Overcome The Challenge Of High Temperature Corrosion

ABSTRACT

To those who are familiar with the subject, corrosion is a natural occurring process that will continue, provided that the necessary prerequisites of thermodynamics - that a product is able to form, and kinetics - that there is time enough for this to happen, are satisfied. High temperature corrosion follows the same rules compounded by the fact that reaction rates are higher, internal penetration into the metal can be extensive, phase changes can occur within the metal, temperature cycling can affect scale adhesion and contribute to thermal fatigue, and the products that form may disappear - as can the material if due diligence is not involved. This paper briefly reviews high-temperature corrosion, summarizes the anticipated performance of selected high-temperature materials, and addresses aspects of reliability that influence the potential for failures that could have been avoided or prevented by better awareness. The intentions of this paper are simple. First, to provide a summary of the subject for those who are less aware. Second, to avert the old maxim that “history will repeat itself.” Third, to demonstrate that the challenge of high-temperature corrosion can often be overcome with improved and effective communication between those who plan (designers), those who provide (material suppliers), those who use (operators), and those who keep things going (maintenance personnel).

INTRODUCTION

Most publications about high-temperature corrosion1-11 focus on the types and mechanisms of corrosion and/or provide guidelines about the range of candidate materials - metals/alloys, coatings, weld overlays, intermetallics and ceramics - that are suitable for the intended application (Figure 1). The options for choice are well summarized in materials suppliers' selection charts - that should be used judiciously - or in their more detailed literature, which is generally available on-line. As with all materials selection processes it is most important to recognize the prevailing circumstances and the human factor that plays an important role in the selection process Once properly installed, materials can be expected to satisfy their intended function - which they often do - provided that service parameters do not change and assuming that maintenance procedures are properly fulfilled. In recent years, improvements in on-line surveillance and monitoring techniques with remote controls and fail-safe devices, etc., have contributed significantly to minimizing the risk of premature and often devastating failure; especially for older plant that may have exceeded the original design life but still have a remaining useful life.14 A better, safer operating plant is always preferred to an uncontrolled, dangerous operation

HIGH-TEMPERATURE CORROSION

Basic Definitions

From personal experience it is necessary to state that high-temperature corrosion is a form of corrosion process. Others have argued to the contrary with ulterior motives that are probably driven by economic undertones, as for example in insurance claims and litigation matters where corrosion - considered as fair wear and tear - is excluded. To paraphrase the basic definition of corrosion15 - high-temperature corrosion is the deterioration of a material, usually a metal that results from a reaction with its environment at high temperatures. Depending on the environment (Figure 1). the products of the reaction may be solid, liquid or gaseous.