Ferrite content effects on corrosion

[Talha Aamir]

Hi,

Why ferrite content should be lower than 0.6 percent or absent in Urea grade materials like 316 UG or 25Cr-22Ni-2Mo. How ferrite content causes corrosion ?

[Amol Betkar]

Dear Tahir,

In urea production environments, materials used for constructing process equipment must be highly resistant to corrosion, particularly from carbamate solutions and other aggressive chemical compounds. For this reason, specific stainless steels such as 316 UG (Urea Grade) or 25Cr-22Ni-2Mo are preferred. These materials are designed to have very low or no ferrite content. Here’s why controlling ferrite content is crucial in these grades and how ferrite can impact corrosion resistance:

Importance of Low Ferrite Content in Urea Grade Materials:

1. Corrosion Resistance:

   • Corrosion Susceptibility: Ferrite phase in stainless steels is generally more susceptible to corrosion in the aggressive environments found in urea production. Ferrite can form galvanic cells with the austenite phase, leading to localized corrosion.
   • Carbamate Corrosion: Urea production involves the formation of ammonium carbamate, which is highly corrosive. Ferrite phases can be more prone to attack by carbamate solutions, leading to pitting and other forms of localized corrosion.

2. Phase Stability:

   • Sigma Phase Formation: Ferrite is a precursor to the formation of sigma phase at high temperatures. Sigma phase is a brittle intermetallic compound that can drastically reduce toughness and corrosion resistance.
   • Chromium Depletion: The formation of sigma phase also causes chromium depletion in the surrounding matrix, further reducing the corrosion resistance of the material.

3. Mechanical Properties:

   • Embrittlement: Ferrite can contribute to embrittlement at low temperatures, which is undesirable in many industrial applications where toughness is required.

How Ferrite Content Causes Corrosion:

1. Galvanic Corrosion:

   • Differential Aeration Cells: Ferrite and austenite phases can form micro-galvanic cells. The anodic (ferrite) areas corrode preferentially, leading to localized corrosion like pitting or crevice corrosion.

2. Decreased Passivity:

   • Chromium Distribution: Ferrite tends to have a different distribution of alloying elements, such as chromium, compared to austenite. This can lead to regions with lower chromium content, reducing the formation of the passive oxide layer that protects stainless steel from corrosion.

3. Carbide Precipitation:

   • Sensitization: In ferritic regions, chromium carbides can precipitate at grain boundaries during thermal cycles, depleting chromium and making these regions more susceptible to intergranular corrosion.

Conclusion:

For urea grade materials such as 316 UG and 25Cr-22Ni-2Mo, maintaining a ferrite content lower than 0.6 percent or ensuring it is absent is essential to maximize corrosion resistance in the harsh environments of urea production. High ferrite content can lead to various forms of corrosion, including pitting, crevice corrosion, and intergranular corrosion, due to the formation of galvanic cells, phase instability, and localized depletion of chromium. Therefore, controlling the microstructure to minimize or eliminate ferrite is critical for ensuring the longevity and reliability of materials used in urea production plants.

On Thu, 30 May 2024 at 19:43, Talha Aamir <aamirt...@gmail.com> wrote:

Hi,

Why ferrite content should be lower than 0.6 percent or absent in Urea grade materials like 316 UG or 25Cr-22Ni-2Mo. How ferrite content causes corrosion ?

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Best regards,

Amol K.B

B.E(Mech), CSWIP 3.2.2, NACE CIP 1 & ASNT LII(4M)