Sanicro® 35 from Alleima bridges the gap between stainless steels and nickel alloys

Bridging the gap

Sanicro® 35 was developed for highly corrosive environments in demanding industries like chemicals and refining, and to support ‘green’ technologies such as bio-refineries for renewable fuel production, biomass, waste gasification, and chemical recycling. With its balanced chemical composition, Sanicro® 35 bridges the corrosion resistance gap between super austenitic stainless steel grades and more expensive high nickel alloys, making it an attractive upgrade material.

Its extremely good pitting and crevice corrosion properties make it suitable for applications where seawater is used for cooling or heating. Sanicro® 35 is used in heat exchanger applications such as refinery overhead condensers, chlorinated hydrocarbon applications, organic acids, and phosphoric acid, to name a few. It also has great potential to replace high nickel alloys in offshore hydraulic and instrumentation systems and high strength fasteners. Sanicro® 35 is available in round bar, seamless tube and pipe, and plates and sheets.

From R&D, to melt, to extrusion of hollows and manufacture of the final tube, bar or pipe, high quality and full traceability are guaranteed. Like all Alleima materials, control of the supply chain ensures independence and security in product delivery.

Standards and approvals

Sanicro^® 35 has an ASME code case, no. 2982, for Boiler & Pressure Vessel Code, section VIII, Division I & II, covering ASTM B163 (tubes), B649 (bars), B677 (pipes), and B625 (plate & sheet). Maximum temperature is currently 450°C, which will be updated in future revisions to 550°C (842°F).

For the EU Pressure Equipment Directive (PED), a Particular Material Appraisal (PMA) is available as TÜV file 1326W043219, covering tube, pipe, bar and sheet & plate products. Maximum temperature is 550°C (842°F).

Sanicro^® 35 complies with NACE MR0175/ISO 15156-3:2015, (Petroleum, Petrochemical, and Natural Gas Industries - Materials for Use in H₂S-Containing Environments in Oil and Gas Production - Part 3: Cracking-Resistant CRAs (Corrosion-Resistant Alloys) and Other Alloys) for Type 4a type 4c materials.

It complies with ANSI/NACE MR0103/ISO 17495-1:2016, (Petroleum, Petrochemical, and Natural Gas Industries - Metallic Materials Resistant to Sulfide Stress Cracking in Corrosive Petroleum Refining Environments) for highly alloyed austenitic stainless steels and nickel alloys.

Summary of properties of Sanicro^® 35

The chemical composition of Sanicro® 35 is shown below.

Chemical composition (nominal), % UNS N08935

Its balanced chemistry gives Sanicro^® 35 a very high structural stability and excellent pitting corrosion resistance with a PREN* » 52. With its excellent pitting and crevice corrosion properties, Sanicro^® 35 is particularly suitable for applications where chlorides are present. It also has a high resistance to general corrosion in acid environments, making it suitable for a variety of applications.

*PREN = %Cr + 3.3×(%Mo+0.5×%W) + 16×%N

Mechanical properties

Sanicro^® 35 has very high mechanical strength compared to other super austenitic and nickel base alloys. The minimum required yield strength for heat exchanger and hydraulic & instrumentation (H&I) tubing is 425 MPa (62 ksi). In addition, Sanicro^® 35 has excellent ductility, resulting in good cold forming properties.

The high mechanical strength together with the excellent ductility also allows for the use of thinner tube walls in pressurised systems. In heat exchangers, this gives weight savings, gives better heat transfer and efficiency of the equipment, and can lower the overall material cost. For large hydraulic & instrumentation installations in for example offshore environments, significant weight savings can be achieved by utilising the materials mechanical properties.

Welding

Weldability is good and has been verified in ASME IX testing, using Alloy 59 as filler metal. A suitable method for fusion welding is TIG welding (GTAW), although Alleima can offer additional welding recommendations.

Sanicro^® 35 can be welded to dissimilar materials such as Alloy 625, Alloy 825, super duplex or super austenitic alloys using different filler metals. This makes it possible to retain existing tube sheets and only replace the tubes in a failing heat exchanger.

General corrosion

Sanicro^® 35 has better resistance to hydrochloric and sulphuric acid compared to stainless steels with a lower chromium and molybdenum content. With its high chromium content, Sanicro^® 35 also exhibits good resistance towards nitric acid.

In organic acids it is beneficial with high chromium and molybdenum content, making Sanicro^® 35 an excellent material for many petrochemical applications. With its excellent resistance towards pitting corrosion, Sanicro^® 35 is also suitable for environments with organic acids contaminated by halides such as chlorides and bromides.

