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article imageQ&A: Solution to the challenge of water phosphate regulations Special

By Tim Sandle     Mar 23, 2020 in Science
Phosphate and phosphonate chemistries can help to control water systems. However, they can lead to dangerous levels of microbial growth. To overcome this, a nutrient-free industrial cooling corrosion control program has been developed.
To learn more about the technology, Digital Journal spoke with Paul Frail, an expert from SUEZ WTS. Frail recently presented the technology at the ACS National Meeting & Expo this March. Frail's technology is a non-phosphate industrial water corrosion control technology as a solution to the challenge of ongoing phosphate regulations.
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Digital Journal: How does water corrosion occur?
Paul Frail: There are numerous metal corrosion mechanisms under aqueous conditions. General, localized, microbiological, under deposit, and flow assisted corrosion are several common examples. The specific type of corrosion depends on the local environment and water composition. There are two general reactions that occur at the metal interface during a corrosion event. The anodic reaction is where the electrons within the metal are trying to find an active site to transfer their electrons to the cathodic reaction. This is partly driven by a metal’s thermodynamic drive to achieve the highest oxidation state possible. At the cathodic reaction, oxygen or an oxidizer (such as bleach) are willing to accept the metal’s electrons converting oxygen or oxidizer into a hydroxide ion for oxygen, chloride for bleach, or other anions for alternative oxidizers. The salts in the aqueous medium act as a bridge for the electron reaction and help drive the reaction by facilitating the dissolution of metal cations.
Chlorides and sulfates are examples of anions that accelerate corrosion reactions due to their ability to form soluble salts with metals. When the anodic and cathodic reaction occur equally on a metal surface this results in general corrosion. The reactions can become localized to a small area on the metal surface resulting in localized corrosion or pitting. Microorganisms often attach to metals surfaces forming biofilms and cause localized corrosion as their respiration process generates chlorides and sulfates. When a deposit forms on a metal surface, it results in a high concentration of anions at just beneath the edge and accelerates the corrosion reactions.
DJ: What are the significance of corrosion?
Frail: Corrosion is a problem that is often tolerated or mitigated in industrial systems. If a corrosion control program results in general corrosion, the life expectancy of the metal can be estimated and appropriately planned for replacement. The most detrimental form of corrosion is localized or pitting. Here, the corrosion process occurs at an accelerated rate in a small area. In a short and unanticipated time, localized corrosion can form sizable holes in metal piping. Metal failure can occur resulting in the potential for unsafe leaks, pipe bursting, or cross contamination within an industrial process. Corrosion reaction products can also form scale.
Scale in industrial systems can result in reduced flow within a pipe or interfere with industrial processes that require heat transfer in a heat exchanger. Scale can also promote microbiological growth that can form a biofilm. Similarly, biofilm can reduce flow and interfere with the heat transfer process. Scale and microbiological growth each promote localized corrosion reactions.
DJ: How does corrosion vary regionally?
Frail:The approach to mitigating corrosion changes regionally since water quality changes as do environmental permits. Water ion composition varies significantly depending on the geographical location, geological environment, and water source. The more chlorides and sulfate present in the water, the more challenging the water is to mitigate corrosion. Similarly, more hardness (Calcium and Magnesium ions) can make the water easier to handle if there is enough carbonate or M-alkalinity in the water. Corrosion control programs are tailored specifically based on the water quality and system. Depending on local permits, one may be able to use a metal like zinc or phosphate or phosphorus-containing chemistries for corrosion control. Zinc and phosphorus restrictions may exist in one region versus another and an alternative approach would need to be adopted for a particular water stream.
DJ: What are the conventional solutions to address corrosion? What are the problems with these?
Frail:For industrial systems, they used heavy metals, such as chromate or molybdate. These were very effective inhibitors, but were eliminated from industrial use to the exposure of their detrimental side effects to the environment and people. The replacement of the use of heavy metals led to the development of anionically charged organic polymers and phosphonate chemistries. Through injection with these treatments, one could use inorganic phosphate as a corrosion inhibitor. With the discovery of sulfonic acid polymers, phosphate-based programs became very effective for industrial systems. The phosphate would interact with the metal surface and form insoluble metal phosphate or the calcium in the water would form passivating film on the surface. Phosphate programs became very effective programs and were considered environmentally friendly and benign to people. The negative aspects of the use of phosphate is that it is a micronutrient that can promote algae blooms or microbiological growth. Additionally, even with advanced polymer technology, conditions exist where scale and fouling from a phosphate program cannot be mitigated effectively.
DJ: How can non-phosphate industrial water corrosion control help?
Frail:When industrial systems made the switch away from chromate programs in the 1970s and 80s, it was a significant advancement. The progression away from phosphate and zinc is a similar achievement for industrial systems. SUEZ has been working steadily on non-phosphorous solutions for industrial systems as early as the 1990s. More recently, SUEZ has been focusing efforts on how passivation films form under various water conditions and chemical treatments. This has led to the development and launch of our E.C.O.Film (Engineered Carboxylate Oxide) technology, which is a non-phosphorusŦ corrosion control program. E.C.O.Film technology does not use phosphate as a corrosion control agent and lowers the amount of phosphorus in the effluent. This reduces the scaling tendency within the system and helps mitigate algal blooms by limiting the micronutrient.
DJ: In what way is this a superior solution?
Frail:SUEZ has gone through several iterations in the development phase of a non-phosphorus corrosion control solution. More recently, efforts were focused on understanding how protective films naturally form on metal surfaces in relation to the water quality and chemical treatments. Using this knowledge, we developed a bottom up approach to non-phosphorus corrosion control. Through our phosphate programs, we learned that chemical treatments play an active role in constructing protective films on metals surfaces that block the corrosion reactions from transferring an electron. We developed scale and corrosion inhibitors that feature only carbon-hydrogen-oxygen elements, or CHO inhibitors, that work with the ions in the water to form the passivation layer.
When the water quality is insufficient to form a passivation film with just a CHO inhibitor, we found a surface film formation catalyst that helps extend the window of water characteristics, such as E.C.O.Film technology, can be applied. This technology provides a solution for customers that have strict discharge regulations for phosphorus and zinc, eliminates fouling associated with metal phosphate salts, and provides the customer environmentally friendly chemistry at a time when customers are more aware of their social responsibilities.
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