Environmental Engineering Reference
In-Depth Information
Such observations suggest that the addition of aluminum within the permissi-
ble limit that does not cause embrittlement to high-temperature alloys should
provide sufficient protection by self-forming oxide layers at temperatures below
1673 K. Chromium is known to be the mildest of the three elements (i.e., Al,
Cr, and Si) insofar as its influence on alloy embrittlement is concerned. But to
guarantee self-forming oxide layers, there is a need to add more than the mini-
mum highest permissible limit (20 wt %). Moreover, chromium as the single
alloying element provides only limited protection above 1273 K, due to formation
of its volatile oxide (CrO
3
). Accordingly, it is desirable to use aluminum (in low
concentrations) in combination with higher contents of chromium to guarantee
protection above 1273 K. Another interesting observation is that at very high
temperatures (above 1773 K), the transport rates of molecular oxygen through
SiO
2
layer are low in comparison with aluminum ion diffusion through Al
2
O
3
.
This provided impetus for the development of silicides and silicon ceramics as
suitable coating materials in oxidizing environments at extremely high tempera-
tures (1773-2073 K). However, it is to be recognized that the element silicon,
apart from causing embrittlement to high-temperature alloys, similar to alumi-
num, has an additional negative effect on the coating-alloy performance. SiO
2
can form low-melting eutectics for which the use of silicon in iron-based alloys
as a stable oxide forming addition is avoided at performance temperatures above
1273 K.
6.8.2 Coating Methods
In the present state of knowledge, the most successful coatings are the metallic
coatings, particularly the intermetallics, wherein one constituent preferentially
oxidizes to produce a protective scale. Under service conditions of metallic mate-
rials, protection by coatings from subsequent degradation is obtained in two dif-
ferent ways:
1.
Constituents of the coating material may react with the corrosive constituents
of the environment under service conditions of the metallic material, forming
product layers of positive protective properties, and
2.
Layers of coating material can be applied over the metallic component that
mechanically isolate the underlying substrate from the corrosive environ-
ment.
Materials used for protective coatings on metal/alloy surfaces are broadly classi-
fied into four groups:
1.
Scaling-resistant metals or alloys, which on reaction with the environment
develop protectiveness
2.
Intermetallic compounds of the type silicides, aluminides, borides, etc.