Localised corrosion

The excellent pitting corrosion resistance of Sanicro^® 35 comes from high levels of chromium, molybdenum, and nitrogen. Its PREN is comparable to Alloy 625 and is significantly higher than for super duplex and 6Mo austenitic grades, where SAF^® 2507 and Alleima 254 SMO^® have minimum PREN values of 42.5. The difference in pitting corrosion resistance in laboratory tests between Sanicro^® 35, 904L and Alleima 254 SMO^®, can be seen in Fig. 1. A comparison of Sanicro^® 35 and Alloy 625 using a slightly modified test method can be seen in Fig. 2.

Figure 1. Comparison of pitting corrosion resistance between 904L, Alleima 254 SMO and Sanicro^® 35. Test according to ASTM G150 with 3 M MgCl₂ as test solution.

Figure 2. Comparison of pitting corrosion resistance between Sanicro^® 35 and Alloy 625. Test according to ASTM G150 with 4.5 M MgCl₂ at pH = 5 as the test solution.

Sanicro^® 35 also hasexcellent crevice corrosion resistance in chloride bearing environments. This has been verified in ASTM G48 testing, where Sanicro^® 35 performs better than Alloy 625 and is at least on par with Alloy C-276 (Fig. 3).

Figure 3. Results from crevice corrosion testing of Sanicro 35 and competing alloys according to ASTM G48 Method F.

Sanicro^® 35 has also been tested in natural and chlorinated seawater. In natural seawater with an active biofilm, it outperforms Alloy 625 if crevices are present. This makes it an interesting alternative to Alloy 625 in for example offshore installations. In chlorinated seawater, Sanicro^® 35 performs at least on par with Alloy 625 and is therefore a good material for seawater cooled heat exchangers in sour service conditions in offshore applications.

Environmental induced cracking

If stainless steel absorbs hydrogen it can severely impact the ductility of the material, making it brittle and prone to cracking (HISC, Hydrogen Induced Stress Cracking). This is especially a problem in subsea and offshore environments when the stainless steel is electrically coupled to sacrificial anodes or carbon steel. In a laboratory test with a constant load at 100% or 120% of the material yield strength, and with an applied potential of -1050 mV_SCE in 3% sodium chloride solution at 4°C (39°F), Sanicro^® 35 showed no cracking after 500 hours. This confirms its high austenitic phase stability and resistance to hydrogen embrittlement.

Chloride stress corrosion cracking (Cl-SCC) can easily lead to catastrophic failures if a pressurised shell or pipe cracks and releases process fluids into the environment. With its high nickel content in combination with molybdenum, chromium and nitrogen, Sanicro^® 35 is expected to have high resistance towards this failure mechanism. Several Cl-SCC tests have shown that Sanicro® 35 has very high resistance to this corrosion mechanism.

A hydrogen sulphide stress cracking test has also been performed on Sanicro^® 35 using welded samples, U-bent tubes with bend radius 1.5×OD, and twisted tubes (heavily cold-worked material). The test was performed according to a modified NACE TM0177 Method A (modified to 6% H₂S) at 90°C for 30 days and resulted in no cracks on any specimen.

Advancing material solutions for refineries

Sanicro^® 35 has excellent resistance to ammonium chloride corrosion and in laboratory testing it has shown similar performance as Alloy 625 (Fig. 4). This makes it a good option for overhead condensers in refineries if super or hyper duplex grades do not give sufficient service life.

Figure 4. Results from testing in static and flowing ammonium chloride environments, show that Sanicro^® 35 is a suitable option to Alloy 625 in these conditions.

In hydrotreater plants, some heat exchangers can suffer from long-term build-up of ammonium chloride deposits, which becomes especially problematic during shutdowns as the salt deposit can absorb moisture when the vessel is open to the atmosphere. Lower alloyed stainless steels such as the stabilised 304 grades TP321 and TP347 can suffer rapid corrosion under these conditions. Traditionally, the standard solution has been to change the tube metallurgy to Alloy 625 as this grade is highly resistant to these kinds of environments, but here Sanicro^® 35 can offer a cost-effective alternative to the nickel base alloy.

For the hydrotreater reactor effluent air cooler (REAC), ammonium bisulphide corrosion is commonly the main concern and Alloy 825 is the standard material used for this application. However, in certain cases, the process stream can contain high amounts of chlorides which can cause pitting and under deposit corrosion of this grade. In such cases, a material upgrade to Alloy 625 is the standard solution. However, Sanicro^® 35 has excellent resistance to ammonium bisulphide corrosion and similar pitting resistance as Alloy 625, making it a cost-effective alternative as a material upgrade.

