Corrosion rates in chemical media usually reduce with
increase in pH values. In alkaline media, the hydrogen ion is available in
small contents. However various metal receive nominal corrosion at some pH,
usually basic and then become quickly corrosive as pH increases. Often
corrosion because of alkaline media causes localized and pitting attack by the
development of cathodic layers and corrosion is extreme at the sensitive anodic
areas. Austenitic grades and various nickel based alloys receive stress
corrosion cracking or general corrosion in hot concentrated caustic.
Addition of nickel in an alloy increases corrosion
resistance in alkaline media. Nickel 200 and various high nickel alloys provide
suitable service in dealing with this media. The commonly used nickel alloys in
alkali media are nickel 200, Monel 400, Inconel 600 and alloy 625.
Nickel 200
The great corrosion resistance characteristic of alloy 200 is its
potential against caustic soda and other alkalies. The alloy is not affected by
anhydrous ammonia or ammonium hydroxide in contents of 1%. Larger contents are
resulted by fast corrosion in the presence of dissolved oxygen.
Nickel 200 offers good resistance to whole concentration percentage of
caustic soda or sodium hydroxide at the different temperature limits including
the melting point. Below 50% concentration, corrosion rates are low, usually
below 0.1 mpy or 0.003 mm/a actually in boiling points. The increased content
and temperature limits result in slightly accelerated corrosion rates.
The major influencing factor of nickel alloy in highly concentrated
caustic soda is the same nature of layer developed while exposing the caustic
soda. In the diverse media, a security layer of black nickel oxide is developed
that results in a considerable reduction in the corrosion rates during prolong
experience. For example, specimens of alloy 201 were exposed to caustic
solutions consisting of 500 cc of water to 2kg of technical caustic flakes
followed by heating in nickel 201 at temperature limit of 790oF – 830oF. The
corrosion rate in the first 24 hours was average 21 mpy. The specimens were
exposed to test without the elimination of oxide layer. During the weekends the
overall corrosion rates decrease to 2.8 mpy or 0.07 mm/a.
The presence of chlorates in caustic soda solutions accelerates the
corrosion of alloy 200 considerably. Taking the damaging effects into account,
it is preferred to avoid chlorates fully prior evaporation at the high
temperatures in the presence of nickel. It is also known that the presence of
oxidizable sulphur compounds increases the corrosivity of caustic for nickel.
This effect is noticed normally with sulphides such as hydrogen sulphide,
mercaptans, sodium sulphide and to a small limit with slightly oxidizing media
containing thiosulfates and sulphates.
The effect of oxidizable sulfur compounds can be
eliminated by the addition of caustic solution of suitable sodium peroxide to
oxidize them to sulfates. Wide peroxide does not appear to alter the corrosiveness
of caustic soda to nickel.
In the other media, when nickel is used at
temperatures above 600oF, the preferred metal is a low carbon alternative for
example Nickel 201.
Monel 400
The high nickel content,
alloy 400 provides good resistance identical to the Pure Nickel alloy 200 to caustic
soda for the several concentration limits, although causing stress corrosion
cracking in the strong alkalis at the elevated temperatures. The corrosion
rates of alloy 400 are higher in concentrated caustic soda and caustic potash
at the high temperatures. Alloy 400 also offers good resistance to anhydrous
ammonia and ammonium hydroxide solutions comprising of 3% alkali concentration
in the absence of dissolved oxygen.
Incoloy Grades
The alloys provide excellent resistance to alkaline
solutions, getting a corrosion rate of 0.5 mpy or 0.01 mm/a in the boiling 50%
sodium hydroxide, their resistance is below that provided by Nickel 200, are
used in alkaline conditions except other corrosive media is included. They can
also get stress corrosion cracking in warm concentrated alkalis. They also
prevent corrosion in ammonia and ammonium hydroxide solutions in the various
service media.
Inconel 600
In the high temperature
caustic conditions containing sulfur, alloy 600 offers superior performance
than alloy 201. However alloy 600 can receive stress corrosion cracking in
specific alkali media. So it should be stress relieved prior service and
service stresses must be used to the minimum. Alloy 600 provides good
resistance to sodium hydroxide at the boiling temperatures in concentrations
about 80%.
For nickel 200, the presence
of chlorates in caustic soda accelerates the corrosion rates. For its con
For Nickel
alloy 200, the availability of chlorates in caustic soda increases the
corrosion rates. Due to its concentration of chromium, Inconel alloy 600 is fully resistant to corrosion by ammonia for a
complete range of concentrations and temperatures.
While offering good corrosion resistance, Inconel
grades are not often required for service in absolute caustic media. They can
be advantageous for use in halide media.
Salt
Salt and water combination offers good conductivity
of salts and therefore they become able to take higher corrosion currents. Therefore
the galvanic effects are more noticeable in the salt solutions instead in pure
water.
The commonly used nickel alloys in the process media
involving sulfate salts are Incoloy 825, Inconel 625, Hastelloy C276 etc. For
media containing chloride salts, the commonly used alloys are alloy 400, alloy
825, alloy 625, etc.
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