Ammonia is an important raw material for the manufacture of nitric acid, ammonium salt and amine. Ammonia is gas at room temperature and can be liquefied under pressure. Most metals such as stainless steel, aluminum, magnesium, titanium, etc. have excellent corrosion resistance to ammonia gas, liquid ammonia and ammonia water, except copper and other copper alloys.
Copper – Zinc alloys including navy brass and aluminum brass are copper alloys that most susceptible to ammonia-induced stress corrosion cracking (NH3SCC). Ammonia stress corrosion cracking in copper alloy heat exchanger tubes is characterized by surface cracking, green/light blue Cu-Ammonia-corrosion complexes (compounds) and the formation of a single or highly branched crack on the tube surface, which can be transgranular or intergranular, which depending on the environment and stress levels. Liquid ammonia stress corrosion is formed when the medium simultaneously meets the following conditions:
- Occasions where liquid ammonia (water content no more than 0.2%) is likely to be polluted by air (oxygen or carbon dioxide);
- The operating temperature is higher than -5℃.
In fact, oxygen and other oxidants such as water are important conditions for stress corrosion of copper. There is a lot of potential corrosion in petroleum refining due to impurities in the original and additives in the process of processing. The types of ammonia-induced cracking corrosion including:
This is mainly determined by the concentration, flow rate and properties of the medium. The higher the concentration of NH3 and H2S, the more serious the corrosion; The higher the flow rate of the fluid in the tube, the stronger the corrosion. The low flow rate leads to ammonium salt deposition and local corrosion; Some media, such as cyanide, aggravate the corrosion, and oxygen (which enters with the injected water) accelerates the corrosion.
Ammonia corrosion of sulfuric acid alkylation tower top
In order to control the excessive corrosion of the column top system in the fractionation section, alkaline washing and washing reactor products are very important to remove acidic impurities. Precedents of neutralizing and film-forming amine inhibitors have sometimes been used in tower top systems. To reduce the corrosion rate and minimize the amount of inhibitor used, neutralizing amines or NH3 can neutralize the tower topwater condensate to a pH of 6 to 7. However, in some cases, NH3 can cause stress corrosion cracking of navy brass tubes in overhead condensers.
Ammonia corrosion of catalytic reforming
There are several types of stress corrosion cracking in catalytic reforming units, one of which is ammonia-induced stress corrosion cracking. NH3 exists in the effluent of the pretreatment reactor and reforming reactor and is dissolved in water to form ammonia, causing rapid stress-corrosion cracking of the copper-based alloy.
Ammonia corrosion of delayed coking unit
The equipment of the delayed coking unit is susceptible to low-temperature corrosion mechanisms, including ammonia-induced stress cracking of copper-based alloy. These corrosion mechanisms play a role in the process of water quenching, steam coke cleaning and air venting. But since all coking towers usually have vent pipes and blowdown tanks, they are almost continuously exposed to wet vent steam and liquid.
Quench and vent vapors and liquids usually contain large amounts of H2S, NH3, NH4Cl, NH4HS, and cyanide, which are released from the thermal cracking reaction of the feed to the coking plant. Due to the presence of NH3 in the coking unit, ammonia-induced stress corrosion cracking occurs in copper alloy tubes at a high pH value.
Ammonia corrosion of sulfur recovery unit
Gas feeds are usually rich in H2S and saturated water vapor, and may also be mixed with hydrocarbons and amines, which can cause H to permeate the metal, so consider the risks of hydrogen-induced cracking (including hydrogen bulging) and sulfide stress cracking (SSC) in gas feeds. In addition, there may be NH3 in the gas feed, which can cause nh3-induced stress corrosion cracking, and cyanide can also accelerate the corrosion rate.
When the mass fraction of Zn is reduced to less than 15%, the corrosion resistance of Cu – Zn alloy is improved. The SCC in the steam environment can sometimes be controlled by means of preventing air from entering. The sensitivity of copper alloys is generally assessed by examining and monitoring the PH value of water samples and NH3. Eddy’s current inspection or visual inspection can be used to judge the cracking of the heat exchanger bundle. In short, copper and its alloys should be avoided in production processes involving ammonia and liquid ammonia.