Eco-Friendly Corrosion Inhibitors

Corrosion inhibition in the cooling water systems of refineries and petrochemical facilities—particularly as environment-friendly treatment programmes gain prominence—has become a subject of discussion. Open cooling water systems consist of critical equipment such as recirculation pumps and heat exchangers, and carbon steel is a material commonly used in this equipment. By nature, carbon steel undergoes corrosion in water, which leads to equipment damage, increased maintenance requirements, and consequently more unplanned downtime at facilities. To prevent unwanted corrosion, cooling water quality must be monitored and treatment programmes containing corrosion inhibitors must be applied. Phosphate, phosphonate, zinc and their combinations have been used for corrosion inhibition for many years. In addition to the effectiveness of these commonly used corrosion inhibitors against corrosion, their environmental impacts when discharged are also very important. These substances have low biodegradability and the release of heavy metals such as zinc into the environment should be prevented. In recent years, increased environmental awareness has imposed much stricter limitations on the discharge of corrosion inhibitors. Kurita Europe has developed a new corrosion inhibitor that complies with these limitations and provides strong protection against corrosion: Modified Phosphorus-based Organic Acid (PMOA). This corrosion inhibitor is based on natural and renewable organic acids and is biodegradable by nature.

Performance Corrosion Inhibition

The development objective of PMOA was to create a molecule with strong protection against corrosion while being environment-friendly. Starting from this basis, the corrosion inhibition performance of PMOA was first compared with standard inhibitors at Kurita's European Research and Development Centre. The results of these tests and the effectiveness of PMOA are shown in Figure 1. Zinc, phosphate and polyacrylic acid were used to represent advanced but low-biodegradability corrosion inhibitors. Additionally, polyaspartic acid was also used and serves as an example of an environment-friendly inhibitor. The quality of the water in which the inhibitors were tested is shown in Table 1. PMOA showed slightly better performance than zinc, and corrosion prevention efficiency exceeded 90%, demonstrating the potential of the newly developed corrosion inhibitor. Polyaspartic acid and polyacrylic acid are known as effective anti-scale agents; however, as expected, their corrosion inhibition capacity is quite low. After confirming the strong corrosion inhibition performance of PMOA, more detailed evaluations were carried out by varying important parameters such as water quality and inhibitor concentration. In the tests conducted, the effects of pH, water hardness, different chloride concentrations and particularly water temperature were observed.

Effect of Calcium Concentration and pH

Cooling water systems operate at calcium concentrations across a very wide range. Sometimes water with low or no hardness, such as desalted water, may be used. On the other hand, feed water with high hardness values is also used, and thus the calcium hardness of cooling water can easily exceed 10 mol/m3 (1,000 g/m3 as CaCO3) or higher. For this reason, corrosion tests were conducted at different Ca2+ concentrations—ranging between 0 and 10 mol/m3. As shown in Figure 2, PMOA demonstrates effectiveness against corrosion even in the absence of calcium. Furthermore, the corrosion inhibition effectiveness of PMOA remains approximately 70–80% up to a Ca2+ concentration of 10 mol/m3. A series of further tests were conducted to examine the corrosion inhibition performance of PMOA at different pH levels. The results of corrosion tests on carbon steel are shown in Figure 3 as a function of the pH value of the test water. Cooling water systems typically operate at pH levels between 7.0 and 9.0, which is why the tests were conducted in this range. The test results demonstrated that the corrosion inhibition effectiveness of PMOA is independent of the classical cooling water pH value range.

Chloride and Temperature Variation

Chloride concentration in cooling water can typically vary in the 200–500 ppm range. Chloride is an aggressive ion and reduces the effectiveness of corrosion inhibitors at high concentrations. Furthermore, as chloride concentrations increase, the corrosion inhibition effectiveness of PMOA decreases. However, corrosion inhibition within the general chloride concentration range is more than adequate. To determine the effect of temperature changes in cooling water, tests were conducted at three different temperatures: 25, 30 and 40°C. According to the results, PMOA demonstrates good performance independent of water temperature.

Chlorine Stability

Since oxidising biocides, particularly chlorine or sodium hypochlorite, are frequently used biocides, the corrosion inhibitor must have high stability against them. A series of tests were conducted to evaluate chlorine stability, comparing PMOA with other standard phosphonic acids—aminotrimethylenephosphonic acid (ATMP), hydroxyethylidenediphosphonic acid (HEDP) and hydroxyphosphonoaceticacid (HPA). The results are shown in Figure 4. In terms of chlorine stability, PMOA demonstrates good performance; for example, after a 4-hour test with 1 ppm Cl2, only 21% PMOA loss was observed.

