Nickel alloys are used by chemical processing industries to handle highly corrosive chemicals in extreme conditions and to meet the challenging environmental requirements. The previous Materials Technology Institute (MTI) Project 269, completed in 2017, had built up a corrosion rate database for five grades of the most commonly used nickel alloys inclusive of Alloy 276 (N10276), Alloy 22 (N06022), Alloy 625 (N06625), Alloy 825 (N08825), and Alloy 600 (N06600).
The documentation from Project 269 has proven extremely useful to many MTI members, and a successive phase 2 project, MTI Project 367, was initiated to extend the database for more grades of nickel alloys. The main goal of Project 367 is to collect and build up a corrosion rate database for 10 grades of commercially available nickel alloys, including Alloy 2000 (N06200), Alloy 59 (N06059), Alloy 686 (N06686), Alloy 2120 (N06058), Alloy 690 (N06690), Alloy G-30 (N06030), Alloy 800 (N08800), Alloy 825CTP (N08827), Alloy B-2 (N10665), and Alloy B-3 (N10675). Project 367 consisted of two parts; Part 1 is to collect the readily available corrosion rate data and to analyze the data statistically, and Part 2 is to acquire test materials from world-renowned material producers and to conduct corrosion tests accordingly.
Existing corrosion rate data for nickel alloys tested by ASTM G28A and G28B, ASTM A262C, and DuPont SW800M standards had been collected comprehensively from research papers, publications, data sheets, and internal test data shared by material producers. In total, 1,629 data points were collected for wrought and welded coupons. However, readers should be aware that corrosion rate data are not suitable for a direct comparison among nickel alloys and the actual service conditions may be very much different from the corrosion test solutions described in this report. The corrosion tests in many cases do not correlate to fitness for use in the intended service.
ITRI had acquired wrought and welded plates/sheets, base metal coupons and welded coupons, and weld filler rods from four world reputed material producers. The as-received plates/sheets were in a mill-annealed condition, and the as-welded coupons were not subject to further heat treatment. All of these coupons were ready for testing after cutting, grinding, and surface preparation. Alloy sheets and filler rods were also used to fabricate some tungsten inert gas (TIG)–welded bead-on-plate test coupons, to which the generally accepted welding procedure was applied. The heat input of manual welding was calculated as 1–1.2 kJ/mm.
Part 2 laboratory corrosion test results showed a similar ranking of corrosion rates to Part 1 collected data. Additionally, laboratory test data showed slightly lower corrosion rates and a smaller scattering of test data than the collected database, which were retrieved from numerous production heats. Except for Alloy 800’s test data, all nickel alloys irrespective of material producer demonstrated similar ranges of corrosion rates tested by a specific test method. Welded coupons did suffer slightly greater weight losses than their wrought counterparts because of the weld’s microstructural features and redistribution of alloying elements. Alloy 800 is a dual-purpose nickel alloy suitable for corrosion applications and high temperature services. The corrosion rates of wrought Alloy 800 plates per ASTM G28A varied with C content. It would be more severely attacked if the C content were higher. A similar finding was obtained from the TIG-welded Alloy 800 coupons with a high C content, which exhibited a severe weld decay at the heat-affected zone.
The specified testing time for ASTM G28A test is 24 or 120 h, depending on alloy grade. The feasibility to assess the low- or no-Mo nickel Alloys G30, 800, 825CTP, and 690 by a shorter 24-h testing was explored. The modified tests showed the corrosion rates were in the same order of magnitude as those data assessed by standard-specified 120-h testing, if uniform corrosion dominated the corrosion morphology. However, it could be very misleading if subject to intergranular corrosion. So, adopting a shorter 24-hour testing time is possible for the purpose of fast screening or simple comparison, but the user should pay careful attention to interpreting the test results. For instance, a careful examination of corrosion morphology at higher magnification is strongly suggested after testing. To determine whether the nickel alloy components are properly processed, it is still recommended to use the standard-specified testing time for performing the G28A test.
The MTI project 367 final report was released in June 2024 and is available exclusively to MTI members in the Technical Resource Library.
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