Materials play an
important part in the serviceability of equipment. In many cases, designers
grasp only the requirements for minimum strength. Their designs are stress
based and sometimes failure to incorporate the influence of corrosion which
is present in just about every service environment. This is particularly
true in applications involving high temperature processes. Many of these
involve combustion which liberates corrosive byproducts such as SO2
or halide gases. Other applications such as found in petrochemicals, involve
the split hydrocarbon molecules to either liberate highly active carbon
and hydrogen or corrosive species such as CO2 and H2S
which can result in serious forms of corrosive attack.
OXIDATION
Carbon and low alloy
steels are the most widely utilized materials for plant construction. The
applications include power generation, waste incineration, oil refining,
petrochemical and chemical processing, and industrial heating. Typical
equipment includes: steam generators, boilers, reactor vessels, incinerators
and flue gas systems. In most cases, the materials used in this equipment
are most commonly limited by the loss of strength due to exposure to high
temperature. This is why Ni additions are commonly utilized. At sufficient
levels, it imposes an austenitic microstructure and minimizes the phenomenon
of "885 F" embrittlement. However, in applications involving
oxidative conditions, additions of Cr tend to promote the formation of
protective films which reduce corrosion rates to acceptable levels. Exhaust
gases which contain H20, CO2, NOx and
O2 can accelerate corrosion and require > 17% Cr and >8
Ni to reduce corrosion to acceptable levels. At very high temperatures
(>1000 C) alternative materials must be utilized which contain high
levels of Ni and Co which impart both enhanced high temperature strength
and resistance to corrosive attack in many environments. In some cases
where particularly corrosive environments are encountered, ceramic materials
either as coatings or monolithic materials may be required.
CARBURIZATION
This form of high
temperature attack results in the formation of carbides in the metal matrix
or in grain boundaries. A typical example is in the deterioration observed
in pyrolysis tubes in ethylene plants. Many of the alloys utilized in these
environments depend on high levels of Cr and Ni (e.g., HK-40 and Alloy
800H) which help the formation of a protective oxide film which minimizes
carburization. For particularly severe conditions at high temperatures,
alloys with additions of Nb, Mo W and Si are utilized due to their high
resistance. The effect of Si is particularly significant since it allows
the formation of an SiO2 layer which provides a significant increase in
carburization resistance.
NITRIDIZATION
This form of attack
is similar in many regards to carburization as nitrogen, like carbon, is
an interstitial atom and can defuse into material at intermediate temperature.
Several applications in the chemical industry involve exposure to reactive
nitrogen-containing environments. These include reducing environments for
the production of ammonia, nitric acid and nylon. Ni additions are most
effective in minimizing nitridization which allow the use of stabilized
stainless steels up to about 600 C. For higher temperatures, nickel or
cobalt based alloys are usually required. For severe conditions, high levels
of Al (>4,5%) or ceramic materials may be required.
HOT SALT / ASH CORROSION
A common form of
corrosive attack found in gas turbines, fireside coal and oil fired boilers,
furnaces and steam super heaters and waster incinerators is related to
the formation of hot salts or ash. In these cases, the single most important
alloying element is Cr to resist the influences of hot sulfur and halide
attack. In many applications at 450 C Cr-Ni-Mo steels (e.g. AISI 310) are
required. Above 650 F, the use of higher Cr alloys is needed up to 50 Cr-50Ni
alloys. Contaminants in the combustion environments such as V, Na and Cl
can greatly add to the severity of corrosion and may require mitigation
by non-alloy methods such as the injection of inhibitors [Mg(OH)2] or blending
of fuels to reduce the levels of Cl, Zn, Pb, K and other constituents.
HALOGEN ATTACK
IMany high temperature
processes such as ore processing, calcining and vinyl chloride manufacturing
involve the formation of volatile low melting point halide corrosion products.
These products lead to the destruction of normally protective oxide corrosion
films and, in turn, result in highly corrosive conditions. For reducing
conditions Mo and W additions are usually used to enhance corrosion resistance.
However, under oxidizing conditions they can be detrimental. In some cases,
pure metals are needed particularly where fluorine is encountered. Cr additions
are often detrimental in contrast to their normal role in oxidizing environment.
SULFIDATION
Sulfur is a common
impurity in many combustion environments which leads to an increased corrosion
rate at high temperatures. It is commonly observed in the combustion of
low grade fuel oil or coal in electric power generation, fuel engines and
oil refining. Such environments can include exposure to H2,
CO, CO2, H2O and H2S under reducing conditions.
In most cases Cr-Mo steels or Cr-Ni-Mo steels are used up to about 500
C. For sulfurous environments under oxidizing conditions, Ni-alloys with
at least 25 % Cr and sometimes Si are needed.
|