A guide to protecting cooling systems

Save to read list Published by Bella Weetch, Editorial Assistant Hydrocarbon Engineering, Wednesday, 11 January 2023 09:00

Cooling systems in many downstream oil and gas plants are comprised mostly of carbon steel piping, while the heat exchange surfaces utilise primarily carbon steel and yellow metals (copper-bearing alloys). As a result, both carbon steel and copper corrosion control are critical to maintaining system reliability and maximising asset life. Fundamentally, the corrosion process drives all metals towards their highest oxidation state, resulting in the formation of an oxide layer on the surface. Over time, this oxide layer slows the electron transfer process until the cationic metal ion dissolves into the cooling water via chlorides or sulfates. Yellow metal corrosion is a notable challenge, as it can not only cause the tube bundles themselves to fail, but it induce aggressive galvanic corrosion pitting on carbon steel surfaces. Environmentally, the release of copper corrosion byproducts into the cooling water can impact discharge compliance.

Initially finding use as a paint additive for corrosion, mold, and microbial control, benzotriazole and azole molecules have long histories as chemical treatments for industrial applications. Industrial water treatment has ubiquitously used benzotriazole derivatives for yellow metal (copper, copper nickel, and admiralty heat exchangers) corrosion control for approximately 75 years.

Azole-based technology does have drawbacks and limitations. The primary negative aspects include elevated aquatic toxicity, the generation of adsorbable organic halides (AOX), and the loss of metal surface passivation when exposed to oxidisers.

To maintain microbiological control in industrial cooling water treatment, it is common practice to feed an oxidiser, either continuously or intermittently. The most-used oxidisers include sodium hypochlorite, bromine, chlorine dioxide, peroxide, or ozone. Unfortunately, oxidisers can circumvent the cathodic reaction in a corrosion cell, drive a higher oxidation potential, and accelerate both general and localised corrosion. When an oxidising biocide is added to industrial cooling water that is treated with an azole, a cross-sectional transmission electron microscopy (TEM) image of the metal surface illustrates that the previously-uniform passivated surface is now porous and discontinuous (see Figure 1).

Figure 1. A: cross-sectional TEM image of a TTA film on an admiralty brass (ADM) surface, formed in the absence of hypochlorous acid. B: cross-sectional TEM image showing the hypochlorous catalysed displacement of TTA on the metal surface.

As a result, there is often a required trade-off between yellow metal corrosion control and microbial control. Despite extensive research into the properties of benzotriazoles and azole compounds as film-forming corrosion inhibitors, industrial water treatment facilities often experience a wide range of unsatisfactory results: yellow metal corrosion rates exceeding the industry standard of maximum 0.2 mpy, dealloying of yellow metal alloys (dezincification), galvanic corrosion, and elevated copper levels in the effluent.

More recently, material availability is another limitation that has complicated traditional azole technology use. Supply chain issues around raw materials, government-imposed tariffs, and shipping delays have affected supply and driven costs up significantly, prompting end users to seek out new technologies that reduce their dependency on this technology.

State-of-the-art surface science technology (e.g. XPS, ToF-SIMS, TEM) facilitates the understanding of how passivation films are constructed at a molecular level on metal surfaces. This has allowed for the development of a novel ‘engineered’ protective system of components that form an electrically-insulating barrier to inhibit corrosion. The system works by forming a dynamic ‘co-film’ that integrates low-level azoles, patent-pending chemistry, and various salt colloids present in cycled water chemistry.

The engineered copper passivation (ECP) system addresses several of the deficiencies of traditional azole programmes, with the added benefit of providing equal or improved asset protection, even under stressed conditions. The tenacious passivation film formed by ECP can handle elevated chlorides (> 1500 ppm), affording end users the ability to use grey water or other recycled water sources in their cooling systems, without sacrificing asset protection. The ‘co-film’ is extremely stable in systems that lean heavily on oxidisers for microbiological control. Field tests in a refinery have demonstrated that the passivation film can withstand free chlorine residuals of up to 100 ppm for several days, without loss of protection.

Environmentally, the additives in this new technology generally have lower aquatic toxicity and have been shown to reduce the generation of AOX by up to 50%. Furthermore, this technology significantly reduces end users’ dependency on azole chemistry, mitigating supply chain issues. Surface analysis of coupons shows an average 80% reduction in nitrogen content on the ECP-treated systems. Nitrogen is a signature element for the presence of azoles on the metal surface, validating the reduction in dependency on azole chemistry for corrosion protection.

In the past, end users have been reluctant to move away from proven chemistries such as tolytriazole, benzotriazole and chlorinated tolytriazole. Even though other technologies for copper corrosion control have been considered, the potential benefits have not exceeded the cost of change enough to prompt action. However, the global supply pressures mentioned above, coupled with downstream oil and gas processing facilities switching to alternative and more challenging water supply sources, have provided additional incentives to justify the re-examination of new technologies.

