Corrosion in chemical processing media often reduces with increase in their pH level. In alkaline solutions, hydrogen ions are usually present in nominal magnitudes. Although, various metals can withstand some pH, often basic in nature but experience severe corrosion with increase in pH level. Usually alkali corrosion is led by pitting and other types of localized corrosion as they develop cathodic layers and corrosion is corrosion is concentrated at sensitive anodic regions. Austenitic stainless steels and other low nickel alloys are prone to stress corrosion cracking and general corrosion in hot caustic solutions.

Increase in nickel concentration significantly improves resistance to alkali corrosion. It is observed in both plant and lab tests from which it is found that Nickel 200 and high nickel alloys offer suitable performance in handling these solutions. Alloys that perform satisfactory in alkali media are Inconel 625, 600, Monel 400 and Nickel 200.

Nickel 200

It shows excellent performance in caustic soda and other alkali media. Nickel 200 is also immune to anhydrous ammonia or ammonium hydroxide in concentrations 1%. Stronger concentrations cause quick corrosion in dissolved oxygen.

Nickel 200 performs suitably in this media at all concentrations and temperatures to melting point. The security layer formed by nickel in the concentrated caustic soda protects the metal from any type of corrosion. commonly black nickel oxide layer is formed that results in a significantly reduction in corrosion rates over the passage of time. The chlorates in caustic soda solutions make the corrosion of Nickel 200 severe. It is recommended to remove chlorates as much as possible before evaporation while using nickel.

The presence of sulfur compounds improve the corrosive behavior of caustic solutions such as hydrogen sulfide, mercaptans and sodium sulfide. In such cases, use of Nickel 201 is recommended.

Monel 400

The performance of Monel 400 in concentrated caustic soda is similar to Nickel 200 however it is susceptible to stress corrosion racking in highly concentrated alkalis at high temperatures. But alloy 400 is immune to corrosion in anhydrous ammonia and ammonium hydroxide up to 3% concentrated without dissolved oxygen.

Incoloy 800 and 825

Incoloy alloys offer supreme performance in alkaline solutions, showing nominal corrosion rate of 0.5 mpy in boiling 50% sodium hydroxide but their service is lower than nickel 200 in alkaline conditions. They may be stress corrosion cracked in hot concentrated alkali solutions.

Inconel 600

Inconel 600 offers superior performance to Nickel 200 in hot caustic solutions in the presence of sulfur. Meanwhile alloy 600 should be stress relieved before using in some alkali conditions. It also resists corrosion in sodium hydroxide up to boiling point even at concentrations up to 80%. Similar to Nickel 200, chlorates in caustic soda accelerate the corrosion rate. But Inconel 600 comprises chromium which makes this alloy fully resistant to ammonia based corrosion at all temperatures and concentration limits.

Inconel 625 and C276

While Inconel 625 and C276 provide supreme corrosion resistance, these alloys are usually not needed in pure caustic solutions. They can offer significant performance in halide containing conditions.