Addressing De-Zincification and Stress Corrosion Cracking in Brass Heat Exchangers

In an exclusive interview with Industry Outlook Magazine, Jyoti ShankarJha, Associate Lead Scientist at Alleima, discusses the risks of de-zincification in brass heat exchangers, its acceleration by aggressive environments, and the benefits of using hyper duplex stainless steel for improved reliability and durability in such conditions. He has over nine years of experience in new product development, material science, and structural integrity. He specializes in material modeling, thermomechanical processing, and mechanical testing, with notable contributions at Bharat Forge Ltd and Sandvik.

Can you elaborate on how de-zincification poses a significant threat to brass heat exchangers, particularly when zinc content exceeds 15%? What are the critical signs engineers should watch for in such scenarios?

De-zincification is a critical issue for brass heat exchangers, especially when the zinc content exceeds 15%. Despite adding inhibitors like antimony, arsenic, or phosphorus, this phenomenon can't be completely avoided. Zinc, being less noble than copper, dissolves preferentially in aggressive environments, leaving behind a brittle, porous copper-rich layer. This severely compromises the mechanical integrity of the tubes.

In one of our investigations with aluminum brass (C68700) tubes used in an oil refinery, we observed significant de-zincification that led to metal loss and premature failure.

To prevent such failures, we recommend engineers to vigilantly monitor for early signs, such as surface discoloration—usually a reddish or brownish surface from zinc depletion—pitting, wall thinning, porosity, or leaks. Detecting these indicators early can help mitigate risks and plan maintenance proactively.

How do aggressive environments containing chlorides, sulfides, and Ammonia accelerate de-zincification and stress corrosion cracking (SCC) in brass components? Are there specific monitoring or mitigation strategies you would recommend?

If chlorides, sulfides, and Ammonia are not effectively controlled, they can create a highly aggressive environment for brass tubes, leading to rapid degradation. Chloride ions, for example, compromise the protective oxide layer, resulting in selective zinc leaching and pitting corrosion. Elevated concentrations of chlorides and sulfides (H₂S) further weaken the material, and under tensile stress, they significantly accelerate stress corrosion cracking (SCC).

Ammonia aggravates the problem by reacting to form ammonium bisulfide, a highly corrosive substance that also causes SCC. Moreover, Ammonia can dissolve in condensed water, leading to external surface corrosion and wall thinning in brass tubes. To mitigate these risks, regular inspections and environmental monitoring are essential.

While protective coatings and corrosion inhibitors can offer some level of protection, upgrading the material of construction (MOC) often proves to be the most effective solution. Switching to more corrosion-resistant materials, such as those from the duplex stainless steel family, like Sanicro® 35 or optimizing tube design to minimize stress can significantly improve durability and extend service life.

Employing a combination of these strategies ensures enhanced reliability and long-term performance of heat exchangers in such challenging environments.

Could you share insights from a case study where aluminum brass tubes experienced rapid failure due to de-zincification and exposure to Cl, S, and Ammonia in an oil refinery environment? What were the key lessons learned?

In an oil refinery, aluminum brass tubes (C68700) used in a heat exchanger were expected to operate for two years but failed within six months. The primary cause of failure was de-zincification, exacerbated by an aggressive environment containing chlorides, sulfides (H2S), and doses of ammonia gas. Visual inspection revealed surface discoloration, wall thinning, pitting, and cracks, while an SEM-EDS analysis confirmed significant zinc depletion, leaving behind a porous copper-rich structure. Sulfur and chlorine accumulation on both the shell and tube sides further accelerated the corrosion process. Cracks along grain boundaries indicated the possibility of ammonia or sulfide stress corrosion cracking (SCC).

The key lessons learned were:

• Brass is not suitable for seawater applications and other aggressive environments such as chlorides, sulfides, and Ammonia.
• Metallurgy should be upgraded. Cupronickel alloys (Cu-Ni 90/10, Cu-Ni 70/30) are alternatives but are highly prone to various forms of corrosion (pitting, SCC, etc.) when exposed to seawater. The hyper duplex grades UNS S32707 (SAF™  2707HD) and Sanicro® 35 are more suitable for such applications, as they can withstand very corrosive environments and have higher SCC resistance.

