A survey of test methods for scale adhesion measurement

Abstract

The spallation of oxide scales from the steam side of tubes in power generation boilers can severely limit component life time. The process involves through scale cracking and crack propagation at or parallel to the scale/alloy interface. Many techniques have been developed in the past two decades to assess the interfacial fracture toughness of brittle coatings on ductile substrates. Analyses from fracture mechanics approaches also allow for quantitative determination of the fracture toughness, which can be used for life time prediction modelling. This paper gives an overview on techniques that have been used on thermally grown oxides, such as Ni, Fe, Ti, Al, and Cr oxides and spinels, and compare the results obtained on each type of oxide system from different techniques. In doing so, the limitations of each technique are illustrated. The methods discussed are flexure, scratch, indentation, tensile and compression tests and those that perform direct analysis of the spallation process. The data on steam grown oxide is sparse, but the test methods discussed could all be applied to these scale types.

Keywords:

• spallation of oxides sales
• scale adhesion measurements