Hardness Stabilization

In addition to having strong corrosion prevention properties, PMOA also demonstrates excellent calcium carbonate inhibition characteristics. Scale inhibition is nearly complete even at extremely low dosages. With this performance, it shows similar results to polyacrylic acid, but significantly better results than polyacrylic acid in terms of biodegradability. Due to its effective corrosion and scale inhibition properties, PMOA can form the basis of treatment programmes applied to cooling systems.     Ecology and Toxicity Profile PMOA is a low-toxicity inhibitor (LD50 mouse >5,000 mg/kg), is non-irritating to eyes and skin, and has no skin sensitization effects. It shows no mutagenic effects. This new corrosion inhibitor has balanced biodegradability characteristics. Being not readily biodegradable biologically, it continues to provide corrosion prevention without significant degradation in cooling systems. However, its biodegradability is sufficient to be degradable under discharge to water or wastewater treatment facilities. Furthermore, the no observed effect concentration (NOEC) applicable to algae and aquatic crustaceans is much higher than expected in surface waters. Consequently, it can be concluded that PMOA application is safe for humans and the environment.

Synergy

To further enhance corrosion inhibition effectiveness, the synergy of PMOA with other corrosion inhibitors was evaluated. For this purpose, tests used PMOA and other phosphate ratios between 0%–100%. Total inhibitor concentration was held constant at 20 g/m3 in all tests. The best corrosion inhibition performance was achieved with a combination of 60% PMOA and 40% standard phosphonic acid. Based on the results of the synergy tests, Product A, a PMOA and standard phosphonic acid-based Aktiphos 4170 formulation, was developed to increase corrosion inhibition effectiveness. Additionally, Product A also contains dispersant and non-ferrous metal inhibitor.

Corrosion Tests in Pilot Facility

The aforementioned formulation was examined in a pilot cooling system to determine corrosion inhibition effectiveness under realistic and critical conditions. The formulation of Product A containing PMOA was compared with two other standard corrosion inhibitors. These are a high-performance corrosion inhibitor HPA and a low-phosphorus product. Both formulations contain dispersant and non-ferrous metal inhibitor. The low-phosphorus product additionally contains ortho-phosphate and amino compounds. According to tests conducted in the pilot facility, Product A demonstrated excellent corrosion inhibition effectiveness after 25 days of use. A corrosion rate of 1.6 mils per year (MPY) was determined, and this result is nearly identical to a high-performance corrosion inhibitor containing 60% more phosphorus. The corrosion coupons used during the test period are shown in Figure 5. It is clearly evident that coupons treated with Product A have extremely low general corrosion rate and minimal pitting corrosion. The low-phosphorus corrosion inhibitor used in the second test was not successful in general corrosion protection and could not prevent pit formation.

Case Study

In an open recirculation cooling system at a refinery in Germany, completely organic treatment has been applied for corrosion control for many years. When the competent authorities began to limit phosphorus concentration in the blowdown system, a product could not be used at a dosage that both met the phosphorus discharge limit and provided good corrosion protection. As a result, the corrosion rate of the completely organic treatment programme could not meet the standard corrosion rate of less than 4 MPY accepted by the customer for carbon steel. Product A is a newly developed PMOA-based product by Kurita with low phosphorus content. By treating the aforementioned system with Product A, excellent corrosion protection was achieved and the system was able to operate at a product concentration that met the strict phosphorus limits. The system data and cooling water quality are shown in Table 2. A comparison of results obtained with two different treatment programmes free of heavy metals is shown in Figure 6. Significant success in corrosion and scale control was achieved with the PMOA-based programme. At the same time, total phosphorus (P) discharge from treatment was reduced by 30%, which significantly facilitated the refinery's wastewater management. Product A significantly enhanced corrosion inhibition, particularly in phosphate contribution to system water. With this product, excellent corrosion inhibition can be achieved without exceeding phosphorus limits in wastewater.

Conclusions

PMOA has opened a new chapter in the development of environment-friendly corrosion inhibitors. By using renewable and natural organic acids and minimal phosphorus contribution, successful corrosion inhibition is possible. PMOA has demonstrated strong performance in corrosion and scale inhibition in laboratory conditions and in difficult-to-treat cooling water systems. It is possible to treat nearly all cooling water qualities at different pH levels and hardness concentrations with PMOA. It also has a good toxicity profile and biodegradable characteristics by nature. Translation: İpek Öztürk / Marketing Manager / Kurita Europe GmbH Translation: Prof. Dr. Ersin Serhatlı / Chemistry Department Faculty of Arts and Sciences / Istanbul Technical University Authors: Dr. Wolfgang Hater Technical Director Kurita Europe GmbH Christian zum Kolk Product Manager Kurita Europe GmbH Ingo Königs Technical Customer Representative Kurita Europe GmbH