The phenomenon of adherence of a growing scale to the surface of a solid substance

Abstract

During the oxidation of metals the growing oxide or sulphide scales sometimes adhere to solid substances when mutual contact takes place. For example, a growing copper oxide scale adheres to quartz glass and platinum at 1050 °C. This phenomenon has not yet been investigated thoroughly. In this paper we present results of studies of this effect when oxide scales growing on copper or iron reach intimate contact with materials such as quartz glass, corundum and platinum. The oxidation process was carried out in air in the temperature range 500–900 °C. The rectangular samples of both oxidizable metals were prepared from Armco iron (purity, 99.9%) and electrocytic copper (purity, 99.9%) sheets. The samples of oxidizable metal and contact material were arranged in specimen sets with initial linear contact between both materials. During the reaction the growing scale gradually covered the surface of the contact material. The oxidation experiments were conducted in isothermal conditions using three standard exposure times: 5, 10 and 24 h. The adherence was evaluated after the reaction had been stopped and the specimen set had been cooled. Adherence is revealed if force is needed to separate the contracting surfaces, or when some of the oxide scale and/or contact material remains stuck on them. Adhesion effects were detected in all systems except for the iron oxides-platinum interface. The exposure time required to obtain adhesion decreased with increase in the oxidation temperature. After the decohesion the contact surface of one material often had remains of the other material, e.g. spalls of quartz glass were detected stuck on the scale growing on iron. This suggests that the adhesion forces created on the interdace are of the same order of magnitude as cohesion forces in the two materials. The formation of oxide scales on iron and copper takes place on the external surfaces (cationic transport scales), so that adhesive contact is made between the surface of the contact material and the newly formed, but not fully ordered, lattice planes of the scale. This may be responsible for the formation of adhesion bonding between the scale and the contact material. Other explanations are also possible, such as a keying effect by the scale tongues which grow into the irregularities of the contact material surface, or a reaction of the scale material with the remains of contamination on the same surface.