External Corrosion and Cracking of Stainless Steel Equipment and Pipes

By MTI Admin posted 12 days ago

  
Download a PDF of the full article from MTI CONNECT 2021, Issue 1.
During the author’s 15-year career as a materials engineer dealing with a large number of corrosion and cracking incidents of process equipment and piping in various Asian chemical sites, there are some commonalities that can be summarized from these incidents. One of the prominent findings is that most of the corrosion and cracking incidents of stainless steel equipment and piping were initiated at the external surface due to chloride concentration from the environment. This article tabulates 24 stainless steel related incidents, which were investigated during a 10-year period from 2007 to 2016. Some real images taken in the field are included to further illustrate these findings. Finally, recommendations and possible solutions to deal with these external corrosion and cracking issues are provided. The goal is that this review will be helpful to industry colleagues who may not be fully aware how frequently and significantly chloride stress corrosion can affect process equipment.

Please note that the failures and incidents described in this article are not directly relevant to the author’s current company. In fact, they do not represent the performance of any specific company, because I collected them from a variety of different chemical sites in Asia during my career. So, no specific companies are identified, and the information provided in this article is applicable to all chemical sites, in general. Also, no proprietary information of any company is disclosed in this article.

The focus of this article is on process equipment and piping made of common austenitic stainless steels, including 304 SS, 304L SS, 316 SS, and 316L SS. These are commonly used in Asian chemical sites. Dual certified stainless steels 304/304L and 316/316L are more commonly used in the US industry, but since they have similar compositions, the findings as shown in this article are also applicable to these dual-certified materials.

The title topic of “corrosion and cracking” was selected to cover examples of general corrosion, localized corrosion, stress induced failure and the combination of both stress and corrosion, such as chloride stress corrosion cracking (SCC).

The environments covered here are in the chemical industry in Asia; however, the findings in this article are also applicable to other process industry environments and all other regions, because chlorides are almost everywhere and anywhere.

Table 1 (refer to the pdf of the full article) lists 24 stainless steel related incidents that were investigated from 2007 to 2016. During this period, the author was providing technical support to 15 chemical sites throughout Asia, including China, India, Indonesia, Japan, Korea, Malaysia, and Singapore. These sites are typically not large, with each site having about 20-50 pieces of stainless steel process equipment. The sites only sent samples for failure analysis when there was a major incident, significant financial loss, prolonged shutdown, or injury resulted. Therefore, based on a rough calculation, about one percent of stainless steel process equipment or piping in each Asian chemical site will get a major failure every year. While this sounds like a small number, it is quite significant. If one site has 100 pieces of stainless steel equipment, there will likely be almost one major incident every year based on this ratio.

As summarized in Figure 1, among all these incidents, almost half of them (11 cases) were caused by external corrosion and cracking, which can be further attributed to chloride concentration at the external surface of the process equipment or piping where the chlorides came from the environment. In Asian chemical sites, not every site has a materials engineer, but there are always process equipment engineers or reliability engineers. They typically have some basic knowledge about the importance of avoiding the use of austenitic stainless steels in chloride-containing or other incompatible process conditions. However, many of them are not aware that the chlorides in the external environment can be just as problematic. And while some may be aware, they may not be sensitive to how significant external surface corrosion or cracking can be. As a result, these engineers may not give enough consideration to the prevention of external corrosion and cracking at the design, fabrication, and maintenance stages. The result is that chloride concentration at the external surface becomes the number one cause of the major stainless steel equipment and piping failures at their sites.

The second largest contributor to the failures is improper material selection or design for the corrosive process, totaling eight cases. Again, because of the lack of material engineering knowledge, the site engineers sometimes select stainless steels for chloride containing processes, HCl containing processes, or the processes that require high strength and high hardness. The incorrect material selection or improper design will likely cause major failures. Therefore, it is very important to consult with material engineers before any new equipment fabrication or piping system construction.

The third cause of major failure in this study is attributed to manufacturing defects, including welding defects, casting defects, or iron contamination, which accounts for four cases. This should raise our attention to focus on the quality control of equipment and piping fabrications. The quality control process should include a thorough vendor audit at the beginning, a detailed prefabrication meeting, a comprehensive inspection and testing plan (ITP), and careful inspections at each hold point or witness point.

There is only one case in Table 1 where the failure was caused by improper operation. The operator used abnormally high-pressure water to clean a stainless steel pipe with one end plugged, and caused the pipe to rupture. Typically, in Asian chemical sites, operators follow the rules and regulations quite strictly. This type of failure is indeed uncommon. The external corrosion and cracking described in this article is actually part of a hot topic being 16discussed by materials engineers during international conferences, which is corrosion under insulation, or CUI. In Table 1, among 11 cases of external corrosion and cracking, eight cases can be defined as CUI. This is because chloride concentration at the metal surface can be accelerated by the presence of insulation. This review provides some real practical evidence to confirm the significance of CUI and its relevant discussions.

The author’s recommendations to deal with these external corrosion and cracking cases include:

  1. Get involved in the design stage and specifying a proper coating to prevent the external corrosion and CUI. If it is critical service and the operating temperature is between 60 and 150 °C, the coating is mandatory for stainless steel equipment and piping. It is often difficult to convince the site managers to do this because of the cost, but it is often the easiest and most cost-effective approach when compared with other solutions such as upgrading materials of construction. You may meet resistance because they simply do not believe that external corrosion or CUI is a problem. Pointing to real examples of damage from external corrosion or CUI (such as those in this article) may be beneficial to help them to begin to understand its significance. Another common pushback from the site personnel is that coatings tend to cause pitting corrosion of stainless steels. This is true for some older types of coating, but for modern coatings, which are specially designed for stainless steels, this is no longer an issue. Of course, it is very important to select and apply the proper coating from qualified suppliers.
  2. Educate the site personnel to recognize the warning signs of CUI and take action at an early stage to prevent the catastrophic failures.
  3. Provide maintenance guidance for stainless steel equipment and piping to reduce the potential for external corrosion and cracking.
In addition to common stainless steels, the author has been conducting failure analyses on many other common materials of construction for chemical sites, including carbon steels, duplex stainless steels, superaustenitic stainless steels, nickel alloys, copper alloys, wear resistant alloys, high temperature materials, and non-metals. Of course, different materials have very different failure mechanisms. The author hopes to have a chance to share these experiences in future articles.

Contributed by HAIHUI (HENRY) YE, PH.D. – MATERIALS ENGINEERING CONSULTANT
THE CHEMOURS CHEMICAL (SHANGHAI) CO. LTD.

The contents of articles and any opinions expressed therein are those of the authors and do not represent those of MTI.
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