Performance of Inconel alloy 625 in industrial corrosive factors

Carburization

Development of stable oxide layer on the surface is a key to the carburization resistance of an alloy. A test was designed to determine the relative high temperature carburization resistance of complex  alloys used in ethylene furnaces. A test included exposing alloy to a mixture of hydrogen and methane. Specimens are regularly eliminated for weighing by pulling the samples to the cold end of the test container and flushed with argon to reduce oxidation. A mass change in H2 – 1% methane at 1000oC for around 1000 hours for different commercial alloys. Alloy 625 offers supreme carburization similar to alloys that are directionally solidified and SS 330 and higher than Incoloy 800 and SS 309. Alloy 625 offers better performance than Incoloy 800 and Inconel 600.

Chloride based conditions

Chlorine is a major factor in various industrial process conditions for example mineral chlorination, ethylene dichloride and vinyl chloride monomer development, aluminum smelting and refining, fuel element reprocessing and heat recovery units. High temperature halide corrosion is noticed in fossil fuel boilers, coal gasification unit, gas turbines and in municipal and chemical waste incinerators.

Municipal waste contains about 0.5% halides on dry basis. Many lab studies imitating waste incineration conditions have been limited to temperatures varying from 120oC to 650oC or 248oF to 1202oF. As nickel and nickel based alloys are widely popular for offering resistance to halogen corrosion, they are successfully used in the lab temperature in simulated HCl conditions. 

The temperature limit chosen was about 593oC – 927oC or 1100oF  - 1700oF. This temperature level is common for flue gas studies. Boiler internals and flue stacks could specifically be expected to attain heat exposure in this temperature limit. The simulated condition chosen for the evaluation was nitrogen, carbon dioxide, oxygen, hydrochloric acid, Hydrogen bromide and sulfur dioxide.

It is found most alloys received negative mass variations resulted by spalling, vaporization losses or both. The rate of metal loss increased with increase in service temperature. High iron alloys showed higher mass changes at 704oC as compare to at 927oC. Alloys containing high iron content received wide internal oxidation. The morphology of corrosion scale at high temperature different from that at lower limits. At high temperature, voids could be commonly seen that were more visible in high iron alloys. Inconel alloy 600 and Inconel 625 wire offered outstanding resistance to corrosion at the more higher temperature.

Alloy 625 containing high nickel, chromium, niobium and molybdenum offers supreme aqueous corrosion resistance in a wide range of vigorous conditions. In various environments, its resistance is similar to Hastelloy G3, C276 and Hastelloy C22. It prevents general corrosion and intergranular corrosion in various environments of chemical processing plants. It also resists chloride and sulfide stress corrosion cracking, hydrogen embrittlement in oil containing tubular items. It also prevents general and localized corrosion in the pulp and paper plants. In addition of it, the alloy’s various other characteristics made it fit for use in heat exchangers.