The Case for Super Austenitic Alloys

316L Fittings Failure Analysis

A Failure Analysis of 316L at a Food Processing Facility

According to Global Market Insights, the world-wide market for beef, poultry, and vegetable broths hit $2.6 billion USD in 2016 and is estimated to realize compound annual growth of over 4% globally through 2024 (2018). That's good news for global companies in the broth business - all of whom have to keep pace with demand while operating competitively and profitably.

But at the same time, NACE International reports that corrosion in the food processing industry has direct costs of approximately $2.1 billion every year in the US alone. Direct costs are to plant operations and replacement equipment. In addition, indirect costs that double the total figure to over $4 billion include lost productivity caused by downtime, plus taxes, operations and indirect service costs (Koch et al, 2012). In other words, corrosion is both an operational and financial problem.

In this article, we outline a case of corrosion in a food processing plant, how it was diagnosed, and how it can be prevented by selecting the proper alloy. Leaks appeared at welds and interior surfaces of a reducer and tee section of a broth processing system. Although visual examination showed where corrosion had occurred, root causes of corrosion required further metallurgical analysis. The company called CSI for diagnostics, and CSI's affiliate labs provide the data required to determine causes and recommend the right processing equipment for the job.

Tee Sample
Tee sample from food processing plant sent in for testing.
Reducer Sample
Reducer sample from food processing plant sent in for testing.

The Food Processing Case: Details

When maintenance engineers at a food processing plant discovered leaking of stainless steel 316L tubing that had been in service for only four years, the company felt the cost of corrosion in ways that statistics only begin to convey.

Initial Findings
  • Unit: Broth manufacturing
  • System: Sterilization System, holding tubes
  • Materials: 316L and 304L stainless steel
  • Time in Service: 4 years, operating 105 hr/week
  • Process Conditions: Chicken, vegetable, and beef broth, with salt content of 10-14%
  • Temperature and Rate: Broths heated to 290°F with no more than 2°F drop at a rate of 60 gal/min
  • Clean-In-Place and Sterilization Steps: Include acids and caustics

CSI Diagnostics

While visual inspection may reveal where corrosion occurs, additional analysis is required to determine the type, extent, and causes of corrosion. In the broth processing case, CSI affiliate labs used a variety of technologies to determine causes of equipment failure. Each technology helps to measure variables that contribute to accurate diagnosis of the problem, its causes, and viable solutions. Accurate diagnosis requires information about the extent and type of corrosion and chemical changes that indicate its causes. The broth case tested for chemical composition of leaking parts, surface topography of damaged materials, chemical composition of processed materials, and manufacturing defects.

Optical Emission Spectroscopy (OES)

OES determines the chemical composition of leaking parts. It's recognized as a reliable technique for analyzing elemental composition of several metals. OES produces metal-specific light signatures for precisely determining the metal composition of test materials. In this case, the pipe sections were fabricated using 304L stainless steel, the tee flange is fabricated using 304L, and the re-melted weld metal is a mixture of 316L/304L.

Scanning Electron Microscopy (SEM)

A scanning electron microscope (SEM) reveals the surface topography of components affected by corrosion, which helps to determine the extent of damage. While many types of corrosion are visible to the naked eye, in some cases corrosion is masked or microscopic. In this case, SEM examination of samples from the reducer and the tee revealed preferential (grooved) corrosion of the weld metal. The dendritic (branching) structure of the weld metal was visible in the regions that had been attacked.

SEM

Energy Dispersive X-Ray Analysis (EDS)

An energy dispersive x-ray analysis explains the chemical composition of materials processed within the system. EDS analysis of the reducer surface near pits revealed the presence of 5% chloride, a well known pitting and cracking agent for 300 series stainless steel.

Reducer EDS
EDS analysis of the reducer surface revealed the presence of chlorides in the vicinity of the pits.

Metallographic Examination

Metallography reveals microscopic flaws such as cracks or unwanted materials that can be introduced during fabrication, welding, heat treating, and coating. Metallographic examination of the tee indicated corrosion pits in the weld metal and stress corrosion cracks in the tubing that were not in the vicinity but were cold work stresses from fabrication.

Metallographic Examination

Stress Corrosion Crack
Stress corrosion cracks from the base metal adjacent to the weld.

In-system sensors can also help with diagnostics, although they were not in use at the time of this case. Sensors for temperature and pressure monitoring, for example, transmit data that facilitates troubleshooting and early warning of problems that may contribute to corrosion.

Summary of Results

When used together, diagnostic tools paint a thorough and detailed picture of the causes and extent of damage from corrosion.

Visual Examination: The reducer showed pitting corrosion of the weld and the heat-affected zone. The tee showed pitting corrosion of the weld.

