Corrosion Resistance in Various Environments

Cast irons find many applications in various environments, and are selected based upon the anticipated chemicals present. Unalloyed and low-alloy cast irons corrosion rates are increased by exposure to sulfur dioxide and similar industrial type atmospheres. They are also readily attacked by chlorides, typical in marine environments. In soils, increased rates of corrosion can be expected in poorly drained areas and where corrosive chemicals are present. The addition of ~3% nickel has been used to increase corrosion resistance of cast irons in poorly drained soils. The corrosion of unalloyed cast irons in water is lower for hard water conditions, where a protective scale of calcium carbonate will develop. Protective scales do not develop well in soft and deionized waters for unalloyed cast irons, resulting in some expected corrosion. Lower pH levels will increase the rate of attack while higher pH levels reduce corrosive effects. High alloy cast irons are not typically used in these environments as their increased cost versus performance does not warrant their use. High-nickel austenitic cast irons have been used for their resistance to pitting in calm seawater conditions. High-silicon cast irons have been used for anodic protection in seawater and brackish water environments.

Cast irons also find applications in many of the common acid and alkali solutions used. They are generally attacked more in mineral acids than the organic acids. The cast irons find uses for varying concentration levels and temperatures, but impurities present can severely degrade their corrosion resistance. The resistance to mineral acids is summarized in Table 45. Unalloyed and low-alloyed cast irons have fair resistance to alkali solutions, but should be kept below 80ºC and 70% concentrations and are highly susceptible to hot solutions of ≥30% concentration levels. The addition of 3-5% nickel increases corrosion resistance to alkali solutions. The high-silicon cast irons have generally the same corrosion resistance as the unalloyed cast irons. They may be used only when impurities are present which reduce the resistance of the unalloyed irons. Highchromium cast irons are more susceptible to alkali solutions and are therefore not recommended.