Dental materials and radiology basics: sensitisation of stainless steel: practice question

A Dentistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Dentistry · Dental materials and radiology basics: sensitisation of stainless steel

Sample item

A stainless steel orthodontic wire is heated to about 650 degrees C during soldering and later shows corrosion along its grain boundaries. What caused this?

AChromium carbide forming at the grain boundariesBWork hardening that produced a brittle martensitic skinCNickel leaching out from the wire surface into the salivaDGalvanic coupling between the steel and the gold solder

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. Chromium carbide forming at the grain boundaries

Between about 400 and 900 degrees C, carbon in stainless steel combines with chromium at the grain boundaries to form chromium carbide. The adjacent zone loses chromium and its passive layer, so the wire is sensitised and prone to intergranular corrosion and weakness. Stabilised steels and brief heating minimise this.

Option by option

A. Chromium carbide forming at the grain boundaries (correct)
Correct. Carbide precipitation (sensitisation) strips chromium from the boundary zone.
B. Work hardening that produced a brittle martensitic skin
Work hardening raises strength and does not by itself cause intergranular attack.
C. Nickel leaching out from the wire surface into the saliva
Nickel release is a surface process that does not preferentially attack grain boundaries.
D. Galvanic coupling between the steel and the gold solder
Galvanic corrosion occurs at the junction of dissimilar metals, not along internal grain boundaries.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Dentistry question

What is the answer to this Dentistry question?

The best answer is A, Chromium carbide forming at the grain boundaries. Between about 400 and 900 degrees C, carbon in stainless steel combines with chromium at the grain boundaries to form chromium carbide.

Why is "Work hardening that produced a brittle martensitic skin" not the answer?

Work hardening raises strength and does not by itself cause intergranular attack.

Why is "Nickel leaching out from the wire surface into the saliva" not the answer?

Nickel release is a surface process that does not preferentially attack grain boundaries.

Why is "Galvanic coupling between the steel and the gold solder" not the answer?

Galvanic corrosion occurs at the junction of dissimilar metals, not along internal grain boundaries.

What concept does this question test?

It tests Dental materials and radiology basics: sensitisation of stainless steel in Dentistry. The adjacent zone loses chromium and its passive layer, so the wire is sensitised and prone to intergranular corrosion and weakness. Stabilised steels and brief heating minimise this.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Dental materials and radiology basics: sensitisation of stainless steel?

Learn the dental formula, eruption ages and tooth structure once, as tables. Then use the flashcards and the other Dentistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.