Because the highest concentration of hydrochloric acid will not exceed thirty percent, which contains a large amount of water, so that the product after the reaction (ferrous chloride) dissolved in water without preventing the reaction to continue.
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The passivation phenomenon of iron and concentrated sulfuric acid (anhydrous sulfuric acid) is because iron and concentrated sulfuric acid will begin to react for a short time, the product is ferrous sulfate, because there is no water in the reactant, and ferrous sulfate is insoluble in sulfuric acid, so ferrous sulfate is covered in the surface of iron, thereby preventing the reaction from continuing.
Hydrochloric acid does not passivate iron because hydrochloric acid is a strong acid that reacts with iron to produce ferric chloride, which is a soluble salt that continues to react with water to form iron and chloride ions. These ions further promote the corrosion of the iron rather than forming a passivation layer. Passivation refers to the formation of a dense oxide layer on the surface of the metal, which prevents further corrosion. Therefore, hydrochloric acid cannot passivate iron.