High Temperature Corrosion

High Temperature Corrosion

Metals exposed to oxidizing gases at elevated temperatures can corrode by direct reaction with the gas in the absence of any liquid electrolyte. This type of corrosion is referred to as tarnishing, high-temperature oxidation or scaling. The rate of such attack is a function of the temperature and it increases substantially with temperature. Surface film at the scale/gas or metal/scale interface typically thickens as a result of reaction due to the movement of cation and anions through the scale, which acts as a solid electrolyte. In case of continuous, nonporous scales, ionic transport through the solid electrolyte becomes the rate controlling factor. The thermodynamic stability, the ionic defect structure and certain morphological features of the scale formed are key factors in the resistivity of an alloy towards a specific environment.

Desired characteristics for a protective oxide scale include:

High thermodynamic stability, so that it forms in preference to other possible reaction products
Low vapor pressure so that the product forms as a solid and does not evaporate into the environment
Pilling-Bedworth ratio > 1.0 so that the product completely covers the metal surface
Low coefficient of diffusion of reactant species so that the scale has a slow growth rate (lower corrosion rate of the metal surface)
High melting temperature
Good adherence to the metal substrate, which usually involves a coefficient of thermal expansion close to that of the metal and sufficient plasticity to resist fracture from differential thermal expansion stresses

Forms of High temperature Corrosion include:

High temperature oxidation
High temperature sulfidation
High temperature carburization
Hydrogen effects
Hot Corrosion

Thermal Cycling in Hydrogen Reformer Tubes

A high temperature tensile/rupture failures can result from cumulative thermal cycles in reformer tubes. These conditions promote high tensile stresses in the axial direction which lead to the formation of highly branched networks of cracks. These cracks are generally oriented in the through thickness direction and link up with time resulting in premature tube failures.