Yellow metal corrosion protection is critical in refineries and petrochemical plants, which use heat exchangers that contain copper-based components for their high thermal conductivity and antifouling properties. Cooling water is inherently corrosive to copper and its alloys, requiring a robust, halogen resistant corrosion inhibition treatment to protect the metal surface.
Continuous disinfection is common in cooling water systems, making corrosion prevention challenging. Ultimately, this could result in the heat exchangers’ failure and cause a loss of production and downtime for replacement.
Triazole chemistries are traditionally the most used corrosion inhibitor for this application. The triazole market has historically been volatile, with individual chemicals experiencing aggressive price swings. In late 2020, key triazoles were hit with a significant anti-trust US import tax increase. Besides being classified as toxic or harmful to the environment, these commodity chemicals usually exhibit high consumption rates due to biocontrol practices.
Nalco Water has developed a novel yellow metal corrosion inhibitor with improved halogen stability, lower consumption rates, and reduced aquatic toxicity than traditional triazole compounds to address these issues. This product is manufactured with primary sourced raw materials, providing consistent supply at predictable costs.
Yellow metal corrosion control
The asset integrity of critical yellow metal heat exchangers is paramount, which means that the key performance indicators (KPIs) of yellow metal corrosion control typically focus on the following:
- Corrosion rates below 0.2 mils/y metal loss (mpy).
- No galvanic corrosion on mild steel.
- Soluble copper below discharge limit.
- Low inhibitor consumption by halogenation.
The amount of soluble copper discharge allowed is dependent on the local regulations and is often based on the sensitivity of the receiving water body to metal toxicity. Some countries may also have regulations in place to drive cooling water treatment practices towards the use of chemistries with the least environmental impact.
Meeting these KPIs is challenging when there are adverse water conditions:
- Process control issues: frequent pH dips or spikes in free residual chlorine (FRC).
- High ammonium levels, e.g. through the use of recycled municipal effluent.
- High halogenation events (organics ingress; leaks or change in surface run-off).
- High cycles and long holding time index (HTI) with a build-up of aggressive ions (sulfate, chloride).
With water scarcity on the rise globally, these adverse water conditions are becoming increasingly common. Therefore, process control is more critical to buffer against the effects of water quality changes.
The new inhibitor developed by the Nalco Water Global Research teams showed consistently improved corrosion when compared to tolyltriazole (TT) and benzotriazole (BZT ) both in benchtop testing, as well as pilot cooling tower (PCT) trials. These tests were run at identical operation conditions and chemical dose rates.
The benchtop gamry test results show the average coupon corrosion rate between brass and copper. The PCT results display the averages by the coupon’s metallurgy, heat transfer tube, and online corrosion probe.
One of the major concerns with triazoles is their chemistry’s aquatic toxicity, particularly in Europe, as the environmental profile is gaining in importance significantly.
All triazoles tend to form halogenated species under typical biocontrol conditions in industrial cooling systems. These compounds tend to display higher aquatic toxicity than the parent triazole. The new inhibitor, however, does not react with chlorine. As such, no halogenated species were detected. An analysis of the recirculating water from PCT studies and industrial towers showed that the chlorinated species were under not detectable.
In addition, the new inhibitor did not generate the characteristic odour known from TT in the cooling tower, and no odour was observed in the indoor PCT studies nor in industrial towers, which means that no volatile chlorinated species were formed.
Industrial cooling tower case histories
Four different industrial cooling systems were used to validate the inhibitor’s performance. During the trials, the industrial cooling water system dynamics increased the stresses associated with corrosion inhibition due to very long holding time indexes, challenging make-up water chemistries, or poor continuous chlorination practices for FRC control. This methodology ensured a challenging treatment during the trials. The results presented a low corrosion level in the industrial cooling towers, demonstrating the new inhibitor’s efficacy.
The industrial trial results showed three to five times better corrosion control and lowered the amount of soluble copper in the recirculating cooling water and blowdown. These findings have been replicated in a wide range of cooling tower conditions as the novel technology is now more widely used across various industries.
Water scarcity and environmental restrictions make it critical to have a better choice for a yellow metal corrosion control program, and new developments are needed to meet KPIs in this challenging environment. Simultaneously, environmental stewardship is increasing in importance. New corrosion inhibitors can help protect both the assets and the water at industrial sites through their low aquatic toxicity and corrosion protection, lowering copper discharge to the environment.
Written by Daniel Meier and Renate Ruitenberg, Nalco Water, an Ecolab Company, USA.
Read the article online at: https://www.hydrocarbonengineering.com/special-reports/10112020/yellow-metal-corrosion-control/
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