Why Nickel based super alloys are recommended over other materials

With their outstanding corrosion resistance and supreme mechanical characteristics, nickel based alloys are popularly used in a broad range of applications in the diverse industries such as chemical, petrochemical processing, pollution control, oil and gas extraction, marine engineering, power production and pulp and paper production. Their versatility and reliability make them the special materials for construction of process vessels, pipes, pumps, valves and various other components designed for operate in aqueous and high temperature conditions.

Different factors influence the functionality of specific material in a specific condition such as:

·         Concentration

·         Temperature

·         Aeration

·         Liquid or gaseous flow rates

·         Contaminants

·         Abrasives

·         Cycling Process

Metallic corrosion in various aqueous conditions can occur by various mechanism offering different results. Commonly occurring corrosion problems are:

a.       General Corrosion

b.      Localized corrosion

c.       Pitting

d.      Crevice Corrosion

e.      Microbially influenced corrosion

f.        Environmental assisted cracking

g.       Corrosion fatigue

h.      Stress corrosion cracking

i.         Liquid Metal Cracking

j.        Erosion Corrosion

k.       Galvanic Corrosion

l.         Hydrogen Embrittlement

m.    Intergranular corrosion

n.      Dealloying

The solution for different corrosion problems is nickel and nickel based alloys:

Nickel is an austenitic, face centered cubic crystal metal that is more noble than iron however more active than copper. Corrosionresistance by Hastelloy C276 bar in reducing ad nominally oxidizing conditions is excellent. It also resists localized attack and stress corrosion cracking. It is widely used in chemical and process plants and for vigorous conditions in the pollution control industry. It is also available in rod, bar, tubing and wire forms.

Other corrosion resistant alloys are: Nickel 200, 201, Monel 400, Inconel 600, Inconel 718, Inconel 625, Incoloy 800 and Incoloy 825.

Corrosion by Acids

Acids can be oxidizing or reducing in nature. Metals that are resistant to reducing acids do not perform well against oxidizing acids. By using nickel base alloys it is possible to resist corrosion in both oxidizing and reducing media. The selection of an alloy for a specific alloy will be based on acid or mixture of acids available, concentration, temperature, aeration, contaminants, flow properties, availability and tightness of crevices, various materials in the system and several other environmental conditions.

Monel alloy 400 is used in handling sulphuric acid solutions in reducing conditions. In air-free solutions the rate is nominal at all temperature limits. Similar to nickel, however to a lower level, aeration increases the corrosion rate for Monel alloy 400.

Nickel 200 can also be used in sulfuric acid solutions at low or moderate temperature. Corrosion rate increases with aeration specifically in dilute acids. In concentration acids, aeration reduces corrosion rates, feasibly due to the development of passive layer however Nickel 200 is rarely used in this process because better resistant materials are available.

Inconel alloy 600 superior performance in sulfuric acid conditions as compare to Nickel 200 or Monel 400. It offers suitable resistance to corrosion by cold, non-aerated sulfuric acid solutions to 70% concentration however resistance is not equivalent to Monel 400.