WATER TREATMENT
Carbon steel treated with 10ppm active FFA for one hour ( above ) & with no treatment ( below )
dissolved gases and water from reaching it . Carbon steel treated with 10 ppm active FFA for one hour showed a significant difference in contact angle of water droplets , signifying a hydrophobic surface over that of an untreated coupon ( pictured )
This unique inhibition mechanism is becoming increasingly appealing to water treaters as it allows the film to be maintained by feeding product at a low residual level . Having a resistant film for corrosion inhibition provides additional leeway for system upsets or times of sub-optimal feeding , unlike neutralising amines , which are quickly consumed by carbonic acid .
The application of FFAs can be tightly controlled when combined with a fluorescent tracer and commercially available FFA residual tests . Their high affinity with the metal surface means that they are able to penetrate existing deposits and corrosion debris quite well .
When applying FFAs to systems previously fouled with deposits or corrosion by-products for the first time , starting with a very low dose and gradually increasing the levels is advised . This will help to avoid excessive sloughing off , which can cause blockages in small diameter passages or low flow areas .
Traditional treatments
Nitrite is one of the most commonly used corrosion inhibitors for closed loop systems as it is relatively inexpensive and the chemistry is well understood . It acts as a mild steel corrosion inhibitor by oxidising the metal surface to form a protective Fe 2
O 3 film .
However , nitrite provides no aluminium protection and can even be aggressive to aluminium surfaces . 3 In lower-temperature (< 60 ° C ) hot water systems , it can become susceptible to microbiological growth from both nitrifying and denitrifying bacteria . Nitrite reacts with oxidising microbicides to form nitrate and , therefore , they should not be used together . Other disadvantages include its relatively high toxicity and strict discharge regulations .
Molybdate , which can be more costly , provides corrosion protection for both aluminium and mild steel . Unlike nitrite , it is not vulnerable to microbiological degradation but it does require the presence of dissolved oxygen to form its protective oxide layer on mild steel . Protecting aluminium with molybdate is most effective in a relatively tight pH range ( 7.8-8.3 ). Above this , aluminium will become more susceptible to corrosion , while below it , mild steel corrosion rates may not be acceptable .
Silicates are very cost-efficient corrosion inhibitors and exhibit some corrosion protection for all metals . Similar to molybdate , they are not degraded by microorganisms and operate in a similar pH range .
While silicates can be used for many applications , they are most commonly used as an adjunct to enhance other treatment programmes .
Experimental testing
Coupon testing was performed using a 38 litre , 3,000-Watt hot water heater , circulation pump and CPVC corrosion coupon rack ( Figure 2 ). Each test run lasted 14 days and was maintained at 71 ° C . The circulation pump was able to provide a flow velocity of 0.3 m / sec through the coupon rack . While this is lower than the recommended 0-1.5 m / sec , relative data is valuable and quantifiable .
Soft water was used to triple-rinse the system prior to each run . The rinse water was circulated for ten minutes at room temperature . Makeup water for each test was prepared separately and was pumped into the water heater . It remained the same for all trials .
FFA technology was also tested for efficacy in open cooling systems using a small circulating water loop with linear polarisation resistance ( LPR ) corrosion probes ( Figure 3 ). Make-up water for each trial was prepared directly in the bulk tank . The entire system was triplerinsed with deionised ( DI ) water between each run .
56.78L of DI water were added to the feed tank prior to salt additions . Salts were left to mix for 30 minutes in the bypass loop before treatment
NOV / DEC 2023 SPECCHEMONLINE . COM
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