Materials of Construction - High Silicon Cast Iron
September 29, 2001

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.

Corrosion Resistance

Specification

Specification of silicon cast iron is covered by ASTM A518 – Standard Specification of Corrosion-Resistant High-Silicon Iron Castings.  The standard describes three grades of silicon cast iron.

All grades are suitable for application in severe corrosive environments.  Grade 1 is suitable for sulphuric acid service.  Grade 2 is particularly suited for strong chloride service.  Grade 3 is recommended for impressed current electrodes.