ABSTRACT

High temperature gaseous environments that promote corrosion attack are present in a variety of industrial environments such as chemical and petrochemical processing, glass manufacturing, waste incineration, cement manufacture, mineral processing and metals heat treatment and finishing. These aggressive environments can involve corrosive attack in the forms of oxidation, sulfidation, nitridation, carburization, metal dusting, chlorination and molten salt corrosion. The paper will discuss the selection of the correct alloy(s) to withstand each type of environment.

INTRODUCTION

Nickel-base alloys are industrially important for their resistance to certain types of high temperature corrosion. For example, they have outstanding resistance to oxidation, and they are generally superior to iron- or cobalt-base alloys in this mode of attack. They have low solubilities for interstitial atoms, which makes them inherently more resistant to carburization and nitridation attack than other alloy bases. They also have good resistance to halogen-containing environments due to the high melting points of the halogen compounds formed. A summary of the nominal compositions of some alloys resistant to various forms of high temperature corrosion is given in Table 1.

The Ni-Cr-Fe family consists of general purpose type materials that are used in a wide variety of applications because of their good resistance to many forms of high temperature corrosion and more reasonable costs. The Ni-Cr-Fe-Al family can be viewed as an upgrade over the Ni-Cr-Fe alloys through the use of Al additions to enhance their resistance to high temperature corrosion by making use of Al2O3 formation. For 601 and 693 alloys, this occurs in the form of sub-surface oxides. On the other hand, enough Al is present in 214TM (A) alloy species. The Ni-Cr-Mo/W alloy family consists of materials which have very good high temperature strength in addition to resistance to certain forms of high temperature corrosion. Therefore, they are used in a great variety of high temperature structural components. Finally, the Ni-Co-Cr-Si category, consisting of HR-160® alloy, represents a unique material that resists high temperature sulfidation as well as environments containing complex mixed salts through critical additions of Co, Cr and Si. This alloy is unusual since nickel-base alloys are not noted for their resistance to sulfidation attack.

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