Sulfuric acid corrosion resistance of high silicon cast iron

Sulfuric acid corrosion resistance of high silicon cast iron

1.Introduction

      Standard ductile cast irons contain approximately 1.8 to 2.8% silicon, while modified ductile cast irons such as Mondi™ contain 3.5% silicon.  Silicon cast iron contains 14.20 to 14.75% silicon.

Silicon cast iron has excellent resistance to sulphuric acid in all concentrations to 100% up to the atmospheric boiling point.  Corrosion rates of less than 5 mpy (0.13 mm/y) are typical except in a region of hot acid from 5% to 55% H2SO4 where corrosion rates may be as high as 20 mpy (0.51 mm/y).  Above 100% sulphuric acid, silicon cast iron is rapidly attacked by free SO3. One disadvantage of silicon cast iron is susceptible to thermal and mechanical shock.

 2.Corrosion Curve

3.Result 

     The typical composition of high silicon iron is 0.95% carbon, 14.5% silicon and the rest is iron. From the composition, it seems to belong to steel, from the structure and properties, it is similar to cast iron, so it is usually called high silicon cast iron. It does not contain valuable alloy elements such as chromium and nickel, and has good corrosion resistance. In sulfuric acid, it has better corrosion resistance than all other engineering alloys in the concentration of 0 ~ 100%, so it is widely used in sulfuric acid medium. However, high silicon cast iron is hard and brittle, difficult to process and can only be cast.

     It is very sensitive to temperature change in use, and it will crack and damage in extreme cold and heat. It is also easy to crack when it is subjected to mechanical vibration or impact. Therefore, special care must be taken in manufacturing, installation and use. However, because it is hard, so it is very wear-resistant, especially suitable for use in sulfuric acid containing solid suspended solids or solid impurities.
       In recent years, with the addition of rare earth elements, the brittleness of high silicon cast iron has been improved, and pumps, valves, sulphuric acid concentrating heaters, sprayers, small containers, tanks, outlets, pipes, electrodes, bubble caps, other parts of the tower and other fittings have been manufactured.
       High silicon cast iron has good corrosion resistance in sulfuric acid up to boiling temperature. However, high silicon cast iron is not suitable for use as common gray cast iron in fuming sulfuric acid with concentration more than 100%. In addition, when the sulfuric acid contains sulfur dioxide and fluoride, the corrosion rate of high silicon cast iron in sulfuric acid will increase significantly. High silicon cast iron can be used at 540 ℃, but slow heating and cooling, shock and vibration must be avoided.