51314-3771-A Study of the Corrosion Properties of Welds and Heat-Affected Zones of a Wrought Nickel Alloy

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Product Number: 51314-3771-SG

ISBN: 3771 2014 CP

Author: Paul Crook

Publication Date: 2014

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The wrought nickel alloys containing high chromium and molybdenum levels are of vital importance to the chemical process industries. Not only do they resist hydrochloric and sulfuric acid which are among the most corrosive chemicals but also they withstand the insidious effects of chloride-induced pitting and crevice corrosion. While the corrosion characteristics of the wrought nickel alloys have been studied extensively there is a dearth of information concerning the corrosion behavior of their welds and heat-affected zones in effect the weak links in any welded structure. In this study welds and heat-affected samples of a relatively new nickel alloy containing 22% molybdenum and 15% chromium were tested in hydrochloric acid sulfuric acid nitric acid and ferric chloride allowing comparison of their uniform corrosion behavior and pitting resistance with those of the wrought base material. The all-weld-metal samples were prepared by gas metal arc (MIG) welding and the simulated heat-affected zone samples were created using resistive heating.Like many wrought nickel alloys containing high levels of chromium and molybdenum the new material exhibits a single phase (gamma) face-centered cubic structure in the solution annealed and quenched condition but has a tendency to form alternate second phases at temperatures between about 500°C and 1100°C where the solubilities of chromium and molybdenum in gamma phase are exceeded and there is sufficient thermal energy for diffusion. Furthermore residual levels of carbon and silicon can also promote the nucleation and growth of second phases during excursions to temperatures in this range as encountered during welding. Molybdenum-rich M6C carbides and mu phase are known to be problematic in materials of this type since they nucleate rapidly at the alloy grain boundaries rendering them susceptible to preferential attack.As micro-castings welds of the nickel alloys are generally more prone to attack due to compositional differences within the grains and the possibility of setting up permanent anodic and cathodic sites on the weld surfaces. In this study the corrosion tests were augmented by metallographic examination of the microstructures of the various simulated weld regions and the forms of corrosive attack.