API 571 Certification (Corrosion & Materials) Practice Test

Which type of cast iron is not susceptible to graphitic corrosion?

1. Gray Cast Iron

2. Ductile Cast Iron

3. Malleable Cast Iron

4. White Iron

The correct answer is: White Iron

Ductile cast iron, also known as nodular cast iron, has a microstructure that includes spherical graphite inclusions. This structure provides greater toughness and resistance to impact compared to gray cast iron, which has flake-shaped graphite that can create stress concentration points, making it more susceptible to graphitic corrosion. Malleable cast iron undergoes heat treatment that transforms its structure, giving it improved ductility over gray cast iron, but it can still be affected by graphitic corrosion under certain conditions. Gray cast iron is particularly vulnerable to this type of corrosion due to its flake-like graphite, which can separate from the iron matrix more easily when exposed to corrosive environments. White iron, on the other hand, has very little to no graphite as it retains its carbide structure. This characteristic makes it significantly less prone to graphitic corrosion, as there is minimal free graphite that could corrode. Therefore, white iron's structure largely contributes to its resistance in this aspect.