Renewable diesel and sustainable aviation fuels

When revamping existing hydrotreater plants or designing new ones, Alloy 625 is extensively used to combat corrosion from ammonium chloride deposits or high temperature, acidic water phases containing chlorides. The main concerns in both cases are pitting corrosion, under deposit corrosion and chloride induced stress corrosion cracking. Alleima has performed several advanced laboratory tests at external, third-party laboratories showing that Sanicro^® 35 has similar resistance to these types of environments as Alloy 625 and Sanicro^® 35 is therefore considered a cost-efficient alternative to Alloy 625.

Sanicro^® 35 is also suitable for several pre-treatment technologies for renewable feedstock, including clean-up of used cooking oil (UCO), treatment of woody residues, waste plastics, etc. Alleima has performed tests in high temperature acidic water solutions with chlorides, showing that the material is not susceptible to pitting corrosion, general corrosion or stress corrosion cracking in these environments. The high chromium and molybdenum content of Sanicro^® 35 also makes it resistant to organic acids and high TAN feedstock.

There are several emerging technologies using water systems at high temperatures and pressures to convert a large variety of waste materials into synthetic crudes and bio-oils. This process almost always contains chlorides and other impurities which in combination with the process parameters can cause rapid corrosion of stainless steels. The high mechanical strength of Sanicro^® 35 in combination with its excellent resistance to pitting corrosion and cracking makes it a very interesting candidate material for these kinds of applications.

Offshore applications

Offshore platforms within the oil & gas industry have large installations of hydraulic & instrumentation systems. In these offshore marine environments, the use of tubes with high corrosion resistance is crucial and super duplex or super austenitic 6Mo grades are the common material choices. For environments that also require sour service resistance, Alloy 625 is the most common material upgrade. Sanicro^® 35 has excellent resistance towards offshore marine environment and sour service, being a Type 4a or 4c material according to NACE MR0175/ISO 15156. Sanicro^® 35 is therefore considered a good alternative material to Alloy 625 when also sour service resistance is required, coming at a significant cost reduction.

Offshore platforms often use seawater to cool crude oil or gas before further processing. A common material for seawater coolers in these environments is titanium Grade 2, although it cannot be used if mercury is present in the crude. Alloy 625 and C-276 are also sometimes used in offshore seawater coolers when resistance to sour service is necessary. Sanicro^® 35 has very good resistance towards seawater, both natural and chlorinated and can be an option for titanium if mercury is present in the crude. With its good sour service resistance and equal or better resistance towards seawater compared to Alloy 625, Sanicro^® 35 is also an option when nickel base alloys are considered.

HISC, Hydrogen Induced Stress Cracking, of precipitation hardened (PH) nickel base fasteners is a difficult problem in offshore environments. When the bolts come in contact with carbon steel or sacrificial anodes in the presence of seawater, hydrogen is formed on the fastener surface. The hydrogen is absorbed into the material which due to its precipitates becomes very susceptible to HISC. Sanicro® 35 has a fully austenitic microstructure without precipitates and is not susceptible to HISC. As the material can be readily cold formed to high mechanical strength it is a suitable solution to the HISC problem of PH nickel base alloys. In addition, the use of Sanicro^® 35 fasteners comes at a large cost reduction compared to PH nickel alloys.

Materials make the difference

Sanicro^® 35 has been successfully installed in several refineries, and chemical, petrochemical and fertilizer plants across the globe. In most cases it has been selected as a material upgrade in failing shell & tube heat exchangers, replacing both super duplex, hyper duplex and 6Mo super austenitic grades. Sanicro® 35 has also been recognised as a good option for nickel base alloys in several green field projects, allowing large cost reductions and making the projects more economically viable.

Moving industry forward

Sanicro^® 35 bridges the properties gap between super austenitic steel grades and more expensive high nickel alloys for the chemical, petrochemical, and refinery industries sectors. Its combination of excellent corrosion resistance and mechanical properties, while still being generally low alloyed compared to competing nickel base alloys, makes it a very interesting and cost-saving alternative for many applications where nickel base alloys are the traditional material solutions.

At Alleima, we always work closely with our customers, and we are eager to discuss material solutions for new technologies as well as finding good alternatives to existing, failing installations. Join us in developing the future!

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