Abstract

A relatively new N06235 alloy has been developed to offer excellent metal dusting resistance by utilizing the effect of copper on inhibiting carbon deposition and graphite formation, and its weld overlay could provide a cost-efficient alternative for metal dusting and other high temperature corrosion protection. The current paper studied metal dusting resistance of the N06235 alloy and its weld overlays, which were applied by Gas Metal Arc Welding (GMAW) or Hot-Wire Laser process on N06230 alloy plate, along with a wrought N06025 alloy for comparison. The metal dusting test was conducted in a 47CO-47H2-2H2O-4CO2 gas with high carbon activity under 18bar pressure at 620°C. The test coupons were periodically removed from the furnace to measure weight change and check on metal dusting attack. After 790h exposure, the N06235 alloy and its weld overlays showed excellent metal dusting resistance, while metal dusting attack was observed on the wrought alloys, N06025 and N06230, after the first 50h exposure. Metallurgical examination after the metal dusting test was performed to characterize the metal dusting attack on the N06235 weld overlay and the wrought alloys N06025 and N06230. Detailed investigation and discussion on the N06235 weld overlays, including oxide scale, surface pit, and subsurface chemistry change, were presented.

Introduction

Many industrial processes contain H2, CO, CO2, and H2O gas mixtures, such as syngas production and processing in hydrogen, ammonia, and methanol plants. These process environments have high carbon activity, i.e. ac > 1, and low oxygen partial pressure at their elevated operating temperatures, such as in the temperature range of 400-800 °C (752-1472 °F).1-3 The high carbon activity could result in a catastrophic material degradation, i.e. metal dusting. The resulting corrosion products consist of carbon or graphite and metal particles, along with possible carbides and oxides, and cause material disintegration.4-8

High carbon activity causes carbon deposit at the gas-metal interface through the following reactions,

equation (1)

equation (2)

equation (3)

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