Know the Limits of Duplex Stainless Steels

Figure 1. After corrosion problems, column was rebuilt using duplex stainless steel but stress corrosion cracking occurred after a few years.

However, under certain conditions — mostly involving chloride-containing fluids and elevated temperatures — even duplex stainless steels are vulnerable to this type of corrosion. This makes thorough failure analysis and corrosion testing imperative. The current condition of the materials used is not only important for safety and availability but also crucial for the profitability and the service life of existing plants.DISTILLATION COLUMN CORROSIONAt a chemical plant, a failure occurred in a distillation column that had been in continuous operation for ten years. The column was made from robust austenitic steel (1.4571, titanium stabilized). Stress corrosion cracking in various areas of the wall caused the failure. The bottom of the column suffered the most severe cracking, but stress corrosion also was found in upper parts of the unit at a height of up to 15 meters. Corrosion was attributed to an acidic oxygen-free aqueous chloride solution with highly oxidizing admixtures that had a temperature of 125°C in the bottom and around 90°C in the upper part of the column.The experts sought an alternative material that is completely immune to stress corrosion cracking. After laboratory analyses, they selected a duplex stainless steel (1.4462) and reconstructed the column in this material (Figure 1). To improve the column’s resistance to stress corrosion cracking even further, its internal surface was treated by applying strong pickling agents to remove a specific amount of material (normally 3–5 µm) from the surface together with all superficial residual stresses.

Figure 2. The inside surface of the column’s duplex-stainless-steel bottom exhibited pits after five years in service.

The column was placed into service and examined for signs of corrosive attacks during each maintenance shutdown. Five years later, the bottom of the column — which had been in direct contact with the fluid — showed the first signs of corrosive attacks. The inspectors found pronounced pits on the inner surface of the metal wall and corrosion grooves along the weld seam (Figures 2 and 3). Electrochemical tests performed using the chloride solution from the bottom part of the distillation column as test fluid revealed the probability of pitting corrosion to be high at temperatures as low as 90°C. Given this, the experts assumed pitting corrosion to be the problem in the case at hand. Over the following years, they monitored and documented the progress of corrosion. They marked selected surface areas and produced surface replicas on film to see whether the area affected by corrosion was growing. In addition, they measured the depth of the most-pronounced defects. A comparison of pit depths with the surface replicas produced at various times revealed that no new pits had developed and existing pits had not become either deeper or bigger. The case at hand obviously was caused by one-time corrosion that did not progress. Pitting corrosion can be tolerated up to a certain degree. However, in combination with tensile stress, it may cause stress corrosion cracking that could significantly reduce component integrity.

Figure 3. The weld seam at the bottom of the column also showed significant signs of attack.

PITS TURN INTO CRACKSAfter another four years of operation, the distillation column had to undergo a gas pressure test. For this reason, to be on the safe side, the experts decided to re-examine some of the corrosion grooves at the circular bottom weld and the longitudinal weld of the bottom part. Dye penetration testing did not produce a clear result. Therefore, the experts carefully sanded and polished the weld surface. To exclude cracking, they repeated the dye penetration test, which now revealed cracks that had been invisible previously (Figure 4). Further measurements of crack depth showed that some cracks had penetrated the entire wall. The experts separated isolated sections from the affected areas and performed metallographic analysis. It became evident that the problem was due to stress corrosion cracking, which was limited almost exclusively to the austenitic grain. This explained the difference in appearance between the defects on the inner surface of the metal wall and those along the weld seam. In the metal wall, the austenitic and ferritic phases run in lines parallel to the surface, therefore the corrosive attacks were flat and pit-like. The cracks were unable to penetrate deeply and were “stopped” at the ferritic lines. In the weld seam, by contrast, austenitic and ferritic phases run almost perpendicular to the surface. Residual tensile stresses from welding act in parallel to the surface where they are strongest. This causes incipient cracking, which then progresses. The cracks generally grow vertically to the tensile stresses acting upon them, which means the cracks in the austenitic phase penetrate relatively deeply into the weld seam and branch out. Some of these cracks were filled with corrosion products. Using coarsely ground samples, the experts succeeded in reproducing these results in the corrosion test. The bottom part of the column was replaced with one made of titanium, which is even more corrosion-resistant — making the column fully functional again.

Figure 4. Some fissures in the austenitic phase (bright; ferritic phase, dark) extended right through the column wall.

EVALUATE THE SUITABILITYThe case study shows not only that stress corrosion cracking can develop in duplex stainless steels but also how it does — depending upon the fluid involved and the temperature. The tensile stresses acting on the material cause different forms of corrosion on the weld seam (cracks) and the metal wall (pits). Because cracking in the weld seam is not always visible, it is important to examine the seam carefully when pitting corrosion is suspected in the metal wall. Corrosion products might cover the cracks in the seams to such an extent that dye penetration testing without prior sanding and polishing of suspicious spots might not deliver valid results. The analysis of corrosion processes like the one discussed here requires careful monitoring, coupled with extensive experience and expertise in materials technology. Most operating companies lack such capabilities. Specialist consultants, such as TÜV SÜD Chemie Service, have experts that can advise about material selection and corrosion protection, carry out monitoring and damage analysis, and create expert reports. A few, e.g., TÜV SÜD Chemie Service, also operate an accredited corrosion laboratory.

HELGA LEONHARD and GERNOT GRÖTSCH are materials experts at Materials Engineering & Testing, TÜV SÜD Chemie Service, Frankfurt am Main, Germany. E-mail them at [email protected] and
[email protected].