In your view, why is it crucial to consider the long-term consequences of material selection, especially in environments prone to de-zincification and SCC?

Considering the long-term implications of material selection is essential, especially in environments prone to de-zincification and stress corrosion cracking (SCC). Brass is an economical choice with excellent heat transfer properties, but its performance is limited to less aggressive environments. In harsh conditions, such as refineries, brass often fails due to its susceptibility to corrosive media. This is analogous to the story of the ‘hare and the tortoise’—stainless steel, despite its slower heat transfer, proves more reliable and durable over time.

Material selection is not merely about reducing initial costs; it’s about ensuring long-term operational reliability, minimizing downtime, and mitigating safety risks. Failures in materials like aluminum brass can result in production losses, expensive maintenance, and safety hazards. Engineers must, therefore, evaluate material compatibility comprehensively, considering chemical exposure, operating parameters, and potential failure mechanisms.

Upgrading to advanced materials such as duplex stainless steels might seem costly upfront but delivers long-term benefits, including extended service life, reduced maintenance needs, and improved safety.

In conclusion, prioritizing long-term durability and compatibility in material selection ensures enhanced reliability, safety, and cost-efficiency, making it a vital practice in challenging environments.

What makes hyper duplex stainless steel (UNS S32707) a promising alternative to brass for environments susceptible to de-zincification and SCC? Could you provide examples of its successful implementation in similar settings?

Hyper duplex stainless steel SAF 2707 HD (UNS S32707) is also known as a highly effective alternative to brass in environments prone to de-zincification and stress corrosion cracking (SCC) thanks to its remarkable combination of corrosion resistance and mechanical strength. Its dual-phase ferrite-austenite microstructure provides excellent resistance to pitting, SCC, and crevice corrosion while maintaining superior mechanical properties. With a Pitting Resistance Equivalent (PRE) number of 48, SAF™ 2707 HD (UNS S32707) demonstrates outstanding performance in aggressive environments containing chlorides, sulfides, and ammonia—conditions where brass often fails prematurely.

A real-world example of its successful application comes from an oil refinery, where aluminum brass tubes in a heat exchanger experienced early failure due to de-zincification and SCC. These tubes were replaced with SAF 2707HD HD (UNS S32707), which has since operated reliably for over a decade. To date, the heat exchanger shows no signs of pitting, de-zincification, or SCC despite enduring the same harsh operating conditions. This case underscores the long-term reliability, durability, and cost-effectiveness of hyper duplex stainless steel in demanding applications.

How can research on material behavior in aggressive environments guide engineers in the process industries to improve equipment reliability and minimize downtime? What factors should they prioritize during material selection?

According to the latest NACE report, corrosion accounts for approximately 4% of global GDP loss annually, highlighting the urgent need for improved corrosion monitoring and advancements in metallurgical solutions for aggressive environments. Research on material behavior plays a critical role in addressing these challenges by identifying failure mechanisms, understanding corrosion processes, and developing effective mitigation strategies. Introducing new materials with enhanced corrosion resistance and superior mechanical properties can be transformative, significantly reducing downtime and improving operational efficiency. There are many such examples where new materials such as Sanicro 35 and SAF™ 2707HD minimized the downtime and improved equipment reliability significantly.

To guide material selection, we recommend adopting the STAR approach, which provides a structured framework for decision-making:

• S: Scenario Evaluation of the aggressive environment, including factors like pitting corrosion, SCC, or de-zincification.
• T: Think about all associated costs, safety risks, and environmental impacts over the material’s lifecycle.
• A: Ask technical experts for insights and guidance on material performance under specific conditions.
• R: Partner with a Reliable supplier who can ensure consistent product quality and long-term support.

By following the STAR approach, engineers can make well-informed material decisions that enhance equipment reliability, minimize unplanned downtime, and ensure sustainable, cost-effective operations, even in the most demanding environments.