Eco-friendly corrosion control

Kai Zhang and Renate Ruitenberg, Nalco Water, an Ecolab Company, USA, discuss an innovative non-phosphorus and non-zinc mild steel corrosion control programme for cooling water systems.

Phosphorus was recognised globally as the primary growth-limiting nutrient for algae in surface water bodies (e.g. lakes and rivers) in 1972.1 Excessive biomass growth due to phosphorus nutrient in lakes and rivers can cause reduced light penetration, death of algae, and subsequent depletion of oxygen in water, which results in the death of fi sh and other aquatic life (known as eutrophication).2 In the last 20 years, the phosphorus discharge restrictions from cooling systems have been reinforced to control algae blooms in many large lakes in

China. In the US, states such as Ohio, Illinois and Iowa are joining a growing list of states to ramp up efforts to reduce phosphorous discharge and control algal blooms in lakes, ponds, and reservoirs.3 Similarly, the use of zinc is restricted in some regions due to its toxic nature, and industrial producers have started to seek more environmentally friendly treatment solutions, both to comply with the changing legislation and achieve their performance targets.

Table 1 summarises the phosphorus (P) and zinc (Zn) restrictions in various countries. More countries and regions have or plan to have tighter control on wastewater discharge to achieve sustainable development. Refi neries and petrochemical plants use evaporative cooling systems to control the temperatures of their critical processes and eliminate excess heat. When water evaporates, the salts will concentrate, and without adequate conditioning, both corrosion and scaling will occur. Water treatment chemistries are very effi cient in controlling the occurrence and cost of corrosion, scaling, and biofouling. Historically chromate was used, but this has since been banned as safety and environmental awareness increased. Phosphate (PO4) combined with zinc took over from chromate as key components in corrosion control to extend the run length and asset life. For effective corrosion control, the phosphate concentration needs to be adjusted based on the pH and concentration of chlorides, sulfates and calcium in the cooling water, as well as heat exchanger skin temperature. Precise control of polymer concentration is required in the stabilised phosphate programmes. Meanwhile, eutrophication can result from these inorganic phosphates if sensitive water bodies receive the cooling water blowdown. Changes in makeup water matrices can lead to severe scale or corrosion events if the stabilised phosphate programme is not adjusted appropriately.

With the increasing responsibility of sustainable development and industrial users adopting a ‘reduce, reuse and recycle’ strategy to save water, wastewater has been reused in cooling applications. This practice leads to signifi cant challenges of corrosion and scaling control with a conventional chemical treatment programme.4

Since the early 2000s, various non-phosphorous programmes have been tried and evaluated for corrosion and scale control in cooling systems. Without phosphate as an anodic corrosion inhibitor, success was achieved at a limited operation window, such as high hardness and high alkalinity. Nalco Water developed a non-phosphorous

programme based on different chemistry innovations in the past few decades.5

This article presents the latest innovation that delivers on both environmental and performance goals at reasonable treatment costs. The new non-phosphorous programme showed signifi cantly improved performance in broad application conditions, from soft water to medium to high calcium and alkalinity waters and water with high chloride content. In addition, this programme application can be further expanded to non-phosphorous and non-zinc application window.

Solution

The novel and cost-effective non-phosphorus cooling water treatment programme was developed to solve the challenges with stabilised phosphate programme. The non-P programme includes a proprietary dual function organic corrosion and scale inhibitor and an inorganic-based corrosion inhibitor. The newly-developed non-phosphorous programme has the following advantages compared to conventional non-phosphorus: Wider application window covering medium to high

Ca/alkalinity water, soft water, and high conductivity water with high chloride. Tolerant to both fluctuating make-up water and tower water matrix. Enable operation at higher (8 – 9) or pH dips to 7 with its robust scale and corrosion control. Able to handle high cycles (10) and long holding time index (HTI) (200 hr) application. Automatic dosage monitoring and control of both inhibitors with fluorescence-based technology.

Case history 1

A major petrochemical plant in China aimed to improve its cooling water treatment both to Table 1. Phosphorus and zinc restrictions by comply with the Total P legislation (<0.5 ppm P as countries and regions phosphorus) and achieve asset protection key Country/region Regulation examples Drivers performance indicators (KPIs) mild steel corrosion rate China <0.5 ppm P legislation, <0.5 mils per year (mpy) and copper corrosion rate <0.1 mpy. non-P cooling Since its start-up in 2012, Nalco Water applied the very treatment required fi rst generation of non-phosphate programme in this system. After nearly 10 years of reliable operation, this cooling system had unexpected contamination of organic compounds and sulfi de compounds, leading to high bacterial counts in the cooling water. The high dosing of bleach required introduced high corrosion rates, causing a lower life span of assets and high maintenance costs. The high effi ciency biocide chlorine dioxide from PURATETM technology was applied to replace bleach, which substantially reduced the chloride level. The PURATE chlorine dioxide programme is designed to maintain and optimise critical water treatment applications.6 Yet, the existing non-phosphate programme was not robust enough to protect critical heat exchangers. An occasional high corrosion rate was observed due to fl uctuation of key parameters, e.g. pH, TOC, conductivity, etc. After switching to the new generation of non-P programme the overall cooling water chemistry was maintained at similar condition with conductivity of approximately 2000 μs/cm (Figure 1). The mild steel corrosion rates were much improved in both online Nalco Corrosion Monitoring (NCM) probe readings (Figure 1) and corrosion coupons (Figure 2). The average corrosion readings by the 3D TRASAR controller NCM probe fell from 1.3 mpy to less than 0.3 mpy. Before the switch to using the new programme, the corrosion coupons showed some localised corrosion. After the switch, general corrosion rate was well controlled below 0.1 mpy without pitting corrosion based on one-month corrosion coupon reading.

