Question

Which of the following steels will have less corrosion rate in boiling 65% nitric acid?

(a) 4.5% chromium steel

(b) 8% chromium and 8% nickel steel

(c) 18% chromium and 8% nickel stainless steel

(d) 18% chromium stainless steel

I have been asked this question in examination.

The above asked question is from Mineral Acids topic in portion Mineral Acids of Corrosion Engineering

Answer 1

The correct answer is:

(c) 18% chromium and 8% nickel stainless steel

Explanation:

In boiling 65% nitric acid, the corrosion resistance of steel is highly influenced by the composition of the steel, especially its chromium and nickel content.

• Chromium is a critical element for corrosion resistance, especially in acidic environments, because it forms a passive oxide layer on the surface of the steel, which helps protect it from further corrosion.
• Nickel enhances the overall corrosion resistance, especially in more aggressive environments like nitric acid, by stabilizing the oxide layer and improving the material's ability to resist pitting and stress corrosion cracking.

Among the options:

• 18% chromium and 8% nickel stainless steel (option c) is highly resistant to nitric acid due to its high chromium and nickel content, making it the best choice for this environment.
• The other steels listed either have lower chromium or do not have the combination of chromium and nickel that provides optimal resistance to nitric acid.