1.4 days. Test duration was seven days. Figure 2 shows the large improvement in LCS corrosion rates when TTA was replaced with HRA. There was also a reduction in the number and depth of pits. In these tests, the ADM corrosion rates were similar for both products.
An additional laboratory test was conducted in the BT units. The water, test parameters, and treatment were the same as in the previously described BT unit tests, but the azoles were shot fed, at a 4.0 ppm dosage, at the beginning of the test and again after 68 hours. Based on retention time in the BT unit, the azole concentration had decayed to about 6 percent of the initial dosage when the second dosage was administered. The overall test duration was 140 hours.
Both ADM and LCS corrosion rates were measured using a Corrosion Rate Meter (CRM). Without continuous treatment feed, ADM corrosion rates increased after chlorination began with both azoles (from < 0.1 mpy to about 1.0 mpy). With TTA, the ADM rates began to increase immediately after chlorination started; however, with HRA, they remained low for 30 hours before increasing. After the second dose of azoles, the ADM corrosion rates recovered in both cases, to 0.4 to 0.8 mpy for TTA and to 0.1 to 0.5 mpy for HRA.
The two azoles exhibited strikingly different LCS corrosion rates. With TTA, the LCS corrosion rate began to rise about 18 hours after chlorination began and remained high for the remainder of the test. With HRA, the LCS corrosion rate remained low for about 30 hours after start of chlorination, and then began to rise. In contrast to the TTA test, the LCS corrosion rates decreased after the second dose of HRA, and then increased gradually over time. This demonstrates the superior film durability of HRA compared to TTA, and HRA`s exceptional ability to recover from upset conditions.
Field Trial Results
A direct comparison of TTA with HRA was made at Carolina Power and Light`s Harris Nuclear Plant. This plant has a cooling system volume of 7,000,000 gallons, a recirculation rate of 350,000 gallons per minute and a T of 20 F. The large hyperbolic tower on site operates at 3.5 to 4 cycles, which generates a retention time of approximately three days (to 75 percent depletion). Chlorine (bleach) is fed intermittently, twice per day (three times per day during the summer), to control microbiological activity. The baseline treatment contained orthophosphate, zinc, polymer and azole. At the time of the trial, the azole (TTA) was being fed at 1.7 ppm active, with about 0.5 ppm being measured in the system. The loss of TTA can be attributed to its observed chemical decomposition during chlorination.8
In order to better compare performance, the TTA feed was increased to 3.0 ppm and a two-week baseline was established, immediately followed by two weeks of HRA fed at 2.0 ppm. During the one-month period, a “Corrator” Corrosion Rate Meter (CRM) continuously monitored Cu:Ni corrosion rates. LCS and Cu:Ni coupons were installed for weight loss and pitting measurements at the beginning of each two-week period. TTA was fed from May 6th to May 20th and HRA from May 21st to June 2nd.
Figure 3 shows the Cu:Ni CRM corrosion rates. The peaks correspond to chlorination feed cycles. Twice each day, when bleach was fed, the corrosion rate increased and then decayed. As Figure 3 shows, there was a dramatic decrease in corrosion rates when HRA was fed. Particularly striking was the 60 percent to 80 percent reduction in the corrosion rate spikes during chlorination. Taking an overall average, the Cu:Ni corrosion rates were 0.19 mpy during the TTA feed and 0.04 mpy during the HRA feed.
The corrosion coupons also exhibited improved performance with HRA. Table 1 shows these results. LCS and Cu:Ni coupon corrosion rates dropped from 4.3 mpy to 3.1 mpy and 0.7 to 0.5 mpy respectively, when HRA was fed. Very significantly, pits were observed on the LCS coupons during the TTA baseline period, with an average pit depth of 100 mm. No pits were observed when HRA was fed. Pits were not observed on the Cu:Ni coupons with either azole.
Daily water analyses demonstrated that HRA was resistant to chlorine-induced decomposition. When TTA was fed at 3.0 ppm, less than 0.5 ppm was typically recovered. In contrast, when HRA was fed at 2.0 ppm, an average of 1.8 ppm was recovered. Also, no odor was observed with HRA. The chlorine level during the intermittent bleach feed averaged 2.0 ppm free and 2.9 ppm total for the TTA baseline period and 2.2 ppm free and 2.9 ppm total for the HRA trial period. Table 2 shows complete water analyses.
This dramatic improvement has produced less copper in the water and lower galvanic corrosion on steel surfaces. Over the two-year TTA application period, the average dissolved copper concentration was 60 ppb. Since HRA has been applied, the dissolved copper concentration has averaged 40 ppb. With less aggressive water, the plant has been able to reduce zinc treatment levels by 33 percent and phosphate concentration by 22 percent, while maintaining comparable steel corrosion rates. LCS coupon corrosion rates averaged 2.10 mpy when TTA was being fed and 2.06 mpy since HRA has been applied.
The plant has also required less bleach. With TTA, the plant had been using 800 to 900 gallons of bleach per day. With HRA, bleach use has been reduced by 100 to 200 gallons per day.
The effluent from the cooling tower at the Harris Plant is discharged into Harris Lake. An on-site LC50 acute toxicity test periodically measures mortality in fathead minnows in effluent dilutions ranging from 0 to 99 percent. Zero mortality in the 99 percent solution means that minnows survive when placed directly into the effluent. With TTA, the mortality rate in the 99 percent solutions was zero over a 14-month period except on three occasions when it was 10, 15 and 30 percent. When HRA was fed for the same length of time, the mortality rate in the 99 percent solutions was zero in all cases. p
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Roger C. May is a research associate, Longchun Cheng is senior research scientist, Kurt M. Given is project engineer and P. Rick Higginbotham is account specialist, all withthe BetzDearborn Water Management Group. Wayne Gurganious is superintendent of environmental and chemistry at Carolina Power & Light`s Harris Nuclear Plant.