Nickel alloy’s resistance performance in Alkali media
Nickel alloy 200
The excellent property of Nickel alloy 200 is its resistance capability against caustic soda and other alkalies (excluding ammonium hydroxide). The alloy is not corroded by anhydrous ammonia or ammonium hydroxide in concentrations about 1%. Higher concentrations can result in quick corrosion in the availability of dissolved oxygen.
Nickel alloy 200 provides outstanding resistance to whole content percentages of caustic soda or sodium hydroxide at the various temperature ranges and including the melting point. Lower than 50% content, corrosion rates are nominal, being often less than 0.1 mpy or 0.003 mm/a in fact in boiling solutions. With increase in content and temperature limits, the corrosion rates increase slightly. The resistance of Nickel alloy 200 is described in the following figure:
The key factor influencing the conduct of nickel alloy in highly concentrated caustic soda is the same nature of the layer produced while exposure to the caustic soda. In the several conditions, a security layer of black nickel oxide is produced that results in a remarkable reduction in the corrosion rates for longer exposure. For instance, samples of nickel alloy 201 were subjected to caustic solutions comprised of 500cc of water to 2kg of technical caustic flakes followed by heating in Nickel alloy 201 at temperature limit of 790oF to 830of or 412 to443oC. The corrosion rate within the initial 24 hours was average 21 mpy or 0.53 mm/a. The samples were subjected to test conditions again without eradication of oxide layer. At the weekend, the net corrosion rates decreased to an average of 2.8 mpy or 0.07 mm/a. The test was continued for second week with the same samples. The average corrosion rate 2.8 mpy or 0.07 mm/a was maintained.
The availability of chlorates in caustic soda solutions increases the corrosion rates of Nickel alloy 200 significantly as shown in the following table:
| Lab corrosion tests in evaporation of caustic soda from 73 to 96% with and without chlorate | ||
| Alloy | Corrosion rate, mpy or mm/a | |
| Nickel alloy 200 | 1.5 or 0.038 | 260 or 6.60 |
| Inconel alloy 600 | 2.2 or 0.056 | 380 or 9.65 |
Considering the depleting effects, it is recommended to prevent chlorates completely on nickel before evaporation in the elevated temperature range.
The availability of oxidizable sulfur compounds tends to increase the corroisvity of caustic toward nickel. This influence is observed basically with the sulfides like hydrogen sulfide, mercaptans, or sodium sulfide and to a nominal limit with partially oxidized materials like thiosulfates and sulfates.See the following table:
Effect of oxidizable sulfur compounds on corrosion of nickel alloy 200 in caustic soda
| Corrosive alkalies | Corrosion rate, mpy or mm/a |
| Commercial sodium hydroxide concentrated from 50 to 75 % NaOH (sulfur concentration at beginning, found as H2S, 0.009%) | 1.7 or 0.04 |
| 75% chemically pure sodium hydroxide | 0.6 or 0.015 |
| 75% chemically pure sodium hydroxide plus 0.75% sodium sulfide | 22.8 or 0.58 |
| 75% chemically pure sodium hydroxide plus 0.75% sodium thiosulfate | 7.9 or 0.20 |
| 75% chemically pure sodium hydroxide plus 0.75% sodium sulfite | 5.2 or 0.13 |
| 75% chemically pure sodium hydroxide plus 0.75% sodium sulfate | 0.6 or 0.015 |
It has been discovered that the influence of oxidizable sulfur compounds can be prevented by inclusions to the caustic solution of appropriate sodium peroxide to oxidize them to sulfates. Abundant of peroxide does not seem to change the corrosiveness of caustic soda toward nickel.
As in other conditions, when nickel is utilized at temperatures more than 600oF or 315oC,the recommended material is low carbon grade Nickel alloy 201.
Monel alloy 400
Due to its high nickel concentration, Monel alloy 400 offers similar resistance as to Nickel alloy 200 to caustic soda over the most concentration limits, however resistant to stress corrosion cracking in strong alkalis at the high temperatures. The corrosion rates of Monel alloy 400 are higher in concentrated caustic soda and caustic potash at the elevated temperatures. Monel alloy 400 offers resistance to anhydrous ammonia and ammonium hydroxide solutions containing 3% alkali content in unavailability of dissolved oxygen.
Incoloy alloy 800, 825, 020, 25-6MO
While these alloys offer outstanding resistance to alkaline solutions, with a corrosion rate of 0.5 mpy or 0.01 mm/a in the boiling 50% sodium hydroxide, their resistance is less than that offered by Nickel alloy 200, are mildly utilized in alkaline conditions unless other corrosive media is included. They can also go through stress corrosion cracking in hot concentrated alkalis. These alloys offer resistance to ammonia and ammonium hydroxide solutions under the several application conditions.
Inconel alloy 600
For the elevated temperature caustic applications that include sulfur, Inconel alloy 600 is recommended over Nickel alloy 201. Although, alloy 600 can get stress corrosion cracking in some alkali conditions. Therefore, alloy 600 components should be stress relieved before using and operating stresses should be used to a minimum. Inconel alloy 600 offers resistance to sodium hydroxide at the boiling temperatures in concentration about 80%. The test outcomes in sodium hydroxide at 572oF or 300oC are described in the following table:
| Alloy | NaOH content | Test time, hours | Corrosion rate, mpy or mm/a |
| Nickel alloy 200 | 50 % | 720 | <1 or <0.025 |
| Monel alloy 400 | 50 % | 720 | <1 or <0.025 |
| Inconel alloy 600 | 50 % | 720 | <1 or <0.025 |
| Incoloy alloy 825 | 50 % | 720 | <1 or <0.025 |
| Inconel alloy 625 | 50 % | 720 | <1 or <0.025 |
| Inconel C-276 | 50 % | 720 | <1 or <0.025 |
| Inconel alloy 600 | 10 % | 504 | Nil |
| Inconel alloy 600 | 20 % | 504 | Nil |
| Inconel alloy 600 | 30 % | 504 | <1 or <0.025 |
| Inconel alloy 600 | 40 % | 504 | <1 or <0.025 |
| Inconel alloy 600 | 50 % | 504 | <1 or <0.025 |
| Inconel alloy 600 | 60 % | 504 | 4.1 or 0.10 |
| Inconel alloy 600 | 70 % | 504 | 2.7 or 0.07 |
| Inconel alloy 600 | 80 % | 504 | <1 or <0.025 |
For Nickel alloy 200, the availability of chlorates in caustic soda increases the corrosion rates. Due its concentration of chromium, Inconel alloy 600 is fully resistant to corrosion by ammonia for a complete range of concentrations and temperatures.
Inconel alloys 622, 625, C276 and 686
While these provide outstanding corrosion resistance, Nickel-chromium-molybdenum alloys are not usually needed for application in pure caustic conditions. They can provide some benefits in halide containing conditions.