Environmental Engineering Reference
In-Depth Information
5
Tarnishing and Scaling Processes
5.1 INTRODUCTION
A metal in a gaseous environment constitutes a very complex chemical system.
In most situations an interpretation of the reaction behavior can be achieved only
with great difficulty. The terms metal oxidation , tarnishing , and scaling are used
in literature whenever oxidizing gases such as oxygen, sulfur vapor, the halogens,
water vapor, CO 2 , etc., attack a metal or an alloy at low, intermediate, or high
temperatures. In general, the oxidation processes at high temperatures leading to
the formation of thick oxide layers are referred to as the scaling process , whereas
at lower temperatures the phenomena of thin oxide film formation are termed
the tarnishing process . Oxidative attack on metals can take place under widely
varied conditions from the ''mild'' oxidizing one that prevails in air at room
temperature to the extremely ''severe'' conditions of mixed environments at ele-
vated temperatures encountered by high-temperature components such as super-
heater and reheater tubes, gas turbine blades and vanes, alloy components in fuel
conversion and power-generating units, reformer tubes, heat exchanger tubes,
and the like, which are often subjected to thermal fluctuations and thermal cy-
clings under actual service conditions. The severity of the operating condition
can be inflated due to thermal fluctuations and sudden high-temperature excur-
sion.
It is a matter of common experience that most of the metals with the exception
of gold and platinum, when exposed to different atmospheres like oxygen, halo-
gen, sulfur vapor, etc., take up the electronegative constituents and form the com-
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