Kinetics of volatilization of high temperature corrosion products and its application to chlorine corrosion

Abstract

Operating atmospheres of many industrial high temperature processes contain a certain amount of halogens, in most cases chlorine. Halogens have the tendency of forming volatile metal chlorides of the general formula MxCly, which are well known to play a critical role in high temperature corrosion processes, when present at a significant amount. Thermodynamic calculations give a valuable hint, whether a reaction product can occur and how volatile such products can potentially be, when the partial pressures are calculated. However, such calculations reflect thermodynamically stable conditions, while in open systems the local kinetics control the process of surface reactions. For each specific condition, the reaction rates and thus the corrosion rates have to be determined. In previous work, we have shown that corrosion kinetics which lead to the formation of volatile products follows different steps. In this work, a mathematical model is developed in which the different steps, which are required for the formation of metal chlorides and their removal from the surface are described. It is assumed that the slowest of such formation or transport steps determines the kinetics of high temperature corrosion. The application of this model on the literature data shows that by this model it becomes possible to identify the slowest and therefore rate determining step.

Corrosion behaviour of Ni and nickel aluminide coatings exposed in a biomass fired power plant for two years

“…The transport of Clions resulting in corrosion in Cl environments has been previously suggested by Folkeson et al[39]. Rammer and Galetz[40] suggest the Cl species dissociate and Cl atoms are adsorbed on the surface and then surface diffusion (or in our case grain boundary diffusion) is a fast process. Cl species can also react with Mn (which is the element having the highest affinity for Cl within the alloy) in the interdiffusion zone above the alloy-Ni interface, resulting in formation of gas porosities containing MnCl 2 at grain boundaries, which in turn could react with sulphur resulting in subsequent formation of Mn sulphides.…”

The influence of NaCl and CaCl2 induced high temperature corrosion on the aqueous corrosion resistance of stainless steels

“…The increase of the test temperature from 650 to 750 °C has led to a significant increase of Δm on both materials. This is related to the fact, that higher temperatures lead to increased reaction rates and larger removal rates of gaseous corrosion products with higher vapor pressures . During the tests without a salt layer no significant Δms have been measured on both alloys, neither at 650, nor at 750 °C.…”

Danish Experiences in Biomass Corrosion and Recent Areas of Research

“…The initial mechanism of chlorine corrosion and what is the rate determining step is still not clear and different theories have been suggested including electrochemical reactions or diffusion of Cl atoms, ions or molecules 64,65 . Measuring surface reactions at high temperatures is very demanding and tailored experiments for example with labelled molecules 13 together with mathematical models 65 would give a better insight into the process.…”