Which one of the following is not true for anodizing process?
- (a)It makes aluminium corrosion resistant.
- (b)Metals like aluminium, titanium and magnesium can be anodized.
- (c)Clean aluminium article is the anode and oxygen gas is evolved at the cathode.
- (d)It is used in aircraft industry.
Correct — C, Clean aluminium article is the anode and oxygen gas is evolved at the cathode. This is the statement that is not true, and it is untrue by exactly one word. The first half is right — the article being treated is made the anode, which is where the process gets its name. The second half puts the oxygen at the wrong electrode. NCERT's Class 10 chemistry says it plainly: 'During anodising, a clean aluminium article is made the anode and is electrolysed with dilute sulphuric acid. The oxygen gas evolved at the anode reacts with aluminium to make a thicker protective oxide layer.' Oxygen appears at the anode because that is where oxidation happens and where negative ions give up electrons; the cathode, being the reduction electrode, gives off hydrogen. The whole point of the process would collapse if the oxygen came off at the far electrode, since it has to meet the aluminium surface in order to thicken the oxide film on it.
- (a)It makes aluminium corrosion resistant. — True, and it is the reason the process exists. Aluminium already grows a thin oxide skin in air; anodising makes that film thicker and more even, and NCERT lists anodising among the standard ways of preventing corrosion.
- (b)Metals like aluminium, titanium and magnesium can be anodized. — True. Aluminium alloys are much the commonest case, but titanium, magnesium, zinc, niobium, tantalum, zirconium and hafnium are all anodised industrially. Iron and carbon steel are the metals this does not work for.
- (d)It is used in aircraft industry. — True, and it is where the industry began. Anodising was first used on a commercial scale in 1923 to protect Duralumin seaplane parts from corrosion, and aerospace remains one of its main markets.
Anodising is electrolysis run on purpose to grow an oxide coat rather than to strip or plate metal. The aluminium article is wired as the anode in a bath of dilute sulfuric acid. Current drives oxidation at that surface, and the oxygen liberated there is taken up by the aluminium beneath it, so the natural oxide film thickens into a hard, adherent, porous layer. Because it is porous it takes dye readily, which is why anodised aluminium comes in colours; sealing afterwards closes the pores and locks in both the dye and the protection.
A 'which is not true' item rewards attacking the longest option first, because a compound statement gives the examiner two places to hide an error. Here three options are single clean claims and the third carries two clauses joined by 'and'. Read them separately: the article is the anode — yes; oxygen comes off at the cathode — no. The reasoning that settles it needs no memory at all. An anode is where oxidation occurs, oxygen is what oxidation delivers, and the coat has to form on the article, so anode and oxygen must be the same electrode. Candidates who half-remember electroplating are the ones caught, because in plating it is the cathode that gains the deposit.
- NCERT Class 10 Science: 'a clean aluminium article is made the anode and is electrolysed with dilute sulphuric acid. The oxygen gas evolved at the anode reacts with aluminium to make a thicker protective oxide layer.'
- Hydrogen, not oxygen, is given off at the cathode.
- The electrolyte in the standard process is dilute sulfuric acid.
- Aluminium, titanium, magnesium, zinc, niobium, tantalum, zirconium and hafnium can be anodised; iron and carbon steel cannot usefully be.
- The first industrial use was in 1923, on Duralumin seaplane components.
Anode and oxygen belong together — the coat has to grow on the article, so the gas has to be released there.
- Carrying electroplating habits across. In plating the article is the cathode; in anodising it is the anode.
- Assuming only aluminium can be anodised. Titanium and magnesium can be, and the colours on titanium jewellery come from exactly this.
- Thinking the coat is something deposited from the bath. It is grown out of the article's own metal.
As a 'which is not true' statement item on anodising, as a one-line question on what layer anodising deposits, or inside a broader corrosion-prevention question alongside galvanising.
Aluminium surfaces are often “anodized”. This means the deposition of a layer of
- (a) chromium oxide
- (b) aluminium oxide
- (c) nickel oxide
- (d) zinc oxide
Answer(b) aluminium oxide
The same process asked for its product rather than its mechanism. Knowing that the film is aluminium's own oxide is what tells you the oxygen has to be released at the aluminium electrode, which is the point this CDS item hangs on.
- practice — not a real PYQ
During the anodising of aluminium, the gas liberated at the cathode is
- (a)oxygen
- (b)hydrogen
- (c)chlorine
- (d)sulfur dioxide
Answer(b) hydrogen — oxygen is released at the anode, where it thickens the oxide film on the article, while reduction at the cathode gives off hydrogen.
- practice — not a real PYQ
Galvanisation protects iron by coating it with
- (a)tin
- (b)chromium
- (c)zinc
- (d)aluminium oxide
Answer(c) zinc — a thin zinc layer protects the steel underneath even after the coating is scratched, which is the contrast with an anodised oxide film.