In certain cases, 304 stainless steel may show signs of rusting even under normal conditions. One significant potential cause is related to ammonia decomposition during specific stages of the cold rolling process. Below is an explanation of how this phenomenon occurs and why it may affect the material’s corrosion resistance.
1. Ammonia Decomposition in the Annealing Process
In the cold rolling production of stainless steel, the annealing process is a critical step designed to restore the material’s mechanical properties and improve its ductility after cold working. To prevent surface oxidation during annealing, a protective atmosphere is typically used, and ammonia decomposition is often involved in this process.
(1) Use of Ammonia Decomposition Gas as a Protective Atmosphere
Ammonia decomposition gas (also known as “cracked ammonia”) is commonly used as a protective atmosphere in annealing furnaces. It is generated by decomposing ammonia (NH₃) into hydrogen (H₂) and nitrogen (N₂) as follows:
2NH₃ → N₂ + 3H₂
- This gas mixture offers two key benefits:
Nitrogen (N₂): Provides an inert environment to prevent oxidation. - Hydrogen (H₂): Acts as a reducing agent, helping to remove surface oxides.
However, if the ammonia decomposition process is incomplete (due to suboptimal temperature control or inefficient catalysts), residual ammonia gas (NH₃) may remain in the furnace. This residual ammonia can react with the steel surface, damaging the passive layer (Cr₂O₃) that protects 304 stainless steel from corrosion.
(2) Incomplete Cleaning After Annealing
If residual ammonia or its byproducts (such as ammonium ions, NH₄⁺) are not thoroughly cleaned from the steel surface after annealing, they can cause localized corrosion when exposed to high humidity or certain environmental conditions. This can lead to rusting, even in what would normally be considered a benign environment for 304 stainless steel.
2.Other Potential Sources of Ammonia in Cold Rolling
Ammonia or ammonia-derived compounds may also be introduced during other stages of the cold rolling process, such as:
- Lubricants or Emulsions: Certain rolling oils or lubricants may contain nitrogen-based compounds or ammonia derivatives. During annealing at high temperatures, these compounds can release ammonia gas, which then decomposes into hydrogen and nitrogen. This can lead to similar issues as described above.
- Acid Pickling Residue: If acid cleaning or pickling stages leave behind residues containing nitrogen compounds, these may decompose under heat, contributing to ammonia gas production and subsequent surface corrosion.
3.Key Issues Caused by Ammonia Decomposition
When ammonia decomposition occurs during the cold rolling process, it may lead to the following issues with 304 stainless steel:
- Damage to the Passive Layer: Hydrogen produced by ammonia decomposition can weaken the passive layer on stainless steel, leaving it vulnerable to rusting and other forms of corrosion.
- Localized Corrosion: Ammonia or its byproducts (e.g., NH₄⁺) can form a corrosive medium when combined with moisture or salts, leading to rust spots or discoloration.
- Inadequate Process Control: If the ammonia decomposition gas ratio, temperature, or flow rate is not carefully controlled, residual ammonia or incomplete decomposition products can exacerbate corrosion risks.
4.Preventive Measures
To reduce the likelihood of rusting caused by ammonia decomposition during production, the following steps can be taken:
- Optimize Protective Atmosphere: Ensure complete ammonia decomposition by using the appropriate catalysts and maintaining optimal temperature control in the annealing furnace.
- Thorough Cleaning: After annealing, thoroughly clean the stainless steel surface to remove any residual ammonia or byproducts.
- Improve Lubricants: Use rolling oils or lubricants that are free of nitrogen-based compounds to minimize ammonia generation during processing.
- Monitor Furnace Atmosphere: Employ real-time monitoring equipment to check hydrogen, nitrogen, and residual ammonia levels in the furnace atmosphere to ensure the protective gas remains effective and non-corrosive.
By addressing these factors, the risk of rust formation on 304 stainless steel caused by ammonia decomposition can be significantly minimized, ensuring the material performs reliably in its intended applications.
