Corrosion resistance of High performance metals alloys in Acetic Acid
Acetic acid is produced in large magnitudes. It is one of the most used organic acids and is widely occurred as contaminant in the process of other chemicals. Acetic acid is also used as a solvent in some organic reactions. So, it is important to understand its corrosive behaviour.
Acetic acid is categorized as a weak acid however the suitable acidity in aqueous streams increases quickly with increase in concentration.
Steels are steeply corroded by all concentrations of acetic acid, in fact at room temperature also. Glacial acetic acid at room temperature is less aggressive than aqueous solutions of the acid however it still attacks at the rate of 0.8 – 1.3 mm/yr. So the use of steel is quite inappropriate in acetic acid applications.
Aluminum offers supreme resistance to almost all concentrations of acetic acid at room temperatures and is widely used for storage and transportation. It provides good performance in acid concentrations of 97% to 99% up to the boiling temperatures, however it receives sharp attack at 100% or in presence of excess (CH3CO)2O. Aluminum is also resistant to pure(CH3CO)2O, it results into contamination of the anhydride due to the development of a white crystalline solid called as Aluminum triacetate that quickly precipitates in the liquid.
In the presence of contaminants, the performance of aluminium in acetic acid solutions is widely affected.
Copper and its alloys show supreme performance in all concentrations of acetic acid up to and in fact above the boiling point if the oxygen or other oxidizing are absent. Copper was also widely used to handle acetic acid however it is now replaced with stainless steel 316 and other high alloys.
Copper is almost resistant to corrosion in pure acetic acid, but a nominal contamination with air during storage under air or entry of air can cause a rapid corrosion of copper components. Inclusion of nickel to copper modifies the effect of oxidants.
Stainless steel types 304 is the lowest grade commonly used in acetic acid solution. This grade is widely used in dilute acetic acid and in the transport and storage of concentrated acetic acid. It is found that glacial acetic acid can be perfectly handled by stainless steel grade 304 up to 80oC temperature. This steel grade can also be used for lower concentrations up to the boiling temperature of acid. At higher temperatures, it is recommended to use 304L for welded structure to prevent intergranular corrosion of the heat affected regions.
Stainless steel grade 31 is also widely used in acetic acid processing systems. It prevents corrosion by glacial acid to boiling temperatures. When acetic acid is completely anhydrous or contains small magnitudes of (CH3CO)2O, the corrosion rate of stainless steel type 316 sharply increases.
On the other hand uses of Hastelloy C276 in oxidizing acetic acid solutions is commonly recommended as this alloy provides supreme performance at all concentrations and normal temperature limits.