Chemical Composition: The optical emission spectroscopy data showed that the reducer was fabricated and welded using 316L. The tee was fabricated using 304L stainless steel and was welded using 316L filler wire.

Scanning Electron Microscopy: SEM images confirmed corrosion of the dendritic (branching) structure of the weld metal.

Energy Dispersive X-Ray Analysis: The EDS data showed the presence of high concentration of chlorides.

Metallographic Examination: Confirmed pitting corrosion of the weld in the reducer, pitting and chloride-induced stress corrosion cracking in the weld and base metal.

Conclusions

Corrosion resistance is the most important consideration when designing from an operational and financial standpoint. The costs of downtime and repair should be factored into decisions about food processing especially when product contains corrosive ingredients -- e.g., foods with pH 3-5 such as fruit juices with citric acids, jams, acidic canned goods, broths, sauces, dressings, and vegetables in 1-3% salt solutions.

The pitting and chloride stress corrosion cracking observed in the reducer and tee section is due to the presence of chlorides at elevated temperatures. Corrosion resistance can fall with increased temperature (Ahmed 2016, p. 6), so identifying the presence of chlorides is only one consideration. Operating temperatures must also be factored into the diagnosis.

The molybdenum contained in 316L is marginal in this service and the non-molybdenum alloy 304L is not acceptable for this service. A more effective design would include corrosion-resistant components using AL-6XN® Super Alloy.

To arrive at the true cost of corrosion add direct and indirect costs. Food processing companies have to weigh both when designing food processing facilities for operational and financial advantage.

Investment Description316LAL-6XN
Initial Investment (Material Cost)$25,369.00$101,479.20
Downtime + Demolition + Replacement Installation Cost*
$96,000.00
Replacement Material Cost*$25,369.00
Totals$146,738.00$101,479.20

*The cost comparison shown above is an accurate approximation. Actual numbers vary greatly by application, number, and extent of corrosion incidents.

Recommendations

Care should always be taken when considering 300-series stainless steel in applications including chloride and high temperatures. AL-6XN® Super Alloy products, also known as high corrosion-resistant alloys, work in extremely corrosive conditions where 316 stainless steel cannot be used. Because of its extra hardness, AL-6XN® is the preferred choice in high-salt, high heat environment or when acid and caustic solutions are used for cleaning in place (CIP).

Given the significant costs of downtime and parts replacement when 300-series parts fail in corrosive applications, low-cost 300-series solutions can be much less cost-effective than they seem.

Reducers and tees using super austenitic alloys containing 6% molybdenum should be considered for this application. CSI has a full range of AL-6XN products in stock. For more information or a corrosion analysis contact CSI.

A complete corrosion management strategy requires working faster, smarter, and better:

Faster

  • Using sensor technologies for performance assessment.
  • Performing complete diagnostics at the first signs of corrosion.

Smarter

  • Calculating true corrosion costs before settling on low-cost alternatives.
  • Learning what you can do about corrosion right now.

Better

  • Improving corrosion management with corrosion-resistant equipment.
  • Designing and managing or corrosion resistance.

UNS N08367 (commonly known as AL-6XN), is an acceptable material for use in hygienic service as per "Part MM" (Metallic Materials) section of the ASME-BPE international standard. The US Food and Drug Administration (FDA), National Sanitation Foundation (NSF), NACE MR0175, and Norsok Standard M-DP-001 have all approved AL-6XN.

References

Ahmed, S.R. (2016). Influence of high temperature on corrosion behavior of 304 stainless steel in chloride solutions. AIP Advances 6, 115301. American Institute of Physics. doi: 10.1063/1.4967204

Global Market Insights, (2018). Worldwide Broth Market work over USD 2.8 Billion by 2024: Global Market Insights, Inc. https://globenewswire.com/news-release/2018/02/26/1387046/0/en/Worldwide-Broth-Market-worth-over-USD-2-8-Billion-by-2024-Global-Market-Insights-Inc.html

Koch, G. H., Brongers, M. P. H., Thompson, N. G., Virmani, Y. P., Payer, V. J. H., (2012). Corrosion Costs and Preventative Strategies In the United States, NCE Publication No. Fhwa-rd-01-156. https://www.nace.org/uploadedFiles/Publications/ccsupp.pdf

ABOUT CSI

Central States Industrial Equipment (CSI) is a leader in distribution of hygienic pipe, valves, fittings, pumps, heat exchangers, and MRO supplies for hygienic industrial processors, with four distribution facilities across the U.S. CSI also provides detail design and execution for hygienic process systems in the food, dairy, beverage, pharmaceutical, biotechnology, and personal care industries. Specializing in process piping, system start-ups, and cleaning systems, CSI leverages technology, intellectual property, and industry expertise to deliver solutions to processing problems. More information can be found at www.csidesigns.com.