Algal blooms: human health and drinking water supply risks Japan Environmental Impact Study (EIS) may result in non-P, non-Zn in certain areas

Tourism and aquaculture protection USA Depending on P-levels from agricultural run-off in water

Eutrophication and phosphate scaling risks European Union Water Framework Directive, variable limits

Risk assessment of receiving water body (quality and use) Saudi Arabia 1 ppm Zn and 1 ppm P discharge limit to Red Sea

Environmental and coral reef protection for tourism Argentina Local restrictions P <1 ppm discharge limit

Figure 1. Online Nalco Corrosion Monitoring (NCM) mild steel corrosion rate and cooling water conductivity. River water protection for drinking water intake Figure 2. Mild steel (MS) coupons before and after NexGen non-P programme.

With the new generation of non-P treatment programme, the plant was able to: Fully comply with total phosphorus discharge limits and reduce wastewater treatment costs by

US$0.5 million/yr. Increase production throughput and profit by

US$3 million/yr. Reduce maintenance cost and extend asset lifespan.

Case history 2

A major integrated refi nery and petrochemical complex in China adopted enhanced sustainability goals to save more water by reusing wastewater into its cooling system. The Total P discharge limit is <0.5 ppm P as phosphorus and for asset protection the KPIs are: Mild steel corrosion rate of <0.5 mpy. Copper corrosion rate of <0.1 mpy.

With the make-up source alternating between river water and reused wastewater, containing high chlorides and high calcium, the assets are posted to high corrosion and scaling stress at the same time. Nalco Water researchers and industrial technical consultants conducted a total plant survey to understand the system challenge including mechanical, operational, and chemical limitations. After a thorough study and pilot cooling tower simulation, a further stretched application window of a novel non-P non-Zn programme was implemented to the system.

The critical component of success on this programme was 3D TRASAR technology for cooling automation platform as a stress management system. This technology could control the key parameters in the cooling water system using online monitoring. Based on the real time monitoring, feeding of chemicals was able to respond to

the dynamic changes in the system. Meanwhile, the cooling water system was remotely monitored by Nalco Water System Assurance Centre, allowing actionable data visibility. After starting the non-P non-Zn programme, the overall cooling water chemistry was maintained at similar condition with conductivity around 2500 μs/cm (Figure 3). The mild steel corrosion rates were progressively decreased to less than 0.2 mpy from previous more than 0.5 mpy (Table 2). Through the onsite automation and remote digital platform, excellent operational reliability in Table 2. New non-P non-Zn treatment results high stress heat exchangers under low fl ow or high heat fl ux Cooling water was achieved with signifi cant savings on total operation parameters cost and productivity increase. The platform was able to: Fully comply with total phosphorus discharge limits. Save freshwater consumption by 1 million tpy. Reduce deposition and fouling in heat exchangers, as well as maintenance costs.

Conclusion

Innovations tailoring the corrosion control treatment to local regulatory requirements have led to a large improvement in both corrosion control and effl uent quality, showing that protecting the environment can coincide with reductions in total cost of operation. The new generation of non-P programme combines a proprietary organic dual function inhibitor (corrosion and scale inhibitor) with inorganic corrosion inhibitor to generate a synergistic programme. It has been proved to be effective over a wide range of water matrices, such as soft water, high conductivity water with high chloride, and medium to high calcium and alkalinity water. It provides reliable corrosion inhibition performance without introducing CaPO4 scale stress or phosphorous nutrient for bio-growth. Furthermore, automated programme control through Nalco Water 3D TRASAR technology and System Assurance Centre can handle a dynamic water matrix including reused wastewater. This advanced total solution also enabled corrosion and scale control with non-P non-Zn application in a highly stressed water matrix, which poses signifi cant challenges for a stabilised phosphate programme.

Target Actual reading

Total phosphorus <0.5 ppm <0.1 ppm Zinc <0.2 ppm Chlorides <600 ppm 300 ppm Cl Calcium, as CaCO3 <700 ppm 500 ppm CaCO3 Mild steel corrosion rate <0.5 mpy <0.2 mpy

Figure 3. Online NCM mild steel corrosion rate and cooling water conductivity.

System challenge is high organic contamination from reused process water

References

1. PELIPE, M., FULMER, D., SANDU, C., GUO, B., and NGUYEN, K., ‘Novel and Efficient Non-Phosphorous Cooing Water Corrosion Inhibitor’,

Cooling Technology Institute, TP 16-50, Houston, Texas, US, (2016). 2. CHISLOCK, M. F., et al., ‘Eutrophication: Causes, Consequences, and

Controls in Aquatic Ecosystems’, Nature Education Knowledge, (2013). 3. MAHER, K., and MCWHIRTER, C., ‘Midwest States Target Algae

Blooms in Waterways’, Wall Street Journal, (28 September 2018), https://www.wsj.com/articles/midwest-states-target-algae-bloomsin-waterways-1465772363 4. GILABERT-ORIOL, G., et al., ‘Wastewater reuse for industrial applications in cooling towers’, EMChIE 2015, Tarragona, Spain, (10 – 12 June 2015). 5. XIE, Y., et al., ‘A Novel Non-Phosphorous Cooling Water Treatment

Program with Robust Scaling and Corrosion Control’, NACE

Corrosion 2019, Paper 13009, , Nashville, Tennessee, US, (24 – 28 March 2019) 6. FALLET, L., and RUITENBERG, R., ‘Biofouling Mitigation’,

Hydrocarbon Engineering, (December 2021), pp. 25 – 28.