Nickel-Copper alloys are extensively used
as corrosion resistant materials in marine industry. The corrosion rates of
these materials steeply decreases with increase in nickel concentration.
Corrosion performance of Monel 400 in naturally aerated and deaerated Gulf
water and 3.5% NaCl solutions was assessed.
Monel 400 is widely used in desalination
industry and other marine fields. Since Monel 400 consists of 60 to 70% nickel,
20 – 29% copper and nominal magnitudes of iron, manganese, silicon and carbon.
Monel 400 has properties of supreme corrosion resistance, good weldability and
good strength. So, it is commonly used in diverse applications for example
chemical processing units, gasoline and fresh water tanks, crude petroleum
still, valves and pumps, propeller shafts, marine components, electrical and
electronic components, deaerating heaters, process vessels and pipes, boiler
fed water heaters and heat exchanger.
Low corrosion rate in running brackish or
seawater combined with corrosive media encouraged the use of Nickel-Copper
alloys in marine applications, oil, chemical processing industries such as
non-oxidizing chloride solutions.
The corrosion of copper-nickel alloys in
natural seawater and chloride environments in different conditions has been
evaluated. It is found that corrosion rate of Copper-Nickel alloys
significantly decreases with increase in Nickel concentration in alloy.
Although corrosion resistant Monel 400 bar contains 70% nickel and has
the potential to withstand vigorous media, it should not be used in stagnant
salt water.
Seawater is a combination of inorganic
mixtures, gases, solids, organic materials and creatures. Gulf seawater
contains salts of sodium, magnesium, potassium, calcium and strontium and has
high concentration of chlorine and sulfate ions. Oxygen dissolved in water also
increases its corrosive nature however in desalination and oil well plants, the
seawater is not much vigorous to many application materials.
Experiment
Monel 400 bar and sheet were used for analysis.
An electrochemical cell with triple electrodes was used for measurements. Monel
rod was used as an electrode, it was grinded and polished and then washed with
double distilled water, degreased with acetone, washed again with distilled
water and dried.
In deaerated media, Monel 400 electrode was
kept after the solution was purged with pure nitrogen for 60 minutes and the
cell was placed purged above the solution before the analysis ends. It was
followed for the entire deaerated experiments to prevent oxygen penetration in
AGW and NaCl solutions during the tests.
Results
Monel 400 was placed in solution for 60
minutes in naturally aerated solution of gulf water and NaCl solutions. Anodic
reaction of alloy 400 is a selective dissolution of nickel, specifically at
high potential points. Increase in applied potential value decreases the development
of passive oxide layer or the accumulation of corrosion products on the surface
of electrode.
Pitting corrosion on Monel in both seawater
and NaCl solutions was due to chlorine ion and selective dissolution of nickel
caused the development of pits. It is found that Gulf seawater is more vigorous
than 3.5% NaCl and deaeration process reduced the corrosion of Monel in both
environments. Pitting corrosion of Monel in AGW was faster than in NaCl
solution.
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