Alkaline solutions comprise hydrogen ion in nominal
concentrations. Usually corrosion by alkalies results into pitting and other
types of localized attack as they develop cathodic layers and corrosion is
concentrated at sensitive anodic areas. Austenitic stainless steels and other
low nickel alloys may experience stress corrosion cracking or general corrosion
in hot concentrated caustic.
Corrosion resistance in alkalies generally enhances with increase
in nickel concentration. Nickel 200 and other high nickel alloys provide satisfactory
performance in these materials. The most commonly used Inconel bars in alkalies are made from Inconel 625 and 600.
Nickel 200
The excellent corrosion resistance of Nickel 200 is its
resistance to caustic soda and other alkalies. Nickel 200 is not corroded by
anhydrous ammonia or ammonium hydroxide in concentrations 1%. Higher
concentrations can cause quick corrosion in the presence of dissolved oxygen.
Nickel 200 provides outstanding resistance to concentrations
of caustic soda at temperatures about as well as melting level. For
concentrations below 50% the corrosion rates are nominal being often below 0.1
mpy even in the boiling solutions. With increase in concentrations and
temperature, corrosion rates nominally increase.
The major factor influencing the performance of nickel in
highly concentrated NaOH is the type of layer developed when kept in its
contact. In various conditions, a security layer of black nickel layer is
developed that causes remarkable reduction in corrosion rates over prolong
exposure. The presence of chlorates in caustic soda solutions increases the
corrosion rates of Nickel 200 significantly.
It has been observed that the influence of oxidizable sulfur
compounds results into increased vigorousness of caustic solutions. This effect
can be prevented by the addition of adequate sodium peroxide in the caustic solution
to oxidize it to sulphates.
Monel 400
Containing high nickel concentration, Monel 400 offers
similar resistance as of Nickel 200 to caustic soda in the most of
concentrations, however susceptible to stress corrosion cracking in strong
alkalies at high temperatures. The corrosion rates of Monel 400 are higher in
highly concentrated caustic soda and caustic potash at high temperatures. Monel
400 resists anhydrous ammonia and to ammonium hydroxide solutions to 3%
concentrations in the absence of dissolved oxygen.
Incoloy 800 and Incoloy 825
While these alloys provide outstanding resistance to
alkaline solutions, the corrosion rate of 0.5 mpy in boiling 50% sodium
hydroxide, they are not as resistant as Nickel 200 and are nominally used in
alkaline solutions if the corrosive materials are not involved. These alloys
may also suffer from stress corrosion cracking in the hot concentrated
alkalies.
Inconel 600
Inconel 600 should be stress relieved before use and
subjecting to stresses should be minimum. Inconel 600 prevents corrosion in
sodium hydroxide at boiling points in concentrations about 80%.
Similar to Nickel 200, exposure to chlorates present in
caustic soda corrodes the alloy. With the presence of chromium concentration,
Inconel 600 is fully resistant to ammonia solutions for a complete range of
concentrations and temperatures.
Inconel 625, C276
These alloys offer advantageous performance in halide based
conditions.
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