In corrosion and passivation studies, in addition to concentrated nitric acid, which other acid can render iron passive by forming a protective oxide layer on its surface?

Introduction / Context:
Passivation is a phenomenon where a metal becomes less reactive by forming a thin protective oxide layer on its surface. Iron, which normally rusts and corrodes, can become passive in the presence of certain strong oxidising acids. Concentrated nitric acid is a classic example used in textbooks. This question asks you to identify another acid, besides nitric acid, that can make iron passive by forming such a protective film.

Given Data / Assumptions:

• The metal considered is iron, which is normally prone to rusting.
• The concept is passivation by acid treatment.
• Options include acetic acid, oxalic acid, chromic acid, and citric acid.
• We focus on strong oxidising behaviour that can create a protective oxide film.

Concept / Approach:
Passivating acids are typically strong oxidising agents that react with the metal surface to produce a tightly adherent, protective oxide layer. Concentrated nitric acid is one such acid, and chromic acid, which contains hexavalent chromium species, is another strong oxidising acid. Both can form passive films on iron, reducing its further reaction. Acetic, oxalic, and citric acids are weaker organic acids and do not generally exhibit the same strong oxidising ability needed for passivation of iron. Therefore, chromic acid is the correct choice among the options.

Step-by-Step Solution:
Step 1: Recall that passivation of iron involves forming a protective oxide film using an oxidising environment. Step 2: Recognise that concentrated nitric acid is a strong oxidising acid and can render iron passive. Step 3: Identify chromic acid as another strong oxidising acid, containing chromium in a high oxidation state. Step 4: Compare with organic acids like acetic, oxalic, and citric acid, which are weak acids without strong oxidising power. Step 5: Conclude that chromic acid is the acid, besides nitric acid, that can render iron passive and select it as the answer.

Verification / Alternative check:
Corrosion and surface treatment references mention that iron and some other metals can become passive in strong oxidising environments, such as in concentrated nitric acid or chromic acid solutions. Chromic acid is used in certain metal finishing processes to create protective films, though its use is now restricted in many places due to toxicity concerns. Organic acids like acetic and citric are more commonly used for cleaning or descaling and lack the strong oxidising properties needed for passivation. This comparison confirms chromic acid as the correct answer.

Why Other Options Are Wrong:
Acetic acid is a weak organic acid found in vinegar and is not a strong oxidising agent; it does not typically passivate iron. Oxalic acid, while capable of forming complexes and sometimes used for cleaning, does not have the strong oxidising ability required to create a stable passive film on iron. Citric acid is another weak organic acid used in food and cleaning applications, again lacking significant oxidising strength. These acids may clean or etch the surface but do not cause true passivation like chromic acid does.

Common Pitfalls:
Students may choose an acid simply because it is familiar or often mentioned in everyday life, such as acetic or citric acid, without considering its oxidising power. Another pitfall is not remembering that passivation by acids usually involves strong oxidisers rather than weak organic acids. To avoid these mistakes, associate passivation of iron with concentrated nitric acid and chromic acid, both of which can form protective oxide layers under appropriate conditions. This memory aid helps in answering related exam questions correctly.

Final Answer:
In addition to nitric acid, chromic acid can render iron passive by forming a protective oxide layer on its surface.