NDA (II) 2023 General Ability Test · Question 91 of 146

Correct — D, Tin. Tin is not an alloying element of stainless steel at all. The standard lists of what goes into stainless grades run through chromium, nickel, molybdenum, nitrogen, carbon, manganese, silicon, vanadium, boron, cobalt, niobium and aluminium — tin does not appear among them. Tin's place in metallurgy is elsewhere: it is the partner of copper in bronze and of lead in soft solder, and it is the coating on a tin-plated steel food can. The other three options are all genuine members of the stainless family, so the odd one out is unambiguous.

• (a)Iron — Iron is the base metal of every steel. Take the iron away and there is no steel left to make stainless, so it cannot be the element that is not essential.
• (b)Nickel — Nickel is a real stainless steel component — it is the second addition in type 304, the austenitic grade every textbook cites as '18/8', meaning about eighteen per cent chromium and eight per cent nickel. It is worth knowing that nickel is not in every stainless steel, since ferritic grades carry very little or none, but it belongs to the alloy family in a way tin never does.
• (c)Chromium — Chromium is the one element a steel cannot do without if it is to be stainless. A chromium content of 10.5 per cent or more is what forms the thin passive oxide film that stops rusting, and that threshold is the working definition of stainless steel.

Stainless steel is iron alloyed with chromium, and it resists corrosion not because chromium is unreactive but because it is very reactive — chromium at the surface oxidises instantly into an extremely thin, tightly bound and self-repairing oxide film that seals the metal underneath. That passivation begins to work reliably once the chromium content reaches about 10.5 per cent. Nickel is then added in the austenitic grades to change the crystal structure, improving toughness and formability.

Strictly speaking, only chromium is indispensable to every stainless steel, so a very demanding reading could argue that nickel is not essential either — ferritic and plain chromium-carbon martensitic grades manage with almost none. The examiner is not testing that nuance. The item is built on the school-level statement that stainless steel is an alloy of iron with chromium and nickel, and it asks which of four metals falls outside that description altogether. Tin does, in every grade and on every reading, which is why the key marks (d) without controversy. The useful habit for exams is to keep a short table of alloys and their components, because the same four or five alloys are asked in rotation.

• Stainless steel resists corrosion because of a chromium content of 10.5 per cent or more, which forms a protective passive film.
• Type 304, the commonest austenitic stainless steel, is the '18/8' chromium-nickel grade.
• Ferritic stainless steels contain very little or no nickel, and plain iron-chromium-carbon martensitic grades contain essentially none.
• Tin is not listed among the alloying elements of stainless steel; it belongs instead to bronze, to soft solder and to tin-plate.

• Mixing up tin and nickel because both appear in alloy questions; tin goes with copper in bronze and with lead in solder.
• Treating iron as an 'additive' rather than the base metal of the alloy.
• Assuming every stainless steel contains nickel — many grades do not, although chromium is always there.

Asked as a negative-form composition item — identify the metal that has no place in the named alloy.

• practice — not a real PYQ

  The corrosion resistance of stainless steel is primarily due to the presence of

  • (a)nickel
  • (b)chromium
  • (c)manganese
  • (d)carbon

  Answer(b) chromium — at 10.5 per cent or more it forms a thin, self-repairing passive oxide film that seals the surface.

• practice — not a real PYQ

  Which one of the following alloys contains tin?

  • (a)Brass
  • (b)Bronze
  • (c)Stainless steel
  • (d)Duralumin

  Answer(b) Bronze — it is an alloy of copper and tin, whereas brass is copper and zinc and duralumin is aluminium based.