Dealloying
De-alloying, or selective leaching, is the selective
corrosion of a specific element in an alloy. This results in the formation of a
porous structure that is not strong enough to support the applied mechanical
loads. The specific type of corrosion that occurs depends on several
factors including metal composition, metal microstructure, environment,
component geometry, stress on the component, contact between metals, and the
manner in which components are joined together. The common examples are
dezincification of brass alloys used for plumbing, where the zinc is leached
out of the alloy forming unstabilized brass. The result of corrosion in such
cases is deteriorated and porous copper.
Intergranular Corrosion
Intergranular corrosion is a chemical or electrochemical
attack on the grain boundaries of the affected metal. It often occurs due to
impurities in the metal, which tend to be present in higher contents near grain
boundaries. These boundaries can be more vulnerable to corrosion than the bulk
of the metal. The result is that the metal grains fall away and the metal is
weakened. The close microstructure of a metal reveals that the grains are
formed during the solidification of the alloy as well as at the grain boundaries
between them. Intergranular corrosion can be caused by impurities present at
these grain boundaries or by the depletion or enrichment of an alloying element
at the grain boundaries. It occurs along or adjacent to these grains, seriously
affecting the mechanical properties of the metal while the bulk of the metal
remains intact. An example of intergranular corrosion is carbide precipitation,
a chemical reaction that can occur when a metal is subjected to very high
temperatures (800 °F - 1650 °F) and/or localized hot work such as welding.
Austenitic stainless steels and precipitation-strengthened aluminum alloys are
examples of metals that can suffer from intergranular corrosion if the alloys
are not properly processed and if they are exposed to corrosive environments.
In stainless steels, during these reactions, carbon consumes the chromium-forming carbides and causes the level of chromium remaining in the alloy to
drop below the 11% needed to sustain the spontaneously-forming passive oxide
layer. The SS 304L and 316L are enhanced versions of 304 and 316 stainless
steel that contain lower levels of carbon providing the best corrosion resistance
to carbide precipitation.
Stress Corrosion Cracking
Stress corrosion cracking (SCC) is a result of the
combination of tensile stress and a corrosive environment, often at elevated
temperatures. In most cases, the stress or environment by itself is
insufficient to cause the degradation of the metal. That is if the stress is below
the metal’s yield strength the metal would not corrode in the specific
environment. It is the net result of external stress such as actual tensile loads
on the metal or expansion/contraction due to rapid temperature changes. It may
also be the result of residual stress imparted during the manufacturing process
such as cold forming, welding, machining, grinding, etc. In stress corrosion,
the majority of the surface usually remains intact; however, fine cracks appear
in the microstructure, making the corrosion hard to detect. The cracks
typically have a brittle appearance and form and spread in a direction
perpendicular to the location of the stress. Selecting proper materials for a
given environment can mitigate the potential for catastrophic failure due to
SCC.
Fatigue or Environmental Cracking Corrosion
Environmental cracking is a corrosion process that can
result from a combination of environmental conditions affecting the metal.
Chemical, temperature, and stress-related conditions can result in the following
types of environmental corrosion:
(i) Stress corrosion cracking.
(ii) Corrosion fatigue.
(iii) Hydrogen-induced cracking.
(iv) Liquid metal embrittlement.
High-Temperature Corrosion
Fuels used in gas turbines, diesel engines, and other
machinery, which contain vanadium or sulfates during combustion can form
compounds with a low melting point. These compounds are very corrosive towards
metal alloys normally resistant to high temperatures and corrosion, including
stainless steel. High-temperature corrosion can also be caused by
high-temperature oxidization, sulfidation, and carbonization. The American
Society of Metals (ASM) classified various corrosion types as given in Table.1.
Table.1: ASM Classifications of Corrosion Types
|
General
Corrosion |
Localized
Corrosion |
Metallurgically
Influenced Corrosion |
Mechanically
Assisted Degradation |
Environmentally
Induced Cracking |
|
Corrosive
attack dominated by uniform thinning |
High rates of
metal penetration of specific sites |
Affected by
alloy chemistry and heat treatment |
Corrosion
with a mechanical component |
Cracking
produced by corrosion, in the presence of stress |
|
• Atmospheric
corrosion • Galvanic
corrosion •
Stray-current corrosion • General
biological corrosion • Molten salt
corrosion • Corrosion
in liquid metals • High–temperature corrosion |
• Crevice
corrosion • Filiform
corrosion • Pitting
corrosion • Localized
biological corrosion |
•
Intergranular corrosion • Dealloying
corrosion |
• Erosion
corrosion • Fretting
corrosion • Cavitations
and water drop impingement • Corrosion
fatigue |
• Stress –
Corrosion Cracking (SCC) • Hydrogen
Damage • Liquid
metal embrittlement • Solid metal-induced embrittlement |