A large refinery/petrochemical complex in southwest US developed and enacted a trial plan to evaluate the benefits of the ECP technology compared to their traditional azole programme. The bank of towers chosen for trial had a history of yellow metal corrosion rates exceeding the targeted 0.2 mpy due to heavy use of sodium hypochlorite to control microbiological activity. The towers utilised a chlorinated triazole for their yellow metal corrosion protection.

The cooling towers selected for trial at the refinery rely heavily on sodium hypochlorite, producing elevated levels of free chlorine to control microbiological fouling for extended periods of time. The long-term free chlorine average approached 2 ppm as chlorine, with daily readings occasionally exceeding 2 ppm as chlorine (see Figure 2). While preventing microbiological contamination and minimising the potential for pathogens, this high free halogen residual would negatively impact the previous azole-based yellow metal treatment and lead to higher-than-desired corrosion rates.

Figure 2. Typical cooling tower free chlorine residuals (ppm).

As the site investigated the ECP technology, the main challenge to address was validating increased protection in the face of continuously high free chlorine and/or cooling towers with chlorine control issues that result in periodic, unpredicted high free chlorine concentrations. A series of bench tests were developed and conducted, followed by a limited-scope field trial, to measure performance improvement under typical operating conditions and ‘stressed’ high free chlorine conditions held for extended periods.

In 2021, the engineered copper passivation trial was initiated on a select tower with adjacent towers running on the traditional azole programme utilised for baseline comparisons. Overall, the transition was smooth, as feed rates were adjusted to phase in the new treatment technology and establish a satisfactory passivation film throughout the open evaporative cooling system. Product compatibility with the traditional azole programme made for an easy transition and minimised the cost of change. Throughout the trial, efforts were made to ensure that variables such as pH, cycles of concentration, chlorine residuals, and concentrations of other treatment programme additives, remained stable. Consistency of these external variables, for both the trial and baseline cooling towers, helped to isolate and validate the impact of the new ECP technology.

Figure 3. Comparison of 30-day admiralty coupon corrosion rates of the ECP treated tower vs a baseline azole treated tower.

Once the new engineered copper passivation film was established, the site began to see improved protection of copper metallurgy. When compared to other cooling towers onsite that are still treated with the traditional azole-based programme (see Figure 3), the ECP-treated tower produced lower admiralty corrosion rates on 30-day coupons in each of the five months during the trial, ranging from 20 to 80% lower. The greatest improvement was seen during months when free chlorine residuals were consistently above 1.5 ppm. This validates the tenacity of the ECP film and its ability to withstand the heavy use of bleach.

The positive impact of the ECP technology could also be seen visually with the inspection of the 30-day admiralty coupons. The engineered film is brighter and shows minimal change to the base metallurgy (see Figure 4). Surface analysis further indicated a 72% reduction in copper content on the coupon surface compared to a traditional azole passivation film. The reduction in copper oxide demonstrates that corrosion rates are lower in ECP-protected systems than azole-protected systems.

Figure 4. Left to right: azole-treated mild steel (1.07 mpy), EPC-treated mild steel (0.29 mpy), azole-treated admiralty (0.74 mpy), EPC-treated admiralty (0.16 mpy).

A secondary benefit of the ECP technology during this trial was a measurable reduction in mild steel corrosion. As stated, the release of soluble copper as a byproduct of yellow metal corrosion can promote mild steel corrosion through galvanic attack. Historically, these towers demonstrated mild steel corrosion rates on 30-day coupons at approximately 1 mpy. During the trial, the mild steel corrosion rates approached 0.3 mpy, which represents a 70% improvement over historical trends.

As a result of the successful trial, other cooling towers are undergoing conversion to the ECP technology where there is justification to do so. It is beneficial for end users to have a choice of technologies to apply as circumstances warrant, improving operating flexibility, and avoiding significant cost increases due to supply chain issues, while delivering equal or better performance. The ease of the transition to the new technology was another positive, allowing for the use of existing injection points, controls, pumps, and tank equipment, further lowering the cost of change.

Based on these early successes in transitioning to the new technology, the site is currently investigating an expanded scope to include how using low or very low hardness water for cooling tower make-up will change corrosion protection. Since the foundation of the technology involves forming protective films using the cooling water's dissolved ions, low hardness waters provide less material for film formation. Another bench study is underway to address this condition, with planned limited field trials to follow. While the study is still underway, early results are positive.

This article was written by Jesse E. Stamp, ExxonMobil, alongside Eric Zubovic and Dr. Paul Frail, Veolia Water Technologies & Solutions.

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/11012023/a-guide-to-protecting-cooling-systems/

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