Corrosion points in a plate heat exchanger

Hidden Dangers: Unmasking Corrosion in Plate Heat Exchangers

"Comprehensive analysis reveals how pitting, crevice corrosion, and fretting cooperate to cause internal leakage in plate heat exchangers, threatening industrial safety and efficiency."


Plate heat exchangers (PHEs) are indispensable in various industries because of their compact design, ease of maintenance, and high efficiency. From dairy and food processing to chemical industries, co-generation power plants, and central cooling systems, PHEs play a vital role in thermal management. However, the corrugated austenitic stainless steel (ASS) plates, the heart of PHEs, face relentless degradation due to harsh environmental conditions.

In China, heating power plants using PHEs are increasingly experiencing internal leakage, jeopardizing both plant safety and operational efficiency. Studies indicate that local corrosion, leading to cracking and perforation, is a primary cause of these leaks. Traditional assumptions pointed to stress corrosion cracking (SCC) in 304-type ASS plates, triggered by cold working-induced stress and chloride ion build-up. Others highlighted pitting corrosion from high chloride content. However, these explanations often fell short of fully explaining the observed failures.

This article explores a comprehensive failure analysis of 316L plates from a co-generation power plant, revealing a complex interplay of factors beyond simple corrosion. By investigating design flaws, manufacturing quality, and operational conditions, we aim to provide insights into preventing PHE failures and ensuring reliable performance.

The Cooperative Corrosion Culprits: Pitting, Crevice Corrosion, and Fretting

Corrosion points in a plate heat exchanger

The investigation focused on bowl-like perforations regularly distributed at crossing contact points (CCPs) between the corrugated plates. While initial assessments leaned towards typical failure mechanisms, the unique characteristics of these perforations prompted a deeper analysis. It was determined that these perforations resulted from the cooperation of pitting, crevice corrosion, and fretting.

The study highlights the significant pressure difference between the high-temperature hot water (HTHW) and low-temperature hot water (LTHW) within the exchanger. This differential creates a narrower crevice and compressive stress at the CCPs on the LTHW side. The confined space accelerates the accumulation of corrosive elements like chloride ions. Further, the fluctuating water pressure induces fretting, mechanically damaging the surface passive film. This combination makes the CCPs especially vulnerable to corrosion.

  • Pitting: Localized corrosion that creates small holes.
  • Crevice Corrosion: Occurs in gaps where stagnant solution can lead to concentrated corrosion.
  • Fretting Corrosion: Corrosion damage accelerated by repetitive surface contact and pressure.
  • Synergistic Effect: Cooperation of pitting, crevice corrosion, and fretting makes corrosion occur.
Once the passive film is compromised by fretting, crevice corrosion and pitting take hold. The combination of these mechanisms accelerates material loss in the circumferential direction, while pitting and crevice corrosion dominate the depth. This results in the distinctive bowl-shaped perforations observed. This process is further accelerated by the presence of martensite, a phase induced by deformation during plate manufacturing, reducing the corrosion resistance of the stainless steel.

Preventing Future Failures: Practical Recommendations

Based on the analysis, several measures can be implemented to mitigate the risk of PHE failures: Strict control of chloride levels in the LTHW, maintaining consistent pressure between HTHW and LTHW to minimize fretting, and implementing regular repassivation treatments. By addressing design vulnerabilities, manufacturing quality, and operational conditions, industries can significantly extend the lifespan and reliability of their plate heat exchangers, ensuring safer and more efficient operations.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1016/j.engfailanal.2018.10.007, Alternate LINK

Title: Internal Leakage Of Plate Heat Exchangers Caused By Cooperation Of Pitting, Crevice Corrosion, And Fretting

Subject: General Engineering

Journal: Engineering Failure Analysis

Publisher: Elsevier BV

Authors: Z.D. Fan, J.S. Du, Z.B. Zhang, Y.C. Ma, S.Y. Cao, K. Niu, C.X. Liu

Published: 2019-02-01

Everything You Need To Know

1

Why are Plate Heat Exchangers so widely used, and what is the primary concern regarding their operation?

Plate Heat Exchangers (PHEs) are widely used in industries like food processing, chemical plants, co-generation power facilities, and cooling systems due to their compact size, ease of maintenance, and efficiency. However, they are susceptible to internal leakage due to corrosion of the corrugated austenitic stainless steel plates, leading to safety and efficiency concerns. This makes understanding the causes and prevention of PHE failures crucial for maintaining operational reliability.

2

What causes the bowl-shaped perforations observed at crossing contact points in Plate Heat Exchangers?

The bowl-shaped perforations at crossing contact points (CCPs) are caused by a combination of pitting, crevice corrosion, and fretting. The pressure difference between the high-temperature hot water (HTHW) and low-temperature hot water (LTHW) creates compressive stress and a narrow crevice. Fluctuating water pressure induces fretting, damaging the passive film. This synergistic effect makes CCPs particularly vulnerable to corrosion, resulting in bowl-shaped perforations.

3

Can you explain the individual roles of pitting, crevice corrosion, and fretting in the overall corrosion process of Plate Heat Exchangers?

Pitting is a localized form of corrosion that creates small holes in the metal. Crevice corrosion occurs in confined spaces, such as gaps between plates, where stagnant solutions can concentrate corrosive elements. Fretting corrosion is accelerated by repetitive surface contact and pressure, mechanically damaging the protective passive film. These corrosion types can occur independently, however they contribute to the failure of Plate Heat Exchangers.

4

What is the role of martensite in accelerating corrosion, and how does it affect the durability of stainless steel plates within Plate Heat Exchangers?

Martensite, a phase induced during plate manufacturing due to deformation, reduces the corrosion resistance of the stainless steel. The presence of martensite accelerates material loss, making the plates more susceptible to corrosion, especially when combined with pitting, crevice corrosion, and fretting. The manufacturing process and material composition play critical roles in determining a Plate Heat Exchangers lifespan.

5

What specific actions can be taken to mitigate the risk of Plate Heat Exchanger failures, and how do these measures improve their reliability?

To prevent future failures, several measures can be implemented including strict control of chloride levels in the LTHW. Maintaining consistent pressure between HTHW and LTHW minimizes fretting. Regular repassivation treatments can help restore the protective passive film. Addressing design vulnerabilities, manufacturing quality, and operational conditions can significantly extend the lifespan and reliability of plate heat exchangers. Industries using Plate Heat Exchangers can implement these processes as part of a preventative maintenance and monitoring program.

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