ABSTRACT:

Metallic materials used in certain areas of the chemical process industry face high temperature attack in the form of oxidation, sulfidation, carburization, halogen attack, etc. One approach to defending metallic materials against high temperature corrosive attack is through the use of protective coatings. Although several coating methods are available, halide-activated pack cementation is a coating process of choice due to its versatility and cost-effectiveness. Traditionally pack cementation has been utilized to apply aluminum, chromium or silicon-rich coatings. The coating process is reviewed and a model to predict the kinetics is outlined. The effective utilization of this process to achieve desired results is discussed.

INTRODUCTION:

The chemical process industry presents a broad range of environments to the materials used in service. Some of these environments combine high temperatures with corrosive conditions resulting in the need for careful materials selection. Two broad strategies may be employed to address this issue - alloy selection or surface modification or a combination. Surface modification, usually achieved by the application of coatings, is a process route typically selected when the geometry of the component, mechanical properties, design issues, availability and economical factors become dominant. The usual mechanism afforded for protection is the formation or application of a stable oxide film that can withstand corrosive attack and maintain mechanical stability. Aluminum, chromium and silicon are examples of elements that are typically incorporated into protective coatings. The oxides of these elements offer protection to the underlying substrate in a wide variety of aggressive process environments. One of the key techniques to apply coatings is by halide- activated pack cementation. In this coating technique, halide vapor species are typically generated in situ in a pack containing the alloy substrate (either buried in, or suspended outside the pack). A detailed review of this process is reviewed in a different article (1). In this paper, some of the thermodynamic and kinetic issues involved in obtaining coatings that may be useful in process engineering conditions are presented. Halide-activated pack cementation basics A typical pack contains a powder blend of an inert “filler” material that provides a scaffold for the pack and typically does not participate in the reactions in the pack; elemental or alloy powders that contain the element(s) to be coated and an activator salt that reacts with the alloy powder to produce the halide vapor species. In the “in-pack” process, the substrate is buried in the blend and the entire assembly sealed with permeable cement and heated to a suitably high temperature. In a pack process reactions between the masteralloy and activator produce halide vapor species which move through the pack, arrive at the surface of the metal or metallic alloy to be coated (the “substrate”), undergoes reactions at the surface to deposit the coating element(s), and finally, the coating element(s) diffuse into the substrate. Thermodynamics and Kinetics of halide-activated pack cementation Thermodynamic equilibrium is assumed to exist between gas and solid or liquid phases within the bulk pack and at the substrate.

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